class Junction

Logical superposition of values

class Junction is Mu { }

A junction is an unordered composite value of zero or more values. Junctions autothread over many operations, which means that the operation is carried out for each junction element (also known as eigenstate), and the result is junction of the return values of all those operators.

Junctions collapse into a single value in boolean context, so when used in a conditional, a negation or an explicit coercion to Bool through the so or ? prefix operators. The semantics of this collapse depend on the junction type, which can be all, any, one or none.

type constructor operator True if ...
all all & no value evaluates to False
any any | at least one value evaluates to True
one one ^ exactly one value evaluates to True
none none no value evaluates to True

Autothreading happens when a junction is bound to a parameter of a code object that doesn't accept values of type Junction. Instead of producing an error, the signature binding is repeated for each value of the junction.


my $j = 1|2;
if 3 == $j + 1 {
    say 'yes';

First autothreads over the infix:<+> operator, producing the Junction 2|3. The next autothreading step is over infix:<==> , which produces False|True. The if conditional evaluates the junction in boolean context, which collapses it to True. So the code prints yes\n.

The type of a Junction does not affect the number of items in the resultant Junction after autothreading. For example, using a one Junction during Hash key lookup, still results in a Junction with several items. It is only in boolean context would the type of the Junction come into play:

my %h = :42foo, :70bar;
say    %h{one <foo meow>}:exists# OUTPUT: «one(True, False)␤» 
say so %h{one <foo meow>}:exists# OUTPUT: «True␤» 
say    %h{one <foo  bar>}:exists# OUTPUT: «one(True, True)␤» 
say so %h{one <foo  bar>}:exists# OUTPUT: «False␤» 

Note that the compiler is allowed, but not required, to parallelize autothreading (and Junction behavior in general), so it is usually an error to autothread junctions over code with side effects.

It is possible that future language versions will include short-circuiting behaviour in Junctions. For example one or more routine calls (a(), b(), or c()) in the code below might not get executed at all, if the result of the conditional has been fully determined from routine calls already performed (only one truthy return value is enough to know the entire Junction is true):

if a() | b() | c() {
    say "At least one of the routines was called and returned a truthy value"

Junctions are meant to be used as matchers in boolean context; introspection of junctions is not supported. If you feel the urge to introspect a junction, use a Set or a related type instead.

Usage examples:

my @list = <1 2 "Great">;
my @bool_or_int = grep Bool|Int@list;
sub is_prime(Int $xreturns Bool {
    # 'so' is for boolean context 
    so $x %% none(2..$x.sqrt);
my @primes_ending_in_1 = grep &is_prime & / 1$ /2..100;
say @primes_ending_in_1;        # OUTPUT: «[11 31 41 61 71]␤» 
my @exclude = <~ .git>;
for dir("."{ say .Str if .Str.ends-with(none @exclude}

Special care should be taken when using all with arguments that may produce an empty list:

my @a = ();
say so all(@a# True, because there are 0 False's 

To express "all, but at least one", you can use @a && all(@a)

my @a = ();
say so @a && all(@a);   # OUTPUT: «False␤» 

Negated operators are special-cased when it comes to autothreading. $a !op $b is rewritten internally as !($a op $b). The outer negation collapses any junctions, so the return value always a plain Bool.

my $word = 'yes';
my @negations = <no none never>;
if $word !eq any @negations {
    say '"yes" is not a negation';

Note that without this special-casing, an expression like $word ne any @words would always evaluate to True for non-trivial lists on one side.

For this purpose, infix:<ne> counts as a negation of infix:<eq> .

In general it is more readable to use a positive comparison operator and a negated junction:

my $word = 'yes';
my @negations = <no none never>;
if $word eq none @negations {
    say '"yes" is not a negation';

Failures and Exceptions

Failures are just values like any other, as far as Junctions are concerned:

my $j = +any "not a number""42""2.1";
(gather $j».take).grep(Numeric).say# OUTPUT: «(42 2.1)␤» 

Above, we've used prefix + operator on a Junction to coerce the strings inside of it to Numeric. Since the operator returns a Failure when Str that doesn't contain a number gets coerced to Numeric, one of the elements in the Junction is a Failure, but same Failure rules as normal apply and the Failure doesn't explode just because it's in a Junction, and we can .grep it out. The exception will be thrown, if you try to use the Failure as a value—just like were this Failure on its own and not part of the Junction:

my $j = +any "not a number""42""2.1";
try say $j == 42;
$! and say "Got exception: $!.^name()"# OUTPUT: «Got exception: X::Str::Numeric␤» 

Note that if an exception gets thrown when any of the values in a Junction get computed, it will be thrown just as if the problematic value were computed on its own and not with a Junction; you can't just compute the values that work while ignoring exceptions:

sub calc ($_{ die when 13 }
my $j = any 1..42;
say try calc $j# OUTPUT: «Nil␤» 

Only one value above causes an exception, but the result of the try block is still a Nil. A possible way around it is to cheat and evaluate the values of the Junction individually and then re-create the Junction from the result:

sub calc ($_{ die when 13 }
my $j = any 1..42;
$j = any (gather $j».take).grep: {Nil !=== try calc $_};
say so $j == 42# OUTPUT: «True␤» 


Note that using Junctions on the right-hand side of ~~ works slightly differently than using Junctions with other operators.

Consider this example:

say 25 == (25 | 42);    # OUTPUT: «any(True, False)␤» – Junction 
say 25 ~~ (25 | 42);    # OUTPUT: «True␤»             – Bool 

The reason is that == (and most other operators) are subject to auto-threading, and therefore you will get a Junction as a result. On the other hand, ~~ will call .ACCEPTS on the right-hand-side (in this case on a Junction) and the result will be a Bool.


method Str

Defined as:

multi method Str(Junction:D:)

Autothreads the .Str method over its elements and returns results as a Junction. Output methods that use .Str method (print and put) are special-cased to autothread junctions, despite being able to accept a Mu type.

method gist

Defined as:

multi method gist(Junction:D:)

Collapses the Junction and returns a Str composed of the type of the junction and the gists of its components:

<a 42 c>.all.say# OUTPUT: «all(a, 42, c)␤» 

method perl

Defined as:

multi method perl(Junction:D:)

Collapses the Junction and returns a Str composed of perls of its components that evaluates to the equivalent Junction with equivalent components:

<a 42 c>.all.perl.put# OUTPUT: «all("a",, "42"), "c")␤» 

infix ~

Defined as:

multi sub infix:<~>(Str:D $aJunction:D $b)
multi sub infix:<~>(Junction:D $aStr:D $b)
multi sub infix:<~>(Junction:D \aJunction:D \b)

The infix ~ concatenation can be used to merge junctions into a single one or merge Junctions with strings. The resulting junction will have all elements merged as if they were joined into a nested loop:

my $odd  = 1|3|5;
my $even = 2|4|6;
my $merged = $odd ~ $even;
say $merged#OUTPUT: «any(12, 14, 16, 32, 34, 36, 52, 54, 56)␤» 
say "Found 34!" if 34 == $merged#OUTPUT: «Found 34!␤» 
my $prefixed = "0" ~ $odd;
say "Found 03" if "03" == $prefixed#OUTPUT: «Found 03!␤» 
my $postfixed = $odd ~ "1";
say "Found 11" if 11 == $postfixed#OUTPUT: «Found 11!␤» 

On the other hand, the version of "~" that uses strings as a prefix or postfix will just concatenate the string to every member of the Junction, creating another Junction with the same number of elements.

See Also

Type Graph

Type relations for Junction
perl6-type-graph Junction Junction Mu Mu Junction->Mu

Stand-alone image: vector