compiler/TypeChecker.cs

@@ -24,7 +24,7 @@ public class TypeChecker
     private readonly NodeDefinitionFunc function;
     private NubType functionReturnType = null!;
     private readonly ModuleGraph moduleGraph;
-    private readonly Scope scope = new();
+    private readonly Stack<Dictionary<string, NubType>> scopes = new();
 
     private TypedNodeDefinitionFunc? CheckFunction(out List<Diagnostic> diagnostics)
     {
@@ -51,7 +51,7 @@ public class TypeChecker
             }
         }
 
-        using (scope.EnterScope())
+        using (EnterScope())
         {
             foreach (var parameter in function.Parameters)
             {
@@ -60,6 +60,7 @@ public class TypeChecker
                 try
                 {
                     parameterType = ResolveType(parameter.Type);
+                    DeclareLocalIdentifier(parameter.Name, parameterType);
                 }
                 catch (CompileException e)
                 {
@@ -68,7 +69,6 @@ public class TypeChecker
                     continue;
                 }
 
-                scope.DeclareIdentifier(parameter.Name.Ident, parameterType);
                 parameters.Add(new TypedNodeDefinitionFunc.Param(parameter.Tokens, parameter.Name, parameterType));
             }
 
@@ -114,7 +114,7 @@ public class TypeChecker
 
     private TypedNodeStatementBlock CheckStatementBlock(NodeStatementBlock statement)
     {
-        using (scope.EnterScope())
+        using (EnterScope())
         {
             var statements = statement.Statements.Select(CheckStatement).ToList();
             return new TypedNodeStatementBlock(statement.Tokens, statements);
@@ -137,8 +137,19 @@ public class TypeChecker
         if (!condition.Type.IsAssignableTo(NubTypeBool.Instance))
             throw BasicError("Condition part of if statement must be a boolean", condition);
 
-        var thenBlock = CheckStatement(statement.ThenBlock);
+        TypedNodeStatement thenBlock;
-        var elseBlock = statement.ElseBlock == null ? null : CheckStatement(statement.ElseBlock);
+
+        using (EnterScope())
+        {
+            thenBlock = CheckStatement(statement.ThenBlock);
+        }
+
+        TypedNodeStatement? elseBlock;
+
+        using (EnterScope())
+        {
+            elseBlock = statement.ElseBlock == null ? null : CheckStatement(statement.ElseBlock);
+        }
 
         return new TypedNodeStatementIf(statement.Tokens, condition, thenBlock, elseBlock);
     }
@@ -175,7 +186,7 @@ public class TypeChecker
 
         type ??= value.Type;
 
-        scope.DeclareIdentifier(statement.Name.Ident, type);
+        DeclareLocalIdentifier(statement.Name, type);
 
         return new TypedNodeStatementVariableDeclaration(statement.Tokens, statement.Name, type, value);
     }
@@ -186,10 +197,12 @@ public class TypeChecker
         if (!condition.Type.IsAssignableTo(NubTypeBool.Instance))
             throw BasicError("Condition part of if statement must be a boolean", condition);
 
+        using (EnterScope())
+        {
             var body = CheckStatement(statement.Body);
-
             return new TypedNodeStatementWhile(statement.Tokens, condition, body);
         }
+    }
 
     private TypedNodeStatementMatch CheckStatementMatch(NodeStatementMatch statement)
     {
@@ -214,14 +227,14 @@ public class TypeChecker
 
             uncoveredCases.Remove(@case.Variant.Ident);
 
-            using (scope.EnterScope())
+            using (EnterScope())
             {
                 if (@case.VariableName != null)
                 {
                     if (variant.Type is null)
                         throw BasicError("Cannot capture variable for enum variant without type", @case.VariableName);
 
-                    scope.DeclareIdentifier(@case.VariableName.Ident, variant.Type);
+                    DeclareLocalIdentifier(@case.VariableName, variant.Type);
                 }
 
                 var body = CheckStatement(@case.Body);
@@ -416,14 +429,14 @@ public class TypeChecker
     {
         if (expression.Sections.Count == 1)
         {
-            var name = expression.Sections[0].Ident;
+            var name = expression.Sections[0];
 
-            var localType = scope.GetIdentifierType(name);
+            var localType = GetIdentifierType(name.Ident);
             if (localType is not null)
-                return new TypedNodeExpressionLocalIdent(expression.Tokens, localType, name);
+                return new TypedNodeExpressionLocalIdent(expression.Tokens, localType, name.Ident);
 
-            if (moduleGraph.TryResolveIdentifier(currentModule, name, true, out var ident))
+            if (moduleGraph.TryResolveIdentifier(currentModule, name.Ident, true, out var ident))
-                return new TypedNodeExpressionGlobalIdent(expression.Tokens, ident.Type, currentModule, name);
+                return new TypedNodeExpressionGlobalIdent(expression.Tokens, ident.Type, currentModule, name.Ident);
         }
         else if (expression.Sections.Count == 2)
         {
@@ -755,19 +768,13 @@ public class TypeChecker
         return new CompileException(Diagnostic.Error(message).At(fileName, node).Build());
     }
 
-    private sealed class Scope
+    public void DeclareLocalIdentifier(TokenIdent name, NubType type)
     {
-        private readonly Stack<Dictionary<string, NubType>> scopes = new();
+        var existing = GetIdentifierType(name.Ident);
+        if (existing is not null)
+            throw BasicError($"Local identifier '{name.Ident}' is already defined", name);
 
-        public IDisposable EnterScope()
+        scopes.Peek().Add(name.Ident, type);
-        {
-            scopes.Push([]);
-            return new ScopeGuard(this);
-        }
-
-        public void DeclareIdentifier(string name, NubType type)
-        {
-            scopes.Peek().Add(name, type);
     }
 
     public NubType? GetIdentifierType(string name)
@@ -783,12 +790,18 @@ public class TypeChecker
         return null;
     }
 
+    public IDisposable EnterScope()
+    {
+        scopes.Push([]);
+        return new ScopeGuard(this);
+    }
+
     private void ExitScope()
     {
         scopes.Pop();
     }
 
-        private sealed class ScopeGuard(Scope owner) : IDisposable
+    private sealed class ScopeGuard(TypeChecker owner) : IDisposable
     {
         public void Dispose()
         {
@@ -796,7 +809,6 @@ public class TypeChecker
         }
     }
 }
-}
 
 public abstract class TypedNode(List<Token> tokens)
 {