What's the difference between -> (skinny arrow operator) and |> (triangle operator)?


#1

I’ve seen this triangle operator |> in some code examples, but couldn’t find any information about it in the reason docs. There’s only -> described as a fast pipe.

I’ve made a primitive demo, to find out what the difference was, but results turned out the same: Link for the demo


#2

To avoid confusion, fast pipe is now called pipe first: https://reasonml.github.io/docs/en/fast-pipe (yeah I know, the url still says otherwise).
And it means the first parameter of the function is the one handled by the first pipe operator -> whereas |> pipes the last parameter of the function.

As you only use one-parameter-functions in your example, it really makes no difference. I adapted it a bit with the separator as a second parameter: Link

HTH


#3

Thanks fham, now it’s clear.

Still very weird, why there’s no mention in docs about |> operator, is it intentional? or is it part of ocaml and it’s assumed that you should read it in ocaml docs?


#4

I got no answer why it isn’t mentioned in the current ReasonML docs, I am still a beginner myself, but I do think I have read about it in the ReasonML documentation a time ago, may it have been removed?

It is called the reverse-application operator, you can read about it in the API-documentation on Pervasives and in the functions section of Rauschmayer’s guide on ReasonML.


#5

@notgiorgi I think the omission is not really intentional, it’s just that they want to encourage usage of Belt over the standard library, and the |> operator does not work well with Belt functions.

Btw, in ReasonTown they call |> the pizza operator, which for me is easier to remember than reverse-application operator. There is also the |., or golf club operator, which has been deprecated in favor of -> but can still appear in .ml files.


#6

it’s just that they want to encourage usage of Belt over the standard library

What’s Belt?

This comes up: https://bucklescript.github.io/bucklescript/api/Belt.html

A stdlib shipped with BuckleScript

If I’m understanding this correctly:

Bucklescript/Reason both used JS implementation of oCaml stdlib before, then Belt was introduced to make stdlib more js-friendly. |> operator was part of oCaml stdlib, while -> is more Belt thing, so now it’s more commonly used.


#7

It still doesn’t support native compilation though, so if you need that then don’t use any part of it or you’ll be stuck.


#8

It is not intentional, I just know that people are busy and everything takes time :smile: I wrote the first documentation for the fast-pipe out of need because it didn’t exist in the reasonml documentation! :smiley:

I know if you want to add it they would be so happy!


#9

It’s a ppx and someone packaged it as a opam package https://github.com/IwanKaramazow/FastPipe


#10

Was speaking of Belt, not PipeFirst. ^.^


#11

Right, the same thing for Belt as well. it’s idiomatic OCaml (the implementation, not public API) so you can extract it into a package. An example of doing that is here https://github.com/jaredly/reason-language-server/tree/master/belt . Someone should package it for OPAM though


#12

It seems like “pipe-first” can be used with an underscore placeholder to pipe a value into any missing parameter? It doesn’t have to be the first parameter. I’m I reading that right from the docs?