Pretty print extensions

[let-def]

With current version, Reason accepts:

%lwt
let x = Lwt.return(5);

But pretty print it as:

[%%lwt let x = Lwt.return(5)];

This PR implements pretty-printing for extension sugars (in structures and in expressions).

[hcarty]

Very cool! Does this work for match, if and try? Those are all used/supported by Lwt and match and if are supported by ppx_let.

[let-def]

Yes, these are supported, I added them to the test.

However, the sugar triggers only in "sequences" (between {/}, so either with a ...; ... or let ... = ...; ...).

So when there is only one expression, it falls back to [%lwt ...].
Compare:

• 4816f92#diff-ffb9c2b127d7d439f65f425eda9d3a28R58
  and
• 4816f92#diff-b7c46d6c2761df27d2ef15c37fe786e1R50

[let-def]

So the question remains what should be the printing for a single expression:

let x = {
  %extend
  switch None {
    | Some(x) => assert(false)
    | None => ()
  };
};

let x = [%extend
  switch None {
    | Some(x) => assert(false)
    | None => ()
  };
];

let x = (%extend
  switch None {
    | Some(x) => assert(false)
    | None => ()
  };
);

let x = %extend
  switch None {
    | Some(x) => assert(false)
    | None => ()
  };

(I am not sure all cases are unambiguous, I can investigate patching the parser).

[chenglou]

Awesome! Though can we do like ocaml and have let%lwt, etc?
Or maybe it's easier to have this one land first?

[let-def]

@chenglou: I agree it would be nice, I will investigate!

[jordwalke]

The convenient thing about OCaml's let%ppx form is that it conveys that the whole let is a ppx extension - completely replacing an AST node, as opposed to a "decorator" attached to another otherwise normal AST node.

For that Reason, I'm thinking that the prefix syntax you have here would make perfect sense for @ attributes:

@foo
let anotherThing = bar;

Even if ppx extensions use the more "embedded" syntax:

let%foo = bar;

For single expressions, the same lean bracketless ppx attributes would also be awesome:

let something = @attr myFunc();

[let-def]

@chenglou: I agree it would be nice, I will investigate!

[let-def]

Ok, the last update add supports for fun%extend, function%extend, let%extend, switch%extend, try%extend, while%extend, for%extend, if%extend (I hope I didn't forget any :P?).

Plus non negligible simplification to the parser (more to come).

[hcarty]

@let-def Looks like it covers if%extend too, correct?

This is really cool! ocamlformat just got extension point sugar support too so pleasantly formatted extension points for everyone!

[let-def]

@hcarty indeed, thanks I forgot to list that one :).

[hcarty]

This looks amazing - I can't wait to try it out!

[jordwalke]

Thank You!
I've wanted this for a long time and so have a lot of other people.