From 64e048dfcd85a6ee4acaaafd67e3c6772e8c2c07 Mon Sep 17 00:00:00 2001
From: tijani <tijani@tijanilawal.com>
Date: Tue, 9 May 2023 23:57:45 +0000
Subject: [PATCH] String concatenation now a thing.

git-svn-id: https://svn.tlawal.org/svn/monkey@64 f6afcba9-9ef1-4bdd-9b72-7484f5705bac
---
 evaluator/evaluator.go      | 16 ++++++++++++++--
 evaluator/evaluator_test.go | 18 ++++++++++++++++++
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/evaluator/evaluator.go b/evaluator/evaluator.go
index 8f8990c..fef25ea 100644
--- a/evaluator/evaluator.go
+++ b/evaluator/evaluator.go
@@ -86,9 +86,8 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		}
 		return apply_function(function, args)
 
-
 	case *ast.StringLiteral:
-		return &object.String { Value: node.Value }
+		return &object.String{Value: node.Value}
 	}
 
 	return nil
@@ -232,6 +231,9 @@ func eval_infix_expression(operator string, left object.Object, right object.Obj
 
 	case left.Type() != right.Type():
 		return new_error("type mismatch: %s %s %s", left.Type(), operator, right.Type())
+
+	case left.Type() == object.STRING_OBJECT && right.Type() == object.STRING_OBJECT:
+		return eval_string_infix_expression(operator, left, right)
 	default:
 		return new_error("unknown operator: %s %s %s", left.Type(), operator, right.Type())
 	}
@@ -287,6 +289,16 @@ func eval_if_expression(ie *ast.IfExpression, env *object.Environment) object.Ob
 	}
 }
 
+func eval_string_infix_expression(operator string, left, right object.Object) object.Object {
+	if operator != "+" {
+		return new_error("unknown operator: %s %s %s", left.Type(), operator, right.Type())
+	}
+
+	left_value := left.(*object.String).Value
+	right_value := right.(*object.String).Value
+	return &object.String{Value: left_value + right_value}
+}
+
 func is_truthy(object object.Object) bool {
 	switch object {
 	case NULL:
diff --git a/evaluator/evaluator_test.go b/evaluator/evaluator_test.go
index eb228af..058ae42 100644
--- a/evaluator/evaluator_test.go
+++ b/evaluator/evaluator_test.go
@@ -56,6 +56,10 @@ func TestErrorHandling(l_test *testing.T) {
        }
       `, "unknown operator: BOOLEAN + BOOLEAN"},
 		{"foobar", "identifier not found: foobar"},
+		{
+			`"Hello" - "World"`,
+			"unknown operator: STRING - STRING",
+		},
 	}
 
 	for _, tt := range tests {
@@ -252,6 +256,20 @@ func TestStringLiteral(l_test *testing.T) {
 	}
 }
 
+func TestStringConcatenation(l_test *testing.T) {
+	input := `"Hello" + " " + "World!"`
+
+	evaluated := test_eval(input)
+	string, ok := evaluated.(*object.String)
+	if !ok {
+		l_test.Fatalf("object is not String. got=%T (%+v)", evaluated, evaluated)
+	}
+
+	if string.Value != "Hello World!" {
+		l_test.Errorf("String has wrong value. got=%q", string.Value)
+	}
+}
+
 // Helpers
 func test_eval(input string) object.Object {
 	l_lexer := lexer.New(input)