diff --git a/src/built_ins.cpp b/src/built_ins.cpp
index 7fead60..bf87eb9 100644
--- a/src/built_ins.cpp
+++ b/src/built_ins.cpp
@@ -24,7 +24,7 @@ proc lisp_object_equal(Lisp_Object* n1, Lisp_Object* n2) -> bool {
     case Lisp_Object_Type::Number:
         return n1->value.number == n2->value.number;
     case Lisp_Object_Type::Pair:
-        create_error(Error_Type::Not_Yet_Implemented, n1->sourceCodeLocation);
+        create_not_yet_implemented_error();
         return false;
     case Lisp_Object_Type::String:
         return string_equal(n1->value.string, n2->value.string);
@@ -51,7 +51,7 @@ proc built_in_load(String* file_name, Environment* env) -> Lisp_Object* {
         }
         return result;
     } else {
-        create_error(Error_Type::File_Not_Found, nullptr);
+        create_generic_error("The file '%s' was not found", file_name);
         return nullptr;
     }
 }
@@ -318,9 +318,8 @@ proc load_built_ins_into_environment(Environment* env) -> void {
                 arguments = eval_arguments(arguments, env, &arguments_length);
             }
 
-            if (arguments_length != 2) {
-                create_error(Error_Type::Wrong_Number_Of_Arguments, arguments->sourceCodeLocation);
-                return nullptr;
+            try {
+                assert(arguments_length == 2);
             }
 
             try {
@@ -342,28 +341,19 @@ proc load_built_ins_into_environment(Environment* env) -> void {
     defun("assert", cLambda {
             int arguments_length;
 
-            try {
-                arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
 
             if (is_truthy(arguments->value.pair.first, env))
                 return Memory::t;
 
-            report_error(Error_Type::Assertion_Error);
+            create_generic_error("Userland assertion.");
             return nullptr;
         });
     defun("define", cLambda {
-            try {
-                arguments_length = list_length(arguments);
-            }
 
-            if (arguments_length < 2) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length >= 2);
 
             Lisp_Object* symbol = arguments->value.pair.first;
             Lisp_Object* value;
@@ -390,9 +380,7 @@ proc load_built_ins_into_environment(Environment* env) -> void {
 
                 Lisp_Object* real_symbol = symbol->value.pair.first;
 
-                try {
-                    assert_type(real_symbol, Lisp_Object_Type::Symbol);
-                }
+                try assert_type(real_symbol, Lisp_Object_Type::Symbol);
 
                 Lisp_Object* fake_lambda = Memory::create_lisp_object_pair(
                     symbol   ->value.pair.rest,
@@ -401,17 +389,12 @@ proc load_built_ins_into_environment(Environment* env) -> void {
                 value = parse_lambda_starting_from_args(fake_lambda, env);
                 symbol = real_symbol;
             } else {
-                if (arguments_length > 2) {
-                    report_error(Error_Type::Wrong_Number_Of_Arguments);
-                }
-                if (symbol->type != Lisp_Object_Type::Symbol) {
-                    report_error(Error_Type::Type_Missmatch);
-                }
+                try assert(arguments_length <= 2);
+                try assert_type(symbol, Lisp_Object_Type::Symbol);
 
                 value = arguments->value.pair.rest->value.pair.first;
-                try {
-                    value = eval_expr(value, env);
-                }
+
+                try value = eval_expr(value, env);
             }
 
             define_symbol(symbol, value, env);
@@ -454,17 +437,14 @@ proc load_built_ins_into_environment(Environment* env) -> void {
     //         return value;
     //     });
     defun("mutate", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length != 2)
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 2);
 
             if (evaluated_arguments->value.pair.first->type == Lisp_Object_Type::Nil ||
+                evaluated_arguments->value.pair.first->type == Lisp_Object_Type::T   ||
                 evaluated_arguments->value.pair.first->type == Lisp_Object_Type::Keyword)
             {
-                report_error(Error_Type::Type_Missmatch);
+                create_generic_error("You cannot mutate nil, t or keywords");
             }
 
