perf improvements

Cargo.lock

@@ -315,6 +315,7 @@ dependencies = [
  "rust_decimal",
  "rust_decimal_macros",
  "rustc-hash",
+ "serde",
  "serde_json",
  "smallvec",
  "smol_str",

Cargo.toml

@@ -2,6 +2,9 @@
 name = "dexpr"
 version = "0.1.0"
 edition = "2021"
+description = "Embeddable expression evaluator and bytecode VM"
+license = "MIT"
+exclude = ["editor/", "wasm/", "docs/", ".vscode/", "benches/", "scripts/", "CLAUDE.md", "flamegraph.svg", "profile.json.gz", "gen.js", "*.dexpr", "*.txt", "src/main.rs"]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
@@ -35,6 +38,7 @@ serde_json = "1"
 
 [dev-dependencies]
 criterion = { version = "0.8.2", features = ["html_reports"] }
+serde = { version = "1", features = ["derive"] }
 
 [[bench]]
 name = "my_benchmark"

benches/my_benchmark.rs

@@ -1,21 +1,39 @@
 use criterion::{criterion_group, criterion_main, Criterion};
 use dexpr::{ast::value::Value, compiler::Compiler, parser, vm::VM};
+use indexmap::IndexMap;
+use rust_decimal::Decimal;
 use rust_decimal_macros::dec;
+use smol_str::SmolStr;
+use std::rc::Rc;
 
+/// Helper: compile source to bytecode
+fn compile(source: &str) -> Vec<u8> {
+  let ast = parser::program(source).unwrap();
+  let mut compiler = Compiler::new();
+  compiler.compile(ast).unwrap()
+}
 
+/// Helper: build a sample object with N fields
+fn sample_object(n: usize) -> Value {
+  let mut map = IndexMap::new();
+  for i in 0..n {
+    map.insert(SmolStr::new(format!("field{}", i)), Value::Number(Decimal::from(i)));
+  }
+  Value::Object(Rc::new(map))
+}
 
 pub fn criterion_benchmark(c: &mut Criterion) {
-  // 1. Parser Benchmark
+  // ── Existing benchmarks ──────────────────────────────────────────────
+
   c.bench_function("parser_long", |b| {
-    let input = include_str!("../src/bench_long.dexpr");
+    let input = include_str!("../examples/bench_long.dexpr");
     b.iter(|| {
       let _ = parser::program(input).unwrap();
     })
   });
 
-  // 2. Compiler Benchmark
   c.bench_function("compiler_long", |b| {
-    let input = include_str!("../src/bench_long.dexpr");
+    let input = include_str!("../examples/bench_long.dexpr");
     let ast = parser::program(input).unwrap();
     b.iter(|| {
       let mut compiler = Compiler::new();
@@ -23,11 +41,8 @@ pub fn criterion_benchmark(c: &mut Criterion) {
     })
   });
 
-  // 3. VM Benchmarks
-
-  // basic_long.dexpr benchmark
   c.bench_function("vm_basic_long", |b| {
-    let input = include_str!("../src/basic_long.dexpr");
+    let input = include_str!("../examples/basic_long.dexpr");
     let ast = parser::program(input).unwrap();
     let mut compiler = Compiler::new();
     let bytecode = compiler.compile(ast).unwrap();
@@ -38,9 +53,8 @@ pub fn criterion_benchmark(c: &mut Criterion) {
     })
   });
 
-  // Long code benchmark (using bench_long.dexpr)
   c.bench_function("vm_long", |b| {
-    let input = include_str!("../src/bench_long.dexpr");
+    let input = include_str!("../examples/bench_long.dexpr");
     let ast = parser::program(input).unwrap();
     let mut compiler = Compiler::new();
     let bytecode = compiler.compile(ast).unwrap();
@@ -50,12 +64,9 @@ pub fn criterion_benchmark(c: &mut Criterion) {
     })
   });
 
-  // Short code benchmark
   c.bench_function("vm_short", |b| {
     let input = "5.12 + test * 1.5";
-    let ast = parser::program(input).unwrap();
+    let bytecode = compile(input);
-    let mut compiler = Compiler::new();
-    let bytecode = compiler.compile(ast).unwrap();
     let test_val = dec!(100);
     b.iter(|| {
       let mut vm = VM::new(&bytecode);
@@ -63,6 +74,220 @@ pub fn criterion_benchmark(c: &mut Criterion) {
       let _ = vm.execute().unwrap();
     })
   });
+
+  // ── #1: Method dispatch clone overhead ───────────────────────────────
+
+  // Object method — clone entire IndexMap per call
+  c.bench_function("vm_object_method_keys", |b| {
+    let bytecode = compile("obj.keys()");
+    let obj = sample_object(20);
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      vm.set_global("obj", obj.clone());
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  c.bench_function("vm_object_method_length", |b| {
+    let bytecode = compile("obj.length()");
+    let obj = sample_object(20);
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      vm.set_global("obj", obj.clone());
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  c.bench_function("vm_object_method_contains", |b| {
+    let bytecode = compile(r#"obj.contains("field10")"#);
+    let obj = sample_object(20);
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      vm.set_global("obj", obj.clone());
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  // StringList method — clone entire Vec per call
+  c.bench_function("vm_strlist_method_length", |b| {
+    let bytecode = compile("items.length()");
+    let items = Value::StringList(Rc::new((0..50).map(|i| SmolStr::new(format!("item{}", i))).collect()));
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      vm.set_global("items", items.clone());
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  c.bench_function("vm_numlist_method_sum", |b| {
+    let bytecode = compile("nums.sum()");
+    let nums = Value::NumberList(Rc::new((0..100).map(Decimal::from).collect()));
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      vm.set_global("nums", nums.clone());
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  // String method — lighter clone (SmolStr)
+  c.bench_function("vm_string_method_upper", |b| {
+    let bytecode = compile(r#"s.upper()"#);
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      vm.set_global("s", Value::String(SmolStr::new("hello world this is a test string")));
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  // ── #2 & #3: Vec alloc in method/external calls ─────────────────────
+
+  c.bench_function("vm_method_call_with_args", |b| {
+    let bytecode = compile(r#"s.replace("hello", "world")"#);
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      vm.set_global("s", Value::String(SmolStr::new("hello world hello")));
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  c.bench_function("vm_external_fn_call", |b| {
+    let bytecode = compile("getRate(a, b)");
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      vm.set_global("a", Value::Number(dec!(10)));
+      vm.set_global("b", Value::Number(dec!(20)));
+      vm.register_function("getRate", |_args| Ok(Value::Number(dec!(34.5))));
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  // ── #4: Value enum size (cache pressure on register ops) ─────────────
+
+  c.bench_function("vm_arithmetic_chain", |b| {
+    // Pure arithmetic — measures register read/write cache performance
+    let bytecode = compile(
+      "a = 1.5\nb = 2.3\nc = a + b\nd = c * a\ne = d - b\nf = e / c\ng = f + d\ng * 2.0",
+    );
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  c.bench_function("vm_comparison_chain", |b| {
+    let bytecode = compile(
+      "a = 10\nb = 20\nc = a < b\nd = b >= a\ne = a == 10\nf = b != 15\nc && d && e && f",
+    );
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  // ── #5: SmolStr alloc per opcode (string table missing) ──────────────
+
+  c.bench_function("vm_global_read_heavy", |b| {
+    // Many LoadGlobal ops → SmolStr alloc per read (split to stay within register limit)
+    let bytecode = compile(
+      "r1 = x1 + x2 + x3\nr2 = x4 + x5 + x6\nr = r1 + r2\nr",
+    );
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      for i in 1..=6 {
+        vm.set_global(&format!("x{}", i), Value::Number(Decimal::from(i)));
+      }
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  c.bench_function("vm_global_write_heavy", |b| {
+    // Many StoreGlobal ops
+    let bytecode = compile(
+      "a = 1\nb = 2\nc = 3\nd = 4\ne = 5\nf = 6\nr = a + b + c\ns = d + e + f\nr + s",
+    );
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  c.bench_function("vm_property_access_chain", |b| {
+    // GetProperty → read_string per access
+    let bytecode = compile("obj.field0 + obj.field1 + obj.field2 + obj.field3 + obj.field4");
+    let obj = sample_object(10);
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      vm.set_global("obj", obj.clone());
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  // ── #8: String concat with format! ───────────────────────────────────
+
+  c.bench_function("vm_string_concat", |b| {
+    let bytecode = compile(
+      r#"a = "hello" + " " + "world"
+b = a + " " + "this"
+c = b + " " + "test"
+c"#,
+    );
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  c.bench_function("vm_string_number_coerce", |b| {
+    let bytecode = compile(
+      r#"a = "value: " + 42
+b = a + " and " + 3.14
+b"#,
+    );
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  // ── #6: Opcode dispatch (overall loop throughput) ────────────────────
+
+  c.bench_function("vm_opcode_throughput", |b| {
+    // Many simple ops to stress the dispatch loop
+    let bytecode = compile(
+      "a = 1\nb = 2\nc = a + b\nd = c * 2\ne = d - 1\nf = e / 3\n\
+       g = f + a\nh = g * b\ni = h - c\nj = i + d\n\
+       k = j * 2\nl = k - 1\nm = l + 3\nn = m / 2\n\
+       o = n + a\np = o * b\np",
+    );
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      let _ = vm.execute().unwrap();
+    })
+  });
+
+  // ── Combined: realistic expression with multiple issue areas ─────────
+
+  c.bench_function("vm_realistic_mixed", |b| {
+    let bytecode = compile(
+      r#"
+      price = 100.50
+      tax = 18
+      discount = 5.5
+      net = price * (1 + tax / 100) - discount
+      label = "Total: " + net
+      if net > 100 then
+        result = label + " (high)"
+      else
+        result = label + " (low)"
+      end
+      result
+      "#,
+    );
+    b.iter(|| {
+      let mut vm = VM::new(&bytecode);
+      let _ = vm.execute().unwrap();
+    })
+  });
 }
 
 criterion_group!(benches, criterion_benchmark);

docs/compiler.md

@@ -96,8 +96,9 @@ Parse ile birlikte pozisyon bilgisi de toplar ve `DebugInfo` üretir.
 2. Sağ operandı register'a derle
 3. Sonuç register'ı ayır
 4. Uygun opcode'u emit et (Add, Sub, Mul, vs.)
-5. **Özel durum:** String + String → `Concat` kullanılır
+5. Operand register'ları serbest bırak
-6. Operand register'ları serbest bırak
+
+> **Not:** String birleştirme derleme zamanında ayırt edilmez. `Op::Add` her zaman `OpCodeByte::Add` emit eder; string birleştirme ve otomatik tip dönüşümü VM tarafından çalışma zamanında (runtime) ele alınır.
 
