Refactor type variable counter to be shared

src/ast/compiler/type_checker/inference.rs

@@ -34,6 +34,11 @@ impl TypeChecker {
         match ty {
             StaticType::TypeVar(n) => {
                 if let Some(resolved) = subst.get(&n) {
+                    // Guard against self-referential substitutions (TypeVar(n) → TypeVar(n))
+                    // which would cause infinite recursion.
+                    if *resolved == StaticType::TypeVar(n) {
+                        return StaticType::TypeVar(n);
+                    }
                     // Follow the chain (handles transitive substitutions)
                     Self::apply_subst(resolved.clone(), subst)
                 } else {

src/ast/compiler/type_checker/mod.rs

@@ -18,8 +18,10 @@ use context::TypeContext;
 
 pub struct TypeChecker {
     pub(crate) root_types: Rc<std::cell::RefCell<Vec<StaticType>>>,
-    /// Monotonic counter for generating unique type variable IDs.
-    pub(crate) var_counter: Cell<u32>,
+    /// Shared monotonic counter for generating unique type variable IDs.
+    /// Points to the `Environment`'s counter so that IDs never collide
+    /// with TypeVars stored in `root_types` from earlier compilation passes.
+    pub(crate) var_counter: Rc<Cell<u32>>,
     /// Global substitution map: TypeVar ID → resolved StaticType.
     /// Shared across all scopes within a single type-checking pass.
     pub(crate) subst: std::cell::RefCell<HashMap<u32, StaticType>>,
@@ -39,11 +41,12 @@ pub struct TypeChecker {
 impl TypeChecker {
     pub fn new(
         root_types: Rc<std::cell::RefCell<Vec<StaticType>>>,
+        var_counter: Rc<Cell<u32>>,
         compiler_hooks: Rc<HashMap<u32, Rc<dyn RtlCompilerHook>>>,
     ) -> Self {
         Self {
             root_types,
-            var_counter: Cell::new(0),
+            var_counter,
             subst: std::cell::RefCell::new(HashMap::new()),
             index_constraints: std::cell::RefCell::new(HashMap::new()),
             compiler_hooks,

src/ast/environment.rs

@@ -5,7 +5,7 @@ use crate::ast::compiler::{CapturePass, TypeChecker, TypedNode};
 use crate::ast::nodes::{AnalyzedPhase, BoundPhase, Symbol, SyntaxKind, SyntaxNode};
 use crate::ast::parser::Parser;
 use crate::ast::vm::{GlobalStore, TracingObserver, VM};
-use std::cell::RefCell;
+use std::cell::{Cell, RefCell};
 use std::collections::{HashMap, HashSet};
 use std::path::{Path, PathBuf};
 use std::rc::Rc;
@@ -71,6 +71,11 @@ pub struct Environment {
     pub function_registry: Rc<RefCell<GlobalFunctionRegistry>>,
     pub typed_function_registry: Rc<RefCell<GlobalAnalyzedRegistry>>,
     pub monomorph_cache: Rc<RefCell<MonoCache>>,
+    /// Shared monotonic counter for generating unique HM type-variable IDs.
+    /// Survives across multiple `compile()` / `run_script()` calls so that
+    /// TypeVar IDs created in one pass never collide with IDs stored in
+    /// `root_types` from earlier passes (prevents `Forall` self-substitution loops).
+    pub(crate) typevar_counter: Rc<Cell<u32>>,
     pub debug_mode: bool,
     pub optimization: bool,
     pub macro_registry: Rc<RefCell<MacroRegistry>>,
@@ -106,6 +111,7 @@ struct RuntimeMacroEvaluator {
     root_scopes: Rc<RefCell<Vec<CompilerScope>>>,
     root_slot_count: Rc<RefCell<u32>>,
     root_types: Rc<RefCell<Vec<StaticType>>>,
+    typevar_counter: Rc<Cell<u32>>,
     globals: GlobalStore,
     root_purity: Rc<RefCell<Vec<Purity>>>,
     compiler_hooks: Rc<HashMap<u32, Rc<dyn RtlCompilerHook>>>,
@@ -129,7 +135,7 @@ impl MacroEvaluator for RuntimeMacroEvaluator {
 
         let (bound_ast, captures, _, _) = Binder::bind_root(initial_scopes, initial_slot_count, node, &mut diag)?;
         let bound_ast = CapturePass::apply(bound_ast, &captures);
-        let checker = TypeChecker::new(self.root_types.clone(), Rc::clone(&self.compiler_hooks));
+        let checker = TypeChecker::new(self.root_types.clone(), Rc::clone(&self.typevar_counter), Rc::clone(&self.compiler_hooks));
         let typed_ast = checker.check(&bound_ast, &[], &mut diag);
 