             Lisp_Object* target = evaluated_arguments->value.pair.first;
@@ -474,55 +454,35 @@ proc load_built_ins_into_environment(Environment* env) -> void {
             return target;
         });
     defun("if", cLambda {
-            try {
-                arguments_length = list_length(arguments);
-            }
-
-            if (arguments_length != 2 && arguments_length != 3) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length == 2 || arguments_length == 3);
 
             Lisp_Object* condition = arguments->value.pair.first;
             Lisp_Object* then_part = arguments->value.pair.rest;
             Lisp_Object* else_part = then_part->value.pair.rest;
 
             bool truthy;
-            try {
-                truthy = is_truthy(condition, env);
-            }
+            try truthy = is_truthy(condition, env);
 
             Lisp_Object* result;
 
             if (truthy)
-                try{
-                    result = eval_expr(then_part->value.pair.first, env);
-                }
+                try result = eval_expr(then_part->value.pair.first, env);
             else if (arguments_length == 3)
-                try {
-                    result = eval_expr(else_part->value.pair.first, env);
-                }
+                try result = eval_expr(else_part->value.pair.first, env);
             else return Memory::nil;
+
             return result;
         });
     defun("quote", cLambda {
-            try {
-                arguments_length = list_length(arguments);
-            }
-
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
-
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length == 1);
             return arguments->value.pair.first;
         });
     defun("quasiquote", cLambda {
-            try {
-                arguments_length = list_length(arguments);
-            }
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length == 1);
 
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
             // print(arguments);
             // printf("\n");
 
@@ -582,107 +542,77 @@ proc load_built_ins_into_environment(Environment* env) -> void {
     defun("and", cLambda {
             bool result = true;
             while (arguments->type != Lisp_Object_Type::Nil) {
-                if (arguments->type != Lisp_Object_Type::Pair) {
-                    report_error(Error_Type::Ill_Formed_List);
-                }
-                try {
-                    result &= is_truthy(arguments->value.pair.first, env);
-                }
-                arguments = arguments->value.pair.rest;
+                try assert_type(arguments, Lisp_Object_Type::Pair);
+                try result &= is_truthy(arguments->value.pair.first, env);
 
+                arguments = arguments->value.pair.rest;
                 if (!result) return Memory::nil;
             }
-
             return Memory::t;
         });
     defun("or", cLambda {
             bool result = false;
             while (arguments->type != Lisp_Object_Type::Nil) {
-                if (arguments->type != Lisp_Object_Type::Pair) {
-                    report_error(Error_Type::Ill_Formed_List);
-                }
-                try {
-                    result |= is_truthy(arguments->value.pair.first, env);
-                }
-                arguments = arguments->value.pair.rest;
+                try assert_type(arguments, Lisp_Object_Type::Pair);
+                try result |= is_truthy(arguments->value.pair.first, env);
 
+                arguments = arguments->value.pair.rest;
                 if (result) return Memory::t;
             }
-
             return Memory::nil;
         });
     defun("not", cLambda {
-            try {
-                arguments_length = list_length(arguments);
-            }
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length == 1);
 
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
             bool truthy;
-            try {
-                truthy = is_truthy(arguments->value.pair.first, env);
-            }
-            if (truthy)
-                return Memory::nil;
-            return Memory::t;
+
+            try truthy = is_truthy(arguments->value.pair.first, env);
+
+            return (truthy) ? Memory::nil : Memory::t;
         });
     defun("while", cLambda {
-            try {
-                arguments_length = list_length(arguments);
-            }
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length >= 2);
 