 ### UnaryOp (Tekli Operasyon)
 1. Operandı register'a derle

docs/opcodes.md

@@ -85,6 +85,25 @@ Bytecode komut setini (instruction set) tanımlar. Her opcode bir `u8` değerine
 
 ---
 
+## Built-in Fonksiyon ID'leri
+
+`default_fn` modülü, built-in fonksiyonlar için sabit ID'ler tanımlar. `CallExternal` opcode'u bu ID'leri kullanarak built-in fonksiyonları çağırır.
+
+| Sabit | ID | Fonksiyon |
+|-------|----|-----------|
+| `ABS` | `1` | Mutlak değer |
+| `MIN` | `2` | Minimum değer |
+| `MAX` | `3` | Maksimum değer |
+| `FLOOR` | `4` | Aşağı yuvarlama |
+| `CEIL` | `5` | Yukarı yuvarlama |
+| `ROUND` | `6` | Yuvarlama |
+| `SQRT` | `7` | Karekök |
+| `LEN` | `8` | Uzunluk |
+| `TO_STRING` | `9` | String'e dönüştür |
+| `TO_NUMBER` | `10` | Sayıya dönüştür |
+
+---
+
 ## Hızlı Lookup Tablosu
 
 `LOOKUP[256]` statik dizisi, O(1) karmaşıklıkta byte-to-opcode dönüşümü sağlar. `from_byte(u8)` metodu bu tabloyu kullanır.

docs/vm.md

@@ -11,7 +11,9 @@ Register tabanlı sanal makine. Bytecode'u çalıştırır, 8 register ve global
 | Dosya | İçerik |
 |-------|--------|
 | `vm/mod.rs` | Modül export'ları |
-| `vm/vm.rs` | Ana VM implementasyonu |
+| `vm/vm.rs` | Ana VM implementasyonu (core çalıştırma döngüsü) |
+| `vm/methods.rs` | Metod dispatch (String, StringList, NumberList, Object metodları) |
+| `vm/builtins.rs` | Built-in fonksiyon implementasyonları (abs, min, max, floor, ceil, round, sqrt, len, toString, toNumber) |
 | `vm/error.rs` | Hata türleri (VMError) |
 | `vm/debug_info.rs` | Bytecode offset → kaynak konum eşleştirme |
 
@@ -120,6 +122,7 @@ struct VM<'a> {
 ### Aritmetik
 - **`binary_op(f, name)`** — Genel handler: iki operand register'ı oku, fonksiyonu uygula, sonucu kaydet
 - Sıfıra bölme kontrolü yapılır
+- **`Add` opcode:** Sayısal toplama yanında string birleştirmeyi de destekler. Otomatik tip dönüşümü (auto-coercion) yapılır: String+String, String+Number, Number+String, String+Boolean kombinasyonları birleştirme olarak çalışır
 - **`handle_neg()`** — Sadece Number tipinde tekli negatif
 
 ### Karşılaştırma
@@ -133,7 +136,7 @@ struct VM<'a> {
 - **`handle_jump_if_false()`** — Register `Boolean(false)` ise atla
 
 ### String, Nesne ve Metodlar
-- **`handle_concat()`** — İki String register'ını birleştir
+- **`handle_concat()`** — İki register'ı birleştir (karışık tip dönüşümü destekler: String, Number, Boolean otomatik olarak String'e dönüştürülür)
 - **`handle_get_property()`** — Object register'ından alan oku, alan yoksa `Null` döndür
 - **`handle_set_property()`** — Object register'ında alan değerini ayarla
 - **`handle_method_call()`** — Nesne register'ı, metod adı, argümanlar
@@ -153,6 +156,18 @@ struct VM<'a> {
 ### Built-in
 - **`handle_log()`** — Register değerini stdout'a yazdır
 - **`rand(min, max)`** — min ile max arasında rastgele tamsayı üret (varsayılan harici fonksiyon)
+- **`abs(n)`** — Mutlak değer
+- **`min(a, b, ...)`** — Verilen değerlerin minimumu
+- **`max(a, b, ...)`** — Verilen değerlerin maksimumu
+- **`floor(n)`** — Aşağı yuvarlama
+- **`ceil(n)`** — Yukarı yuvarlama
+- **`round(n[, places])`** — Yuvarlama (opsiyonel ondalık basamak sayısı)
+- **`sqrt(n)`** — Karekök
+- **`len(v)`** — Değerin uzunluğu (String, List, Object)
+- **`toString(v)`** — Değeri String'e dönüştür
+- **`toNumber(v)`** — Değeri Number'a dönüştür
+
+> **Not:** Built-in fonksiyon implementasyonları `vm/builtins.rs` dosyasında, sabit ID tanımları `src/opcodes.rs` içindeki `default_fn` modülündedir.
 
 ---
 

gen.js

@@ -1,74 +0,0 @@
-function randomBetween(min, max) {
-  min = Math.round(min);
-  max = Math.round(max);
-  return Math.floor(Math.random() * (max - min + 1) + min);
-}
-
-function shuffleArray(array) {
-  for (let i = array.length - 1; i > 0; i--) {
-    const j = Math.floor(Math.random() * (i + 1));
-    [array[i], array[j]] = [array[j], array[i]];
-  }
-}
-
-function generateRandomArithmeticExpression() {
-  const operators = ['+', '-', '*', '/']; // Array of arithmetic operators
-  const maxDecimalPlaces = 5;
-  const numOperands = randomBetween(10, 20);
-  const numParentheses = randomBetween(4, 8);
-
-  // Generate random numbers with two decimal places
-  let nums = [];
-  for (let i = 0; i < numOperands; i++) {
-    nums.push((Math.random() * 100).toFixed(randomBetween(1, maxDecimalPlaces)));
-  }
-
-  let res = [];
-
-  shuffleArray(nums);
-
-  let numRemainingOperands = nums.length;
-  let openParentheses = 0;
-
-  for (let i = 0; i < nums.length; i++) {
-    // Open a parenthesis if there are enough operands remaining
-    if (openParentheses < numParentheses && numRemainingOperands > 1 && Math.random() < 0.5) {
-      res.push('(');
-      openParentheses++;
-    }
-
-    res.push(nums[i]);
-
-    // Close a parenthesis if there are enough operands preceding it
-    if (openParentheses > 0 && numRemainingOperands > 2 && Math.random() < 0.5) {
-      res.push(')');
-      openParentheses--;
-    }
-
-    if (i < nums.length - 1) {
-      res.push(operators[randomBetween(0, 3)]);
-    }
-
-    numRemainingOperands--;
-  }
-
-  // Close any remaining open parentheses
-  while (openParentheses > 0) {
-    res.push(')');
-    openParentheses--;
-  }
-
-  return res.join('');
-}
-
-let res = '';
-
-for (let i = 0; i < 100; i++) {
-  let expr = generateRandomArithmeticExpression();
-  let val = eval(expr);
-
-  // res += `("${expr}", "${val}"),\n`;
-  res += `"${expr}",\n`;
-}
-
-console.log(res);

justfile

@@ -12,6 +12,12 @@ bench:
 run:
     cargo run --release
 
+# --- Publish ---
+
+# Publish to Gitea cargo registry
+publish:
+    cargo publish --registry gitea --allow-dirty
+
 # --- WASM ---
 
 # Build wasm package (web target)

src/ast/value.rs

@@ -2,6 +2,7 @@ use indexmap::IndexMap;
 use rust_decimal::Decimal;
 use smol_str::SmolStr;
 use std::fmt;
+use std::rc::Rc;
 
 /// Value type for the dExpr language
 #[derive(Debug, Clone, PartialEq, Default)]
@@ -11,9 +12,9 @@ pub enum Value {
   Number(Decimal),
   String(SmolStr),
   Boolean(bool),
-  NumberList(Vec<Decimal>),
+  NumberList(Rc<Vec<Decimal>>),
-  StringList(Vec<SmolStr>),
+  StringList(Rc<Vec<SmolStr>>),
-  Object(IndexMap<SmolStr, Value>),
+  Object(Rc<IndexMap<SmolStr, Value>>),
 }
 
 /// Type tag constants for serialization
@@ -120,7 +121,7 @@ impl Value {
         bytes.push((list.len() >> 8) as u8);
         bytes.push(list.len() as u8);
         // List items
-        for n in list {
+        for n in list.iter() {
           bytes.extend_from_slice(&n.serialize());
         }
       }
@@ -129,7 +130,7 @@ impl Value {
         bytes.push((list.len() >> 8) as u8);
         bytes.push(list.len() as u8);
         // List items
-        for s in list {
+        for s in list.iter() {
           // String length (2 bytes)
           bytes.push((s.len() >> 8) as u8);
           bytes.push(s.len() as u8);
@@ -142,7 +143,7 @@ impl Value {
         bytes.push((map.len() >> 8) as u8);
         bytes.push(map.len() as u8);
         // Entries: key (string) + value (recursive)
-        for (key, val) in map {
+        for (key, val) in map.iter() {
           bytes.push((key.len() >> 8) as u8);
           bytes.push(key.len() as u8);
           bytes.extend_from_slice(key.as_bytes());
@@ -209,19 +210,19 @@ impl From<SmolStr> for Value {
 
 impl From<Vec<Decimal>> for Value {
   fn from(v: Vec<Decimal>) -> Self {
-    Value::NumberList(v)
+    Value::NumberList(Rc::new(v))
   }
 }
 
 impl From<Vec<SmolStr>> for Value {
   fn from(v: Vec<SmolStr>) -> Self {
-    Value::StringList(v)
+    Value::StringList(Rc::new(v))
   }
 }
 
 impl From<IndexMap<SmolStr, Value>> for Value {
   fn from(m: IndexMap<SmolStr, Value>) -> Self {
-    Value::Object(m)
+    Value::Object(Rc::new(m))
   }
 }
 
@@ -292,7 +293,7 @@ impl Value {
           list.push(Decimal::deserialize(decimal_bytes));
         }
 
-        Ok((Value::NumberList(list), pos))
+        Ok((Value::NumberList(Rc::new(list)), pos))
       }
       TYPE_STRING_LIST => {
         if bytes.len() < pos + 2 {
@@ -321,7 +322,7 @@ impl Value {
           list.push(s.into());
         }
 
-        Ok((Value::StringList(list), pos))
+        Ok((Value::StringList(Rc::new(list)), pos))
       }
       TYPE_OBJECT => {
         if bytes.len() < pos + 2 {
@@ -354,7 +355,7 @@ impl Value {
           map.insert(key.into(), val);
         }
 
-        Ok((Value::Object(map), pos))
+        Ok((Value::Object(Rc::new(map)), pos))
       }
       _ => Err(format!("Unknown type tag: {}", type_tag)),
     }
@@ -407,7 +408,7 @@ impl Value {
       serde_json::Value::String(s) => Ok(Value::String(SmolStr::new(s))),
       serde_json::Value::Array(arr) => {
         if arr.is_empty() {
-          return Ok(Value::StringList(Vec::new()));
+          return Ok(Value::StringList(Rc::new(Vec::new())));
         }
         // Check if all elements are the same type
         let first = &arr[0];
@@ -418,12 +419,12 @@ impl Value {
               nums.push(n);
             }
           }
-          Ok(Value::NumberList(nums))
+          Ok(Value::NumberList(Rc::new(nums)))
         } else if first.is_string() && arr.iter().all(|v| v.is_string()) {
           let strings: Vec<SmolStr> = arr.iter()
             .filter_map(|v| v.as_str().map(SmolStr::new))
             .collect();
-          Ok(Value::StringList(strings))
+          Ok(Value::StringList(Rc::new(strings)))
         } else {
           Err("Arrays must contain all numbers or all strings".to_string())
         }
@@ -433,7 +434,7 @@ impl Value {
         for (k, v) in obj {
           map.insert(SmolStr::new(k), Self::from_json_value(v)?);
         }
-        Ok(Value::Object(map))
+        Ok(Value::Object(Rc::new(map)))
       }
     }
   }

src/bytecode.rs

@@ -126,9 +126,9 @@ impl<'a> BytecodeReader<'a> {
     self.read_byte()
   }
 
-  /// Read a string
+  /// Read a string as a borrowed slice (zero-copy from bytecode)
   #[inline(always)]
-  pub fn read_string(&mut self) -> Result<SmolStr, String> {
+  pub fn read_str(&mut self) -> Result<&'a str, String> {
     let length = self.read_u16()? as usize;
 
     if self.position + length > self.bytecode.len() {
@@ -139,7 +139,13 @@ impl<'a> BytecodeReader<'a> {
       .map_err(|_| "Invalid UTF-8 string".to_string())?;
     self.position += length;
 
-    Ok(s.into())
+    Ok(s)
+  }
+
+  /// Read a string as SmolStr (allocating — use read_str when possible)
+  #[inline(always)]
+  pub fn read_string(&mut self) -> Result<SmolStr, String> {
+    self.read_str().map(SmolStr::from)
   }
 
   /// Read a value

src/bytecode_dump.rs

@@ -223,6 +223,7 @@ pub fn disassemble_bytecode(bytecode: &[u8]) -> Vec<String> {
         Ok(reg) => format!("{:04x}: SetResult r{}", start_position, reg),
         Err(_) => format!("{:04x}: SetResult (truncated)", start_position),
       },
+      OpCodeByte::ClearResult => format!("{:04x}: ClearResult", start_position),
       OpCodeByte::End => format!("{:04x}: End", start_position),
     };
 

src/compiler.rs

@@ -170,6 +170,7 @@ impl Compiler {
     let expr_reg = self.compile_expr(expr)?;
     self.emit_store_global(name, expr_reg);
     self.free_register(expr_reg);
+    self.emit_byte(OpCodeByte::ClearResult.to_byte());
     Ok(())
   }
 
