Re: [Caml-list] Information for writting a ppx

[Xavier Van de Woestyne <xavier.vdw@gmail.com>]

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Thanks for the point !  
  
Xavier Van de Woestyne

http://xvw.github.io

  
![](https://link.nylas.com/open/5y0wh7okaezysfzysovho20yh/local-37964069-61f2?r=Y2FtbC1saXN0QGlucmlhLmZy)

On juin 20 2016, at 5:32 pm, Marc Lasson <marc.lasson@lexifi.com> wrote:  

> To paraphrase the code, I just advised Xavier to use a mapper that does  
> recursively not call itself.

>

> > (* Mapper for all transformation *)  
> > let general_mapper = Ast_mapper.{ default_mapper with structure =  
> > structure }  
> >  
> > (* New Mapper only for the toplevel *)  
> > let toplevel_mapper =  
> > Ast_mapper.{  
> > default_mapper with  
> > structure = fun _ str ->  
> > let pl = Ast_helper.Str.eval (Ppx.Fabric.print_endline "Hello  
> > World") in  
> > pl :: (general_mapper.structure general_mapper str)  
> > }  
> > ```

>

> Note that if you do not plan to extend general_mapper with a mapper that  
> actually does something you could simply write :

>

> let toplevel_mapper =  
>   Ast_mapper.{  
>     default_mapper with  
>     structure = fun _ str ->  
>       let pl = Ast_helper.Str.eval (Ppx.Fabric.print_endline "Hello  
> World") in  
>       pl :: str  
>   }

>

> Also, if the ppx is also applied to compile mli interfaces, then this  
> will be structures occurring within the toplevel signature (eg. in the  
> payloads of attributes). So to be "safe", you should probably do :

>

> let toplevel_mapper =  
>   Ast_mapper.{  
>     default_mapper with  
>     signature = fun _ -> default_mapper.signature default_mapper;  
>     structure = fun _ str ->  
>       let pl = Ast_helper.Str.eval (Ppx.Fabric.print_endline "Hello  
> World") in  
>       pl :: str  
>   }

>

> \--  
> Marc.

>

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