-            if (arguments_length < 2) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
             Lisp_Object* condition_part = arguments->value.pair.first;
             Lisp_Object* condition;
             Lisp_Object* then_part = arguments->value.pair.rest;
             Lisp_Object* result = Memory::nil;
 
             while (true) {
-                try {
-                    condition = eval_expr(condition_part, env);
-                }
-                if (condition->type == Lisp_Object_Type::Nil) {
+                try condition = eval_expr(condition_part, env);
+
+                if (condition->type == Lisp_Object_Type::Nil)
                     break;
-                }
-                try {
-                    result = eval_expr(then_part->value.pair.first, env);
-                }
+
+                try result = eval_expr(then_part->value.pair.first, env);
             }
             return result;
 
         });
     defun("let", cLambda {
             // (let ((a 10)(b 20)) (body1) (body2))
-            try {
-                arguments_length = list_length(arguments);
-            }
-
-            if (arguments_length < 1)
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length >= 1);
 
             Environment* let_env = Memory::create_child_environment(env);
             Lisp_Object* bindings = arguments->value.pair.first;
             while (true) {
                 if (bindings->type == Lisp_Object_Type::Nil) {
                     break;
-                } else if (bindings->type != Lisp_Object_Type::Pair) {
-                    report_error(Error_Type::Ill_Formed_Arguments);
                 }
 
+                try assert_type(bindings, Lisp_Object_Type::Pair);
+
                 Lisp_Object* sym = bindings->value.pair.first->value.pair.first;
-                if(sym->type != Lisp_Object_Type::Symbol) {
-                    report_error(Error_Type::Ill_Formed_Arguments);
-                }
+
+                try assert_type(sym, Lisp_Object_Type::Symbol);
+
                 Lisp_Object* rest_sym = bindings->value.pair.first->value.pair.rest;
-                if (rest_sym->type != Lisp_Object_Type::Pair) {
-                    report_error(Error_Type::Ill_Formed_Arguments);
-                }
-                if (rest_sym->value.pair.rest->type != Lisp_Object_Type::Nil) {
-                    report_error(Error_Type::Ill_Formed_Arguments);
-                }
+
+                try assert_type(rest_sym,                  Lisp_Object_Type::Pair);
+                try assert_type(rest_sym->value.pair.rest, Lisp_Object_Type::Nil);
 
                 Lisp_Object* value = eval_expr(rest_sym->value.pair.first, env);
 
@@ -697,9 +627,7 @@ proc load_built_ins_into_environment(Environment* env) -> void {
             arguments = arguments->value.pair.rest;
 
             Lisp_Object* evaluated_arguments;
-            try {
-                evaluated_arguments = eval_arguments(arguments, let_env, &arguments_length);
-            }
+            try evaluated_arguments = eval_arguments(arguments, let_env, &arguments_length);
 
             if (evaluated_arguments->type == Lisp_Object_Type::Nil)
                 return evaluated_arguments;
@@ -719,56 +647,35 @@ proc load_built_ins_into_environment(Environment* env) -> void {
              *  (lambda ())
              *  (lambda (x d) (+ 1 2) (- 1 2) (* 1 2))
              */
-            try {
-                arguments_length = list_length(arguments);
-            }
-
-            if (arguments_length == 0)
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-
-            Lisp_Object* function = parse_lambda_starting_from_args(arguments, env);
-            // parse lambda starting from arguments
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length != 0);
 
+            Lisp_Object* function = parse_lambda_starting_from_args(arguments, env, false);
 
             return function;
         });
     defun("special-lambda", cLambda {
-            /*
-             *  (special-lambda ())
-             *  (special-lambda (x d) (+ 1 2) (- 1 2) (* 1 2))
-             */
-            try {
-                arguments_length = list_length(arguments);
-            }
-
-            if (arguments_length == 0)
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length != 0);
 
