# Ppxlib: Type check the generated code? **URL:** \ **Category:** Learning\ **Created:** [May 6, 2021, 7:25am UTC](https://discuss.ocaml.org/t/ppxlib-type-check-the-generated-code/7792 "2021-05-06T07:25:52Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![vch9](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/vch9/32/2885_2.png) [@vch9](https://discuss.ocaml.org/u/vch9)\ **Post date:** [May 6, 2021, 7:25am UTC](https://discuss.ocaml.org/t/ppxlib-type-check-the-generated-code/7792/1 "2021-05-06T07:25:52Z")
Hi everyone. I am currently trying to develop a ppx rewriter that inline code based on a string payload: ```ocaml let f x y = .. [@@foo {| payload |}] ``` In my ppx, it tries to apply f with arguments, and can sometimes produce ill-typed expressions, for example: ```ocaml let f x y = x + y let _ = f 42 "hello" ``` And then produce an error: ```ocaml File "_none_", line 1: Error: This expression has type string but an expression was expected of type int ``` I was wondering if there was a possibility to type check the expression `f 42 "foo"` before, in order to generate an error message based on the generated code instead of `File "_none_"`. ---
**Author:** ![EduardoRFS](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/eduardorfs/32/2259_2.png) [@EduardoRFS](https://discuss.ocaml.org/u/EduardoRFS)\ **Post date:** [May 6, 2021, 7:34am UTC](https://discuss.ocaml.org/t/ppxlib-type-check-the-generated-code/7792/2 "2021-05-06T07:34:36Z")
The error message is related how you handle locations, almost always `_none_` happens because you’re using `Location.none` somewhere ---
**Author:** ![vch9](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/vch9/32/2885_2.png) [@vch9](https://discuss.ocaml.org/u/vch9)\ **Post date:** [May 6, 2021, 7:56am UTC](https://discuss.ocaml.org/t/ppxlib-type-check-the-generated-code/7792/3 "2021-05-06T07:56:52Z")
Indeed thank you, I was using `Location.none` **everywhere**. Now that I use the location of my attribute I can produce that kind of error: ```ocaml File "foo.ml" 1 | let f x y = x + y [@@something {| payload |}] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Error: This expression has type string but an expression was expected of type int ``` This second question might be a naive as the previous one but, is there a possibility to create a location on the generated code in order to show the error ? ---
**Author:** ![EduardoRFS](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/eduardorfs/32/2259_2.png) [@EduardoRFS](https://discuss.ocaml.org/u/EduardoRFS)\ **Post date:** [May 6, 2021, 8:03am UTC](https://discuss.ocaml.org/t/ppxlib-type-check-the-generated-code/7792/4 "2021-05-06T08:03:34Z")
> [@vch9](#): > > This second question might be a naive as the previous one but, is there a possibility to create a location on the generated code in order to show the error ? You can in theory, but most of the time if you just want to specificy something, you can use a more specific loc. It seems like you’re using the loc of the entire structure\_item, probably you want to use the loc of the attribute. ---
**Author:** ![vch9](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/vch9/32/2885_2.png) [@vch9](https://discuss.ocaml.org/u/vch9)\ **Post date:** [May 6, 2021, 8:22am UTC](https://discuss.ocaml.org/t/ppxlib-type-check-the-generated-code/7792/5 "2021-05-06T08:22:34Z")
In theory I would want to create a location on the generated code: ```ocaml let f x y = x + y let _ f "42" foo ^^^^ Error: This expression has type string but an expression was expected of type int ``` But that doesn’t seems to be possible, I might use my attribute location as you suggested. Thank you very much for the help !