What it does

ppx_partial provides syntax for building single argument functions that look and behave similar to partial applications:

something_that_returns_a_string ()
|> Base.String.drop_suffix __ 1
|> Stdio.Out_channel.write_all "/tmp/z" ~data:__

is turned into:

something_that_returns_a_string ()
|> (fun x -> Base.String.drop_prefix x 1)
|> (fun x -> Stdio.Out_channel.write_all "/tmp/z" ~data:x)

In general, the ppx ensures that parameters are executed exactly once, just like with a regular partial application. For instance, these two expressions have the exact same performance:

List.filter (Re.execp (Re.compile re) __) l
List.filter (Re.execp (Re.compile re)) l

This syntax may appear unnecessary at first, given that that ocaml functions are curried, but note that neither occurrence of __ in the first example can be replaced by a partial application.

As a slight generalization, field accesses and sum constructors are allowed: List.map __.field l, List.map (Some __) l.

As an other slight generalization, it is possible to omit the function instead of an argument: Option.iter o ~f:(__ ()) which means Option.iter o ~f:(fun f -> f ()).

What it doesn't do

This is not a general lighter syntax for short anonymous functions. If you want a lightweight syntax for (fun x -> f (g x)) or (fun x -> x * 2 + 1), this ppx isn't providing such things.

Only a single placeholder is supported, so things like f __ __ e1 are not supported. There are probably no technical obstacles, but I haven't seen a compelling reason to do this.

Why

The purpose is simply convenience. To be more specific:

• __.field and Some __ should be self-explanatory.

• there is a tension between 1) easy pipelining (meaning "main parameter" last) 2) t parameter first consistently 3) no excessive labelling of parameters: you can't get all three consistently.

  • Base.List.take list int is a function that picks 2) and 3) but drops 1).
  • Base.Or_error.tag error ~tag is a function that picks 1) and 2) but drops 3).
  • Stdlib.Map.add key value map is a function that picks 1) and 3) but drops 2).

  ppx_partial provides 1) for every parameter of every function, thus function signatures only have to provide 2) and 3).

• partial application of some infix operators can be very misleading. List.filter ((>) 0) can easily be read as List.filter (fun -> x > 0), when it actually means List.filter (fun x -> 0 > x). Writing List.filter (__ > 0) is clearer.

• Very occasionaly, this can be used to drop optional parameters by "eta-expansion", like so:

     match flavor with
     | `Xml -> Markup.parse_xml __ (* wouldn't type without __, due to optional parameters *)
     | `Html -> Markup.parse_html __

  or it can be used to omit a parameter that's not the usual one, say List.iter __ l or List.iter ~f:__ l.