             Lisp_Object* function = parse_lambda_starting_from_args(arguments, env, true);
-            // parse lambda starting from arguments
-
 
             return function;
         });
     defun("eval", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+
             Lisp_Object* result;
-            try {
-                result = eval_expr(evaluated_arguments->value.pair.first, env);
-            }
+
+            try result = eval_expr(evaluated_arguments->value.pair.first, env);
+
             return result;
         });
 
 
     defun("prog", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
 
             if (evaluated_arguments->type == Lisp_Object_Type::Nil)
                 return evaluated_arguments;
@@ -784,84 +691,60 @@ proc load_built_ins_into_environment(Environment* env) -> void {
             return evaluated_arguments->value.pair.first;
         });
     defun("list", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
             return evaluated_arguments;
         });
     defun("pair", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length != 2) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
-            return Memory::create_lisp_object_pair(evaluated_arguments->value.pair.first, evaluated_arguments->value.pair.rest->value.pair.first);
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 2);
+            return Memory::create_lisp_object_pair(
+                evaluated_arguments->value.pair.first,
+                evaluated_arguments->value.pair.rest->value.pair.first);
         });
     defun("first", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+
             if (evaluated_arguments->value.pair.first->type == Lisp_Object_Type::Nil)
                 return Memory::nil;
-            if (evaluated_arguments->value.pair.first->type != Lisp_Object_Type::Pair)
-                report_error(Error_Type::Type_Missmatch);
+
+            try assert_type(evaluated_arguments->value.pair.first, Lisp_Object_Type::Pair);
 
             return evaluated_arguments->value.pair.first->value.pair.first;
         });
     defun("rest", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+
             if (evaluated_arguments->value.pair.first->type == Lisp_Object_Type::Nil)
                 return Memory::nil;
-            if (evaluated_arguments->value.pair.first->type != Lisp_Object_Type::Pair)
-                report_error(Error_Type::Type_Missmatch);
+
+            try assert_type(evaluated_arguments->value.pair.first, Lisp_Object_Type::Pair);
 
             return evaluated_arguments->value.pair.first->value.pair.rest;
         });
     defun("set-type", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-            if (arguments_length != 2) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 2);
 
             Lisp_Object* object = evaluated_arguments->value.pair.first;
             Lisp_Object* type = evaluated_arguments->value.pair.rest->value.pair.first;
 
-            try {
-                assert_type(type, Lisp_Object_Type::Keyword);
-            }
+            try assert_type(type, Lisp_Object_Type::Keyword);
 
             evaluated_arguments->value.pair.first->userType = type;
             return type;
         });
     defun("delete-type", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+
             evaluated_arguments->value.pair.first->userType = nullptr;
-            return  Memory::t;
+            return Memory::t;
         });
     defun("type", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
 
             if (evaluated_arguments->value.pair.first->userType) {
                 return evaluated_arguments->value.pair.first->userType;
@@ -891,13 +774,8 @@ proc load_built_ins_into_environment(Environment* env) -> void {
             return Memory::get_or_create_lisp_object_keyword("unknown");
         });
     defun("info", cLambda {
-            try {
-                arguments_length = list_length(arguments);
-            }
-
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length == 1);
 
             print(arguments->value.pair.first);
 
@@ -971,15 +849,9 @@ proc load_built_ins_into_environment(Environment* env) -> void {
             return Memory::nil;
         });
     defun("show", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
-            if (evaluated_arguments->value.pair.first->type != Lisp_Object_Type::Function) {
-                report_error(Error_Type::Type_Missmatch);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+            try assert_type(evaluated_arguments->value.pair.first, Lisp_Object_Type::Function);
 
             puts("body:\n");
             print(evaluated_arguments->value.pair.first->value.function.body);
@@ -988,24 +860,16 @@ proc load_built_ins_into_environment(Environment* env) -> void {
             return Memory::nil;
         });
     defun("print", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+
             print(evaluated_arguments->value.pair.first);
-            // printf("\n");
+
             return Memory::nil;
         });
     defun("read", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length > 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length <= 1);
 
