Merlin vs Ocaml-lsp

Learning
,
19

This is actually a great tip. Thanks @mseri. I tried this out and it definitely works.

Here is an example to make this concrete for people who want more information about -dsource.

Lets say I’m compiling the dream framework.

$ git clone https://github.com/aantron/dream.git
$ cd dream
$ opam install . --deps-only -with-test
$ dune build -p dream
# Lets see the transformation of the src/http/http.ml file
$ cd _build/default/src/http
# ocamlc dumps transformed source on stderr
$ ocamlc -dsource http.pp.ml 2>http.pp.out.ml
# meld is a great difftool. There are many others
$ meld http.ml http.pp.out.ml

A lot of de-sugaring has happened by the time you see http.pp.out.ml

e.g. fun ~a ~b ~c ->... becomes fun ~a -> (fun ~b -> (fun ~c -> ...))) , comments are stripped out etc. So it does become a bit difficult to compare the original with the transformed source.

However the good news is that ppx-es are indeed expanded as advertised.

Something like

let%lwt response = user's_dream_handler request in ...

Now becomes:

 let __ppx_lwt_0 = user's_dream_handler request in
                let module Reraise =
                  struct external reraise : exn -> 'a = "%reraise" end in
                  Lwt.backtrace_bind
                    (fun exn -> try Reraise.reraise exn with | exn -> exn)
                    __ppx_lwt_0
                    (fun response -> ...