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100 lines
1.9 KiB
Markdown
100 lines
1.9 KiB
Markdown
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[wuthep, `?-`, %wthp](#wthp)
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============================
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Switch
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`?-` is a synthetic hoon that selects a case in `q` for the value of
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`p`. The labels in `q` must match the [icon]() of `p`. The list of
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cases, `q` must be terminated by `==`.
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See also
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--------
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[wutlus, `?+`, %wtls](#wtls)
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============================
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Produces
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--------
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Twig: `[%wthp p=wing q=tine]`
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Sample
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------
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`p` is a [`++wine`](). `q` is a [`++tine`]().
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Tall form
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---------
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Kingside:
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?- p
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p.i.q q.i.q
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p.i.t.q q.i.t.q
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p.i.t.t.q q.i.t.t.q
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==
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Queenside:
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?- p
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p.i.q
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q.i.q
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p.i.t.q
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q.i.t.q
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p.i.t.t.q
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q.i.t.t.q
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==
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Wide form
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---------
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?-(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)
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Irregular form
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--------------
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None
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Examples
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--------
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~zod/try=>
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=cor |= typ=$?(%a %b)
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?- typ
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%a 1
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%b 2
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==
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new var %cor
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~zod/try=>
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(cor %a)
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1
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~zod/try=>
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(cor %b)
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2
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Here is a simple example of `?-` showing that its input must have a well
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defined type for which all of the cases are covered. We create a core,
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`cor` that takes an input `typ` which must be either `%a` or `%b` with
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[`$%`](). Calling `cor` with valid arguments selects one of our cases.
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~zod/try=>
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?- 'a'
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%a 0
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%b 1
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==
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! /~zod/try/~2014.11.2..16.56.40..fca2:<[1 1].[4 7]>
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! -lost.@t
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! mint-lost
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~zod/try=>
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?- (?(%a %b) 0)
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%a 'a'
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%b 'b'
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==
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'b'
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Here we can see a common failure case with `?-`. In the first example
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all of our possible input cases are not covered when we pass in a `@t`,
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so we fail with `mint-vain` – a parsing error. When we construct a well
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typed input and select a case from it `?-` in fact has all of the cases
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covered and produces correct output.
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