Implemented `mutate'

src/ast.c

@@ -124,6 +124,7 @@ typedef enum {
     Built_In_Rest,
     Built_In_Load,
     Built_In_Define,
+    Built_In_Mutate,
     Built_In_Lambda,
     Built_In_Eval,
     Built_In_Quote,
@@ -159,6 +160,7 @@ char* Built_In_Name_to_string(Built_In_Name name) {
     case Built_In_List:           return "list";
     case Built_In_Load:           return "load";
     case Built_In_Multiplication: return "*";
+    case Built_In_Mutate:         return "mutate";
     case Built_In_Not:            return "not";
     case Built_In_Or:             return "or";
     case Built_In_Pair:           return "pair";
@@ -261,6 +263,7 @@ Ast_Node* create_ast_node_built_in_function(char* name) {
     else if (string_equal(name, "rest"))   type = Built_In_Rest;
     else if (string_equal(name, "load"))   type = Built_In_Load;
     else if (string_equal(name, "define")) type = Built_In_Define;
+    else if (string_equal(name, "mutate")) type = Built_In_Mutate;
     else if (string_equal(name, "lambda")) type = Built_In_Lambda;
     else if (string_equal(name, "eval"))   type = Built_In_Eval;
     else if (string_equal(name, "quote"))  type = Built_In_Quote;

src/eval.c

@@ -595,10 +595,25 @@ Ast_Node* eval_expr(Ast_Node* node, Environment* env) {
             case Built_In_Equal: {
                 return built_in_equals(evaluated_arguments);
             }
-            case Built_In_Load: {
-                try {
-                    arguments_length = list_length(arguments);
+            case Built_In_Mutate: {
+                if (arguments_length != 2)
+                    report_error(Error_Type_Wrong_Number_Of_Arguments);
+
+                if (arguments->value.pair->first->type == Ast_Node_Type_Nil ||
+                    arguments->value.pair->first->type == Ast_Node_Type_Keyword)
+                {
+                    report_error(Error_Type_Type_Missmatch);
                 }
+
+                Ast_Node* target = evaluated_arguments->value.pair->first;
+                Ast_Node* source = evaluated_arguments->value.pair->rest->value.pair->first;
+
+                target->type = source->type;
+                target->value = source->value;
+
+                return target;
+            }
+            case Built_In_Load: {
                 if (arguments_length != 1)
                     report_error(Error_Type_Wrong_Number_Of_Arguments);