Evaluating Hash Index Value

evaluator/evaluator.go

@@ -298,6 +298,8 @@ func evalIndexExpression(left, index object.Object) object.Object {
 	switch {
 	case left.Type() == object.ArrayObj && index.Type() == object.IntegerObj:
 		return evalArrayIndexExpression(left, index)
+	case left.Type() == object.HashObj:
+		return evalHashIndexExpression(left, index)
 	default:
 		return newError("index operator not supported: %s", left.Type())
 	}
@@ -313,6 +315,20 @@ func evalArrayIndexExpression(array, index object.Object) object.Object {
 	}
 	return arrayObj.Elements[idx]
 }
+func evalHashIndexExpression(hash, index object.Object) object.Object {
+	hashObj := hash.(*object.Hash)
+
+	key, ok := index.(object.Hashtable)
+	if !ok {
+		return newError("unusable as hash key: %s", index.Type())
+	}
+
+	pair, ok := hashObj.Pairs[key.HashKey()]
+	if !ok {
+		return NULL
+	}
+	return pair.Value
+}
 func evalHashLiteral(node *ast.HashLiteral, env *object.Environment) object.Object {
 	pairs := make(map[object.HashKey]object.HashPair)
 

evaluator/evaluator_test.go

@@ -170,6 +170,10 @@ func TestErrorHandling(t *testing.T) {
 			`"Hello" - "World"`,
 			"unknown operator: STRING - STRING",
 		},
+		{
+			`{"name": "Monkey"}[fn(x){x}]`,
+			"unusable as hash key: FUNCTION",
+		},
 	}
 
 	for _, tt := range tests {
@@ -469,3 +473,48 @@ func TestHashLiterals(t *testing.T) {
 		testIntegerObject(t, pair.Value, expectedValue)
 	}
 }
+
+func TestHashIndexExpression(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected interface{}
+	}{
+		{
+			`{"foo":5}["foo"]`,
+			5,
+		},
+		{
+			`{"foo":5}["bar"]`,
+			nil,
+		},
+		{
+			`let key = "foo"; {"foo":5}[key]`,
+			5,
+		},
+		{
+			`{}["foo"]`,
+			nil,
+		},
+		{
+			`{5:5}[5]`,
+			5,
+		},
+		{
+			`{true: 5}[true]`,
+			5,
+		},
+		{`{false:5}[false]`,
+			5,
+		},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		integer, ok := tt.expected.(int)
+		if ok {
+			testIntegerObject(t, evaluated, int64(integer))
+		} else {
+			testNullObject(t, evaluated)
+		}
+	}
+}