         if diag.has_errors() {
@@ -162,6 +168,7 @@ impl Environment {
             function_registry: Rc::new(RefCell::new(HashMap::new())),
             typed_function_registry: Rc::new(RefCell::new(HashMap::new())),
             monomorph_cache: Rc::new(RefCell::new(HashMap::new())),
+            typevar_counter: Rc::new(Cell::new(0)),
             debug_mode: false,
             optimization: true,
             macro_registry: Rc::new(RefCell::new(MacroRegistry::new())),
@@ -254,6 +261,7 @@ impl Environment {
             function_registry: Rc::new(RefCell::new(HashMap::new())),
             typed_function_registry: Rc::new(RefCell::new(HashMap::new())),
             monomorph_cache: Rc::new(RefCell::new(HashMap::new())),
+            typevar_counter: Rc::new(Cell::new(0)),
             debug_mode: false,
             optimization: true,
             macro_registry: Rc::new(RefCell::new(MacroRegistry::new())),
@@ -321,6 +329,7 @@ impl Environment {
             root_scopes: self.root_scopes.clone(),
             root_slot_count: self.root_slot_count.clone(),
             root_types: self.root_types.clone(),
+            typevar_counter: Rc::clone(&self.typevar_counter),
             globals: self.global_store(),
             root_purity: self.root_purity.clone(),
             compiler_hooks: Rc::clone(&self.rtl.compiler_hooks),
@@ -447,7 +456,7 @@ impl Environment {
 
     /// Type-checks a bound AST and collects doc comments.
     fn type_check(&self, bound_ast: &Node<BoundPhase>, diagnostics: &mut Diagnostics) -> TypedNode {
-        let checker = TypeChecker::new(self.root_types.clone(), Rc::clone(&self.rtl.compiler_hooks));
+        let checker = TypeChecker::new(self.root_types.clone(), Rc::clone(&self.typevar_counter), Rc::clone(&self.rtl.compiler_hooks));
         let typed = checker.check(bound_ast, &[], diagnostics);
         self.collect_doc_comments(&typed);
         typed
@@ -766,13 +775,14 @@ impl Environment {
         let root_purity = self.root_purity.clone();
         let optimization = self.optimization;
         let compiler_hooks = Rc::clone(&self.rtl.compiler_hooks);
+        let typevar_counter = Rc::clone(&self.typevar_counter);
 
         let compiler = Rc::new(
             move |func_template: Rc<Node<AnalyzedPhase>>,
                   arg_types: &[StaticType]|
                   -> Result<(Value, StaticType), String> {
                 let mut diag = Diagnostics::new();
-                let checker = TypeChecker::new(root_types.clone(), Rc::clone(&compiler_hooks));
+                let checker = TypeChecker::new(root_types.clone(), Rc::clone(&typevar_counter), Rc::clone(&compiler_hooks));
 
                 // Monomorphization: re-type-check the function template with the concrete
                 // call-site argument types, producing a specialized TypedNode.

src/ast/rtl/prelude.myc

@@ -1,8 +1,6 @@
 ;; Myc System Library
 ;; Standard macros and core utilities.
 
-(def func1 (fn [] (print "Hello World")))
-
 (macro while [cond body]
   `((fn [] (if ~cond
              (do ~body (again))

tests/repl.rs

@@ -0,0 +1,57 @@
+use myc::ast::environment::Environment;
+use myc::ast::types::Value;
+
+#[test]
+fn repl_preserves_variable_bindings() {
+    let env = Environment::new();
+
+    let r1 = env.run_script("(def x 42)");
+    assert!(r1.is_ok(), "def should succeed: {:?}", r1.err());
+
+    let r2 = env.run_script("(+ x 1)");
+    assert_eq!(r2.unwrap(), Value::Int(43));
+}
+
+#[test]
+fn repl_preserves_function_bindings() {
+    let env = Environment::new();
+
+    let r1 = env.run_script("(def double (fn [n] (* n 2)))");
+    assert!(r1.is_ok(), "def fn should succeed: {:?}", r1.err());
+
+    let r2 = env.run_script("(double 21)");
+    assert_eq!(r2.unwrap(), Value::Int(42));
+}
+
+#[test]
+fn repl_function_uses_prior_variable() {
+    let env = Environment::new();
+
+    env.run_script("(def x 10)").unwrap();
+    env.run_script("(def double (fn [n] (* n 2)))").unwrap();
+
+    let result = env.run_script("(double x)");
+    assert_eq!(result.unwrap(), Value::Int(20));
+}
+
+#[test]
+fn repl_reassign_variable() {
+    let env = Environment::new();
+
+    env.run_script("(def x 1)").unwrap();
+    env.run_script("(assign x 99)").unwrap();
+
+    let result = env.run_script("x");
+    assert_eq!(result.unwrap(), Value::Int(99));
+}
+
+#[test]
+fn repl_macro_from_prior_call() {
+    let env = Environment::new();
+
+    env.run_script("(macro unless [c body] `(if ~c ... ~body))")
+        .unwrap();
+
+    let result = env.run_script(r#"(unless false "yes")"#);
+    assert_eq!(result.unwrap(), Value::Text("yes".into()));
+}