feat: add Test effect and native testing framework

- Add Test effect with operations: assert, assertEqual, assertNotEqual,
  assertTrue, assertFalse, fail
- Implement Test effect handlers in interpreter with TestResults tracking
- Add values_equal method for comparing Value types in tests
- Update lux test command to discover and run test_* functions
- Create example test files: test_math.lux, test_lists.lux
- Add TESTING_DESIGN.md documentation
- Fix AST mismatches in C backend and compiler.rs for compatibility

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/TESTING_DESIGN.md

@@ -0,0 +1,307 @@
+# Lux Testing Framework Design
+
+## Overview
+
+Lux needs a native testing framework that leverages its effect system. The key insight: **testing is an effect** - assertions, test discovery, and reporting are all side effects.
+
+## Survey of Testing Approaches
+
+### 1. Rust Style (`#[test]` attributes)
+```rust
+#[test]
+fn test_addition() {
+    assert_eq!(2 + 2, 4);
+}
+```
+**Pros:** Familiar, built-in, zero runtime overhead
+**Cons:** Requires attribute/macro system
+
+### 2. Elm Style (describe/test/expect)
+```elm
+suite = describe "Math"
+    [ test "addition" <| \_ -> Expect.equal 4 (2 + 2)
+    , test "subtraction" <| \_ -> Expect.equal 0 (2 - 2)
+    ]
+```
+**Pros:** Readable, hierarchical
+**Cons:** Verbose, needs custom runner
+
+### 3. Zig Style (inline test blocks)
+```zig
+test "addition" {
+    try std.testing.expect(2 + 2 == 4);
+}
+```
+**Pros:** Inline with code, comptime evaluated
+**Cons:** Needs language support
+
+### 4. Property-Based (QuickCheck/Hypothesis)
+```haskell
+prop_reverse :: [Int] -> Bool
+prop_reverse xs = reverse (reverse xs) == xs
+```
+**Pros:** Finds edge cases automatically
+**Cons:** Complex to implement
+
+### 5. Expect Tests (OCaml/Jane Street)
+```ocaml
+let%expect_test "hello" =
+  print_endline "Hello";
+  [%expect {| Hello |}]
+```
+**Pros:** Self-updating, snapshot testing
+**Cons:** Requires tooling
+
+## Recommended: Effect-Based Testing
+
+Since Lux has effects, testing should be an effect. This is idiomatic and enables powerful features.
+
+### Design
+
+```lux
+// Built-in Test effect
+effect Test {
+    fn assert(condition: Bool, message: String): Unit
+    fn assertEqual<T>(expected: T, actual: T): Unit
+    fn assertNotEqual<T>(a: T, b: T): Unit
+    fn fail(message: String): Unit
+    fn skip(reason: String): Unit
+    fn todo(description: String): Unit
+}
+
+// Test declaration syntax (new keyword)
+test "addition works" {
+    Test.assertEqual(4, 2 + 2)
+}
+
+test "string concatenation" {
+    let result = "hello" + " " + "world"
+    Test.assertEqual("hello world", result)
+}
+
+// Grouped tests
+describe "List operations" {
+    test "map doubles values" {
+        let input = [1, 2, 3]
+        let output = List.map(input, fn(x) => x * 2)
+        Test.assertEqual([2, 4, 6], output)
+    }
+
+    test "filter keeps matches" {
+        let input = [1, 2, 3, 4, 5]
+        let output = List.filter(input, fn(x) => x > 2)
+        Test.assertEqual([3, 4, 5], output)
+    }
+}
+```
+
+### Why This Design?
+
+1. **Idiomatic**: Testing is just another effect
+2. **Mockable**: Can provide different Test handlers for different scenarios
+3. **Parallel-safe**: Effect isolation prevents test interference
+4. **Extensible**: Users can define custom assertion effects
+5. **Property testing**: Can add `Test.forAll` for property-based testing
+
+### Test Runner
+
+```bash
+# Run all tests in a file
+lux test examples/math_test.lux
+
+# Run all tests in directory
+lux test tests/
+
+# Run specific test
+lux test tests/math.lux --filter "addition"
+
+# Watch mode
+lux test --watch
+```
+
+### Alternative: Annotation-Based (Simpler)
+
+If we want simpler implementation:
+
+```lux
+// Functions prefixed with test_ are tests
+fn test_addition(): Unit with {Test} = {
+    Test.assertEqual(4, 2 + 2)
+}
+
+fn test_strings(): Unit with {Test} = {
+    Test.assertEqual("hello", "hel" + "lo")
+}
+```
+
+Run with:
+```bash
+lux test file.lux  # Finds all test_* functions
+```
+
+## Chosen Approach: Hybrid
+
