partial_named_pipes.ml

let namedf ~first:a ~second:b = (a,b);;
let f a b = (a,b);;
(* 
val namedf : first:'a -> second:'b -> 'a * 'b = <fun>
val f : 'a -> 'b -> 'a * 'b = <fun> 
*)

let partialnamedf = namedf ~first:33;;
let partialf = f 33;;
(*
val partialnamedf : second:'_weak2 -> int * '_weak2 = <fun>
val partialf : '_weak3 -> int * '_weak3 = <fun>
*)

let named_tuple_appl = partialnamedf 42;;
let tuple_appl = partialf 42;;
(* 
val named_tuple_appl : int * int = (33, 42)
val tuple_appl : int * int = (33, 42)
*)

let tuple_pipe = 42 |> partialf;;
(* val tuple_pipe : int * int = (33, 42) *)
let named_tuple_pipe = 42 |> partialnamedf;;
(* 
val tuple_pipe : int * int = (33, 42)
File "mycode.ml", line 43, characters 29-42:
43 | let named_tuple_pipe = 42 |> partialnamedf;;
                                  ^^^^^^^^^^^^^
Error: This expression has type second:int -> int * int
       but an expression was expected of type int -> 'a 
*)

Turns out we cannot elide labels when passing to higher order functions like |>

From https://caml.inria.fr/pub/docs/manual-ocaml/lablexamples.html When a function is passed as an argument to a higher-order function, labels must match in both types. Neither adding nor removing labels are allowed.

Workarounds:

1. Use a lambda:

42 |> (fun x -> partialnamedf ~second:f)

2. Use https://github.com/janestreet/ppx_pipebang