Ppxlib: Type check the generated code?

Learning
1

Hi everyone.

I am currently trying to develop a ppx rewriter that inline code based
on a string payload:

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:

let f x y = x + y
let _ = f 42 "hello"

And then produce an error:

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_".

2

The error message is related how you handle locations, almost always _none_ happens because you’re using Location.none somewhere

3

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:

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 ?

4

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.

5

In theory I would want to create a location on the generated code:

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 !