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Perl6-Bible-0.24
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NAME

Synopsis 13 - Overloading

AUTHOR

Larry Wall <larry@wall.org>

VERSION

Maintainer: Larry Wall <larry@wall.org>
Date: 2 Nov 2004
Last Modified: 21 Aug 2005
Number: 13
Version: 3

Overview

This synopsis discusses those portions of Apocalypse 12 that ought to have been in Apocalypse 13.

Multiple dispatch

The overloading mechanism of Perl 5 has been superseded by Perl 6's multiple dispatch mechanism. Nearly all internal functions are defined as multi subs or multi methods on generic types. Built-in operators are merely oddly named functions with an alternate call syntax. All you have to do to overload them is to define your own multi subs and methods that operate on arguments with more specific types.

For unary operators, this makes little effective difference, but for binary operators, multiple dispatch fixes the Perl 5 problem of paying attention only to the type of the left argument. Since both argument types are used in deciding which routine to call, there is no longer any trickery involving swapping the arguments to use the right argument's type instead of the left one. And there's no longer any need to examine a special flag to see if the arguments were reversed.

For much more about multiple dispatch, see A12.

Syntax

There is no longer any special use overload syntax separate from the declarations of the multi routines themselves. To overload an existing built-in sub, say something like:

multi sub *uc (TurkishStr $s) {...}

Now if you call uc() on any Turkish string, it will call your function rather than the built-in one. Putting the multi into the * namespace makes it show up in everyone's packages, but as long as no one else defines a version of uc on TurkishStr, there's no collision. The types of the invocants are included in the "long name" of any multi sub or method.

If you want to overload string concatenation for Arabic strings so you can handle various ligatures, you can say:

multi sub *infix:<~>(ArabicStr $s1, ArabicStr $s2) {...}
multi sub *infix:<~>(Str $s1, ArabicStr $s2) {...}
multi sub *infix:<~>(ArabicStr $s1, Str $s2) {...}

Ordinary methods can be turned into multi methods within the class definition:

    class MyNum {
	multi method abs(MyNum $x) {...}
	...
    }

Likewise operators on your new type can appear in the class:

    class MyNum {
	multi method prefix:<+> (MyNum $x) {...} # what we do in numeric context
	multi method prefix:<~> (MyNum $x) {...} # what we do in string context
	multi method prefix:<?> (MyNum $x) {...} # what we do in boolean context
	...
    }

Binary operators may be declared as commutative:

multi sub infix:<+> (Us $us, Them $them) is commutative { myadd($us,$them) }

That's equivalent to:

multi sub infix:<+> (Us $us, Them $them) { myadd($us,$them) }
multi sub infix:<+> (Them $them, Us $us) { myadd($us,$them) }

Note the lack of * on those definitions. That means this definition of addition is only in effect within the scope of the package in which infix:<+> is defined. Similar constraints apply to lexically scoped multi subs. Generally you want to put your multi subs into the * space, however, so that they work everywhere.

The use overload syntax had one benefit over Perl 6's syntax in that it was easy to alias several different operators to the same service routine. This can easily be handled with Perl 6's aliasing:

multi sub unimpl (MyFoo $x, MyFoo $y) { upchuck(); }
&infix:<+> ::= &unimpl;
&infix:<-> ::= &unimpl;
&infix:<*> ::= &unimpl;
&infix:</> ::= &unimpl;

Fallbacks

Dispatch is based on a routine's signature declaration without regard to whether the routine is defined yet. If an attempt is made to dispatch to a declared but undefined routine, Perl will redispatch to AUTOSUBDEF or AUTOMETHDEF as appropriate to define the routine. This provides a run-time mechanism for fallbacks. By default, these declarations are taken at face value and do not specify any underlying semantics. As such, they're a "shallow" interpretation.

However, sometimes you want to specify a "deep" interpretation of your operators. That is, you're specify the abstract operation, which may be used by various shallow operators. Any deep multi declarations will be "amplified" into all the shallow operators that can be logically based on it. If you say:

multi sub infix:<%> (Us $us, Them $them) is deep { mymod($us,$them) }

then

multi sub infix:<%=> (Us $us, Them $them) { $us = $us % $them }

is also generated for you (unless you define it yourself). The mappings of magical names to sub definitions is controlled by the %?DEEPMAGIC compiler hash. Pragmas can influence the contents of this hash over a lexical scope, so you could have different policies on magical autogeneration. The default mappings correspond to the standard fallback mappings of Perl 5 overloading.