Provided by: libdata-alias-perl_1.28-1_amd64 
NAME
Data::Alias - Comprehensive set of aliasing operations
SYNOPSIS
use Data::Alias;
alias {
# aliasing instead of copying whenever possible
};
alias $x = $y; # alias $x to $y
alias @x = @y; # alias @x to @y
alias $x[0] = $y; # similar for array and hash elements
alias push @x, $y; # push alias to $y onto @x
$x = alias [ $y, $z ]; # construct array of aliases
alias my ($x, $y) = @_; # named aliases to arguments
alias { ($x, $y) = ($y, $x) }; # swap $x and $y
alias { my @t = @x; @x = @y; @y = @t }; # swap @x and @y
use Data::Alias qw/ alias copy /;
alias { copy $x = $y }; # force copying inside alias-BLOCK
use Data::Alias qw/ deref /;
my @refs = (\$x, \@y, \%z);
foo(deref @refs) # same as foo($x, @y, %z)
DESCRIPTION
Aliasing is the phenomenon where two different expressions actually refer to the same
thing. Modifying one will modify the other, and if you take a reference to both, the two
values are the same.
Aliasing occurs in Perl for example in for-loops and sub-calls:
for $var ($x) {
# here $var is an alias to $x
}
foo($y);
sub foo {
# here $_[0] is an alias to $y
}
Data::Alias is a module that allows you to apply "aliasing semantics" to a section of
code, causing aliases to be made wherever Perl would normally make copies instead. You
can use this to improve efficiency and readability, when compared to using references.
The exact details of aliasing semantics are below under "DETAILS".
Perl 5.22 added some support for aliasing to the Perl core. It has a different syntax,
and a different set of operations, from that supplied by this module; see "Assigning to
References" in perlref. The core's aliasing facilities are implemented more robustly than
this module and are better supported. If you can rely on having a sufficiently recent
Perl version, you should prefer to use the core facility rather than use this module. If
you are already using this module and are now using a sufficiently recent Perl, you should
attempt to migrate to the core facility.
SYNTAX
alias EXPR | alias BLOCK
Exported by default.
Enables aliasing semantics within the expression or block. Returns an alias to the
expression, or the block's return value.
"alias" is context-transparent, meaning that whichever context it is placed in (list,
scalar, void), the expression/block is evaluated in the same context.
copy EXPR | copy BLOCK
Restores normal (copying) semantics within the expression or block, and makes a copy of
the result value (unless in void context).
Like "alias", "copy" is context-transparent.
deref LIST
Accepts a list of references to scalars, arrays, or hashes. Applies the applicable
dereferencing operator to each. This means that:
deref $scalarref, $arrayref, $hashref
behaves like:
$$scalarref, @$arrayref, %$hashref
Where an array or hash reference is given, the returned list does not include the array or
hash as an lvalue; the array/hash is expanded and the list includes its elements.
Scalars, including the elements of an array/hash, are treated as lvalues, and can be
enreferenced using the "\" operator or aliased to using the "alias" operator. This is
slightly different from what you'd get using the built-in dereference operators:
@$arrayref references the array as an lvalue, so "\" or "alias" can operate on the array
itself rather than just its elements.
EXAMPLES
A common usage of aliasing is to make an abbreviation for an expression, to avoid having
to repeat that (possibly verbose or ugly) expression over and over:
alias my $fi = $self->{FrobnitzIndex};
$fi = $fi > 0 ? $fi - $adj : $fi + $adj;
sub rc4 {
alias my ($i, $j, $S) = @_;
my $a = $S->[($i += 1) &= 255];
my $b = $S->[($j += $S->[$i]) &= 255];
$S->[(($S->[$j] = $a) + ($S->[$i] = $b)) & 255]
}
In the second example, the rc4 function updates its first two arguments (two state values)
in addition to returning a value.
Aliasing can also be used to avoid copying big strings. This example would work fine
without "alias" but would be much slower when passed a big string:
sub middlesection ($) {
alias my $s = shift;
substr $s, length($s)/4, length($s)/2
}
You can also apply aliasing semantics to an entire block. Here this is used to swap two
arrays in O(1) time:
alias {
my @temp = @x;
@x = @y;
@y = @temp;
};
The "copy" function is typically used to temporarily reinstate normal semantics, but can
also be used to explicitly copy a value when perl would normally not do so:
my $ref = \copy $x;
DETAILS
This section describes exactly what the aliasing semantics are of operations. Anything
not listed below has unaltered behaviour.
scalar assignment to variable or element.