             if (arguments_length == 1) {
                 Lisp_Object* prompt = evaluated_arguments->value.pair.first;
@@ -1014,25 +878,22 @@ proc load_built_ins_into_environment(Environment* env) -> void {
                 /* else */
                 print(evaluated_arguments->value.pair.first);
             }
+
+            // TODO(Felix): make read_line return a String*
             char* line = read_line();
+            defer {
+                free(line);
+            };
             String* strLine = Memory::create_string(line);
-            free(line);
             return Memory::create_lisp_object_string(strLine);
         });
     defun("exit", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length > 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length <= 1);
 
             if (arguments_length == 1) {
                 Lisp_Object* error_code = evaluated_arguments->value.pair.first;
-                if (error_code->type != Lisp_Object_Type::Number)
-                    report_error(Error_Type::Type_Missmatch);
-
+                try assert_type(error_code, Lisp_Object_Type::Number);
                 exit((int)error_code->value.number);
             }
             exit(0);
@@ -1047,13 +908,8 @@ proc load_built_ins_into_environment(Environment* env) -> void {
             return Memory::nil;
         });
     defun("try", cLambda {
-            try {
-                arguments_length = list_length(arguments);
-            }
-
-            if (arguments_length != 2) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try arguments_length = list_length(arguments);
+            try assert(arguments_length == 2);
 
             Lisp_Object* try_part   = arguments->value.pair.first;
             Lisp_Object* catch_part = arguments->value.pair.rest->value.pair.first;
@@ -1069,15 +925,9 @@ proc load_built_ins_into_environment(Environment* env) -> void {
             return result;
         });
     defun("load", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length != 1)
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-
-            if (evaluated_arguments->value.pair.first->type != Lisp_Object_Type::String)
-                report_error(Error_Type::Type_Missmatch);
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+            try assert_type(evaluated_arguments->value.pair.first, Lisp_Object_Type::String);
 
             Lisp_Object* result;
             try {
@@ -1090,17 +940,15 @@ proc load_built_ins_into_environment(Environment* env) -> void {
 
             // TODO(Felix): if we are copying string nodes, then
             // shouldn't the string itself also get copied??
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
 
-            if (arguments_length != 1)
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
 
             if (evaluated_arguments->value.pair.first->type == Lisp_Object_Type::Nil ||
+                evaluated_arguments->value.pair.first->type == Lisp_Object_Type::T ||
                 evaluated_arguments->value.pair.first->type == Lisp_Object_Type::Keyword)
             {
-                report_error(Error_Type::Type_Missmatch);
+                create_generic_error("The values of 'nil', 't', and keywords can't be copied.");
             }
 
             Lisp_Object* target = Memory::create_lisp_object();
@@ -1110,87 +958,54 @@ proc load_built_ins_into_environment(Environment* env) -> void {
             return target;
         });
     defun("error", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-            if (arguments_length != 0) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
-            report_error(Error_Type::Unknown_Error);
+            // TODO(Felix): make the error function useful
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 0);
+            create_generic_error("Userlanderror");
+            return nullptr;
         });
     defun("symbol->keyword", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+            try assert_type(evaluated_arguments->value.pair.first, Lisp_Object_Type::Symbol);
 
             Lisp_Object* source = evaluated_arguments->value.pair.first;
 
-            if (source->type != Lisp_Object_Type::Symbol) {
-                report_error(Error_Type::Type_Missmatch);
-            }
-
             return Memory::get_or_create_lisp_object_keyword(source->value.identifier);
         });
     defun("string->symbol", cLambda {
-
             // TODO(Felix): do some sanity checks on the string. For
             // example, numbers are not valid symbols.
 