@@ -229,6 +230,7 @@ impl Compiler {
     // Store root back to global
     self.emit_store_global(root, root_reg);
     self.free_register(root_reg);
+    self.emit_byte(OpCodeByte::ClearResult.to_byte());
     Ok(())
   }
 
@@ -319,13 +321,7 @@ impl Compiler {
     let result_reg = self.allocate_register()?;
 
     let opcode = match op {
-      Op::Add => {
+      Op::Add => OpCodeByte::Add,
-        if self.is_string_concatenation(left, right) {
-          OpCodeByte::Concat
-        } else {
-          OpCodeByte::Add
-        }
-      }
       Op::Sub => OpCodeByte::Sub,
       Op::Mul => OpCodeByte::Mul,
       Op::Div => OpCodeByte::Div,
@@ -359,12 +355,6 @@ impl Compiler {
     Ok(result_reg)
   }
 
-  /// Check if binary operation is string concatenation
-  fn is_string_concatenation(&self, left: &Expr, right: &Expr) -> bool {
-    matches!(left, Expr::Value(Value::String(_)))
-      || matches!(right, Expr::Value(Value::String(_)))
-  }
-
   /// Compile a unary operation
   fn compile_unary_op(&mut self, op: &Op, operand: &Expr) -> Result<u8, CompileError> {
     let operand_reg = self.compile_expr(operand)?;

src/language_info.rs

@@ -11,6 +11,7 @@
 //! use indexmap::IndexMap;
 //! use smol_str::SmolStr;
 //! use rust_decimal_macros::dec;
+//! use std::rc::Rc;
 //!
 //! let mut info = LanguageInfo::builtin();
 //!
@@ -24,7 +25,7 @@
 //! let mut customer = IndexMap::new();
 //! customer.insert(SmolStr::new("name"), Value::String("Alice".into()));
 //! customer.insert(SmolStr::new("age"), Value::Number(dec!(30)));
-//! info.add_value("customer", &Value::Object(customer), None);
+//! info.add_value("customer", &Value::Object(Rc::new(customer)), None);
 //! info.add_value("price", &Value::Number(dec!(100)), None);
 //!
 //! let json = info.to_json();

src/main.rs

@@ -2,7 +2,7 @@ use dexpr::{ast::value::Value, compiler::Compiler, parser, vm::VM};
 use rust_decimal_macros::dec;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-  let input = include_str!("basic_long.dexpr");
+  let input = include_str!("../examples/basic_long.dexpr");
 
   let ast = parser::program(input)?;
 

src/opcodes.rs

@@ -5,10 +5,31 @@ pub struct Register(pub u8);
 /// Default (built-in) function IDs for CallDefault opcode
 pub mod default_fn {
   pub const RAND: u8 = 0;
-  // Future: ABS = 1, MIN = 2, MAX = 3, FLOOR = 4, CEIL = 5, ROUND = 6, ...
+  pub const ABS: u8 = 1;
+  pub const MIN: u8 = 2;
+  pub const MAX: u8 = 3;
+  pub const FLOOR: u8 = 4;
+  pub const CEIL: u8 = 5;
+  pub const ROUND: u8 = 6;
+  pub const SQRT: u8 = 7;
+  pub const LEN: u8 = 8;
+  pub const TO_STRING: u8 = 9;
+  pub const TO_NUMBER: u8 = 10;
 
-  /// Lookup table: function name → ID
+  /// Lookup table: function name <EFBFBD><EFBFBD><EFBFBD> ID
-  pub const NAMES: &[(&str, u8)] = &[("rand", RAND)];
+  pub const NAMES: &[(&str, u8)] = &[
+    ("rand", RAND),
+    ("abs", ABS),
+    ("min", MIN),
+    ("max", MAX),
+    ("floor", FLOOR),
+    ("ceil", CEIL),
+    ("round", ROUND),
+    ("sqrt", SQRT),
+    ("len", LEN),
+    ("toString", TO_STRING),
+    ("toNumber", TO_NUMBER),
+  ];
 
   /// Get function name by ID
   pub fn name(id: u8) -> Option<&'static str> {
@@ -77,7 +98,8 @@ pub enum OpCodeByte {
   CallDefault = 0xA2,  // Call default (built-in) function by ID
 
   // Result
-  SetResult = 0xB0, // Set expression result (for return value)
+  SetResult = 0xB0,   // Set expression result (for return value)
+  ClearResult = 0xB1, // Clear expression result (assignment resets last result)
 
   // End marker
   End = 0xFF, // End of program
@@ -128,6 +150,7 @@ impl OpCodeByte {
         0xA1 => Some(OpCodeByte::CallExternal),
         0xA2 => Some(OpCodeByte::CallDefault),
         0xB0 => Some(OpCodeByte::SetResult),
+        0xB1 => Some(OpCodeByte::ClearResult),
         0xFF => Some(OpCodeByte::End),
         _ => None,
       };
@@ -178,6 +201,7 @@ impl OpCodeByte {
       OpCodeByte::CallExternal => "CallExternal",
       OpCodeByte::CallDefault => "Rand",
       OpCodeByte::SetResult => "SetResult",
+      OpCodeByte::ClearResult => "ClearResult",
       OpCodeByte::End => "End",
     }
   }

src/sample_test_asm.txt

@@ -1,23 +0,0 @@
-fib:
-  push rbp
-  movrr rbp, rsp
-  movsr rbp, r1
-  cmpsi rbp, 0
-  jg .L0
-  movri eax, 0
-  ret
-.L0:
-  cmpsi rbp, 2
-  jg .L1
-  movri eax, 1
-  ret
-.L1:
-  movrs r1, rbp
-  
-  
-  
-main:
-  add rsp, 16
-  movsi 16(byte) rbp, 10
-  movrs r1, rbp
-  call fib

src/vm/builtins.rs

@@ -0,0 +1,229 @@
+use crate::ast::value::Value;
+use crate::opcodes::default_fn;
+use rust_decimal::{prelude::ToPrimitive, Decimal, MathematicalOps};
+
+use super::error::VMError;
+use super::vm::VM;
+
+impl<'a> VM<'a> {
+  /// Dispatch a built-in (default) function call by ID
+  pub(super) fn dispatch_builtin(
+    &mut self,
+    dest: usize,
+    fn_id: u8,
+    arg_regs: &[usize],
+  ) -> Result<(), VMError> {
+    match fn_id {
+      default_fn::RAND => self.builtin_rand(dest, arg_regs),
+      default_fn::ABS => self.builtin_abs(dest, arg_regs),
+      default_fn::MIN => self.builtin_min(dest, arg_regs),
+      default_fn::MAX => self.builtin_max(dest, arg_regs),
+      default_fn::FLOOR => self.builtin_floor(dest, arg_regs),
+      default_fn::CEIL => self.builtin_ceil(dest, arg_regs),
+      default_fn::ROUND => self.builtin_round(dest, arg_regs),
+      default_fn::SQRT => self.builtin_sqrt(dest, arg_regs),
+      default_fn::LEN => self.builtin_len(dest, arg_regs),
+      default_fn::TO_STRING => self.builtin_to_string(dest, arg_regs),
+      default_fn::TO_NUMBER => self.builtin_to_number(dest, arg_regs),
+      _ => {
+        let name = default_fn::name(fn_id)
+          .map(|s| s.to_string())
+          .unwrap_or_else(|| format!("unknown({})", fn_id));
+        Err(VMError::RuntimeError(format!(
+          "Unknown default function: {}",
+          name
+        )))
+      }
+    }
+  }
+
+  fn builtin_rand(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    use rand::RngExt;
+    if arg_regs.len() < 2 {
+      return Err(VMError::RuntimeError(
+        "rand() requires two arguments (min, max)".to_string(),
+      ));
+    }
+    match (&self.registers[arg_regs[0]], &self.registers[arg_regs[1]]) {
+      (Value::Number(min), Value::Number(max)) => {
+        let min_i64 = min.to_i64().ok_or_else(|| {
+          VMError::RuntimeError("rand() min must be an integer".to_string())
+        })?;
+        let max_i64 = max.to_i64().ok_or_else(|| {
+          VMError::RuntimeError("rand() max must be an integer".to_string())
+        })?;
+        if min_i64 > max_i64 {
+          return Err(VMError::RuntimeError(
+            "rand() min must be <= max".to_string(),
+          ));
+        }
+        let mut rng = rand::rng();
+        let result = rng.random_range(min_i64..=max_i64);
+        self.registers[dest] = Value::Number(Decimal::from(result));
+        Ok(())
+      }
+      _ => Err(VMError::RuntimeError(
+        "rand() requires number arguments".to_string(),
+      )),
+    }
+  }
+
+  fn builtin_abs(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    require_args("abs", arg_regs, 1)?;
+    let n = extract_number("abs", &self.registers[arg_regs[0]])?;
+    self.registers[dest] = Value::Number(n.abs());
+    Ok(())
+  }
+
+  fn builtin_min(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    if arg_regs.is_empty() {
+      return Err(VMError::RuntimeError(
+        "min() requires at least one argument".to_string(),
+      ));
+    }
+    let mut result = extract_number("min", &self.registers[arg_regs[0]])?;
+    for &reg in &arg_regs[1..] {
+      let n = extract_number("min", &self.registers[reg])?;
+      if n < result {
+        result = n;
+      }
+    }
+    self.registers[dest] = Value::Number(result);
+    Ok(())
+  }
+
+  fn builtin_max(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    if arg_regs.is_empty() {
+      return Err(VMError::RuntimeError(
+        "max() requires at least one argument".to_string(),
+      ));
+    }
+    let mut result = extract_number("max", &self.registers[arg_regs[0]])?;
+    for &reg in &arg_regs[1..] {
+      let n = extract_number("max", &self.registers[reg])?;
+      if n > result {
+        result = n;
+      }
+    }
+    self.registers[dest] = Value::Number(result);
+    Ok(())
+  }
+
+  fn builtin_floor(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    require_args("floor", arg_regs, 1)?;
+    let n = extract_number("floor", &self.registers[arg_regs[0]])?;
+    self.registers[dest] = Value::Number(n.floor());
+    Ok(())
+  }
+
+  fn builtin_ceil(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    require_args("ceil", arg_regs, 1)?;
+    let n = extract_number("ceil", &self.registers[arg_regs[0]])?;
+    self.registers[dest] = Value::Number(n.ceil());
+    Ok(())
+  }
+
+  fn builtin_round(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    require_args("round", arg_regs, 1)?;
+    let n = extract_number("round", &self.registers[arg_regs[0]])?;
+    // Optional second argument: decimal places (default 0)
+    let places = if arg_regs.len() > 1 {
+      extract_number("round", &self.registers[arg_regs[1]])?
+        .to_u32()
+        .unwrap_or(0)
+    } else {
+      0
+    };
+    self.registers[dest] = Value::Number(n.round_dp(places));
+    Ok(())
+  }
+
+  fn builtin_sqrt(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    require_args("sqrt", arg_regs, 1)?;
+    let n = extract_number("sqrt", &self.registers[arg_regs[0]])?;
+    if n.is_sign_negative() {
+      return Err(VMError::RuntimeError(
+        "sqrt() argument must be non-negative".to_string(),
+      ));
+    }
+    let result = n.sqrt().ok_or_else(|| {
+      VMError::RuntimeError("sqrt() failed to compute".to_string())
+    })?;
+    self.registers[dest] = Value::Number(result);
+    Ok(())
+  }
+
+  fn builtin_len(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    require_args("len", arg_regs, 1)?;
+    let len = match &self.registers[arg_regs[0]] {
+      Value::String(s) => Decimal::from(s.len()),
+      Value::StringList(l) => Decimal::from(l.len()),
+      Value::NumberList(l) => Decimal::from(l.len()),
+      Value::Object(m) => Decimal::from(m.len()),
+      other => {
+        return Err(VMError::RuntimeError(format!(
+          "len() not supported for type {}",
+          other.type_name()
+        )));
+      }
+    };
+    self.registers[dest] = Value::Number(len);
+    Ok(())
+  }
+
+  fn builtin_to_string(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    require_args("toString", arg_regs, 1)?;
+    let s = super::vm::value_to_string(&self.registers[arg_regs[0]]);
+    self.registers[dest] = Value::String(s.into_owned().into());
+    Ok(())
+  }
+
+  fn builtin_to_number(&mut self, dest: usize, arg_regs: &[usize]) -> Result<(), VMError> {
+    require_args("toNumber", arg_regs, 1)?;
+    let result = match &self.registers[arg_regs[0]] {
+      Value::Number(n) => *n,
+      Value::String(s) => s.parse::<Decimal>().map_err(|_| {
+        VMError::RuntimeError(format!("toNumber() cannot parse '{}'", s))
+      })?,
+      Value::Boolean(b) => {
+        if *b {
+          Decimal::from(1)
+        } else {
+          Decimal::from(0)
+        }
+      }
+      other => {
+        return Err(VMError::RuntimeError(format!(
+          "toNumber() not supported for type {}",
+          other.type_name()
+        )));
+      }
+    };
+    self.registers[dest] = Value::Number(result);
+    Ok(())
+  }
+}
+
+/// Helper: check minimum argument count
+fn require_args(name: &str, arg_regs: &[usize], min: usize) -> Result<(), VMError> {
+  if arg_regs.len() < min {
+    Err(VMError::RuntimeError(format!(
+      "{}() requires at least {} argument(s)",
+      name, min
+    )))
+  } else {
+    Ok(())
+  }
+}
+
+/// Helper: extract a Decimal from a Value or return an error
+fn extract_number(name: &str, val: &Value) -> Result<Decimal, VMError> {
+  match val {
+    Value::Number(n) => Ok(*n),
+    other => Err(VMError::RuntimeError(format!(
+      "{}() requires a number argument, got {}",
+      name,
+      other.type_name()
+    ))),
+  }
+}

