From 87f73abf5e7dcec6a9f5ebd213f4a9eb0a5d45db Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Sun, 4 Oct 2015 17:26:27 +0100
Subject: Error if trying to assign to variable not in scope

---
 .../zwobble/couscous/interpreter/Environment.java    | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

(limited to 'src/main/java/org/zwobble/couscous/interpreter/Environment.java')

diff --git a/src/main/java/org/zwobble/couscous/interpreter/Environment.java b/src/main/java/org/zwobble/couscous/interpreter/Environment.java
index ced87c8..5985a94 100644
--- a/src/main/java/org/zwobble/couscous/interpreter/Environment.java
+++ b/src/main/java/org/zwobble/couscous/interpreter/Environment.java
@@ -2,6 +2,7 @@ package org.zwobble.couscous.interpreter;
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Optional;
 
 import org.zwobble.couscous.Project;
 import org.zwobble.couscous.ast.VariableNode;
@@ -9,24 +10,25 @@ import org.zwobble.couscous.values.ConcreteType;
 import org.zwobble.couscous.values.InterpreterValue;
 
 public class Environment {
-    private final Map<Integer, InterpreterValue> stackFrame;
+    private final Map<Integer, Optional<InterpreterValue>> stackFrame;
     private final Project project;
 
-    public Environment(Project project, Map<Integer, InterpreterValue> stackFrame) {
+    public Environment(Project project, Map<Integer, Optional<InterpreterValue>> stackFrame) {
         this.project = project;
         this.stackFrame = new HashMap<>(stackFrame);
     }
 
     public InterpreterValue get(int referentId) {
-        return stackFrame.get(referentId);
+        return stackFrame.get(referentId).get();
     }
 
     public void put(int variableId, InterpreterValue value) {
-        stackFrame.put(variableId, value);
+        checkVariable(variableId);
+        stackFrame.put(variableId, Optional.of(value));
     }
 
     public void put(VariableNode variable, InterpreterValue value) {
-        stackFrame.put(variable.getId(), value);
+        put(variable.getId(), value);
     }
 
     public ConcreteType<?> findClass(String className) {
@@ -34,7 +36,13 @@ public class Environment {
         return project.findClass(className);
     }
 
-    public Environment withStackFrame(Map<Integer, InterpreterValue> stackFrame) {
+    public Environment withStackFrame(Map<Integer, Optional<InterpreterValue>> stackFrame) {
         return new Environment(project, stackFrame);
     }
+
+    private void checkVariable(int variableId) {
+        if (!stackFrame.containsKey(variableId)) {
+            throw new VariableNotInScope(variableId);
+        }
+    }
 }
-- 
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