+Combine both for flexibility:
+
+### Phase 1: Function-Based (Simple, Immediate)
+
+```lux
+// Any function starting with test_ is a test
+fn test_addition(): Unit with {Test} =
+    Test.assertEqual(4, 2 + 2)
+
+fn test_factorial(): Unit with {Test} = {
+    Test.assertEqual(1, factorial(0))
+    Test.assertEqual(1, factorial(1))
+    Test.assertEqual(120, factorial(5))
+}
+
+// Helper for multiple assertions
+fn test_list_operations(): Unit with {Test} = {
+    // Each assertion is independent
+    Test.assertEqual([2, 4], List.map([1, 2], fn(x) => x * 2))
+    Test.assertEqual([2, 3], List.filter([1, 2, 3], fn(x) => x > 1))
+    Test.assertEqual(6, List.fold([1, 2, 3], 0, fn(a, b) => a + b))
+}
+```
+
+### Phase 2: Block Syntax (Later)
+
+```lux
+test "descriptive name" {
+    Test.assertEqual(4, 2 + 2)
+}
+
+describe "module name" {
+    test "feature 1" { ... }
+    test "feature 2" { ... }
+}
+```
+
+## Implementation Plan
+
+### Step 1: Test Effect
+
+Add built-in `Test` effect to the type system:
+
+```rust
+// In types.rs, add Test effect
+EffectDef {
+    name: "Test".to_string(),
+    operations: vec![
+        EffectOpDef { name: "assert", params: vec![("cond", Bool), ("msg", String)], return_type: Unit },
+        EffectOpDef { name: "assertEqual", params: vec![("expected", T), ("actual", T)], return_type: Unit },
+        EffectOpDef { name: "fail", params: vec![("msg", String)], return_type: Unit },
+    ],
+}
+```
+
+### Step 2: Test Handler (Interpreter)
+
+```rust
+// In interpreter.rs
+fn handle_test_effect(&mut self, op: &str, args: &[Value]) -> Result<Value, RuntimeError> {
+    match op {
+        "assert" => {
+            let cond = args[0].as_bool()?;
+            let msg = args[1].as_string()?;
+            if !cond {
+                self.test_failures.push(TestFailure { message: msg, .. });
+            }
+            Ok(Value::Unit)
+        }
+        "assertEqual" => {
+            let expected = &args[0];
+            let actual = &args[1];
+            if expected != actual {
+                self.test_failures.push(TestFailure {
+                    message: format!("Expected {:?}, got {:?}", expected, actual),
+                    ..
+                });
+            }
+            Ok(Value::Unit)
+        }
+        // ...
+    }
+}
+```
+
+### Step 3: Test Discovery
+
+```rust
+// Find all test_* functions in a file
+fn discover_tests(program: &Program) -> Vec<&FunctionDecl> {
+    program.declarations.iter().filter_map(|d| {
+        if let Declaration::Function(f) = d {
+            if f.name.name.starts_with("test_") {
+                return Some(f);
+            }
+        }
+        None
+    }).collect()
+}
+```
+
+### Step 4: Test Runner CLI
+
+```bash
+lux test file.lux
+```
+
+Output:
+```
+Running tests in file.lux...
+
+  test_addition ... OK
+  test_factorial ... OK
+  test_strings ... FAIL
+    Expected: "hello world"
+    Actual:   "helloworld"
+    at file.lux:15
+
+Results: 2 passed, 1 failed
+```
+
+## Property-Based Testing (Future)
+
+```lux
+// Properties are functions that return Bool
+fn prop_reverse_twice<T>(xs: List<T>): Bool =
+    List.reverse(List.reverse(xs)) == xs
+
+fn prop_sort_idempotent(xs: List<Int>): Bool =
+    List.sort(List.sort(xs)) == List.sort(xs)
+
+// Test runner generates random inputs
+test "reverse is involution" {
+    Test.forAll(prop_reverse_twice)
+}
+```
+
+## Expect Tests (Future)
+
+```lux
+test "json output" {
+    let data = { name: "Alice", age: 30 }
+    let json = Json.stringify(data)
+
+    // This gets auto-updated by the test runner
+    Test.expect(json, |||
+        {"name":"Alice","age":30}
+    |||)
+}
+```
+
+## Summary
+
+| Phase | Feature | Effort |
+|-------|---------|--------|
+| 1 | Test effect + test_* discovery | 1-2 days |
+| 2 | Test runner CLI (`lux test`) | 1 day |
+| 3 | Nice output formatting | 0.5 day |
+| 4 | `test "name" {}` syntax | 1 day |
+| 5 | `describe` blocks | 0.5 day |
+| 6 | Property-based testing | 2-3 days |
+| 7 | Expect/snapshot tests | 2 days |
+
+**Recommendation:** Start with Phase 1-3 (function-based tests with CLI) for immediate value.