Makes the left-side of the assignment an alias to the right-side expression, which can
be anything.
alias my $lexvar = $foo;
alias $pkgvar = $foo;
alias $array[$i] = $foo;
alias $hash{$k} = $foo;
An attempt to do alias-assignment to an element of a tied (or "magical") array or hash
will result in a "Can't put alias into tied array/hash" error.
scalar assignment to dereference
If $ref is a reference or undef, this simply does "$ref = \$foo". Otherwise, the
indicated package variable (via glob or symbolic reference) is made an alias to the
right-side expression.
alias $$ref = $foo;
scalar assignment to glob
Works mostly the same as normal glob-assignment, however it does not set the import-
flag. (If you don't know what this means, you probably don't care)
alias *glob = $reference;
scalar assignment to anything else
Not supported.
alias substr(...) = $foo; # ERROR!
alias lvalsub() = $foo; # ERROR!
conditional scalar assignment
Here $var (and $var2) are aliased to $foo if the applicable condition is satisfied.
$bool and $foo can be any expression. $var and $var2 can be anything that is valid on
the left-side of an alias-assignment.
alias $bool ? $var : $var2 = $foo;
alias $var &&= $foo;
alias $var ||= $foo;
alias $var //= $foo; # (perl 5.9.x or later)
whole aggregate assignment from whole aggregate
This occurs where the expressions on both sides of the assignment operator are purely
complete arrays or hashes. The entire aggregate is aliased, not merely the contents.
This means for example that "\@lexarray == \@foo".
alias my @lexarray = @foo;
alias my %lexhash = %foo;
alias @pkgarray = @foo;
alias %pkghash = %foo;
Making the left-side a dereference is also supported:
alias @$ref = @foo;
alias %$ref = %foo;
and analogously to assignment to scalar dereference, these will change $ref to
reference the aggregate, if $ref was undef or already a reference. If $ref is a
string or glob, the corresponding package variable is aliased.
Anything more complex than a whole-aggregate expression on either side, even just
enclosing the aggregate expression in parentheses, will prevent the assignment
qualifying for this category. It will instead go into one of the following two
categories. Parenthesisation is the recommended way to avoid whole-aggregate aliasing
where it is unwanted. If you want to merely replace the contents of the left-side
aggregate with aliases to the contents of the right-side aggregate, parenthesise the
left side.
whole aggregate assignment from list
If the left-side expression is purely a complete array or hash, and the right-side
expression is not purely a matching aggregate, then a new aggregate is implicitly
constructed. This means:
alias my @lexfoo = (@foo);
alias my @array = ($x, $y, $z);
alias my %hash = (x => $x, y => $y);
is translated to:
alias my @lexfoo = @{ [@foo] };
alias my @array = @{ [$x, $y, $z] };
alias my %hash = %{ {x => $x, y => $y} };
If you want to merely replace the contents of the aggregate with aliases to the
contents of another aggregate, rather than create a new aggregate, you can force list-
assignment by parenthesizing the left side, see below.
list assignment
List assignment is any assignment where the left-side is an array-slice, hash-slice,
or list in parentheses. This behaves essentially like many scalar assignments in
parallel.
alias my (@array) = ($x, $y, $z);
alias my (%hash) = (x => $x, y => $y);
alias my ($x, $y, @rest) = @_;
alias @x[0, 1] = @x[1, 0];
Any scalars that appear on the left side must be valid targets for scalar assignment.
When an array or hash appears on the left side, normally as the last item, its
contents are replaced by the list of all remaining right-side elements. "undef" can
also appear on the left side to skip one corresponding item in the right-side list.
Beware when putting a parenthesised list on the left side. Just like Perl parses
"print (1+2)*10" as "(print(1+2))*10", it would parse "alias ($x, $y) = ($y, $x)" as
"(alias($x, $y)) = ($y, $x)" which does not do any aliasing, and results in the
"Useless use of alias" warning, if warnings are enabled.
