Operators: Pipe Operator

The Pipe operator allows for a more concise, fluid syntax for chaining together expressions. It looks like this:

f() |> g($$) |> h(5, $$)

which conceptually means:

h(5, g(f()))

That is, evaluate the left hand side (LHS) of the pipe, then evaluate the RHS of the pipe with the result of the LHS bound to the placeholder $$.

Runtime execution is explicitly left-to-right - a pipe's LHS is always executed before its RHS. More precisely, the LHS of a Pipe is evaluated and its result stored in a temporary, hidden variable. Then the RHS is evaluated where $$ refers to that hidden variable. This means you can use $$ more than once in the RHS but the LHS will still be evaluated only once. For example:

f() |> g1($$) + g2($$) |> h(5, $$)

roughly means:

h(5, g1(f()) + g2(f()))

except that f():

  • is evaluated only once
  • is evaluated before both g1 and g2.

Hack will enforce that you do use the $$ of each pipe at least once (if you didn't, it's probably a bug or you don't need the pipe anyway).




namespace Hack\UserDocumentation\Operators\Pipe\Examples\MapFilterCountNested;

function nested_example(array<int> $arr): int {
  return \count(
      \array_map($x ==> $x * $x, $arr),
      $x ==> $x % 2 == 0

var_dump(nested_example(range(1, 10)));

With Pipe:


namespace Hack\UserDocumentation\Operators\Pipe\Examples\MapFilterCountPiped;

function piped_example(array<int> $arr): int {
  return $arr
    |> \array_map($x ==> $x * $x, $$)
    |> \array_filter($$, $x ==> $x % 2 == 0)
    |> \count($$);

var_dump(piped_example(range(1, 10)));