string like array access

evaluator/evaluator.go

@@ -376,6 +376,8 @@ func unwrapReturnValue(obj object.Object) object.Object {
 
 func evalIndexExpression(left, index object.Object) object.Object {
 	switch {
+	case left.Type() == object.STRING_OBJ && index.Type() == object.INTEGER_OBJ:
+		return evalStringIndexExpression(left, index)
 	case left.Type() == object.ARRAY_OBJ && index.Type() == object.INTEGER_OBJ:
 		return evalArrayIndexExpression(left, index)
 	case left.Type() == object.HASH_OBJ:
@@ -385,6 +387,18 @@ func evalIndexExpression(left, index object.Object) object.Object {
 	}
 }
 
+func evalStringIndexExpression(str, index object.Object) object.Object {
+	stringObject := str.(*object.String)
+	idx := index.(*object.Integer).Value
+	max := int64((len(stringObject.Value)) - 1)
+
+	if idx < 0 || idx > max {
+		return &object.String{Value: ""}
+	}
+
+	return &object.String{Value: string(stringObject.Value[idx])}
+}
+
 func evalHashIndexExpression(hash, index object.Object) object.Object {
 	hashObject := hash.(*object.Hash)
 

evaluator/evaluator_test.go

@@ -647,3 +647,53 @@ func TestExamples(t *testing.T) {
 		testEval(string(b))
 	}
 }
+
+func TestStringIndexExpressions(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected interface{}
+	}{
+		{
+			`"abc"[0]`,
+			"a",
+		},
+		{
+			`"abc"[1]`,
+			"b",
+		},
+		{
+			`"abc"[2]`,
+			"c",
+		},
+		{
+			`let i = 0; "abc"[i];`,
+			"a",
+		},
+		{
+			`"abc"[1 + 1];`,
+			"c",
+		},
+		{
+			`let myString = "abc"; myString[0] + myString[1] + myString[2];`,
+			"abc",
+		},
+		{
+			`"abc"[3]`,
+			"",
+		},
+		{
+			`"foo"[-1]`,
+			"",
+		},
+	}
+
+	for _, tt := range tests {
+		evaluated := testEval(tt.input)
+		str, ok := tt.expected.(string)
+		if ok {
+			testStringObject(t, evaluated, string(str))
+		} else {
+			testNullObject(t, evaluated)
+		}
+	}
+}