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- Introduction
- Strings, Arrays, Hashes;
- Types
- Basic Control Structures
- Subroutines and Signatures
- Objects and Classes
- Contexts
- Regexes (also called "rules")
- Junctions
- Comparing and Matching
- Containers and Values
- Where we are now - an update
- Changes to Perl 5 Operators
- Laziness
- Custom Operators
- The MAIN sub
- Twigils
- Enums
- Unicode
- Scoping
- Regexes strike back
- A grammar for (pseudo) XML
- Subset Types
- The State of the implementations
- Quoting and Parsing
- The Reduction Meta Operator
- The Cross Meta Operator
- Exceptions and control exceptions
- Common Perl 6 data processing idioms
- Currying
Sat, 27 Sep 2008
Junctions
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NAME
"Perl 5 to 6" Lesson 07 - Junctions
SYNOPSIS
my $x = 4; if $x == 3|4 { say '$x is either 3 or 4' } say ((2|3|4)+7).perl # (9|10|11)
DESCRIPTION
Junctions are superpositions of unordered values. Operations on junctions are executed for each item of the junction separately (and maybe even in parallel), and the results are assembled in a junction of the same type.
The junction types only differ when evaluated in boolean context. The types are any
, all
, one
and none
.
Type Infix operator any | one ^ all &
1 | 2 | 3
is the same as any(1..3)
.
my Junction $weekday = any <Monday Tuesday Wednesday Thursday Friday Saturday Sunday> if $day eq $weekday { say "See you on $day"; }
In this example the eq
operator is called with each pair $day, 'Monday'
, $day, 'Tuesday'
etc. and the result is put into an any
-junction again. As soon as the result is determined (in this case, as soon as one comparison returns True
) it can abort the execution of the other comparisons.
This works not only for operators, but also for routines:
if 2 == sqrt(4 | 9 | 16) { say "YaY"; }
To make this possible, junctions stand outside the normal type hierarchy (a bit):
Mu / \ / \ Any Junction / | \ All other types
If you want to write a sub that takes a junction and doesn't autothread over it, you have to declare the type of the parameter either as Mu or Junction
sub dump_yaml(Junction $stuff) { # we hope that YAML can represent junctions ;-) .... }
MOTIVATION
Perl aims to be rather close to natural languages, and in natural language you often say things like "if the result is $this or $that" instead of saying "if the result is $this or the result is $that". Most programming languages only allow (a translation of) the latter, which feels a bit clumsy. With junctions Perl 6 allows the former as well.
It also allows you to write many comparisons very easily that otherwise require loops.
As an example, imagine an array of numbers, and you want to know if all of them are non-negative. In Perl 5 you'd write something like this:
# Perl 5 code: my @items = get_data(); my $all_non_neg = 1; for (@items){ if ($_ < 0) { $all_non_neg = 0; last; } } if ($all_non_neg) { ... }
Or if you happen to know about List::MoreUtils
use List::MoreUtils qw(all); my @items = get_data; if (all { $_ >= 0 } @items) { ... }
In Perl 6 that is short and sweet:
my @items = get_data(); if all(@items) >= 0 { ... }
A Word of Warning
Many people get all excited about junctions, and try to do too much with them.
Junctions are not sets; if you try to extract items from a junction, you are doing it wrong, and should be using a Set instead.
It is a good idea to use junctions as smart conditions, but trying to build a solver for equations based on the junction autothreading rules is on over-extortion and usually results in frustration.