examples/test_lists.lux

@@ -0,0 +1,57 @@
+// List Operations Test Suite
+// Run with: lux test examples/test_lists.lux
+
+fn test_list_length(): Unit with {Test} = {
+    Test.assertEqual(0, List.length([]))
+    Test.assertEqual(1, List.length([1]))
+    Test.assertEqual(3, List.length([1, 2, 3]))
+}
+
+fn test_list_head(): Unit with {Test} = {
+    Test.assertEqual(Some(1), List.head([1, 2, 3]))
+    Test.assertEqual(None, List.head([]))
+}
+
+fn test_list_tail(): Unit with {Test} = {
+    Test.assertEqual(Some([2, 3]), List.tail([1, 2, 3]))
+    Test.assertEqual(None, List.tail([]))
+}
+
+fn test_list_get(): Unit with {Test} = {
+    let xs = [10, 20, 30]
+    Test.assertEqual(Some(10), List.get(xs, 0))
+    Test.assertEqual(Some(20), List.get(xs, 1))
+    Test.assertEqual(Some(30), List.get(xs, 2))
+    Test.assertEqual(None, List.get(xs, 3))
+}
+
+fn test_list_map(): Unit with {Test} = {
+    let double = fn(x: Int): Int => x * 2
+    Test.assertEqual([2, 4, 6], List.map([1, 2, 3], double))
+    Test.assertEqual([], List.map([], double))
+}
+
+fn test_list_filter(): Unit with {Test} = {
+    let isEven = fn(x: Int): Bool => x % 2 == 0
+    Test.assertEqual([2, 4], List.filter([1, 2, 3, 4, 5], isEven))
+    Test.assertEqual([], List.filter([1, 3, 5], isEven))
+}
+
+fn test_list_fold(): Unit with {Test} = {
+    let sum = fn(acc: Int, x: Int): Int => acc + x
+    Test.assertEqual(6, List.fold([1, 2, 3], 0, sum))
+    Test.assertEqual(10, List.fold([1, 2, 3], 4, sum))
+    Test.assertEqual(0, List.fold([], 0, sum))
+}
+
+fn test_list_reverse(): Unit with {Test} = {
+    Test.assertEqual([3, 2, 1], List.reverse([1, 2, 3]))
+    Test.assertEqual([], List.reverse([]))
+    Test.assertEqual([1], List.reverse([1]))
+}
+
+fn test_list_concat(): Unit with {Test} = {
+    Test.assertEqual([1, 2, 3, 4], List.concat([1, 2], [3, 4]))
+    Test.assertEqual([1, 2], List.concat([1, 2], []))
+    Test.assertEqual([3, 4], List.concat([], [3, 4]))
+}

examples/test_math.lux

@@ -0,0 +1,51 @@
+// Math Test Suite
+// Run with: lux test examples/test_math.lux
+
+fn test_addition(): Unit with {Test} = {
+    Test.assertEqual(4, 2 + 2)
+    Test.assertEqual(0, 0 + 0)
+    Test.assertEqual(100, 50 + 50)
+}
+
+fn test_subtraction(): Unit with {Test} = {
+    Test.assertEqual(0, 2 - 2)
+    Test.assertEqual(5, 10 - 5)
+    Test.assertEqual(-5, 0 - 5)
+}
+
+fn test_multiplication(): Unit with {Test} = {
+    Test.assertEqual(6, 2 * 3)
+    Test.assertEqual(0, 0 * 100)
+    Test.assertEqual(100, 10 * 10)
+}
+
+fn test_division(): Unit with {Test} = {
+    Test.assertEqual(2, 6 / 3)
+    Test.assertEqual(0, 0 / 5)
+    Test.assertEqual(5, 25 / 5)
+}
+
+fn test_comparisons(): Unit with {Test} = {
+    Test.assertTrue(5 > 3)
+    Test.assertTrue(3 < 5)
+    Test.assertTrue(5 >= 5)
+    Test.assertTrue(5 <= 5)
+    Test.assertFalse(3 > 5)
+    Test.assertFalse(5 < 3)
+}
+
+fn test_equality(): Unit with {Test} = {
+    Test.assertTrue(42 == 42)
+    Test.assertFalse(42 == 43)
+    Test.assertTrue(42 != 43)
+    Test.assertFalse(42 != 42)
+}
+
+fn test_boolean_logic(): Unit with {Test} = {
+    Test.assertTrue(true && true)
+    Test.assertFalse(true && false)
+    Test.assertTrue(true || false)
+    Test.assertFalse(false || false)
+    Test.assertTrue(!false)
+    Test.assertFalse(!true)
+}

src/codegen/c_backend.rs

@@ -5,7 +5,7 @@
 //! no garbage collector needed with Perceus-style reference counting.
 
 use crate::ast::*;
-use std::collections::{HashMap, HashSet};
+use std::collections::HashSet;
 use std::fmt::Write;
 
 /// C code generation errors
@@ -81,8 +81,8 @@ impl CBackend {
                 Declaration::Function(f) => {
                     self.emit_function(f)?;
                 }
-                Declaration::Let { name, value, .. } => {
+                Declaration::Let(let_decl) => {
-                    self.emit_global_let(name, value)?;
+                    self.emit_global_let(&let_decl.name, &let_decl.value)?;
                 }
                 _ => {}
             }
@@ -170,18 +170,18 @@ impl CBackend {
         self.types_emitted.insert(name.clone());
 
         match &type_decl.definition {
-            TypeDefinition::Record(fields) => {
+            TypeDef::Record(fields) => {
                 self.writeln(&format!("typedef struct {} {{", name));
                 self.indent += 1;
                 for field in fields {
-                    let c_type = self.type_to_c(&field.field_type)?;
+                    let c_type = self.type_to_c(&field.typ)?;
                     self.writeln(&format!("{} {};", c_type, field.name.name));
                 }
                 self.indent -= 1;
                 self.writeln(&format!("}} {};", name));
                 self.writeln("");
             }
-            TypeDefinition::Adt(variants) => {
+            TypeDef::Enum(variants) => {
                 // Emit tag enum
                 self.writeln(&format!("typedef enum {}_Tag {{", name));
                 self.indent += 1;
@@ -195,13 +195,25 @@ impl CBackend {
 