src/vm/methods.rs

@@ -0,0 +1,500 @@
+use crate::ast::value::Value;
+use rust_decimal::{prelude::ToPrimitive, Decimal};
+use smol_str::{SmolStr, StrExt};
+use std::rc::Rc;
+
+use super::error::VMError;
+use super::vm::VM;
+
+impl<'a> VM<'a> {
+  /// Dispatch a method call on a value.
+  ///
+  /// Uses `std::mem::take` to temporarily move the value out of the register,
+  /// avoiding clones when dispatching to type-specific handlers.
+  pub(super) fn dispatch_method(
+    &mut self,
+    dest: usize,
+    obj: usize,
+    method: &str,
+    args: &[Value],
+  ) -> Result<(), VMError> {
+    // Take the value out to avoid borrow conflicts (register read + write).
+    let obj_val = std::mem::take(&mut self.registers[obj]);
+
+    let result = match &obj_val {
+      Value::String(_) => self.dispatch_string_method_inner(&obj_val, method, args),
+      Value::StringList(_) => self.dispatch_string_list_method_inner(&obj_val, method, args),
+      Value::NumberList(_) => self.dispatch_number_list_method_inner(&obj_val, method, args),
+      Value::Object(_) => self.dispatch_object_method_inner(&obj_val, method, args),
+      _ => {
+        // Try external methods for any type
+        let type_name: SmolStr = obj_val.type_name().into();
+        let key = (type_name, SmolStr::from(method));
+        if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
+          ext_method(&obj_val, args).map_err(VMError::RuntimeError)
+        } else {
+          Err(VMError::MethodNotFound {
+            type_name: obj_val.type_name(),
+            method: SmolStr::from(method),
+          })
+        }
+      }
+    };
+
+    // Put the object back, then set dest (if dest == obj, result overwrites it — that's fine).
+    self.registers[obj] = obj_val;
+    self.registers[dest] = result?;
+    Ok(())
+  }
+
+  fn dispatch_string_method_inner(
+    &self,
+    obj_val: &Value,
+    method: &str,
+    args: &[Value],
+  ) -> Result<Value, VMError> {
+    let s = match obj_val {
+      Value::String(s) => s,
+      _ => unreachable!(),
+    };
+
+    match method {
+      "upper" => Ok(Value::String(s.to_uppercase_smolstr())),
+      "lower" => Ok(Value::String(s.to_lowercase_smolstr())),
+      "trim" => Ok(Value::String(SmolStr::new(s.trim()))),
+      "trimStart" => Ok(Value::String(SmolStr::new(s.trim_start()))),
+      "trimEnd" => Ok(Value::String(SmolStr::new(s.trim_end()))),
+      "split" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError(
+            "split() requires a delimiter argument".to_string(),
+          ));
+        }
+        match &args[0] {
+          Value::String(delim) => {
+            let parts: Vec<SmolStr> = s.split(delim.as_str()).map(SmolStr::new).collect();
+            Ok(Value::StringList(Rc::new(parts)))
+          }
+          _ => Err(VMError::RuntimeError(
+            "split() requires a string delimiter".to_string(),
+          )),
+        }
+      }
+      "replace" => {
+        if args.len() < 2 {
+          return Err(VMError::RuntimeError(
+            "replace() requires two arguments (old, new)".to_string(),
+          ));
+        }
+        match (&args[0], &args[1]) {
+          (Value::String(old), Value::String(new)) => {
+            Ok(Value::String(SmolStr::new(s.replace(old.as_str(), new.as_str()))))
+          }
+          _ => Err(VMError::RuntimeError(
+            "replace() requires string arguments".to_string(),
+          )),
+        }
+      }
+      "startsWith" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError(
+            "startsWith() requires a prefix argument".to_string(),
+          ));
+        }
+        match &args[0] {
+          Value::String(prefix) => Ok(Value::Boolean(s.starts_with(prefix.as_str()))),
+          _ => Err(VMError::RuntimeError(
+            "startsWith() requires a string prefix".to_string(),
+          )),
+        }
+      }
+      "endsWith" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError(
+            "endsWith() requires a suffix argument".to_string(),
+          ));
+        }
+        match &args[0] {
+          Value::String(suffix) => Ok(Value::Boolean(s.ends_with(suffix.as_str()))),
+          _ => Err(VMError::RuntimeError(
+            "endsWith() requires a string suffix".to_string(),
+          )),
+        }
+      }
+      "contains" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError(
+            "contains() requires a substring argument".to_string(),
+          ));
+        }
+        match &args[0] {
+          Value::String(substr) => Ok(Value::Boolean(s.contains(substr.as_str()))),
+          _ => Err(VMError::RuntimeError(
+            "contains() requires a string substring".to_string(),
+          )),
+        }
+      }
+      "length" => Ok(Value::Number(Decimal::from(s.len()))),
+      "charAt" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError(
+            "charAt() requires an index argument".to_string(),
+          ));
+        }
+        match &args[0] {
+          Value::Number(idx) => {
+            let index = idx.to_u64().unwrap_or(u64::MAX) as usize;
+            match s.chars().nth(index) {
+              Some(c) => Ok(Value::String(SmolStr::new(c.to_string()))),
+              None => Ok(Value::Null),
+            }
+          }
+          _ => Err(VMError::RuntimeError(
+            "charAt() requires a number index".to_string(),
+          )),
+        }
+      }
+      "substring" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError(
+            "substring() requires at least a start index".to_string(),
+          ));
+        }
+        match &args[0] {
+          Value::Number(start_idx) => {
+            let start = start_idx.to_usize().unwrap_or(0);
+            let chars: Vec<char> = s.chars().collect();
+            let end = if args.len() > 1 {
+              match &args[1] {
+                Value::Number(end_idx) => end_idx.to_usize().unwrap_or(chars.len()),
+                _ => chars.len(),
+              }
+            } else {
+              chars.len()
+            };
+
+            if start >= chars.len() || start >= end {
+              Ok(Value::String(SmolStr::new("")))
+            } else {
+              let end = end.min(chars.len());
+              let result: String = chars[start..end].iter().collect();
+              Ok(Value::String(SmolStr::new(result)))
+            }
+          }
+          _ => Err(VMError::RuntimeError(
+            "substring() requires a number start index".to_string(),
+          )),
+        }
+      }
+      _ => {
+        let key = (SmolStr::new_static("String"), SmolStr::from(method));
+        if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
+          ext_method(obj_val, args).map_err(VMError::RuntimeError)
+        } else {
+          Err(VMError::MethodNotFound {
+            type_name: "String",
+            method: SmolStr::from(method),
+          })
+        }
+      }
+    }
+  }
+
+  fn dispatch_string_list_method_inner(
+    &self,
+    obj_val: &Value,
+    method: &str,
+    args: &[Value],
+  ) -> Result<Value, VMError> {
+    let list = match obj_val {
+      Value::StringList(l) => l,
+      _ => unreachable!(),
+    };
+
+    match method {
+      "length" | "len" => Ok(Value::Number(Decimal::from(list.len()))),
+      "isEmpty" => Ok(Value::Boolean(list.is_empty())),
+      "first" => Ok(list.first().map(|s| Value::String(s.clone())).unwrap_or(Value::Null)),
+      "last" => Ok(list.last().map(|s| Value::String(s.clone())).unwrap_or(Value::Null)),
+      "get" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError("get() requires an index".to_string()));
+        }
+        match &args[0] {
+          Value::Number(idx) => {
+            let index = idx.to_usize().unwrap_or(usize::MAX);
+            Ok(list.get(index).map(|s| Value::String(s.clone())).unwrap_or(Value::Null))
+          }
+          _ => Err(VMError::RuntimeError("get() requires a number index".to_string())),
+        }
+      }
+      "contains" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError("contains() requires an argument".to_string()));
+        }
+        match &args[0] {
+          Value::String(s) => Ok(Value::Boolean(list.contains(s))),
+          _ => Err(VMError::RuntimeError("contains() requires a string argument".to_string())),
+        }
+      }
+      "indexOf" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError("indexOf() requires an argument".to_string()));
+        }
+        match &args[0] {
+          Value::String(s) => {
+            let idx = list.iter().position(|item| item == s);
+            Ok(idx.map(|i| Value::Number(Decimal::from(i))).unwrap_or(Value::Number(Decimal::from(-1))))
+          }
+          _ => Err(VMError::RuntimeError("indexOf() requires a string argument".to_string())),
+        }
+      }
+      "slice" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError("slice() requires at least a start index".to_string()));
+        }
+        match &args[0] {
+          Value::Number(start_idx) => {
+            let start = start_idx.to_usize().unwrap_or(0).min(list.len());
+            let end = if args.len() > 1 {
+              match &args[1] {
+                Value::Number(end_idx) => end_idx.to_usize().unwrap_or(list.len()).min(list.len()),
+                _ => list.len(),
+              }
+            } else {
+              list.len()
+            };
+            Ok(Value::StringList(Rc::new(list[start..end].to_vec())))
+          }
+          _ => Err(VMError::RuntimeError("slice() requires a number index".to_string())),
+        }
+      }
+      "reverse" => {
+        let mut reversed = list.to_vec();
+        reversed.reverse();
+        Ok(Value::StringList(Rc::new(reversed)))
+      }
+      "sort" => {
+        let mut sorted = list.to_vec();
+        sorted.sort();
+        Ok(Value::StringList(Rc::new(sorted)))
+      }
+      "join" => {
+        let delim = if args.is_empty() {
+          ""
+        } else {
+          match &args[0] {
+            Value::String(s) => s.as_str(),
+            _ => return Err(VMError::RuntimeError("join() requires a string delimiter".to_string())),
+          }
+        };
+        let result: String = list.iter().map(|s| s.as_str()).collect::<Vec<_>>().join(delim);
+        Ok(Value::String(SmolStr::new(result)))
+      }
+      _ => {
+        let key = (SmolStr::new_static("StringList"), SmolStr::from(method));
+        if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
+          ext_method(obj_val, args).map_err(VMError::RuntimeError)
+        } else {
+          Err(VMError::MethodNotFound {
+            type_name: "StringList",
+            method: SmolStr::from(method),
+          })
+        }
+      }
+    }
+  }
+
+  fn dispatch_number_list_method_inner(
+    &self,
+    obj_val: &Value,
+    method: &str,
+    args: &[Value],
+  ) -> Result<Value, VMError> {
+    let list = match obj_val {
+      Value::NumberList(l) => l,
+      _ => unreachable!(),
+    };
+
+    match method {
+      "length" | "len" => Ok(Value::Number(Decimal::from(list.len()))),
+      "isEmpty" => Ok(Value::Boolean(list.is_empty())),
+      "first" => Ok(list.first().map(|n| Value::Number(*n)).unwrap_or(Value::Null)),
+      "last" => Ok(list.last().map(|n| Value::Number(*n)).unwrap_or(Value::Null)),
+      "get" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError("get() requires an index".to_string()));
+        }
+        match &args[0] {
+          Value::Number(idx) => {
+            let index = idx.to_usize().unwrap_or(usize::MAX);
+            Ok(list.get(index).map(|n| Value::Number(*n)).unwrap_or(Value::Null))
+          }
+          _ => Err(VMError::RuntimeError("get() requires a number index".to_string())),
+        }
+      }
+      "contains" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError("contains() requires an argument".to_string()));
+        }
+        match &args[0] {
+          Value::Number(n) => Ok(Value::Boolean(list.contains(n))),
+          _ => Err(VMError::RuntimeError("contains() requires a number argument".to_string())),
+        }
+      }
+      "indexOf" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError("indexOf() requires an argument".to_string()));
+        }
+        match &args[0] {
+          Value::Number(n) => {
+            let idx = list.iter().position(|item| item == n);
+            Ok(idx.map(|i| Value::Number(Decimal::from(i))).unwrap_or(Value::Number(Decimal::from(-1))))
+          }
+          _ => Err(VMError::RuntimeError("indexOf() requires a number argument".to_string())),
+        }
+      }
+      "slice" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError("slice() requires at least a start index".to_string()));
+        }
+        match &args[0] {
+          Value::Number(start_idx) => {
+            let start = start_idx.to_usize().unwrap_or(0).min(list.len());
+            let end = if args.len() > 1 {
+              match &args[1] {
+                Value::Number(end_idx) => end_idx.to_usize().unwrap_or(list.len()).min(list.len()),
+                _ => list.len(),
+              }
+            } else {
+              list.len()
+            };
+            Ok(Value::NumberList(Rc::new(list[start..end].to_vec())))
+          }
+          _ => Err(VMError::RuntimeError("slice() requires a number index".to_string())),
+        }
+      }
+      "reverse" => {
+        let mut reversed = list.to_vec();
+        reversed.reverse();
+        Ok(Value::NumberList(Rc::new(reversed)))
+      }
+      "sort" => {
+        let mut sorted = list.to_vec();
+        sorted.sort();
+        Ok(Value::NumberList(Rc::new(sorted)))
+      }
+      "sum" => {
+        let sum: Decimal = list.iter().sum();
+        Ok(Value::Number(sum))
+      }
+      "avg" => {
+        if list.is_empty() {
+          Ok(Value::Null)
+        } else {
+          let sum: Decimal = list.iter().sum();
+          let avg = sum / Decimal::from(list.len());
+          Ok(Value::Number(avg))
+        }
+      }
+      "min" => Ok(list.iter().min().map(|n| Value::Number(*n)).unwrap_or(Value::Null)),
+      "max" => Ok(list.iter().max().map(|n| Value::Number(*n)).unwrap_or(Value::Null)),
+      _ => {
+        let key = (SmolStr::new_static("NumberList"), SmolStr::from(method));
+        if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
+          ext_method(obj_val, args).map_err(VMError::RuntimeError)
+        } else {
+          Err(VMError::MethodNotFound {
+            type_name: "NumberList",
+            method: SmolStr::from(method),
+          })
+        }
+      }
+    }
+  }
+
+  fn dispatch_object_method_inner(
+    &self,
+    obj_val: &Value,
+    method: &str,
+    args: &[Value],
+  ) -> Result<Value, VMError> {
+    let map = match obj_val {
+      Value::Object(m) => m,
+      _ => unreachable!(),
+    };
+
+    match method {
+      "keys" => {
+        let keys: Vec<SmolStr> = map.keys().cloned().collect();
+        Ok(Value::StringList(Rc::new(keys)))
+      }
+      "values" => {
+        let vals: Vec<Value> = map.values().cloned().collect();
+        if vals.is_empty() {
+          Ok(Value::StringList(Rc::new(Vec::new())))
+        } else if vals.iter().all(|v| matches!(v, Value::String(_))) {
+          let strings: Vec<SmolStr> = vals
+            .into_iter()
+            .map(|v| match v {
+              Value::String(s) => s,
+              _ => unreachable!(),
+            })
+            .collect();
+          Ok(Value::StringList(Rc::new(strings)))
+        } else if vals.iter().all(|v| matches!(v, Value::Number(_))) {
+          let numbers: Vec<Decimal> = vals
+            .into_iter()
+            .map(|v| match v {
+              Value::Number(n) => n,
+              _ => unreachable!(),
+            })
+            .collect();
+          Ok(Value::NumberList(Rc::new(numbers)))
+        } else {
+          Err(VMError::RuntimeError(
+            "values() only works when all values are the same type (String or Number)".to_string(),
+          ))
+        }
+      }
+      "length" | "len" => Ok(Value::Number(Decimal::from(map.len()))),
+      "contains" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError(
+            "contains() requires a key argument".to_string(),
+          ));
+        }
+        match &args[0] {
+          Value::String(key) => Ok(Value::Boolean(map.contains_key(key))),
+          _ => Err(VMError::RuntimeError(
+            "contains() requires a string key".to_string(),
+          )),
+        }
+      }
+      "get" => {
+        if args.is_empty() {
+          return Err(VMError::RuntimeError(
+            "get() requires a key argument".to_string(),
+          ));
+        }
+        match &args[0] {
+          Value::String(key) => Ok(map.get(key).cloned().unwrap_or(Value::Null)),
+          _ => Err(VMError::RuntimeError(
+            "get() requires a string key".to_string(),
+          )),
+        }
+      }
+      _ => {
+        let key = (SmolStr::new_static("Object"), SmolStr::from(method));
+        if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
+          ext_method(obj_val, args).map_err(VMError::RuntimeError)
+        } else {
+          Err(VMError::MethodNotFound {
+            type_name: "Object",
+            method: SmolStr::from(method),
+          })
+        }
+      }
+    }
+  }
+}

