Re: [Caml-list] PPX metaquotation question

[Alain Frisch <alain.frisch@lexifi.com>]

Hi Jonathan,

Extension nodes (i.e. [%...] stuff) are not supported by OCaml's syntax 
in the context of names in type declarations, so the metaquotation in 
ppx_tools cannot support it.

It would be possible to support something like:

let name = "some_type" in
let typ = [%type: string] in
[%stri type [@meta.name] name = [%t typ] ]

When ppx_metaquot detects a specific attribute on type declarations, it 
would interpret the name of the declaration as a "host" variable, not a 
static name.   It could allow passing an expression that compute the 
name and then discard the concrete name:

let typ = [%type: string] in
[%stri type [@meta.name "some_type"] t = [%t typ] ]

The same technique could be used to make other component of the type 
declaration "dynamic" (recursive flag, list of type parameters, type 
kind, etc).

But I'm not convinced this a very good direction: it makes the 
"metaquotation" syntactic DSL more complex.  It might just be better to 
drop the concrete syntax and use directly Ast_helper (or an extra 
lighter convenience layer) to build Parsetree fragments.


Alain


On 11/05/2016 20:30, Jonathan DiLorenzo wrote:
> Hey all!
>
> I'm trying to construct a type declaration structure item using the
> metaquotation in ppx_tools and I have two variables containing a string
> with the name of the type and a Parsetree.core_type, like so:
>
> let name = "some_type" in
> let typ = [%type: string] in
> [%stri type [%??? name] = [%t typ] ]
>
> Is there a ??? that lets me do this? Am I going about it all wrong
> somehow? Or do I need to do something like:
>
> open Ast_helper
>
> let name = "some_type" in
> let typ = [%type: string] in
> let type_decl = Type.mk ~manifest:typ { txt=name ; loc = !default_loc} in
> Str.type_  Recursive [type_decl]
>
> Best,
> Jonathan