reassign values

evaluator/evaluator.go

@@ -45,6 +45,26 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 		}
 		return &object.ReturnValue{Value: val}
 
+	case *ast.AssignmentStatement:
+		ident := evalIdentifier(node.Name, env)
+		if isError(ident) {
+			return ident
+		}
+
+		val := Eval(node.Value, env)
+		if isError(val) {
+			return val
+		}
+
+		obj, ok := ident.(object.Mutable)
+		if !ok {
+			return newError("cannot assign to %s", ident.Type())
+		}
+
+		obj.Set(val)
+
+		return val
+
 	case *ast.LetStatement:
 		val := Eval(node.Value, env)
 		if isError(val) {

evaluator/evaluator_test.go

@@ -238,6 +238,26 @@ func TestErrorHandling(t *testing.T) {
 	}
 }
 
+func TestAssignmentStatements(t *testing.T) {
+	tests := []struct {
+		input    string
+		expected int64
+	}{
+		{"let a = 0; a = 5;", 5},
+		{"let a = 0; a = 5; a;", 5},
+		{"let a = 0; a = 5 * 5;", 25},
+		{"let a = 0; a = 5 * 5; a;", 25},
+		{"let a = 0; a = 5; let b = 0; b = a;", 5},
+		{"let a = 0; a = 5; let b = 0; b = a; b;", 5},
+		{"let a = 0; a = 5; let b = 0; b = a; let c = 0; c = a + b + 5;", 15},
+		{"let a = 0; a = 5; let b = 0; b = a; let c = 0; c = a + b + 5; c;", 15},
+	}
+
+	for _, tt := range tests {
+		testIntegerObject(t, testEval(tt.input), tt.expected)
+	}
+}
+
 func TestLetStatements(t *testing.T) {
 	tests := []struct {
 		input    string
@@ -560,10 +580,8 @@ func TestWhileExpressions(t *testing.T) {
 		{"while (false) { }", nil},
 		{"let n = 0; while (n < 10) { let n = n + 1 }; n", 10},
 		{"let n = 10; while (n > 0) { let n = n - 1 }; n", 0},
-		// FIXME: let is an expression statement and bind new values
-		//        there is currently no assignment expressions :/
-		{"let n = 0; while (n < 10) { let n = n + 1 }", nil},
-		{"let n = 10; while (n > 0) { let n = n - 1 }", nil},
+		{"let n = 0; while (n < 10) { n = n + 1 }", 10},
+		{"let n = 10; while (n > 0) { n = n - 1 }", 0},
 	}
 
 	for _, tt := range tests {