                 // Emit variant structs
                 for variant in variants {
-                    if !variant.fields.is_empty() {
+                    let has_fields = !matches!(&variant.fields, VariantFields::Unit);
+                    if has_fields {
                         self.writeln(&format!("typedef struct {}_{}_Data {{", name, variant.name.name));
                         self.indent += 1;
-                        for (i, field) in variant.fields.iter().enumerate() {
+                        match &variant.fields {
+                            VariantFields::Tuple(fields) => {
+                                for (i, field) in fields.iter().enumerate() {
                                     let c_type = self.type_to_c(field)?;
                                     self.writeln(&format!("{} field{};", c_type, i));
                                 }
+                            }
+                            VariantFields::Record(fields) => {
+                                for field in fields {
+                                    let c_type = self.type_to_c(&field.typ)?;
+                                    self.writeln(&format!("{} {};", c_type, field.name.name));
+                                }
+                            }
+                            VariantFields::Unit => {}
+                        }
                         self.indent -= 1;
                         self.writeln(&format!("}} {}_{}_Data;", name, variant.name.name));
                         self.writeln("");
@@ -215,7 +227,8 @@ impl CBackend {
                 self.writeln("union {");
                 self.indent += 1;
                 for variant in variants {
-                    if !variant.fields.is_empty() {
+                    let has_fields = !matches!(&variant.fields, VariantFields::Unit);
+                    if has_fields {
                         self.writeln(&format!("{}_{}_Data {};", name, variant.name.name, variant.name.name.to_lowercase()));
                     }
                 }
@@ -225,7 +238,7 @@ impl CBackend {
                 self.writeln(&format!("}} {};", name));
                 self.writeln("");
             }
-            TypeDefinition::Alias(_) => {
+            TypeDef::Alias(_) => {
                 // Type aliases are handled during type resolution
             }
         }
@@ -270,7 +283,7 @@ impl CBackend {
         }
 
         let param_strs: Result<Vec<_>, _> = params.iter().map(|p| {
-            let c_type = self.type_expr_to_c(&p.param_type)?;
+            let c_type = self.type_expr_to_c(&p.typ)?;
             Ok(format!("{} {}", c_type, p.name.name))
         }).collect();
 
@@ -301,13 +314,10 @@ impl CBackend {
                     BinaryOp::Ge => ">=",
                     BinaryOp::And => "&&",
                     BinaryOp::Or => "||",
-                    BinaryOp::Concat => {
+                    BinaryOp::Pipe => {
-                        return Ok(format!("lux_string_concat({}, {})", l, r));
+                        // Pipe operator - for now, just call the right side with left as argument
+                        return Ok(format!("{}({})", r, l));
                     }
-                    _ => return Err(CGenError {
-                        message: format!("Unsupported binary operator: {:?}", op),
-                        span: None,
-                    }),
                 };
 
                 Ok(format!("({} {} {})", l, op_str, r))
@@ -404,19 +414,8 @@ impl CBackend {
                 Ok(format!("{}.{}", obj, field.name))
             }
 
-            Expr::Match { expr, arms, .. } => {
+            Expr::Match { scrutinee, arms, .. } => {
-                self.emit_match(expr, arms)
+                self.emit_match(scrutinee, arms)
-            }
-
-            Expr::Constructor { name, args, .. } => {
-                // ADT constructor - need to determine the type
-                // For now, assume it's a simple constructor call
-                if args.is_empty() {
-                    Ok(format!("/* {} */ 0", name.name))
-                } else {
-                    let arg_strs: Result<Vec<_>, _> = args.iter().map(|a| self.emit_expr(a)).collect();
-                    Ok(format!("/* {}({}) */", name.name, arg_strs?.join(", ")))
-                }
             }
 
             _ => Err(CGenError {
@@ -462,9 +461,9 @@ impl CBackend {
     fn pattern_to_condition(&self, pattern: &Pattern, scrutinee: &str) -> Result<String, CGenError> {
         match pattern {
             Pattern::Wildcard(_) => Ok("1".to_string()),
-            Pattern::Var(ident, _) => Ok(format!("(1) /* bind {} = {} */", ident.name, scrutinee)),
+            Pattern::Var(ident) => Ok(format!("(1) /* bind {} = {} */", ident.name, scrutinee)),
-            Pattern::Literal(lit, _) => {
+            Pattern::Literal(lit) => {
-                let lit_val = self.emit_literal_value(lit)?;
+                let lit_val = self.emit_literal_value(&lit.kind)?;
                 Ok(format!("{} == {}", scrutinee, lit_val))
             }
             Pattern::Constructor { name, .. } => {
@@ -504,7 +503,7 @@ impl CBackend {
 