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+            try assert_type(evaluated_arguments->value.pair.first, Lisp_Object_Type::String);
 
             Lisp_Object* source = evaluated_arguments->value.pair.first;
 
-            if (source->type != Lisp_Object_Type::String) {
-                report_error(Error_Type::Type_Missmatch);
-            }
-
             return Memory::get_or_create_lisp_object_symbol(Memory::duplicate_string(source->value.string));
         });
     defun("symbol->string", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length != 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length == 1);
+            try assert_type(evaluated_arguments->value.pair.first, Lisp_Object_Type::Symbol);
 
             Lisp_Object* source = evaluated_arguments->value.pair.first;
 
-            if (source->type != Lisp_Object_Type::Symbol) {
-                report_error(Error_Type::Type_Missmatch);
-            }
             return Memory::create_lisp_object_string(Memory::duplicate_string(source->value.identifier));
         });
     defun("concat-strings", cLambda {
-            try {
-                evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
-            }
-
-            if (arguments_length < 1) {
-                report_error(Error_Type::Wrong_Number_Of_Arguments);
-            }
+            try evaluated_arguments = eval_arguments(arguments, env, &arguments_length);
+            try assert(arguments_length >= 1);
 
             int resulting_string_len = 0;
 
             Lisp_Object* head = evaluated_arguments;
 
             while (head->type == Lisp_Object_Type::Pair) {
-                try {
-                    assert_type(head->value.pair.first, Lisp_Object_Type::String);
-                }
-                resulting_string_len += head->value.pair.first->value.string->length;
+                try assert_type(head->value.pair.first, Lisp_Object_Type::String);
 
+                resulting_string_len += head->value.pair.first->value.string->length;
                 head = head->value.pair.rest;
             }
 
diff --git a/src/defines.cpp b/src/defines.cpp
index 4340b14..9375f44 100644
--- a/src/defines.cpp
+++ b/src/defines.cpp
@@ -20,16 +20,6 @@ constexpr bool is_debug_build = false;
 # define if_linux   if (1)
 #endif
 
-#define assert(cond)                                                    \
-    if_debug {                                                          \
-        if (!cond) {                                                    \
-            if (log_level == Log_Level::Debug) {                        \
-                printf("Assertion failed: %s %d", __FILE__, __LINE__);  \
-            }                                                           \
-            debug_break();                                              \
-        }                                                               \
-    } else {}                                                           \
-
 #define concat_( a, b) a##b
 #define label(prefix, lnum) concat_(prefix,lnum)
 #define try                                                             \
@@ -139,6 +129,9 @@ struct {
 #define create_generic_error(...)               \
     __create_error("generic", __VA_ARGS__)
 
+#define create_not_yet_implemented_error()      \
+    __create_error("not-yet-implemented", "This feature has not yet been implemented.")
+
 #define create_parsing_error(...)                       \
     __create_error("parsing-error", __VA_ARGS__)
 
@@ -160,10 +153,27 @@ struct {
     do {                                                                \
         if (_node->type != _type) {                                     \
             create_type_missmatch_error("symbol", Lisp_Object_Type_to_string(_node->type)); \
-            return nullptr;                                             \
         }                                                               \
     } while(0)
 
+#define assert(condition)                                               \
+    do {                                                                \
+        if (!(condition)) {                                             \
+            create_generic_error("Assertion-error.");                   \
+        }                                                               \
+    } while(0)
+
+// #define assert(cond)                                                    \
+//     if_debug {                                                          \
+//         if (!cond) {                                                    \
+//             if (log_level == Log_Level::Debug) {                        \
+//                 printf("Assertion failed: %s %d", __FILE__, __LINE__);  \
+//             }                                                           \
+//             debug_break();                                              \
+//         }                                                               \
+//     } else {}                                                           \
+
+
 #define console_normal "\x1B[0m"
 #define console_red    "\x1B[31m"
 #define console_green  "\x1B[32m"
diff --git a/src/eval.cpp b/src/eval.cpp
index 7964cc1..ff0c35b 100644
--- a/src/eval.cpp
+++ b/src/eval.cpp
@@ -296,19 +296,19 @@ proc parse_argument_list(Lisp_Object* arguments, Function* function) -> void {
         string_equal(arguments->value.pair.first->value.identifier, "rest"))
     {
         arguments = arguments->value.pair.rest;
-        if (arguments->type != Lisp_Object_Type::Pair ||
+        if (// arguments->type != Lisp_Object_Type::Pair ||
             arguments->value.pair.first->type != Lisp_Object_Type::Symbol)
         {
-            create_error(Error_Type::Ill_Formed_Lambda_List, arguments->sourceCodeLocation);
+            create_parsing_error("After the 'rest' marker there must follow a symbol.");
             return;
         }
         function->rest_argument = arguments->value.pair.first->value.identifier;
         if (arguments->value.pair.rest->type != Lisp_Object_Type::Nil) {
-            create_error(Error_Type::Ill_Formed_Lambda_List, arguments->sourceCodeLocation);
+            create_parsing_error("The lambda list must end after the rest symbol");
         }
     } else {
         printf("this should not happen?");
-        create_error(Error_Type::Unknown_Error, arguments->sourceCodeLocation);
+        create_generic_error("What is happening?");
     }
 }
 
