From ecbe7b4a85c23ba0520d5ff6e2682efd889848e9 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Sun, 16 Jul 2023 18:01:21 +0100
Subject: Check that the variable is not already defined

---
 .../compiler/typechecker/TypeCheckerContext.java   | 33 ++++++++++++++--------
 .../errors/VariableIsAlreadyDefinedError.java      | 17 +++++++++++
 2 files changed, 38 insertions(+), 12 deletions(-)
 create mode 100644 src/main/java/org/shedlang/compiler/typechecker/errors/VariableIsAlreadyDefinedError.java

(limited to 'src/main/java')

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerContext.java b/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerContext.java
index 8e9aa206..6f626725 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerContext.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeCheckerContext.java
@@ -5,6 +5,7 @@ import org.pcollections.PMap;
 import org.shedlang.compiler.ast.typed.FreeVariables;
 import org.shedlang.compiler.sources.Source;
 import org.shedlang.compiler.typechecker.errors.UnknownVariableError;
+import org.shedlang.compiler.typechecker.errors.VariableIsAlreadyDefinedError;
 import org.shedlang.compiler.types.*;
 import org.shedlang.compiler.util.PCollectors;
 import org.shedlang.compiler.util.POrderedMap;
@@ -130,20 +131,28 @@ public class TypeCheckerContext {
         Type type,
         Source source
     ) {
-        if (!this.environment.containsKey(name)) {
+        var variable = this.environment.get(name);
+
+        if (variable == null) {
             throw new UnknownVariableError(name, source);
         }
-        // TODO: check if already defined
-        // TODO: check is not defined
-        // TODO: check is defined in this scope
-        return new TypeCheckerContext(
-            this.environment.plus(name, new Variable.Defined(type)),
-            this.effect,
-            this.returnType,
-            this.boundVariables,
-            this.freeVariables,
-            this.namespaceTypeInfo
-        );
+
+        switch (variable) {
+            case Variable.Defined defined -> {
+                throw new VariableIsAlreadyDefinedError(name, source);
+            }
+            case Variable.Undefined undefined -> {
+                // TODO: check is declared in this scope
+                return new TypeCheckerContext(
+                    this.environment.plus(name, new Variable.Defined(type)),
+                    this.effect,
+                    this.returnType,
+                    this.boundVariables,
+                    this.freeVariables,
+                    this.namespaceTypeInfo
+                );
+            }
+        }
     }
 
     public Effect effect() {
diff --git a/src/main/java/org/shedlang/compiler/typechecker/errors/VariableIsAlreadyDefinedError.java b/src/main/java/org/shedlang/compiler/typechecker/errors/VariableIsAlreadyDefinedError.java
new file mode 100644
index 00000000..e781d398
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/typechecker/errors/VariableIsAlreadyDefinedError.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 VariableIsAlreadyDefinedError extends SourceError {
+    private final String variableName;
+
+    public VariableIsAlreadyDefinedError(String variableName, Source source) {
+        super("Variable is already defined: " + variableName, source);
+        this.variableName = variableName;
+    }
+
+    public String variableName() {
+        return variableName;
+    }
+}
-- 
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