urbit/pub/docs/dev/hoon/runes/wt/wthp.md

wuthep, `?-`, %wthp
============================

Switch

`?-` is a synthetic hoon that selects a case in `q` for the value of
`p`. The labels in `q` must match the [icon]() of `p`. The list of
cases, `q` must be terminated by `==`.

See also
--------

wutlus, `?+`, %wtls
============================

Produces
--------

Twig: `[%wthp p=wing q=tine]`

Sample
------

`p` is a [`++wine`](). `q` is a [`++tine`]().

Tall form
---------

Kingside:

    ?-  p
      p.i.q      q.i.q
      p.i.t.q    q.i.t.q
      p.i.t.t.q  q.i.t.t.q
    ==

Queenside:

    ?-    p
        p.i.q      
      q.i.q
        p.i.t.q    
      q.i.t.q
        p.i.t.t.q  
      q.i.t.t.q
    ==

Wide form
---------

    ?-(p p.i.q q.i.q, p.i.t.q q.i.t.q, p.i.t.t.q q.i.t.t.q)

Irregular form
--------------

None

Examples
--------

    ~zod/try=> 
      =cor  |=  typ=$?(%a %b)
            ?-  typ
            %a  1
            %b  2
            ==
    new var %cor
    ~zod/try=> 
      (cor %a)
    1
    ~zod/try=> 
      (cor %b)
    2

Here is a simple example of `?-` showing that its input must have a well
defined type for which all of the cases are covered. We create a core,
`cor` that takes an input `typ` which must be either `%a` or `%b` with
[`$%`](). Calling `cor` with valid arguments selects one of our cases.

    ~zod/try=> 
      ?-  'a'
          %a  0
          %b  1
          ==
    ! /~zod/try/~2014.11.2..16.56.40..fca2:<[1 1].[4 7]>
    ! -lost.@t
    ! mint-lost
    ~zod/try=> 
      ?-  (?(%a %b) 0)
          %a  'a'
          %b  'b'
          ==
    'b'

Here we can see a common failure case with `?-`. In the first example
all of our possible input cases are not covered when we pass in a `@t`,
so we fail with `mint-vain` – a parsing error. When we construct a well
typed input and select a case from it `?-` in fact has all of the cases
covered and produces correct output.