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

bardot, |., %brdt

Trap

|., is a synthetic hoon that produces a dry %gold trap. A trap is a door with one only arm $, the empty name.

The default action performed on a trap is kicking it by pulling the arm $. |. is similar to |= with no arguments. You can think of |. as a function that takes no inputs.

See also

barhep, |-, %brhp bartis, |=, %brts

Produces

Twig: [%brdt p=twig]

Sample

p is a ++twig.

Tall form

|.  p

Wide form

|.(p)

Irregular form

None

Examples

/~zod/try=> 
=a  |.(42)
changed %a
/~zod/try=> 
a
< 1.yln
  [   a
    < 2.yqy
      [   a
        < 3.kii
          [   a
            < 2.wvx
              [ a=<2.kqf 2.hng 250.qyy 41.raw 414.hhh 100.xkc 1.ypj %164>
                <2.hng 250.qyy 41.raw 414.hhh 100.xkc 1.ypj %164>
              ]
            >
            <2.hng 250.qyy 41.raw 414.hhh 100.xkc 1.ypj %164>
          ]
        >
        <2.hng 250.qyy 41.raw 414.hhh 100.xkc 1.ypj %164>
      ]
    >
    <2.hng 250.qyy 41.raw 414.hhh 100.xkc 1.ypj %164>
  ]
>
/~zod/try=> 
(a)
42
/~zod/try=> 
$:a
42

This is a simple example. We assign a shell variable a to be a trap that simply produces the atom 42. Printing a prints the core and its context. Calling a using (, the irregular form of %-, produces its value. As does pulling the arm $ from inside it using $:a.

/~zod/try=> 
=a  10
changed %a
/~zod/try=> 
=b  |.  (add a 2)
changed %b
/~zod/try=> 
(b)
12

In this case we assign a variable a to be 10, and create a trap b to add 2 to it. This is a trivial example, but is meant to show that traps are useful when you need a gate that only operates on values that are already in its context.

/~zod/try=> 
=loop  =+  reps=10
       =+  step=0
       =+  outp=0
       |.
       ?:  =(step reps)
         outp
       $(outp (add outp 2), step +(step))
changed %loop
/~zod/try=> (loop)
20

Expanding on our previous example, we create a trap with three local variables, reps, step, and outp. We use a trap to create a loop, testing each time if step is equal to reps, if so producing outp otherwise calling our trap again with outp replaced with outp+2, and step incremented.