Docs / Language Manual / PipeLast

Pipe Last

We discourage the usage of the |> / pipe-last operator in BuckleScript codebases. Using it may sometimes have a runtime cost due to currying. Please use the pipe-first operator instead.

Pipe-last is mainly used in native / OCaml codebases or in functional programming libraries aiming for a data last approach. You'll also find it in older Reason codebases and libraries.

|> is an infix operator for piping a value to the last argument position of a function.


let personSays = (person, msg) => person ++ " says: " ++ msg; "This will be put in the msg arg" |> personSays("Alpaca"); /* Returns: "Alpaca says: This will be put in the msg arg */

Differences between pipe-last and pipe-first

  • Pipe-last is a binding to a compiler intrinsic, it will optimize some currying, but not all. Pipe-first (->) is implemented as syntactic sugar within BuckleScript and guarantees zero extra currying overhead.

  • Pipe-last can't pipe into a variant constructor (see 1->Some)

For a more thorough discussion on the rationale and differences between the two operators, please refer to the Data-first and Data-last comparison by Javier Chávarri

Note on Precedence

Mixing both pipe operators might lead to confusion, since the pipe-first operator has a higher precedence. For example:

let maybeGreeting = Some("hello"); maybeGreeting |> Belt.Option.isSome -> Js.log;

will not compile, since the compiler will process the -> first (Belt.Option.isSome -> Js.log) before applying maybeGreeting to Belt.Option.isSome. A fix for the example above would involve using parenthesis, like this:

let maybeGreeting = Some("hello"); (maybeGreeting |> Belt.Option.isSome) -> Js.log;