Check for duplicate variable names

2 files changed, 33 insertions, 6 deletions

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
index feafa5f4..bf09b84b 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
@@ -69,20 +69,20 @@ public class TypeChecker {
         });
     }
 
-    private static List<String> definedVariableNames(UntypedFunctionStatementNode statement) {
+    private static List<UntypedTargetNode.Variable> targetVariables(UntypedFunctionStatementNode statement) {
         return switch (statement) {
             case UntypedExpressionStatementNode expressionStatement ->
                 List.of();
 
             case UntypedValNode val ->
-                definedVariableNames(val.target());
+                targetVariables(val.target());
         };
     }
 
-    private static List<String> definedVariableNames(UntypedTargetNode target) {
+    private static List<UntypedTargetNode.Variable> targetVariables(UntypedTargetNode target) {
         return switch (target) {
             case UntypedTargetNode.Variable variable ->
-                List.of(variable.name());
+                List.of(variable);
         };
     }
 
@@ -259,6 +259,7 @@ public class TypeChecker {
 
         var typedReturnType = typeCheckTypeLevelExpression(TypeSets.META_TYPE, untyped.returnType(), context);
 
+        // TODO: check for duplicate param names
         var bodyEnvironment = typedParams.all()
             .map(param -> Map.entry(
                 param.name(),
@@ -267,8 +268,17 @@ public class TypeChecker {
             .collect(PCollectors.toMap());
 
         for (var statement : untyped.body()) {
-            for (var definedVariableName : definedVariableNames(statement)) {
-                bodyEnvironment = bodyEnvironment.plus(definedVariableName, TypeCheckerContext.undefined());
+            for (var targetVariable : targetVariables(statement)) {
+                if (bodyEnvironment.containsKey(targetVariable.name())) {
+                    throw new DuplicateVariableName(
+                        targetVariable.name(),
+                        targetVariable.source()
+                    );
+                }
+                bodyEnvironment = bodyEnvironment.plus(
+                    targetVariable.name(),
+                    TypeCheckerContext.undefined()
+                );
             }
         }
 
diff --git a/src/main/java/org/shedlang/compiler/typechecker/errors/DuplicateVariableName.java b/src/main/java/org/shedlang/compiler/typechecker/errors/DuplicateVariableName.java
new file mode 100644
index 00000000..21771987
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/typechecker/errors/DuplicateVariableName.java
@@ -0,0 +1,17 @@
+package org.shedlang.compiler.typechecker.errors;
+
+import org.shedlang.compiler.errors.SourceError;
+import org.shedlang.compiler.sources.Source;
+
+public class DuplicateVariableName extends SourceError  {
+    private final String variableName;
+
+    public DuplicateVariableName(String variableName, Source source) {
+        super("Duplicate variable name: " + variableName, source);
+        this.variableName = variableName;
+    }
+
+    public String variableName() {
+        return variableName;
+    }
+}