urbit/pub/doc/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.