src/vm/mod.rs

@@ -1,5 +1,7 @@
+mod builtins;
 mod debug_info;
 pub mod error;
+mod methods;
 mod vm;
 
 pub use debug_info::DebugInfo;

src/vm/vm.rs

@@ -1,9 +1,10 @@
 use crate::{ast::value::Value, bytecode::BytecodeReader, opcodes::OpCodeByte};
-use bumpalo::Bump;
+use std::rc::Rc;
 use micromap::Map;
-use rust_decimal::{prelude::ToPrimitive, Decimal, MathematicalOps};
+use rust_decimal::{Decimal, MathematicalOps};
 use rustc_hash::FxHashMap;
-use smol_str::{SmolStr, StrExt};
+use smallvec::SmallVec;
+use smol_str::SmolStr;
 
 /// Type alias for external (host) functions
 pub type ExternalFn = Box<dyn Fn(&[Value]) -> Result<Value, String>>;
@@ -33,36 +34,25 @@ macro_rules! log_debug {
 
 /// Virtual Machine for executing dExpr bytecode
 pub struct VM<'a> {
-  bytecode: &'a [u8],         // Bytecode to execute
+  bytecode: &'a [u8],
-  reader: BytecodeReader<'a>, // Bytecode reader
+  pub(super) reader: BytecodeReader<'a>,
-  pc: usize,                  // Program counter
+  pc: usize,
 
-  // Registers for computation
+  pub(super) registers: [Value; MAX_REGISTERS],
-  registers: [Value; MAX_REGISTERS],
 
-  // Global variables
   globals: Map<SmolStr, Value, 64>,
 
-  // Last expression result (returned by execute)
   last_result: Value,
 
-  // External (host) functions — lazily allocated
+  pub(super) external_functions: Option<FxHashMap<SmolStr, ExternalFn>>,
-  external_functions: Option<FxHashMap<SmolStr, ExternalFn>>,
 
-  // External (host) methods per type — lazily allocated
+  pub(super) external_methods: Option<FxHashMap<(SmolStr, SmolStr), ExternalMethod>>,
-  external_methods: Option<FxHashMap<(SmolStr, SmolStr), ExternalMethod>>,
 
-  // Heap for complex data types
-  heap: Bump,
-
-  // Debug info for error messages
   debug_info: Option<&'a DebugInfo>,
 
-  // Debug flag
   #[cfg(debug_assertions)]
-  debug: bool,
+  pub(super) debug: bool,
 
-  // Profiling counts
   #[cfg(debug_assertions)]
   opcode_counts: [usize; 256],
 }
@@ -79,7 +69,6 @@ impl<'a> VM<'a> {
       last_result: Value::Null,
       external_functions: None,
       external_methods: None,
-      heap: Bump::new(),
       debug_info: None,
       #[cfg(debug_assertions)]
       debug: false,
@@ -153,7 +142,6 @@ impl<'a> VM<'a> {
     self.registers = [const { Value::Null }; MAX_REGISTERS];
     self.last_result = Value::Null;
     // Preserve globals
-    self.heap = Bump::new();
   }
 
   /// Execute the bytecode program and return the last expression result
@@ -195,7 +183,7 @@ impl<'a> VM<'a> {
         OpCodeByte::StoreLocal => self.handle_store_local(),
         OpCodeByte::LoadGlobal => self.handle_load_global(),
         OpCodeByte::StoreGlobal => self.handle_store_global(),
-        OpCodeByte::Add => self.binary_op(|a, b| Ok(a + b), "add"),
+        OpCodeByte::Add => self.handle_add(),
         OpCodeByte::Sub => self.binary_op(|a, b| Ok(a - b), "subtract"),
         OpCodeByte::Mul => self.binary_op(|a, b| Ok(a * b), "multiply"),
         OpCodeByte::Div => self.binary_op(
@@ -240,6 +228,11 @@ impl<'a> VM<'a> {
         OpCodeByte::CallExternal => self.handle_call_external(),
         OpCodeByte::CallDefault => self.handle_call_default(),
         OpCodeByte::SetResult => self.handle_set_result(),
+        OpCodeByte::ClearResult => {
+          self.last_result = Value::Null;
+          log_debug!(self, "ClearResult");
+          Ok(())
+        }
         OpCodeByte::End => {
           log_debug!(self, "End of program");
           #[cfg(debug_assertions)]
@@ -406,12 +399,12 @@ impl<'a> VM<'a> {
   #[inline]
   fn handle_load_global(&mut self) -> Result<(), VMError> {
     let reg = self.read_register_checked()?;
-    let name = self.reader.read_string().map_err(VMError::BytecodeError)?;
+    let name = self.reader.read_str().map_err(VMError::BytecodeError)?;
 
     let value = self
       .globals
-      .get(&name)
+      .get(name)
-      .ok_or_else(|| VMError::UndefinedVariable(name.clone()))?;
+      .ok_or_else(|| VMError::UndefinedVariable(SmolStr::from(name)))?;
     self.registers[reg] = value.clone();
 
     log_debug!(self,
@@ -424,12 +417,12 @@ impl<'a> VM<'a> {
   /// Handle StoreGlobal opcode - store register to global variable
   #[inline]
   fn handle_store_global(&mut self) -> Result<(), VMError> {
-    let name = self.reader.read_string().map_err(VMError::BytecodeError)?;
+    let name = self.reader.read_str().map_err(VMError::BytecodeError)?;
     let reg = self.read_register_checked()?;
 
     self
       .globals
-      .insert(name.clone(), self.registers[reg].clone());
+      .insert(SmolStr::from(name), self.registers[reg].clone());
 
     log_debug!(self,
       "StoreGlobal global.{} = r{} ({})",
@@ -464,6 +457,54 @@ impl<'a> VM<'a> {
     Ok(())
   }
 