         // Check for top-level run expressions
         let has_run = program.declarations.iter().any(|d| {
-            matches!(d, Declaration::Let { value, .. } if matches!(value.as_ref(), Expr::Run { .. }))
+            matches!(d, Declaration::Let(let_decl) if matches!(&let_decl.value, Expr::Run { .. }))
         });
 
         if has_main || has_run {
@@ -513,9 +512,9 @@ impl CBackend {
 
             // Execute top-level let bindings with run expressions
             for decl in &program.declarations {
-                if let Declaration::Let { name, value, .. } = decl {
+                if let Declaration::Let(let_decl) = decl {
-                    if matches!(value.as_ref(), Expr::Run { .. }) {
+                    if matches!(&let_decl.value, Expr::Run { .. }) {
-                        if let Expr::Run { expr, .. } = value.as_ref() {
+                        if let Expr::Run { expr, .. } = &let_decl.value {
                             if let Expr::Call { func, .. } = expr.as_ref() {
                                 if let Expr::Var(fn_name) = func.as_ref() {
                                     self.writeln(&format!("{}();", fn_name.name));
@@ -549,18 +548,23 @@ impl CBackend {
                     other => Ok(other.to_string()),
                 }
             }
-            TypeExpr::Generic { name, .. } => {
+            TypeExpr::App(base, _) => {
                 // For now, use void* for generic types
+                if let TypeExpr::Named(name) = base.as_ref() {
                     match name.name.as_str() {
                         "List" => Ok("void*".to_string()),
                         "Option" => Ok("void*".to_string()),
                         _ => Ok("void*".to_string()),
                     }
+                } else {
+                    Ok("void*".to_string())
                 }
+            }
+            TypeExpr::Unit => Ok("void".to_string()),
+            TypeExpr::Versioned { base, .. } => self.type_expr_to_c(base),
             TypeExpr::Function { .. } => Ok("void*".to_string()),
             TypeExpr::Tuple(_) => Ok("void*".to_string()),
             TypeExpr::Record(_) => Ok("void*".to_string()),
-            _ => Ok("void*".to_string()),
         }
     }
 

src/codegen/mod.rs

@@ -7,4 +7,5 @@
 
 pub mod c_backend;
 
+#[allow(unused_imports)]
 pub use c_backend::CBackend;

src/compiler.rs

@@ -465,12 +465,13 @@ fn compile_expr(
         }
 
         expr => {
+            use crate::ast::LiteralKind;
             let expr_type = match expr {
-                Expr::Literal(lit) => match lit {
+                Expr::Literal(lit) => match &lit.kind {
-                    Literal::String(_) => "String literal",
+                    LiteralKind::String(_) => "String literal",
-                    Literal::Float(_) => "Float literal",
+                    LiteralKind::Float(_) => "Float literal",
-                    Literal::Char(_) => "Char literal",
+                    LiteralKind::Char(_) => "Char literal",
-                    Literal::Unit => "Unit literal",
+                    LiteralKind::Unit => "Unit literal",
                     _ => "Literal",
                 },
                 Expr::EffectOp { effect, operation, .. } => {
@@ -485,17 +486,8 @@ fn compile_expr(
                 Expr::List { .. } => "List literal",
                 Expr::Record { .. } => "Record literal",
                 Expr::Tuple { .. } => "Tuple literal",
-                Expr::Index { .. } => "Index access",
                 Expr::Run { .. } => "Run expression (effects)",
-                Expr::Handle { .. } => "Handle expression (effects)",
                 Expr::Resume { .. } => "Resume expression (effects)",
-                Expr::Pipe { .. } => "Pipe operator",
-                Expr::Interpolation { .. } => "String interpolation",
-                Expr::Constructor { name, .. } => {
-                    return Err(CompileError {
-                        message: format!("ADT constructor '{}' - algebraic data types are not supported in JIT", name.name),
-                    });
-                }
                 _ => "Unknown expression",
             };
             Err(CompileError {

src/interpreter.rs

@@ -177,6 +177,40 @@ impl Value {
             _ => None,
         }
     }
+
+    /// Compare two values for equality (for testing)
+    /// Returns true if the values are structurally equal
+    pub fn values_equal(a: &Value, b: &Value) -> bool {
+        match (a, b) {
+            (Value::Int(x), Value::Int(y)) => x == y,
+            (Value::Float(x), Value::Float(y)) => (x - y).abs() < f64::EPSILON,
+            (Value::Bool(x), Value::Bool(y)) => x == y,
+            (Value::String(x), Value::String(y)) => x == y,
+            (Value::Char(x), Value::Char(y)) => x == y,
+            (Value::Unit, Value::Unit) => true,
+            (Value::List(xs), Value::List(ys)) => {
+                xs.len() == ys.len() && xs.iter().zip(ys.iter()).all(|(x, y)| Value::values_equal(x, y))
+            }
+            (Value::Tuple(xs), Value::Tuple(ys)) => {
+                xs.len() == ys.len() && xs.iter().zip(ys.iter()).all(|(x, y)| Value::values_equal(x, y))
+            }
+            (Value::Record(xs), Value::Record(ys)) => {
+                xs.len() == ys.len() && xs.iter().all(|(k, v)| {
+                    ys.get(k).map(|yv| Value::values_equal(v, yv)).unwrap_or(false)
+                })
+            }
+            (Value::Constructor { name: n1, fields: f1 }, Value::Constructor { name: n2, fields: f2 }) => {
+                n1 == n2 && f1.len() == f2.len() && f1.iter().zip(f2.iter()).all(|(x, y)| Value::values_equal(x, y))
+            }
+            (Value::Versioned { type_name: t1, version: v1, value: val1 },
+             Value::Versioned { type_name: t2, version: v2, value: val2 }) => {
+                t1 == t2 && v1 == v2 && Value::values_equal(val1, val2)
+            }
+            (Value::Json(j1), Value::Json(j2)) => j1 == j2,
+            // Functions and handlers cannot be compared for equality
+            _ => false,
+        }
+    }
 }
 
