Scope::Upper - Act on upper scopes.
Version 0.12
"reap", "localize", "localize_elem", "localize_delete" and "WORDS" :
package Scope; use Scope::Upper qw/reap localize localize_elem localize_delete :words/; sub new { my ($class, $name) = @_; localize '$tag' => bless({ name => $name }, $class) => UP; reap { print Scope->tag->name, ": end\n" } UP; } # Get the tag stored in the caller namespace sub tag { my $l = 0; my $pkg = __PACKAGE__; $pkg = caller $l++ while $pkg eq __PACKAGE__; no strict 'refs'; ${$pkg . '::tag'}; } sub name { shift->{name} } # Locally capture warnings and reprint them with the name prefixed sub catch { localize_elem '%SIG', '__WARN__' => sub { print Scope->tag->name, ': ', @_; } => UP; } # Locally clear @INC sub private { for (reverse 0 .. $#INC) { # First UP is the for loop, second is the sub boundary localize_delete '@INC', $_ => UP UP; } } ... package UserLand; { Scope->new("top"); # initializes $UserLand::tag { Scope->catch; my $one = 1 + undef; # prints "top: Use of uninitialized value..." { Scope->private; eval { require Cwd }; print $@; # prints "Can't locate Cwd.pm in @INC (@INC contains:) at..." } require Cwd; # loads Cwd.pm } } # prints "top: done"
"unwind" and "want_at" :
package Try; use Scope::Upper qw/unwind want_at :words/; sub try (&) { my @result = shift->(); my $cx = SUB UP; # Point to the sub above this one unwind +(want_at($cx) ? @result : scalar @result) => $cx; } ... sub zap { try { my @things = qw/a b c/; return @things; # returns to try() and then outside zap() # not reached }; # not reached } my @stuff = zap(); # @stuff contains qw/a b c/ my $stuff = zap(); # $stuff contains 3
This module lets you defer actions at run-time that will take place when the control flow returns into an upper scope. Currently, you can:
hook an upper scope end with "reap" ;
localize variables, array/hash values or deletions of elements in higher contexts with respectively "localize", "localize_elem" and "localize_delete" ;
return values immediately to an upper level with "unwind", and know which context was in use then with "want_at".
In all those functions, $context refers to the target scope.
$context
You have to use one or a combination of "WORDS" to build the $context passed to these functions. This is needed in order to ensure that the module still works when your program is ran in the debugger. The only thing you can assume is that it is an absolute indicator of the frame, which means that you can safely store it at some point and use it when needed, and it will still denote the original scope.
reap $callback, $context
Adds a destructor that calls $callback (in void context) when the upper scope represented by $context ends.
$callback
localize $what, $value, $context
Introduces a local delayed to the time of first return into the upper scope denoted by $context. $what can be :
local
$what
A glob, in which case $value can either be a glob or a reference. "localize" follows then the same syntax as local *x = $value. For example, if $value is a scalar reference, then the SCALAR slot of the glob will be set to $$value - just like local *x = \1 sets $x to 1.
$value
local *x = $value
SCALAR
$$value
local *x = \1
$x
1
A string beginning with a sigil, representing the symbol to localize and to assign to. If the sigil is '$', "localize" follows the same syntax as local $x = $value, i.e. $value isn't dereferenced. For example,
'$'
local $x = $value
localize '$x', \'foo' => HERE;
will set $x to a reference to the string 'foo'. Other sigils ('@', '%', '&' and '*') require $value to be a reference of the corresponding type.
'foo'
'@'
'%'
'&'
'*'
When the symbol is given by a string, it is resolved when the actual localization takes place and not when "localize" is called. Thus, if the symbol name is not qualified, it will refer to the variable in the package where the localization actually takes place and not in the one where the "localize" call was compiled. For example,
{ package Scope; sub new { localize '$tag', $_[0] => UP } } { package Tool; { Scope->new; ... } }
will localize $Tool::tag and not $Scope::tag. If you want the other behaviour, you just have to specify $what as a glob or a qualified name.
$Tool::tag
$Scope::tag
Note that if $what is a string denoting a variable that wasn't declared beforehand, the relevant slot will be vivified as needed and won't be deleted from the glob when the localization ends. This situation never arises with local because it only compiles when the localized variable is already declared. Although I believe it shouldn't be a problem as glob slots definedness is pretty much an implementation detail, this behaviour may change in the future if proved harmful.
localize_elem $what, $key, $value, $context
Introduces a local $what[$key] = $value or local $what{$key} = $value delayed to the time of first return into the upper scope denoted by $context. Unlike "localize", $what must be a string and the type of localization is inferred from its sigil. The two only valid types are array and hash ; for anything besides those, "localize_elem" will throw an exception. $key is either an array index or a hash key, depending of which kind of variable you localize.
local $what[$key] = $value
local $what{$key} = $value
$key
If $what is a string pointing to an undeclared variable, the variable will be vivified as soon as the localization occurs and emptied when it ends, although it will still exist in its glob.
localize_delete $what, $key, $context
Introduces the deletion of a variable or an array/hash element delayed to the time of first return into the upper scope denoted by $context. $what can be:
A glob, in which case $key is ignored and the call is equivalent to local *x.
local *x
A string beginning with '@' or '%', for which the call is equivalent to respectiveley local $a[$key]; delete $a[$key] and local $h{$key}; delete $h{$key}.
local $a[$key]; delete $a[$key]
local $h{$key}; delete $h{$key}
A string beginning with '&', which more or less does undef &func in the upper scope. It's actually more powerful, as &func won't even exists anymore. $key is ignored.
undef &func
&func
exists
unwind @values, $context
Returns @values from the context pointed by $context, i.e. from the subroutine, eval or format at or just above $context, and immediately restart the program flow at this point - thus effectively returning to an upper scope.