+  // ============================================================================
+  // Opcode Handlers - Add (Number + String coercion)
+  // ============================================================================
+
+  /// Handle Add opcode - number addition or string concatenation with auto-coercion
+  #[inline]
+  fn handle_add(&mut self) -> Result<(), VMError> {
+    let dest = self.read_register_checked()?;
+    let a = self.read_register_checked()?;
+    let b = self.read_register_checked()?;
+
+    match (&self.registers[a], &self.registers[b]) {
+      (Value::Number(a_num), Value::Number(b_num)) => {
+        self.registers[dest] = Value::Number(*a_num + *b_num);
+      }
+      (Value::String(a_str), Value::String(b_str)) => {
+        let mut result = String::with_capacity(a_str.len() + b_str.len());
+        result.push_str(a_str);
+        result.push_str(b_str);
+        self.registers[dest] = Value::String(result.into());
+      }
+      (Value::String(a_str), other) => {
+        let b_cow = value_to_string(other);
+        let mut result = String::with_capacity(a_str.len() + b_cow.len());
+        result.push_str(a_str);
+        result.push_str(&b_cow);
+        self.registers[dest] = Value::String(result.into());
+      }
+      (other, Value::String(b_str)) => {
+        let a_cow = value_to_string(other);
+        let mut result = String::with_capacity(a_cow.len() + b_str.len());
+        result.push_str(&a_cow);
+        result.push_str(b_str);
+        self.registers[dest] = Value::String(result.into());
+      }
+      (a_val, b_val) => {
+        return Err(VMError::InvalidOperation {
+          operation: "add",
+          left_type: a_val.type_name(),
+          right_type: b_val.type_name(),
+        });
+      }
+    }
+
+    log_debug!(self, "Add r{} = r{} + r{}", dest, a, b);
+    Ok(())
+  }
+
   // ============================================================================
   // Opcode Handlers - Boolean Operations
   // ============================================================================
@@ -614,26 +655,19 @@ impl<'a> VM<'a> {
   // Opcode Handlers - String Operations
   // ============================================================================
 
-  /// Handle Concat opcode - string concatenation
+  /// Handle Concat opcode - string concatenation with auto-coercion
   #[inline]
   fn handle_concat(&mut self) -> Result<(), VMError> {
     let dest = self.read_register_checked()?;
     let a = self.read_register_checked()?;
     let b = self.read_register_checked()?;
 
-    match (&self.registers[a], &self.registers[b]) {
+    let a_cow = value_to_string(&self.registers[a]);
-      (Value::String(a_str), Value::String(b_str)) => {
+    let b_cow = value_to_string(&self.registers[b]);
-        let result = format!("{}{}", a_str, b_str);
+    let mut result = String::with_capacity(a_cow.len() + b_cow.len());
-        self.registers[dest] = Value::String(result.into());
+    result.push_str(&a_cow);
-      }
+    result.push_str(&b_cow);
-      (a_val, b_val) => {
+    self.registers[dest] = Value::String(result.into());
-        return Err(VMError::InvalidOperation {
-          operation: "concat",
-          left_type: a_val.type_name(),
-          right_type: b_val.type_name(),
-        });
-      }
-    }
 
     log_debug!(self, "Concat r{} = r{} + r{}", dest, a, b);
     Ok(())
@@ -643,11 +677,11 @@ impl<'a> VM<'a> {
   fn handle_get_property(&mut self) -> Result<(), VMError> {
     let dest = self.read_register_checked()?;
     let obj = self.read_register_checked()?;
-    let prop = self.reader.read_string().map_err(VMError::BytecodeError)?;
+    let prop = self.reader.read_str().map_err(VMError::BytecodeError)?;
 
     match &self.registers[obj] {
       Value::Object(map) => {
-        let value = map.get(&prop).cloned().unwrap_or(Value::Null);
+        let value = map.get(prop).cloned().unwrap_or(Value::Null);
         self.registers[dest] = value;
       }
       other => {
@@ -665,13 +699,13 @@ impl<'a> VM<'a> {
   /// Handle SetProperty opcode - set a field on an Object (in-place on register)
   fn handle_set_property(&mut self) -> Result<(), VMError> {
     let obj = self.read_register_checked()?;
-    let prop = self.reader.read_string().map_err(VMError::BytecodeError)?;
+    let prop = self.reader.read_str().map_err(VMError::BytecodeError)?;
     let val = self.read_register_checked()?;
 
     let value = self.registers[val].clone();
     match &mut self.registers[obj] {
       Value::Object(map) => {
-        map.insert(prop.clone(), value);
+        Rc::make_mut(map).insert(SmolStr::from(prop), value);
       }
       other => {
         return Err(VMError::RuntimeError(format!(
@@ -689,538 +723,21 @@ impl<'a> VM<'a> {
   fn handle_method_call(&mut self) -> Result<(), VMError> {
     let dest = self.read_register_checked()?;
     let obj = self.read_register_checked()?;
-    let method = self.reader.read_string().map_err(VMError::BytecodeError)?;
+    let method = self.reader.read_str().map_err(VMError::BytecodeError)?;
 
     // Read argument registers
     let arg_count = self
       .reader
       .read_byte()
       .map_err(VMError::BytecodeError)? as usize;
-    let mut args = Vec::with_capacity(arg_count);
+    let mut args: SmallVec<[Value; 4]> = SmallVec::with_capacity(arg_count);
 
     for _ in 0..arg_count {
       let reg = self.read_register_checked()?;
       args.push(self.registers[reg].clone());
     }
 