 /// Trait for extracting typed values from Value
@@ -545,6 +579,24 @@ pub struct Interpreter {
     http_server: Arc<Mutex<Option<tiny_http::Server>>>,
     /// Current HTTP request being handled (stored for respond operation)
     current_http_request: Arc<Mutex<Option<tiny_http::Request>>>,
+    /// Test results for the Test effect
+    test_results: RefCell<TestResults>,
+}
+
+/// Results from running tests
+#[derive(Debug, Clone, Default)]
+pub struct TestResults {
+    pub passed: usize,
+    pub failed: usize,
+    pub failures: Vec<TestFailure>,
+}
+
+/// A single test failure
+#[derive(Debug, Clone)]
+pub struct TestFailure {
+    pub message: String,
+    pub expected: Option<String>,
+    pub actual: Option<String>,
 }
 
 impl Interpreter {
@@ -567,9 +619,20 @@ impl Interpreter {
             in_handler_depth: 0,
             http_server: Arc::new(Mutex::new(None)),
             current_http_request: Arc::new(Mutex::new(None)),
+            test_results: RefCell::new(TestResults::default()),
         }
     }
 
+    /// Get the test results
+    pub fn get_test_results(&self) -> TestResults {
+        self.test_results.borrow().clone()
+    }
+
+    /// Reset test results for a new test run
+    pub fn reset_test_results(&self) {
+        *self.test_results.borrow_mut() = TestResults::default();
+    }
+
     /// Set the initial value for the built-in State effect
     pub fn set_state(&self, value: Value) {
         *self.builtin_state.borrow_mut() = value;
@@ -3466,6 +3529,111 @@ impl Interpreter {
                 Ok(Value::Unit)
             }
 
+            // Test effect for testing framework
+            ("Test", "assert") => {
+                let condition = match request.args.first() {
+                    Some(Value::Bool(b)) => *b,
+                    _ => false,
+                };
+                let message = match request.args.get(1) {
+                    Some(Value::String(s)) => s.clone(),
+                    _ => "Assertion failed".to_string(),
+                };
+
+                if condition {
+                    self.test_results.borrow_mut().passed += 1;
+                } else {
+                    self.test_results.borrow_mut().failed += 1;
+                    self.test_results.borrow_mut().failures.push(TestFailure {
+                        message,
+                        expected: None,
+                        actual: None,
+                    });
+                }
+                Ok(Value::Unit)
+            }
+            ("Test", "assertEqual") => {
+                let expected = request.args.first().cloned().unwrap_or(Value::Unit);
+                let actual = request.args.get(1).cloned().unwrap_or(Value::Unit);
+
+                if Value::values_equal(&expected, &actual) {
+                    self.test_results.borrow_mut().passed += 1;
+                } else {
+                    self.test_results.borrow_mut().failed += 1;
+                    self.test_results.borrow_mut().failures.push(TestFailure {
+                        message: "Values not equal".to_string(),
+                        expected: Some(format!("{}", expected)),
+                        actual: Some(format!("{}", actual)),
+                    });
+                }
+                Ok(Value::Unit)
+            }
+            ("Test", "assertNotEqual") => {
+                let a = request.args.first().cloned().unwrap_or(Value::Unit);
+                let b = request.args.get(1).cloned().unwrap_or(Value::Unit);
+
+                if !Value::values_equal(&a, &b) {
+                    self.test_results.borrow_mut().passed += 1;
+                } else {
+                    self.test_results.borrow_mut().failed += 1;
+                    self.test_results.borrow_mut().failures.push(TestFailure {
+                        message: "Values should not be equal".to_string(),
+                        expected: Some(format!("not {}", a)),
+                        actual: Some(format!("{}", b)),
+                    });
+                }
+                Ok(Value::Unit)
+            }
+            ("Test", "assertTrue") => {
+                let condition = match request.args.first() {
+                    Some(Value::Bool(b)) => *b,
+                    _ => false,
+                };
+
+                if condition {
+                    self.test_results.borrow_mut().passed += 1;
+                } else {
+                    self.test_results.borrow_mut().failed += 1;
+                    self.test_results.borrow_mut().failures.push(TestFailure {
+                        message: "Expected true".to_string(),
+                        expected: Some("true".to_string()),
+                        actual: Some("false".to_string()),
+                    });
+                }
+                Ok(Value::Unit)
+            }
+            ("Test", "assertFalse") => {
+                let condition = match request.args.first() {
+                    Some(Value::Bool(b)) => *b,
+                    _ => true,
+                };
+
+                if !condition {
+                    self.test_results.borrow_mut().passed += 1;
+                } else {
+                    self.test_results.borrow_mut().failed += 1;
+                    self.test_results.borrow_mut().failures.push(TestFailure {
+                        message: "Expected false".to_string(),
+                        expected: Some("false".to_string()),
+                        actual: Some("true".to_string()),
+                    });
+                }
+                Ok(Value::Unit)
+            }
+            ("Test", "fail") => {
+                let message = match request.args.first() {
+                    Some(Value::String(s)) => s.clone(),
+                    _ => "Test failed".to_string(),
+                };
+                self.test_results.borrow_mut().failed += 1;
+                self.test_results.borrow_mut().failures.push(TestFailure {
+                    message,
+                    expected: None,
+                    actual: None,
+                });
+                Ok(Value::Unit)
+            }
+
             _ => Err(RuntimeError {
                 message: format!(
                     "Unhandled effect operation: {}.{}",

src/main.rs

@@ -1,6 +1,7 @@
 //! Lux - A functional programming language with first-class effects
 
 mod ast;
+mod codegen;
 mod compiler;
 mod debugger;
 mod diagnostics;
@@ -364,80 +365,208 @@ fn run_tests(args: &[String]) {
     // Find test files
     let pattern = args.first().map(|s| s.as_str());
 
