urbit/base/pub/doc/hoon/runes/br/brts.md

bartis, |=, %brts

Dry gate

|=, is a synthetic rune that produces a dry %gold gate with sample $*(p), arm q. A gate is a core with one arm, $, the empty name. |= checks its input sample against its tile, p.

|= is similar to a function that takes a defined input and produces the result of some computation. |= differs from |* in that its typechecking occurs at compile-time to ensure that all inputs match its sample tile.

See also

bartar, |*, %brtr

Produces

Twig: [%brts p=tile q=twig]

Sample

p is a tile. q is a twig.

Tall form

|=  p
    q

Wide form

|=(p q)

Irregular form

None

Examples

~zod/try=> =inc |=(a=@ +(a))
~zod/try=> (inc 20)
21

Here we create a very simple gate that increments its sample, a. You can think of |= as similar to a straightforward function that takes arguments.

++  add                                                 ::  add
      ~/  %add
      |=  [a=@ b=@]
      ^-  @
      ?:  =(0 a)
        b
      $(a (dec a), b +(b))

In ++add, from hoon.hoon, |= creates a gate whose sample takes two atoms labeled a and b, and whose arm evaluates an expression that produces the sum of the atoms by decrementing a until it is 0 and incrementing b at each step. Here, $ is used for recursion, calling the gate produced by |= with a replaced by (dec a) and b replaced by +(b).