-    // Dispatch method call
+    self.dispatch_method(dest, obj, method, &args)?;
-    match &self.registers[obj] {
-      Value::String(s) => match method.as_str() {
-        "upper" => {
-          let result = s.to_uppercase_smolstr();
-          self.registers[dest] = Value::String(result);
-        }
-        "lower" => {
-          let result = s.to_lowercase_smolstr();
-          self.registers[dest] = Value::String(result);
-        }
-        "trim" => {
-          let result = SmolStr::new(s.trim());
-          self.registers[dest] = Value::String(result);
-        }
-        "trimStart" => {
-          let result = SmolStr::new(s.trim_start());
-          self.registers[dest] = Value::String(result);
-        }
-        "trimEnd" => {
-          let result = SmolStr::new(s.trim_end());
-          self.registers[dest] = Value::String(result);
-        }
-        "split" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError(
-              "split() requires a delimiter argument".to_string(),
-            ));
-          }
-          match &args[0] {
-            Value::String(delim) => {
-              let parts: Vec<SmolStr> = s.split(delim.as_str()).map(SmolStr::new).collect();
-              self.registers[dest] = Value::StringList(parts);
-            }
-            _ => {
-              return Err(VMError::RuntimeError(
-                "split() requires a string delimiter".to_string(),
-              ));
-            }
-          }
-        }
-        "replace" => {
-          if args.len() < 2 {
-            return Err(VMError::RuntimeError(
-              "replace() requires two arguments (old, new)".to_string(),
-            ));
-          }
-          match (&args[0], &args[1]) {
-            (Value::String(old), Value::String(new)) => {
-              let result = SmolStr::new(s.replace(old.as_str(), new.as_str()));
-              self.registers[dest] = Value::String(result);
-            }
-            _ => {
-              return Err(VMError::RuntimeError(
-                "replace() requires string arguments".to_string(),
-              ));
-            }
-          }
-        }
-        "startsWith" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError(
-              "startsWith() requires a prefix argument".to_string(),
-            ));
-          }
-          match &args[0] {
-            Value::String(prefix) => {
-              let result = s.starts_with(prefix.as_str());
-              self.registers[dest] = Value::Boolean(result);
-            }
-            _ => {
-              return Err(VMError::RuntimeError(
-                "startsWith() requires a string prefix".to_string(),
-              ));
-            }
-          }
-        }
-        "endsWith" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError(
-              "endsWith() requires a suffix argument".to_string(),
-            ));
-          }
-          match &args[0] {
-            Value::String(suffix) => {
-              let result = s.ends_with(suffix.as_str());
-              self.registers[dest] = Value::Boolean(result);
-            }
-            _ => {
-              return Err(VMError::RuntimeError(
-                "endsWith() requires a string suffix".to_string(),
-              ));
-            }
-          }
-        }
-        "contains" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError(
-              "contains() requires a substring argument".to_string(),
-            ));
-          }
-          match &args[0] {
-            Value::String(substr) => {
-              let result = s.contains(substr.as_str());
-              self.registers[dest] = Value::Boolean(result);
-            }
-            _ => {
-              return Err(VMError::RuntimeError(
-                "contains() requires a string substring".to_string(),
-              ));
-            }
-          }
-        }
-        "length" => {
-          let len = Decimal::from(s.len());
-          self.registers[dest] = Value::Number(len);
-        }
-        "charAt" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError(
-              "charAt() requires an index argument".to_string(),
-            ));
-          }
-          match &args[0] {
-            Value::Number(idx) => {
-              let index = idx.to_u64().unwrap_or(u64::MAX) as usize;
-              match s.chars().nth(index) {
-                Some(c) => {
-                  self.registers[dest] = Value::String(SmolStr::new(c.to_string()));
-                }
-                None => {
-                  self.registers[dest] = Value::Null;
-                }
-              }
-            }
-            _ => {
-              return Err(VMError::RuntimeError(
-                "charAt() requires a number index".to_string(),
-              ));
-            }
-          }
-        }
-        "substring" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError(
-              "substring() requires at least a start index".to_string(),
-            ));
-          }
-          match &args[0] {
-            Value::Number(start_idx) => {
-              let start = start_idx.to_usize().unwrap_or(0);
-              let chars: Vec<char> = s.chars().collect();
-              let end = if args.len() > 1 {
-                match &args[1] {
-                  Value::Number(end_idx) => end_idx.to_usize().unwrap_or(chars.len()),
-                  _ => chars.len(),
-                }
-              } else {
-                chars.len()
-              };
-
-              if start >= chars.len() || start >= end {
-                self.registers[dest] = Value::String(SmolStr::new(""));
-              } else {
-                let end = end.min(chars.len());
-                let result: String = chars[start..end].iter().collect();
-                self.registers[dest] = Value::String(SmolStr::new(result));
-              }
-            }
-            _ => {
-              return Err(VMError::RuntimeError(
-                "substring() requires a number start index".to_string(),
-              ));
-            }
-          }
-        }
-        _ => {
-          let key = (SmolStr::new_static("String"), method.clone());
-          if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
-            let obj_val = &self.registers[obj];
-            let result = ext_method(obj_val, &args).map_err(VMError::RuntimeError)?;
-            self.registers[dest] = result;
-          } else {
-            return Err(VMError::MethodNotFound {
-              type_name: "String",
-              method,
-            });
-          }
-        }
-      },
-      Value::StringList(list) => match method.as_str() {
-        "length" | "len" => {
-          self.registers[dest] = Value::Number(Decimal::from(list.len()));
-        }
-        "isEmpty" => {
-          self.registers[dest] = Value::Boolean(list.is_empty());
-        }
-        "first" => {
-          self.registers[dest] = list
-            .first()
-            .map(|s| Value::String(s.clone()))
-            .unwrap_or(Value::Null);
-        }
-        "last" => {
-          self.registers[dest] = list
-            .last()
-            .map(|s| Value::String(s.clone()))
-            .unwrap_or(Value::Null);
-        }
-        "get" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("get() requires an index".to_string()));
-          }
-          match &args[0] {
-            Value::Number(idx) => {
-              let index = idx.to_usize().unwrap_or(usize::MAX);
-              self.registers[dest] = list
-                .get(index)
-                .map(|s| Value::String(s.clone()))
-                .unwrap_or(Value::Null);
-            }
-            _ => return Err(VMError::RuntimeError("get() requires a number index".to_string())),
-          }
-        }
-        "contains" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("contains() requires an argument".to_string()));
-          }
-          match &args[0] {
-            Value::String(s) => {
-              self.registers[dest] = Value::Boolean(list.contains(s));
-            }
-            _ => return Err(VMError::RuntimeError("contains() requires a string argument".to_string())),
-          }
-        }
-        "indexOf" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("indexOf() requires an argument".to_string()));
-          }
-          match &args[0] {
-            Value::String(s) => {
-              let idx = list.iter().position(|item| item == s);
-              self.registers[dest] = idx
-                .map(|i| Value::Number(Decimal::from(i)))
-                .unwrap_or(Value::Number(Decimal::from(-1)));
-            }
-            _ => return Err(VMError::RuntimeError("indexOf() requires a string argument".to_string())),
-          }
-        }
-        "slice" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("slice() requires at least a start index".to_string()));
-          }
-          match &args[0] {
-            Value::Number(start_idx) => {
-              let start = start_idx.to_usize().unwrap_or(0).min(list.len());
-              let end = if args.len() > 1 {
-                match &args[1] {
-                  Value::Number(end_idx) => end_idx.to_usize().unwrap_or(list.len()).min(list.len()),
-                  _ => list.len(),
-                }
-              } else {
-                list.len()
-              };
-              self.registers[dest] = Value::StringList(list[start..end].to_vec());
-            }
-            _ => return Err(VMError::RuntimeError("slice() requires a number index".to_string())),
-          }
-        }
-        "reverse" => {
-          let mut reversed = list.clone();
-          reversed.reverse();
-          self.registers[dest] = Value::StringList(reversed);
-        }
-        "sort" => {
-          let mut sorted = list.clone();
-          sorted.sort();
-          self.registers[dest] = Value::StringList(sorted);
-        }
-        "join" => {
-          let delim = if args.is_empty() {
-            ""
-          } else {
-            match &args[0] {
-              Value::String(s) => s.as_str(),
-              _ => return Err(VMError::RuntimeError("join() requires a string delimiter".to_string())),
-            }
-          };
-          let result: String = list.iter().map(|s| s.as_str()).collect::<Vec<_>>().join(delim);
-          self.registers[dest] = Value::String(SmolStr::new(result));
-        }
-        _ => {
-          let key = (SmolStr::new_static("StringList"), method.clone());
-          if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
-            let obj_val = &self.registers[obj];
-            let result = ext_method(obj_val, &args).map_err(VMError::RuntimeError)?;
-            self.registers[dest] = result;
-          } else {
-            return Err(VMError::MethodNotFound {
-              type_name: "StringList",
-              method,
-            });
-          }
-        }
-      },
-      Value::NumberList(list) => match method.as_str() {
-        "length" | "len" => {
-          self.registers[dest] = Value::Number(Decimal::from(list.len()));
-        }
-        "isEmpty" => {
-          self.registers[dest] = Value::Boolean(list.is_empty());
-        }
-        "first" => {
-          self.registers[dest] = list
-            .first()
-            .map(|n| Value::Number(*n))
-            .unwrap_or(Value::Null);
-        }
-        "last" => {
-          self.registers[dest] = list
-            .last()
-            .map(|n| Value::Number(*n))
-            .unwrap_or(Value::Null);
-        }
-        "get" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("get() requires an index".to_string()));
-          }
-          match &args[0] {
-            Value::Number(idx) => {
-              let index = idx.to_usize().unwrap_or(usize::MAX);
-              self.registers[dest] = list
-                .get(index)
-                .map(|n| Value::Number(*n))
-                .unwrap_or(Value::Null);
-            }
-            _ => return Err(VMError::RuntimeError("get() requires a number index".to_string())),
-          }
-        }
-        "contains" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("contains() requires an argument".to_string()));
-          }
-          match &args[0] {
-            Value::Number(n) => {
-              self.registers[dest] = Value::Boolean(list.contains(n));
-            }
-            _ => return Err(VMError::RuntimeError("contains() requires a number argument".to_string())),
-          }
-        }
-        "indexOf" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("indexOf() requires an argument".to_string()));
-          }
-          match &args[0] {
-            Value::Number(n) => {
-              let idx = list.iter().position(|item| item == n);
-              self.registers[dest] = idx
-                .map(|i| Value::Number(Decimal::from(i)))
-                .unwrap_or(Value::Number(Decimal::from(-1)));
-            }
-            _ => return Err(VMError::RuntimeError("indexOf() requires a number argument".to_string())),
-          }
-        }
-        "slice" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("slice() requires at least a start index".to_string()));
-          }
-          match &args[0] {
-            Value::Number(start_idx) => {
-              let start = start_idx.to_usize().unwrap_or(0).min(list.len());
-              let end = if args.len() > 1 {
-                match &args[1] {
-                  Value::Number(end_idx) => end_idx.to_usize().unwrap_or(list.len()).min(list.len()),
-                  _ => list.len(),
-                }
-              } else {
-                list.len()
-              };
-              self.registers[dest] = Value::NumberList(list[start..end].to_vec());
-            }
-            _ => return Err(VMError::RuntimeError("slice() requires a number index".to_string())),
-          }
-        }
-        "reverse" => {
-          let mut reversed = list.clone();
-          reversed.reverse();
-          self.registers[dest] = Value::NumberList(reversed);
-        }
-        "sort" => {
-          let mut sorted = list.clone();
-          sorted.sort();
-          self.registers[dest] = Value::NumberList(sorted);
-        }
-        "sum" => {
-          let sum: Decimal = list.iter().sum();
-          self.registers[dest] = Value::Number(sum);
-        }
-        "avg" => {
-          if list.is_empty() {
-            self.registers[dest] = Value::Null;
-          } else {
-            let sum: Decimal = list.iter().sum();
-            let avg = sum / Decimal::from(list.len());
-            self.registers[dest] = Value::Number(avg);
-          }
-        }
-        "min" => {
-          self.registers[dest] = list
-            .iter()
-            .min()
-            .map(|n| Value::Number(*n))
-            .unwrap_or(Value::Null);
-        }
-        "max" => {
-          self.registers[dest] = list
-            .iter()
-            .max()
-            .map(|n| Value::Number(*n))
-            .unwrap_or(Value::Null);
-        }
-        _ => {
-          let key = (SmolStr::new_static("NumberList"), method.clone());
-          if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
-            let obj_val = &self.registers[obj];
-            let result = ext_method(obj_val, &args).map_err(VMError::RuntimeError)?;
-            self.registers[dest] = result;
-          } else {
-            return Err(VMError::MethodNotFound {
-              type_name: "NumberList",
-              method,
-            });
-          }
-        }
-      },
-      Value::Object(map) => match method.as_str() {
-        "keys" => {
-          let keys: Vec<SmolStr> = map.keys().cloned().collect();
-          self.registers[dest] = Value::StringList(keys);
-        }
-        "values" => {
-          // Returns a StringList if all values are strings, NumberList if all numbers, otherwise error
-          let vals: Vec<Value> = map.values().cloned().collect();
-          if vals.is_empty() {
-            self.registers[dest] = Value::StringList(Vec::new());
-          } else if vals.iter().all(|v| matches!(v, Value::String(_))) {
-            let strings: Vec<SmolStr> = vals.into_iter().map(|v| match v {
-              Value::String(s) => s,
-              _ => unreachable!(),
-            }).collect();
-            self.registers[dest] = Value::StringList(strings);
-          } else if vals.iter().all(|v| matches!(v, Value::Number(_))) {
-            let numbers: Vec<Decimal> = vals.into_iter().map(|v| match v {
-              Value::Number(n) => n,
-              _ => unreachable!(),
-            }).collect();
-            self.registers[dest] = Value::NumberList(numbers);
-          } else {
-            return Err(VMError::RuntimeError(
-              "values() only works when all values are the same type (String or Number)".to_string(),
-            ));
-          }
-        }
-        "length" | "len" => {
-          self.registers[dest] = Value::Number(Decimal::from(map.len()));
-        }
-        "contains" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("contains() requires a key argument".to_string()));
-          }
-          match &args[0] {
-            Value::String(key) => {
-              self.registers[dest] = Value::Boolean(map.contains_key(key));
-            }
-            _ => return Err(VMError::RuntimeError("contains() requires a string key".to_string())),
-          }
-        }
-        "get" => {
-          if args.is_empty() {
-            return Err(VMError::RuntimeError("get() requires a key argument".to_string()));
-          }
-          match &args[0] {
-            Value::String(key) => {
-              self.registers[dest] = map.get(key).cloned().unwrap_or(Value::Null);
-            }
-            _ => return Err(VMError::RuntimeError("get() requires a string key".to_string())),
-          }
-        }
-        _ => {
-          let key = (SmolStr::new_static("Object"), method.clone());
-          if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
-            let obj_val = &self.registers[obj];
-            let result = ext_method(obj_val, &args).map_err(VMError::RuntimeError)?;
-            self.registers[dest] = result;
-          } else {
-            return Err(VMError::MethodNotFound {
-              type_name: "Object",
-              method,
-            });
-          }
-        }
-      },
-      _ => {
-        // Try external methods for any type
-        let obj_val = &self.registers[obj];
-        let type_name: SmolStr = obj_val.type_name().into();
-        let key = (type_name.clone(), method.clone());
-        if let Some(ext_method) = self.external_methods.as_ref().and_then(|m| m.get(&key)) {
-          let result = ext_method(obj_val, &args).map_err(VMError::RuntimeError)?;
-          self.registers[dest] = result;
-        } else {
-          return Err(VMError::MethodNotFound {
-            type_name: obj_val.type_name(),
-            method,
-          });
-        }
-      }
-    }
 
     log_debug!(self, "MethodCall r{} = r{}.{}(...)", dest, obj, method);
     Ok(())
@@ -1232,48 +749,16 @@ impl<'a> VM<'a> {
 
   /// Handle CallDefault opcode - call a default (built-in) function by ID
   fn handle_call_default(&mut self) -> Result<(), VMError> {
-    use crate::opcodes::default_fn;
-
     let dest = self.read_register_checked()?;
     let fn_id = self.reader.read_byte().map_err(VMError::BytecodeError)?;
     let arg_count = self.reader.read_byte().map_err(VMError::BytecodeError)? as usize;
 
-    let mut arg_regs = Vec::with_capacity(arg_count);
+    let mut arg_regs: SmallVec<[usize; 4]> = SmallVec::with_capacity(arg_count);
     for _ in 0..arg_count {
       arg_regs.push(self.read_register_checked()?);
     }
 
-    match fn_id {
+    self.dispatch_builtin(dest, fn_id, &arg_regs)?;
-      default_fn::RAND => {
-        use rand::RngExt;
-        if arg_regs.len() < 2 {
-          return Err(VMError::RuntimeError("rand() requires two arguments (min, max)".to_string()));
-        }
-        match (&self.registers[arg_regs[0]], &self.registers[arg_regs[1]]) {
-          (Value::Number(min), Value::Number(max)) => {
-            let min_i64 = min.to_i64().ok_or_else(|| {
-              VMError::RuntimeError("rand() min must be an integer".to_string())
-            })?;
-            let max_i64 = max.to_i64().ok_or_else(|| {
-              VMError::RuntimeError("rand() max must be an integer".to_string())
-            })?;
-            if min_i64 > max_i64 {
-              return Err(VMError::RuntimeError("rand() min must be <= max".to_string()));
-            }
-            let mut rng = rand::rng();
-            let result = rng.random_range(min_i64..=max_i64);
-            self.registers[dest] = Value::Number(Decimal::from(result));
-          }
-          _ => return Err(VMError::RuntimeError("rand() requires number arguments".to_string())),
-        }
-      }
-      _ => {
-        let name = default_fn::name(fn_id)
-          .map(|s| s.to_string())
-          .unwrap_or_else(|| format!("unknown({})", fn_id));
-        return Err(VMError::RuntimeError(format!("Unknown default function: {}", name)));
-      }
-    }
 
     log_debug!(self, "CallDefault r{} = fn#{}(...)", dest, fn_id);
     Ok(())
@@ -1282,20 +767,21 @@ impl<'a> VM<'a> {
   /// Handle CallExternal opcode - call host function by name
   fn handle_call_external(&mut self) -> Result<(), VMError> {
     let dest = self.read_register_checked()?;
-    let name = self.reader.read_string().map_err(VMError::BytecodeError)?;
+    let name = self.reader.read_str().map_err(VMError::BytecodeError)?;
     let arg_count = self.reader.read_byte().map_err(VMError::BytecodeError)? as usize;
 
-    let mut args = Vec::with_capacity(arg_count);
+    let mut args: SmallVec<[Value; 4]> = SmallVec::with_capacity(arg_count);
     for _ in 0..arg_count {
       let reg = self.read_register_checked()?;
       args.push(self.registers[reg].clone());
     }
 