-    // Look for test files in current directory and tests/ subdirectory
+    // Look for test files in multiple locations
     let mut test_files = Vec::new();
 
-    for entry in fs::read_dir(".").into_iter().flatten().flatten() {
+    // Current directory
-        let path = entry.path();
+    collect_test_files(".", pattern, &mut test_files);
-        if path.extension().map(|e| e == "lux").unwrap_or(false) {
+
-            if let Some(name) = path.file_name().and_then(|n| n.to_str()) {
+    // tests/ subdirectory
-                if name.starts_with("test_") || name.ends_with("_test.lux") {
+    if Path::new("tests").is_dir() {
-                    if pattern.map(|p| name.contains(p)).unwrap_or(true) {
+        collect_test_files("tests", pattern, &mut test_files);
-                        test_files.push(path);
-                    }
-                }
-            }
-        }
     }
 
-    if Path::new("tests").is_dir() {
+    // examples/ subdirectory (for example tests)
-        for entry in fs::read_dir("tests").into_iter().flatten().flatten() {
+    if Path::new("examples").is_dir() {
-            let path = entry.path();
+        collect_test_files("examples", pattern, &mut test_files);
-            if path.extension().map(|e| e == "lux").unwrap_or(false) {
-                if let Some(name) = path.file_name().and_then(|n| n.to_str()) {
-                    if pattern.map(|p| name.contains(p)).unwrap_or(true) {
-                        test_files.push(path);
-                    }
-                }
     }
+
+    // If a specific file is given, use that
+    if let Some(p) = pattern {
+        if Path::new(p).is_file() {
+            test_files.clear();
+            test_files.push(std::path::PathBuf::from(p));
         }
     }
 
     if test_files.is_empty() {
         println!("No test files found.");
         println!("Test files should be named test_*.lux or *_test.lux");
+        println!("Or contain functions named test_*");
         return;
     }
 
-    let mut passed = 0;
+    println!("Running tests...\n");
-    let mut failed = 0;
+
+    let mut total_passed = 0;
+    let mut total_failed = 0;
+    let mut all_failures = Vec::new();
 
     for test_file in &test_files {
         let path_str = test_file.to_string_lossy().to_string();
-        print!("Testing {}... ", path_str);
 
-        // Run the test file
+        // Read and parse the file
-        let result = std::process::Command::new(std::env::current_exe().unwrap())
+        let source = match fs::read_to_string(test_file) {
-            .arg(&path_str)
+            Ok(s) => s,
-            .output();
+            Err(e) => {
+                println!("  {} ... ERROR: {}", path_str, e);
+                total_failed += 1;
+                continue;
+            }
+        };
 
-        match result {
+        let program = match Parser::parse_source(&source) {
-            Ok(output) if output.status.success() => {
+            Ok(p) => p,
-                println!("OK");
+            Err(e) => {
-                passed += 1;
+                println!("  {} ... PARSE ERROR: {}", path_str, e);
+                total_failed += 1;
+                continue;
+            }
+        };
+
+        // Type check
+        let mut checker = typechecker::TypeChecker::new();
+        if let Err(errors) = checker.check_program(&program) {
+            println!("  {} ... TYPE ERROR", path_str);
+            for err in errors {
+                eprintln!("    {}", err);
+            }
+            total_failed += 1;
+            continue;
+        }
+
+        // Find test functions (functions starting with test_)
+        let test_funcs: Vec<_> = program.declarations.iter().filter_map(|d| {
+            if let ast::Declaration::Function(f) = d {
+                if f.name.name.starts_with("test_") {
+                    return Some(f.name.name.clone());
+                }
+            }
+            None
+        }).collect();
+
+        if test_funcs.is_empty() {
+            // No test functions, run the whole file
+            let mut interp = Interpreter::new();
+            interp.reset_test_results();
+
+            match interp.run(&program) {
+                Ok(_) => {
+                    let results = interp.get_test_results();
+                    if results.failed == 0 && results.passed == 0 {
+                        // No Test assertions, just check it runs
+                        println!("  {} ... OK (no assertions)", path_str);
+                        total_passed += 1;
+                    } else if results.failed == 0 {
+                        println!("  {} ... OK ({} assertions)", path_str, results.passed);
+                        total_passed += results.passed;
+                    } else {
+                        println!("  {} ... FAILED ({} passed, {} failed)", path_str, results.passed, results.failed);
+                        total_passed += results.passed;
+                        total_failed += results.failed;
+                        for failure in &results.failures {
+                            all_failures.push((path_str.clone(), "".to_string(), failure.clone()));
                         }
-            Ok(output) => {
-                println!("FAILED");
-                if !output.stderr.is_empty() {
-                    eprintln!("{}", String::from_utf8_lossy(&output.stderr));
                     }
-                failed += 1;
                 }
                 Err(e) => {
-                println!("ERROR: {}", e);
+                    println!("  {} ... RUNTIME ERROR: {}", path_str, e);
-                failed += 1;
+                    total_failed += 1;
+                }
+            }
+        } else {
+            // Run individual test functions
+            println!("  {}:", path_str);
+
+            for test_name in &test_funcs {
+                let mut interp = Interpreter::new();
+                interp.reset_test_results();
+
+                // First run the file to define all functions
+                if let Err(e) = interp.run(&program) {
+                    println!("    {} ... ERROR: {}", test_name, e);
+                    total_failed += 1;
+                    continue;
+                }
+
+                // Call the test function
+                let call_source = format!("let testResult = run {}() with {{}}", test_name);
+                let call_program = match Parser::parse_source(&call_source) {
+                    Ok(p) => p,
+                    Err(e) => {
+                        println!("    {} ... ERROR: {}", test_name, e);
+                        total_failed += 1;
+                        continue;
+                    }
+                };
+
+                match interp.run(&call_program) {
+                    Ok(_) => {
+                        let results = interp.get_test_results();
+                        if results.failed == 0 {
+                            println!("    {} ... OK", test_name);
+                            total_passed += 1;
+                        } else {
+                            println!("    {} ... FAILED", test_name);
+                            total_failed += 1;
+                            for failure in &results.failures {
+                                all_failures.push((path_str.clone(), test_name.clone(), failure.clone()));
+                            }
+                        }
+                    }
+                    Err(e) => {
+                        println!("    {} ... ERROR: {}", test_name, e);
+                        total_failed += 1;
+                    }
+                }
             }
         }
     }
 
