4.2 Data Types (string concatentation)

cedrickchee · cedrickchee · commit 8afb5f238081 · 2020-04-01T13:06:00.000+08:00
diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
@@ -249,6 +249,8 @@ func evalInfixExpression(
 		// The check for integer operands has to be higher up in the switch
 		// statement.
 		return evalIntegerInfixExpression(operator, left, right)
+	case left.Type() == object.STRING_OBJ && right.Type() == object.STRING_OBJ:
+		return evalStringInfixExpression(operator, left, right)
 	case operator == "==":
 		// Using pointer comparison to check for equality between booleans.
 		return nativeBoolToBooleanObject(left == right)
@@ -294,6 +296,23 @@ func evalIntegerInfixExpression(
 	}
 }
 
+func evalStringInfixExpression(
+	operator string,
+	left, right object.Object,
+) object.Object {
+	// Check for the correct operator.
+	if operator != "+" {
+		return newError("unknown operator: %s %s %s",
+			left.Type(), operator, right.Type())
+	}
+
+	// Unwrap the string objects and construct a new string that's a
+	// concatenation of both operands.
+	leftVal := left.(*object.String).Value
+	rightVal := right.(*object.String).Value
+	return &object.String{Value: leftVal + rightVal}
+}
+
 func evalIfExpression(
 	ie *ast.IfExpression,
 	env *object.Environment,
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
@@ -206,6 +206,10 @@ func TestErrorHandling(t *testing.T) {
 			"5; true + false; 5",
 			"unknown operator: BOOLEAN + BOOLEAN",
 		},
+		{
+			`"Hello" - "World"`,
+			"unknown operator: STRING - STRING",
+		},
 		{
 			"if (10 > 1) { true + false; }",
 			"unknown operator: BOOLEAN + BOOLEAN",
@@ -345,6 +349,20 @@ func TestStringLiteral(t *testing.T) {
 	}
 }
 
+func TestStringConcatenation(t *testing.T) {
+	input := `"Hello" + " " + "World!"`
+
+	evaluated := testEval(input)
+	str, ok := evaluated.(*object.String)
+	if !ok {
+		t.Fatalf("object is not String. got=%T (%+v)", evaluated, evaluated)
+	}
+
+	if str.Value != "Hello World!" {
+		t.Errorf("String has wrong value. got=%q", str.Value)
+	}
+}
+
 func testEval(input string) object.Object {
 	l := lexer.New(input)
 	p := parser.New(l)