+    let name_key = SmolStr::from(name);
     let func = self
       .external_functions
       .as_ref()
-      .and_then(|m| m.get(&name))
+      .and_then(|m| m.get(&name_key))
-      .ok_or_else(|| VMError::RuntimeError(format!("Undefined function: {}", name)))?;
+      .ok_or_else(|| VMError::RuntimeError(format!("Undefined function: {}", name_key)))?;
 
     let result = func(&args).map_err(VMError::RuntimeError)?;
     self.registers[dest] = result;
@@ -1415,3 +901,14 @@ impl<'a> VM<'a> {
     Ok(())
   }
 }
+
+/// Convert a Value to its string representation for concatenation (no quotes around strings)
+pub(super) fn value_to_string(val: &Value) -> std::borrow::Cow<'_, str> {
+  match val {
+    Value::String(s) => std::borrow::Cow::Borrowed(s.as_str()),
+    Value::Number(n) => std::borrow::Cow::Owned(n.to_string()),
+    Value::Boolean(b) => std::borrow::Cow::Borrowed(if *b { "true" } else { "false" }),
+    Value::Null => std::borrow::Cow::Borrowed("null"),
+    other => std::borrow::Cow::Owned(format!("{}", other)),
+  }
+}

tests/data_driven_tests.rs

@@ -0,0 +1,144 @@
+use dexpr::{ast::value::Value, compiler::Compiler, parser, vm::VM};
+use indexmap::IndexMap;
+use rust_decimal::Decimal;
+use smol_str::SmolStr;
+use std::collections::HashMap;
+use std::rc::Rc;
+use std::str::FromStr;
+
+#[derive(serde::Deserialize)]
+struct TestCase {
+    name: String,
+    code: String,
+    #[serde(default)]
+    globals: HashMap<String, ValueDef>,
+    expected: ValueDef,
+}
+
+#[derive(serde::Deserialize)]
+struct ValueDef {
+    #[serde(rename = "type")]
+    typ: String,
+    #[serde(default)]
+    value: Option<serde_json::Value>,
+}
+
+fn value_def_to_value(def: &ValueDef) -> Value {
+    match def.typ.as_str() {
+        "null" => Value::Null,
+        "number" => {
+            let s = def.value.as_ref().unwrap().as_str().unwrap();
+            Value::Number(Decimal::from_str(s).unwrap())
+        }
+        "string" => {
+            let s = def.value.as_ref().unwrap().as_str().unwrap();
+            Value::String(s.into())
+        }
+        "boolean" => {
+            let b = def.value.as_ref().unwrap().as_bool().unwrap();
+            Value::Boolean(b)
+        }
+        "object" => {
+            fn json_obj_to_value(obj: &serde_json::Map<String, serde_json::Value>) -> Value {
+                let mut map = IndexMap::new();
+                for (k, v) in obj {
+                    let val = match v {
+                        serde_json::Value::String(s) => {
+                            if let Ok(d) = Decimal::from_str(s) {
+                                Value::Number(d)
+                            } else {
+                                Value::String(SmolStr::from(s.as_str()))
+                            }
+                        }
+                        serde_json::Value::Bool(b) => Value::Boolean(*b),
+                        serde_json::Value::Object(nested) => json_obj_to_value(nested),
+                        _ => Value::String(SmolStr::from(v.to_string())),
+                    };
+                    map.insert(SmolStr::from(k.as_str()), val);
+                }
+                Value::Object(Rc::new(map))
+            }
+            let obj = def.value.as_ref().unwrap().as_object().unwrap();
+            json_obj_to_value(obj)
+        }
+        other => panic!("Unknown type: {other}"),
+    }
+}
+
+#[test]
+fn test_all_cases() {
+    let json = include_str!("test_cases.json");
+    let cases: Vec<TestCase> =
+        serde_json::from_str(json).expect("Failed to parse test_cases.json");
+
+    let mut failures = Vec::new();
+    let total = cases.len();
+
+    for case in &cases {
+        let ast = match parser::program(&case.code) {
+            Ok(ast) => ast,
+            Err(e) => {
+                failures.push(format!(
+                    "FAIL: {}\n  code:  {}\n  error: parse failed: {e}",
+                    case.name,
+                    case.code.replace('\n', "\\n"),
+                ));
+                continue;
+            }
+        };
+
+        let mut compiler = Compiler::new();
+        let bytecode = match compiler.compile(ast) {
+            Ok(bc) => bc,
+            Err(e) => {
+                failures.push(format!(
+                    "FAIL: {}\n  code:  {}\n  error: compile failed: {e}",
+                    case.name,
+                    case.code.replace('\n', "\\n"),
+                ));
+                continue;
+            }
+        };
+
+        let mut vm = VM::new(&bytecode);
+
+        for (name, def) in &case.globals {
+            vm.set_global(name, value_def_to_value(def));
+        }
+
+        let result = match vm.execute() {
+            Ok(v) => v,
+            Err(e) => {
+                failures.push(format!(
+                    "FAIL: {}\n  code:  {}\n  error: execute failed: {e}",
+                    case.name,
+                    case.code.replace('\n', "\\n"),
+                ));
+                continue;
+            }
+        };
+
+        let expected = value_def_to_value(&case.expected);
+
+        if result != expected {
+            failures.push(format!(
+                "FAIL: {}\n  code:     {}\n  expected: {:?}\n  got:      {:?}",
+                case.name,
+                case.code.replace('\n', "\\n"),
+                expected,
+                result
+            ));
+        }
+    }
+
+    if !failures.is_empty() {
+        panic!(
+            "\n{} / {} test cases failed:\n\n{}\n",
+            failures.len(),
+            total,
+            failures.join("\n\n")
+        );
+    }
+
+    eprintln!("All {total} test cases passed.");
+}

tests/integration_tests.rs

@@ -3,6 +3,7 @@ use indexmap::IndexMap;
 use rust_decimal::prelude::ToPrimitive;
 use rust_decimal_macros::dec;
 use smol_str::SmolStr;
+use std::rc::Rc;
 
 /// Helper to run code and get the value of "result" variable
 fn run_and_get_result(code: &str) -> Value {
@@ -628,7 +629,7 @@ fn test_numberlist_contains() {
   let mut compiler = Compiler::new();
   let bytecode = compiler.compile(ast).expect("compile");
   let mut vm = VM::new(&bytecode);
-  vm.set_global("nums", Value::NumberList(vec![dec!(1), dec!(2), dec!(3), dec!(4)]));
+  vm.set_global("nums", Value::NumberList(Rc::new(vec![dec!(1), dec!(2), dec!(3), dec!(4)])));
   vm.execute().expect("execute");
   assert_eq!(vm.get_global("result").unwrap(), &Value::Boolean(true));
 }
@@ -639,7 +640,7 @@ fn test_numberlist_indexof() {
   let mut compiler = Compiler::new();
   let bytecode = compiler.compile(ast).expect("compile");
   let mut vm = VM::new(&bytecode);
-  vm.set_global("nums", Value::NumberList(vec![dec!(1), dec!(2), dec!(3)]));
+  vm.set_global("nums", Value::NumberList(Rc::new(vec![dec!(1), dec!(2), dec!(3)])));
   vm.execute().expect("execute");
   assert_eq!(vm.get_global("result").unwrap(), &Value::Number(dec!(2)));
 }
@@ -650,7 +651,7 @@ fn test_numberlist_slice() {
   let mut compiler = Compiler::new();
   let bytecode = compiler.compile(ast).expect("compile");
   let mut vm = VM::new(&bytecode);
-  vm.set_global("nums", Value::NumberList(vec![dec!(10), dec!(20), dec!(30), dec!(40)]));
+  vm.set_global("nums", Value::NumberList(Rc::new(vec![dec!(10), dec!(20), dec!(30), dec!(40)])));
   vm.execute().expect("execute");
   // slice(1,3) = [20, 30], sum = 50
   assert_eq!(vm.get_global("result").unwrap(), &Value::Number(dec!(50)));
@@ -662,7 +663,7 @@ fn test_numberlist_reverse() {
   let mut compiler = Compiler::new();
   let bytecode = compiler.compile(ast).expect("compile");
   let mut vm = VM::new(&bytecode);
-  vm.set_global("nums", Value::NumberList(vec![dec!(1), dec!(2), dec!(3)]));
+  vm.set_global("nums", Value::NumberList(Rc::new(vec![dec!(1), dec!(2), dec!(3)])));
   vm.execute().expect("execute");
   assert_eq!(vm.get_global("result").unwrap(), &Value::Number(dec!(3)));
 }
@@ -673,7 +674,7 @@ fn test_numberlist_sort() {
   let mut compiler = Compiler::new();
   let bytecode = compiler.compile(ast).expect("compile");
   let mut vm = VM::new(&bytecode);
-  vm.set_global("nums", Value::NumberList(vec![dec!(30), dec!(10), dec!(20)]));
+  vm.set_global("nums", Value::NumberList(Rc::new(vec![dec!(30), dec!(10), dec!(20)])));
   vm.execute().expect("execute");
   assert_eq!(vm.get_global("result").unwrap(), &Value::Number(dec!(10)));
 }
@@ -684,7 +685,7 @@ fn test_numberlist_isempty() {
   let mut compiler = Compiler::new();
   let bytecode = compiler.compile(ast).expect("compile");
   let mut vm = VM::new(&bytecode);
-  vm.set_global("nums", Value::NumberList(vec![]));
+  vm.set_global("nums", Value::NumberList(Rc::new(vec![])));
   vm.execute().expect("execute");
   assert_eq!(vm.get_global("result").unwrap(), &Value::Boolean(true));
 }
@@ -959,27 +960,19 @@ result = x % y"#;
 }
 
 #[test]
-fn test_error_location_type_mismatch() {
+fn test_string_number_auto_coercion() {
+  // string + number now auto-coerces to string concatenation
   let code = r#"x = "hello"
 y = 5
-result = x + y"#;
+x + y"#;
 
+  let ast = parser::program(code).unwrap();
   let mut compiler = Compiler::new();
-  let (bytecode, debug_info) = compiler
+  let bytecode = compiler.compile(ast).unwrap();
-    .compile_from_source(code)
-    .expect("Failed to compile");
-
   let mut vm = VM::new(&bytecode);
-  vm.set_debug_info(&debug_info);
+  let result = vm.execute().unwrap();
 
-  let err = vm.execute().unwrap_err();
+  assert_eq!(result, Value::String("hello5".into()));
-  let err_msg = err.to_string();
-
-  assert!(
-    err_msg.contains("line 3"),
-    "Error should contain line 3, got: {}",
-    err_msg
-  );
 }
 
 #[test]
@@ -1036,7 +1029,7 @@ fn make_object(entries: Vec<(&str, Value)>) -> Value {
   for (k, v) in entries {
     map.insert(SmolStr::new(k), v);
   }
-  Value::Object(map)
+  Value::Object(Rc::new(map))
 }
 
 #[test]
@@ -1121,7 +1114,7 @@ fn test_object_method_keys() {
   let result = run_expr_with_globals("person.keys()", vec![("person", obj)]);
   assert_eq!(
     result,
-    Value::StringList(vec![SmolStr::new("name"), SmolStr::new("age")])
+    Value::StringList(Rc::new(vec![SmolStr::new("name"), SmolStr::new("age")]))
   );
 }
 
@@ -1185,7 +1178,7 @@ fn test_object_method_values_strings() {
   let result = run_expr_with_globals("obj.values()", vec![("obj", obj)]);
   assert_eq!(
     result,
-    Value::StringList(vec![SmolStr::new("x"), SmolStr::new("y")])
+    Value::StringList(Rc::new(vec![SmolStr::new("x"), SmolStr::new("y")]))
   );
 }
 
@@ -1196,7 +1189,7 @@ fn test_object_method_values_numbers() {
     ("b", Value::Number(dec!(2))),
   ]);
   let result = run_expr_with_globals("obj.values()", vec![("obj", obj)]);
-  assert_eq!(result, Value::NumberList(vec![dec!(1), dec!(2)]));
+  assert_eq!(result, Value::NumberList(Rc::new(vec![dec!(1), dec!(2)])));
 }
 
 #[test]