@@ -317,10 +317,7 @@ proc list_length(Lisp_Object* node) -> int {
     if (node->type == Lisp_Object_Type::Nil)
         return 0;
 
-    if (node->type != Lisp_Object_Type::Pair) {
-        create_error(Error_Type::Type_Missmatch, node->sourceCodeLocation);
-        return 0;
-    }
+    assert_type(node, Lisp_Object_Type::Pair);
 
     int len = 0;
     while (node->type == Lisp_Object_Type::Pair) {
@@ -330,7 +327,7 @@ proc list_length(Lisp_Object* node) -> int {
             return len;
     }
 
-    create_error(Error_Type::Ill_Formed_List, node->sourceCodeLocation);
+    create_parsing_error("Can't calculate length of ill formed list.");
     return 0;
 }
 
@@ -367,7 +364,7 @@ proc eval_arguments(Lisp_Object* arguments, Environment* env, int *out_arguments
         } else if (current_head->type == Lisp_Object_Type::Nil) {
             evaluated_arguments_head->value.pair.rest = current_head;
         } else {
-            create_error(Error_Type::Ill_Formed_Arguments, arguments->sourceCodeLocation);
+            create_parsing_error("Attempting to evaluate ill formed argument list.");
             return nullptr;
         }
         ++my_out_arguments_length;
@@ -432,7 +429,7 @@ proc eval_expr(Lisp_Object* node, Environment* env) -> Lisp_Object* {
         }
     }
     default: {
-        create_error(Error_Type::Not_A_Function, node->sourceCodeLocation);
+        create_generic_error("Not a function.");
         return nullptr;
     }
     }
@@ -455,7 +452,7 @@ proc interprete_file (char* file_name) -> Lisp_Object* {
 
     char* file_content = read_entire_file(file_name);
     if (!file_content) {
-        create_error(Error_Type::Unknown_Error, nullptr);
+        create_generic_error("The file '%s' could not be read.", file_name);
     }
 
     load_built_ins_into_environment(env);
diff --git a/src/slime.h b/src/slime.h
index 0fd0694..edf6f4f 100644
--- a/src/slime.h
+++ b/src/slime.h
@@ -12,7 +12,7 @@
 #undef _CRT_SECURE_NO_DEPRECATE
 #undef _CRT_SECURE_NO_WARNINGS
 
-// namespace Slime {
+namespace Slime {
 #include "./defines.cpp"
 #include "./structs.cpp"
 #include "./forward_decls.cpp"
@@ -26,4 +26,4 @@
 #include "./built_ins.cpp"
 #include "./testing.cpp"
 #include "./undefines.cpp"
-// }
+}
diff --git a/src/testing.cpp b/src/testing.cpp
index 62a809e..e173e38 100644
--- a/src/testing.cpp
+++ b/src/testing.cpp
@@ -478,7 +478,7 @@ proc test_singular_t_and_nil() -> testresult {
     return pass;
 }
 
-proc test_file(char* file) -> testresult {
+proc test_file(const char* file) -> testresult {
     Memory::reset();
     Environment* env = Memory::create_built_ins_environment();