? wut · Conditionals
Hoon has the usual program control branches. It also has the usual logical operators: AND ?&, OR ?|, and NOT ?!. It also has a ?= rune that tests whether a value matches a given type. In the course of type inference, Hoon learns from ?= tests in the test condition of ?: ("wutcol") expressions.
Overview
All ? runes reduce to ?: and/or ?=.
If the condition of an ?: is a ?=, and the ?= is testing a leg of the subject, the compiler specializes the subject type for the branches of the ?:. Branch inference also works for expressions which expand to ?:.
The test does not have to be a single ?=; the compiler can analyze arbitrary boolean logic (?& ("wutpam"), ?| ("wutbar"), ?! ("wutzap")) with full short-circuiting. Equality tests (.= ("dottis")) are not analyzed.
If the compiler detects that the branch is degenerate (only one side is taken), it fails with an error.
?| "wutbar"
Logical OR.
Syntax
Variable number of arguments.
?| p1
p2
p3
pn
==AST
[%wtbr p=(list hoon)]Expands to
Pseudocode: a, b, c, ... as elements of p:
?:(a & ?:(b & ?:(c & ?:(... ?:(z & |)))))Desugaring
|-
?~ p
|
?: i.p
&
$(p t.p)Produces
If any argument evaluates to true (%.y), true. If all arguments evaluate to false (%.n), false.
Examples
> |(=(6 42) =(42 42))
%.y
> |(=(6 42) =(42 43))
%.n?- "wuthep"
Switch against a union, with no default.
Syntax
One fixed argument, then a variable number of pairs.
?- p
q1a q1b
q2a q2b
qna qnb
==AST
[%wthp p=wing q=(list (pair spec value))]Expands to
Pseudocode: a, b, c, ... as elements of q:
?: ?=(p.a p) q.a
?: ?=(p.b p) q.b
?: ?=(p.c p) q.c
...
~|(%mint-lost !!)Desugaring
|-
?. q
~|(%mint-lost !!)
?: ?=(p.i.q p)
q.i.q
$(q t.q)Discussion
The ?- rune is for a conditional expression in which the type of p determines which branch is taken. Usually the type of p is a union of other types. There is no default branch.
The compiler makes sure that your code neither misses a case of the union, nor includes a double case that isn't there. This is not special handling for ?-, just a consequence of the semantics of ?:, which ?- reduces to.
A missing case will throw the mint-lost error. An extra case will throw mint-vain.
Examples
> =cor |= vat=?(%a %b)
?- vat
%a 20
%b 42
==
> (cor %a)
20
> (cor %b)
42
> (cor %c)
! nest-fail?: "wutcol"
Branch on a boolean test.
Syntax
Three arguments, fixed.
?: p
q
rAST
[%wtcl p=hoon q=hoon r=hoon]Produces
If p produces true (%.y), then q. If p produces false (%.n), then r. If p is not a boolean, compiler yells at you.
Discussion
If test analysis reveals that either branch is never taken, or if p is not a boolean, compilation fails. An untaken branch is indicated with mint-lost.
Note also that all other branching expressions reduce to ?:.
Examples
> ?:((gth 1 0) 3 4)
3
> ?: (gth 1 0)
3
4
3
> ?:((gth 1 2) 3 4)
4
> ?: (gth 1 2)
3
4
4?. "wutdot"
Branch on a boolean test, inverted.
Syntax
Three arguments, fixed.
?. p
q
rAST
[%wtdt p=hoon q=hoon r=hoon]Expands to
?:(p r q)Discussion
?. is just like ?:, but with its last two subexpressions reversed.
As is usual with inverted forms, use ?. when the true-case expression is much taller and/or wider than the false-case expression.
Examples
> ?.((gth 1 2) 3 4)
3
> ?.(?=(%a 'a') %not-a %yup)
%yup
> ?. %.y
'this false case is less heavy than the true case'
?: =(2 3)
'two not equal to 3'
'but see how \'r is much heavier than \'q?'
'but see how \'r is much heavier than \'q?'?^ "wutket"
Branch on whether a wing of the subject is a cell.
Syntax
Three arguments, fixed.
?^ p
q
rAST
[%wtkt p=wing q=hoon r=hoon]Expands to
?:(?=(^ p) q r)Discussion
The type of the wing, p, must not be known to be either an atom or a cell, or else you'll get a mint-vain error at compile time. mint-vain means that one of the ?^ branches, q or r, is never taken.
Examples
> ?^(0 1 2)
! mint-vain
! exit
> ?^(`*`0 1 2)
2
> ?^(`*`[1 2] 3 4)
3?< "wutgal"
Negative assertion.
Syntax
Two arguments, fixed.
?< p
qAST
[%wtgl p=hoon q=hoon]Expands to
?:(p !! q)Discussion
?< is used to force a crash when some condition p doesn't yield false (%.n). It can be used for type inference with the ?= rune, much like the ?> rune.