+    // Print failure details
+    if !all_failures.is_empty() {
+        println!("\n--- Failures ---\n");
+        for (file, test, failure) in &all_failures {
+            if test.is_empty() {
+                println!("{}:", file);
+            } else {
+                println!("{} - {}:", file, test);
+            }
+            println!("  {}", failure.message);
+            if let Some(expected) = &failure.expected {
+                println!("  Expected: {}", expected);
+            }
+            if let Some(actual) = &failure.actual {
+                println!("  Actual:   {}", actual);
+            }
             println!();
-    println!("Results: {} passed, {} failed", passed, failed);
+        }
+    }
 
-    if failed > 0 {
+    println!("Results: {} passed, {} failed", total_passed, total_failed);
+
+    if total_failed > 0 {
         std::process::exit(1);
     }
 }
 
+fn collect_test_files(dir: &str, pattern: Option<&str>, files: &mut Vec<std::path::PathBuf>) {
+    use std::fs;
+
+    for entry in fs::read_dir(dir).into_iter().flatten().flatten() {
+        let path = entry.path();
+        if path.extension().map(|e| e == "lux").unwrap_or(false) {
+            if let Some(name) = path.file_name().and_then(|n| n.to_str()) {
+                if name.starts_with("test_") || name.ends_with("_test.lux") {
+                    if pattern.map(|p| name.contains(p)).unwrap_or(true) {
+                        files.push(path);
+                    }
+                }
+            }
+        }
+    }
+}
+
 fn watch_file(path: &str) {
     use std::time::{Duration, SystemTime};
     use std::path::Path;

src/typechecker.rs

@@ -1532,7 +1532,7 @@ impl TypeChecker {
         }
 
         // Built-in effects are always available
-        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http", "HttpServer"];
+        let builtin_effects = ["Console", "Fail", "State", "Reader", "Random", "Time", "File", "Process", "Http", "HttpServer", "Test"];
         let is_builtin = builtin_effects.contains(&effect.name.as_str());
 
         // Track this effect for inference

src/types.rs

@@ -1123,6 +1123,56 @@ impl TypeEnv {
             },
         );
 
+        // Add Test effect for test framework
+        env.effects.insert(
+            "Test".to_string(),
+            EffectDef {
+                name: "Test".to_string(),
+                type_params: Vec::new(),
+                operations: vec![
+                    EffectOpDef {
+                        name: "assert".to_string(),
+                        params: vec![
+                            ("condition".to_string(), Type::Bool),
+                            ("message".to_string(), Type::String),
+                        ],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "assertEqual".to_string(),
+                        params: vec![
+                            ("expected".to_string(), Type::Var(0)),
+                            ("actual".to_string(), Type::Var(0)),
+                        ],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "assertNotEqual".to_string(),
+                        params: vec![
+                            ("a".to_string(), Type::Var(0)),
+                            ("b".to_string(), Type::Var(0)),
+                        ],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "assertTrue".to_string(),
+                        params: vec![("condition".to_string(), Type::Bool)],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "assertFalse".to_string(),
+                        params: vec![("condition".to_string(), Type::Bool)],
+                        return_type: Type::Unit,
+                    },
+                    EffectOpDef {
+                        name: "fail".to_string(),
+                        params: vec![("message".to_string(), Type::String)],
+                        return_type: Type::Unit,
+                    },
+                ],
+            },
+        );
+
         // Add Some and Ok, Err constructors
         // Some : fn(a) -> Option<a>
         let a = Type::var();