@values
The upper context isn't coerced onto @values, which is hence always evaluated in list context. This means that
my $num = sub { my @a = ('a' .. 'z'); unwind @a => HERE; # not reached }->();
will set $num to 'z'. You can use "want_at" to handle these cases.
$num
'z'
want_at $context
Like wantarray, but for the subroutine/eval/format at or just above $context.
wantarray
The previous example can then be "corrected" :
my $num = sub { my @a = ('a' .. 'z'); unwind +(want_at(HERE) ? @a : scalar @a) => HERE; # not reached }->();
will rightfully set $num to 26.
26
SU_THREADSAFE
True iff the module could have been built when thread-safety features.
TOP
Returns the context that currently represents the highest scope.
HERE
The context of the current scope.
For any of those functions, $from is expected to be a context. When omitted, it defaults to the the current context.
$from
UP $from
The context of the scope just above $from.
SUB $from
The context of the closest subroutine above $from. Note that $from is returned if it is already a subroutine context ; hence SUB SUB == SUB.
SUB SUB == SUB
EVAL $from
The context of the closest eval above $from. Note that $from is returned if it is already an eval context ; hence EVAL EVAL == EVAL.
EVAL EVAL == EVAL
Here, $level should denote a number of scopes above the current one. When omitted, it defaults to 0 and those functions return the same context as "HERE".
$level
0
SCOPE $level
The $level-th upper context, regardless of its type.
CALLER $level
The context of the $level-th upper subroutine/eval/format. It kind of corresponds to the context represented by caller $level, but while e.g. caller 0 refers to the caller context, CALLER 0 will refer to the top scope in the current context.
caller $level
caller 0
CALLER 0
Where "reap" fires depending on the $cxt :
$cxt
sub { eval { sub { { reap \&cleanup => $cxt; ... } # $cxt = SCOPE(0), or HERE ... }->(); # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0) ... }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1) ... }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2) ...
Where "localize", "localize_elem" and "localize_delete" act depending on the $cxt :
sub { eval { sub { { localize '$x' => 1 => $cxt; # $cxt = SCOPE(0), or HERE ... } # $cxt = SCOPE(1), or UP, or SUB, or CALLER, or CALLER(0) ... }->(); # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1) ... }; # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2) ... }->(); # $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP ...
Where "unwind" and "want_at" point to depending on the $cxt:
sub { eval { sub { { unwind @things => $cxt; ... } ... }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0) ... }; # $cxt = SCOPE(2), or UP UP, or UP SUB, or EVAL, or CALLER(1) ... }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2) ...
The functions "reap", "localize", "localize_elem", "localize_delete", "unwind" and "want_at" are only exported on request, either individually or by the tags ':funcs' and ':all'.
':funcs'
':all'
The constant "SU_THREADSAFE" is also only exported on request, individually or by the tags ':consts' and ':all'.
':consts'
Same goes for the words "TOP", "HERE", "UP", "SUB", "EVAL", "SCOPE" and "CALLER" that are only exported on request, individually or by the tags ':words' and ':all'.
':words'
Be careful that local variables are restored in the reverse order in which they were localized. Consider those examples:
local $x = 0; { reap sub { print $x } => HERE; local $x = 1; ... } # prints '0' ... { local $x = 1; reap sub { $x = 2 } => HERE; ... } # $x is 0
The first case is "solved" by moving the local before the reap, and the second by using "localize" instead of "reap".
reap
The effects of "reap", "localize" and "localize_elem" can't cross BEGIN blocks, hence calling those functions in import is deemed to be useless. This is an hopeless case because BEGIN blocks are executed once while localizing constructs should do their job at each run. However, it's possible to hook the end of the current scope compilation with B::Hooks::EndOfScope.
BEGIN
import
Some rare oddities may still happen when running inside the debugger. It may help to use a perl higher than 5.8.9 or 5.10.0, as they contain some context-related fixes.
XSLoader (standard since perl 5.006).
"local" in perlfunc, "Temporary Values via local()" in perlsub.
Alias, Hook::Scope, Scope::Guard, Guard.
Continuation::Escape is a thin wrapper around Scope::Upper that gives you a continuation passing style interface to "unwind". It's easier to use, but it requires you to have control over the scope where you want to return.
Scope::Escape.
Vincent Pit, <perl at profvince.com>, http://www.profvince.com.
<perl at profvince.com>
You can contact me by mail or on irc.perl.org (vincent).
irc.perl.org
Please report any bugs or feature requests to bug-scope-upper at rt.cpan.org, or through the web interface at http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Scope-Upper. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.
bug-scope-upper at rt.cpan.org
You can find documentation for this module with the perldoc command.
perldoc Scope::Upper
Tests code coverage report is available at http://www.profvince.com/perl/cover/Scope-Upper.
Inspired by Ricardo Signes.
Thanks to Shawn M. Moore for motivation.
Copyright 2008,2009,2010 Vincent Pit, all rights reserved.
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
To install Scope::Upper, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Scope::Upper
CPAN shell
perl -MCPAN -e shell install Scope::Upper
For more information on module installation, please visit the detailed CPAN module installation guide.