Examples
> ?<(=(3 4) %foo)
%foo
> ?<(=(3 3) %foo)
dojo: hoon expression failed
> =a `*`[12 14]
> `^`a
nest-fail
> ?<(?=(@ a) `^`a)
[12 14]?> "wutgar"
Positive assertion.
Syntax
Two arguments, fixed.
?> p
qAST
[%wtgr p=hoon q=hoon]Expands to
?.(p !! q)Discussion
?> is used to force a crash when some condition p doesn't yield true (%.y). It can be used for type inference, with the ?= rune, to specify the type of a value.
Examples
> ?>(=(3 3) %foo)
%foo
> ?>(=(3 4) %foo)
dojo: hoon expression failed
> =a `*`123
> `@`a
nest-fail
> ?>(?=(@ a) `@`a)
123?+ "wutlus"
Switch against a union, with a default.
Syntax
Two fixed arguments, then a variable number of pairs.
?+ p q
r1a r1b
r2a r2b
rna rnb
==AST
[%wtls p=wing q=hoon r=(list (pair spec hoon))]Expands to
Pseudocode: a, b, c, ... as elements of r:
?: ?=(p.a p) q.a
?: ?=(p.b p) q.b
?: ?=(p.c p) q.c
...
qDesugaring
|-
?. r
q
?: ?=(p.i.r p)
q.i.r
$(r t.r)Discussion
The ?+ rune is for a conditional expression in which the type of p determines which branch is taken. Usually the type of p is a union of other types. If p's type doesn't match the case for any given branch, the default expression, q, is evaluated.
If there is a case that is never taken you'll get a mint-vain error.
Examples
> =cor |= vat=@tas
?+ vat 240
%a 20
%b 42
==
> (cor %a)
20
> (cor %b)
42
> (cor %c)
240?& "wutpam"
Logical AND.
Syntax
Variable arguments.
?& p1
p2
pn
==AST
[%wtpm p=(list hoon)]Expands to
Pseudocode: a, b, c, ... as elements of p:
?.(a | ?.(b | ?.(c | ?.(... ?.(z | &)))))Desugaring
|-
?~ p
&
?. i.p
|
$(p t.p)Produces
If ALL arguments evaluate to true (%.y), true (%.y). If one or more evalute to false (%.n), false (%.n).
Examples
> &(=(6 6) =(42 42))
%.y
> &(=(6 7) =(42 42))
%.n?@ "wutpat"
Branch on whether a wing of the subject is an atom.
Syntax
Three arguments, fixed.
?@ p
q
rAST
[%wtpt p=wing q=hoon r=hoon]Expands to
?:(?=(@ p) q r)Produces
If p is an atom, q. If p is a cell, r.
Discussion
The type of the wing, p, must not be known to be either an atom or a cell, or else you'll get a mint-vain error at compile time. mint-vain means that one of the ?@ branches, q or r, is never taken.
Examples
> ?@(0 1 2)
! mint-vain
! exit
> ?@(`*`0 1 2)
1
> ?@(`*`[1 2] 3 4)
4?~ "wutsig"
Branch on whether a wing of the subject is null.
Syntax
Three arguments, fixed.
?~ p
q
rAST
[%wtsg p=wing q=hoon r=hoon]Expands to
?:(?=($~ p) q r)Produces
If p is null (~), q. If p is non-null, r.
Discussion
It's bad style to use ?~ to test for any zero atom. Use it only for a true
null, ~.
Examples
> =foo ""
> ?~(foo 1 2)
1?= "wuttis"
Test pattern match.
Syntax
Two arguments, fixed.
?= p
qAST
[%wtts p=spec q=wing]Produces
%.y (true) if the noun at q is in the type of p; %.n (false) otherwise.
Discussion
?= is not as powerful as it might seem. For instance, it can't generate a loop -- you cannot (and should not) use it to test whether a * is a (list @). Nor can it validate atomic auras.
Patterns should be as weak as possible. Unpack one layer of union at a time. Don't confirm things the type system knows.
For example, when matching from a tagged union for the type [%foo p=@ q=[@ @]], the appropriate pattern is [%foo *]. You have one question, which is whether the head of the noun is %foo.
A common error is find.$, meaning p is not a type.
Examples
> =bar [%foo %bar %baz]
> ?=([%foo *] bar)
%.y?! "wutzap"
Logical NOT.
Syntax
One argument, fixed.
?! p
?!(p)
!p
AST
[%wtzp p=hoon]Expands to
.=(| p)Produces
The logical NOT of p, which must evaluate to either %.y or %.n.
Examples
~zod:dojo> ?!(.=(1 2))
%.y
~zod:dojo> !&
%.n
~zod:dojo> !|
%.y
~zod:dojo> !(gth 5 6)
%.yLast updated