To circumvent this issue, you can either one of the following:
alias +($x, $y) = ($y, $x);
alias { ($x, $y) = ($y, $x) };
Anonymous aggregate constructors
Return a reference to a new anonymous array or hash, populated with aliases. This
means that for example "\$hashref->{x} == \$x".
my $arrayref = alias [$x, $y, $z];
my $hashref = alias {x => $x, y => $y};
Note that this also works:
alias my $arrayref = [$x, $y, $z];
alias my $hashref = {x => $x, y => $y};
but this makes the lhs an alias to the temporary, and therefore read-only, reference
made by "[]" or "{}". Therefore later attempts to assign to $arrayref or $hashref
results in an error. The anonymous aggregate that is referenced behaves the same in
both cases obviously.
Array insertions
These work as usual, except the inserted elements are aliases.
alias push @array, $foo;
alias unshift @array, $foo;
alias splice @array, 1, 2, $foo;
An attempt to do any of these on tied (or "magical") array will result in a "Can't
push/unshift/splice alias onto tied array" error.
Returning an alias
Returns aliases from the current "sub" or "eval". Normally this only happens for
lvalue subs, but "alias return" can be used in any sub. Lvalue subs only work for
scalar return values, but "alias return" can handle a list of return values.
A sub call will very often copy the return value(s) immediately after they have been
returned. "alias return" can't prevent that. To pass an alias through a sub return
and into something else, the call site must process the return value using an aliasing
operation, or at least a non-copying one. For example, ordinary assignment with the
sub call on the right hand side will copy, but if the call site is in the scope of an
"alias" pragma then the assignment will instead alias the return value.
When alias-returning a list of values from a subroutine, each individual value in the
list is aliased. The list as a whole is not aliasable; it is not an array. At the
call site, a list of aliases can be captured into separate variables or into an array,
by an aliasing list assignment.
Subroutines and evaluations
Placing a subroutine or "eval STRING" inside "alias" causes it to be compiled with
aliasing semantics entirely. Additionally, the return from such a sub or eval,
whether explicit using "return" or implicitly the last statement, will be an alias
rather than a copy.
alias { sub foo { $x } };
my $subref = alias sub { $x };
my $xref1 = \foo;
my $xref2 = \alias eval '$x';
my $xref3 = \$subref->();
Explicitly returning an alias can also be done using "alias return" inside any
subroutine or evaluation.
sub foo { alias return $x; }
my $xref = \foo;
Localization
Use of local inside "alias" usually behaves the same as local does in general, however
there is a difference if the variable is tied: in this case, Perl doesn't localise
the variable at all but instead preserves the tie by saving a copy of the current
value, and restoring this value at end of scope.
alias local $_ = $string;
The aliasing semantics of "local" avoids copying by always localizing the variable
itself, regardless of whether it is tied.
IMPLEMENTATION
This module does not use a source filter, and is therefore safe to use within eval STRING.
Instead, Data::Alias hooks into the Perl parser, and replaces operations within the scope
of "alias" by aliasing variants.
For those familiar with perl's internals: it triggers on a ck_rv2cv which resolves to the
imported "alias" sub, and does a parser hack to allow the "alias BLOCK" syntax. When the
ck_entersub is triggered that corresponds to it, the op is marked to be found later. The
actual work is done in a peep-hook, which processes the marked entersub and its children,
replacing the pp_addrs with aliasing replacements. The peep hook will also take care of
any subs defined within the lexical (but not dynamical) scope between the ck_rv2cv and the
ck_entersub.
KNOWN ISSUES
Lexical variables
When aliasing existing lexical variables, the effect is limited in scope to the
current subroutine and any closures create after the aliasing is done, even if the
variable itself has wider scope. While partial fixes are possible, it cannot be fixed
in any reliable or consistent way, and therefore I'm keeping the current behaviour.
When aliasing a lexical that was declared outside the current subroutine, a compile-
time warning is generated "Aliasing of outer lexical variable has limited scope"
(warnings category "closure").
ACKNOWLEDGEMENTS
Specials thanks go to Elizabeth Mattijsen, Juerd Waalboer, and other members of the
Amsterdam Perl Mongers, for their valuable feedback.
AUTHOR
Matthijs van Duin <xmath@cpan.org> developed the module originally, and maintained it
until 2007. Andrew Main (Zefram) <zefram@fysh.org> updated it to work with Perl versions
5.11.0 and later.
LICENSE
Copyright (C) 2003-2007 Matthijs van Duin. Copyright (C) 2010, 2011, 2013, 2015, 2017
Andrew Main (Zefram) <zefram@fysh.org>. All rights reserved. This program is free
software; you can redistribute it and/or modify it under the same terms as Perl itself.