Pipe Apply (|> and <|)

Pipe-apply operators call functions with one argument, but let you choose a convenient reading order.

Overview

• value |> f is equivalent to f(value)
• f <| value is equivalent to f(value)

Both operators require the function side to evaluate to a function value; anything else raises Pipe apply (|>) requires a function on the right-hand side (or the matching left-hand-side error for <|).

There is no |< operator — the backward form is <|.

Forward apply (|>)

Read left-to-right:

import std:println

trim = |s| s::trim()
upper = |s| s::upper()

result = "  hello  " |> trim |> upper
println(result)  # HELLO

Backward apply (<|)

Read right-to-left. <| is right-associative, so the parentheses are optional:

import std:println

trim = |s| s::trim()
upper = |s| s::upper()

println <| upper <| trim <| "  hello  "   # HELLO

Precedence

<| has the lowest precedence of the two, just above assignment; |> binds slightly tighter, and both bind looser than arithmetic. That means the arithmetic on the left is evaluated before the value is piped:

import std:println

inc = |x| x + 1

println(1 + 2 |> inc)  # 4 - the same as (1 + 2) |> inc

Notes

• |> / <| always apply one argument. They do not “insert into a parameter list”, and there is no partial application syntax — write a lambda returning a lambda if you need that.
• If you want to transform collections, prefer list methods:

import std:println

numbers = [1, 2, 3, 4, 5]
result = numbers::filter(|x| x % 2 == 0)::map(|x| x * x)::sum()

println(result)  # 20

• If you want to build a reusable function instead of calling one right away, use the composition operators >> / << (see Function Composition).
• If you want stdin/stdout piping between stages, use the | operator (see Pipe).

See Also

• Pipe (|)
• Function Composition (>> and <<)
• Operators overview