Generating infix extension nodes with Ppxlib

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1

Hi! I’m working on a PPX deriver for property-based testing that produces testing code utilizing Jane Street’s Core.Quickcheck library. Specifically, I would like my PPX to produce monadic bind expressions that use the library’s let%bind syntax (the infix form of the extension node [%bind ...]), such as the following:

(* sample an int from a monadic Quickcheck generator [gen_int], 
  using [ppx_jane] *)
let%bind x = gen_int in ...

Right now, my PPX deriver produces the expression above by calling the function pexp_extension (in Ppxlib.Ast_builder.Default) and using Metaquot quotations to produce a Pexp_extension AST node:

pexp_extension ({txt = "bind", ...},
      PStr [ pstr_eval ~loc [%expr let x = gen_int in ...] ...]) ...

This produces the following:

[%bind let x = gen_int in ...]

I understand from the OCaml language manual that the let%bind ... infix form desugars to [%bind ...].

I was wondering if it would be possible for my PPX deriver to directly produce the let%bind syntactic form via Ppxlib? I understand if this is infeasible since this desugaring is typically done by another PPX (ppx_jane).

Thanks!

2

let%bind p = e1 in e2 and [%bind let p = e1 in e2] are represented as the same AST, which you are generating correctly, so there’s nothing different you can do. It’s the same as how 1 + 2 and (+) 1 2 have the same AST.

Maybe your question is “when I pretty-print that AST, how I can get let%bind .. instead of [%bind let ...] ?”. For that, usually you can’t choose a specific concrete syntax, but it probably depends on the pretty printer. I don’t know what the compiler printer does. I think ocamlformat looks at AST positions to distinguish ASTs from let%bind ... vs ASTs from [%bind ...], and display the same syntax as the source code.

As a side remark, if you’re using metaquot, you should be able to use it some more to simplify your code further. All these should generate the same AST:

pexp_extension ~loc ({txt = "bind"; loc},
                     PStr [ pstr_eval ~loc [%expr let x = gen_int in ()] []])
pexp_extension ~loc ({txt = "bind"; loc},
                     PStr [ [%stri let x = gen_int in ()]])
pexp_extension ~loc ({txt = "bind"; loc},
                     PStr [%str let x = gen_int in ()])
[%expr let%bind x = gen_int in ()]

(and you could also use Ppx_let_expander.expand to directly expand the %bind code instead of requiring users of your ppx to also list ppx_let in their dune file, with the downside that the Ppx_let_expander API presumably gets breaking changes from times to times).

3

Thanks for the reply, especially the tips on using Metaquot to make the PPX code more succinct!
I’ll look into using the Ppx_let_expander API, and if that doesn’t work I’ll just make my PPX generate monadic code that uses the infix >>= operator (instead of let%bind ...).