Scope::Upper - Act on upper scopes.
VERSION
- Version 0.02
+ Version 0.12
SYNOPSIS
- package X;
+ "reap", "localize", "localize_elem", "localize_delete" and "WORDS" :
- use Scope::Upper qw/reap localize localize_elem/;
+ package Scope;
- sub desc { shift->{desc} }
+ use Scope::Upper qw/reap localize localize_elem localize_delete :words/;
- sub set_tag {
- my ($desc) = @_;
+ sub new {
+ my ($class, $name) = @_;
- # First localize $x so that it gets destroyed last
- localize '$x' => bless({ desc => $desc }, __PACKAGE__) => 1;
+ localize '$tag' => bless({ name => $name }, $class) => UP;
- reap sub {
- my $pkg = caller;
- my $x = do { no strict 'refs'; ${$pkg.'::x'} }; # Get the $x in the scope
- print $x->desc . ": done\n";
- } => 1;
+ 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 {
- my $pkg = caller;
- my $x = do { no strict 'refs'; ${$pkg.'::x'} }; # Get the $x in the scope
- CORE::warn($x->desc . ': ' . join('', @_));
- } => 1;
+ print Scope->tag->name, ': ', @_;
+ } => UP;
}
- package Y;
+ # 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;
{
- X::set_tag('pie');
- # $x is now a X object
- warn 'what'; # warns "pie: what at ..."
- ...
- } # "pie: done" is printed
+ 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
DESCRIPTION
- This module lets you defer actions that will take place when the control
- flow returns into an upper scope. Currently, you can hook an upper scope
- end, or localize variables and array/hash values in higher contexts.
+ 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".
FUNCTIONS
- "reap $callback, $level"
- Add a destructor that calls $callback when the $level-th upper scope
- ends, where 0 corresponds to the current scope.
+ In all those functions, $context refers to the target scope.
- "localize $what, $value, $level"
- A "local" delayed to the time of first return into the $level-th upper
- scope. $what can be :
+ 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.
+
+ "localize $what, $value, $context"
+ Introduces a "local" delayed to the time of first return into the upper
+ scope denoted by $context. $what can be :
* 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
syntax as "local $x = $value", i.e. $value isn't dereferenced. For
example,
- localize '$x', \'foo' => 0;
+ localize '$x', \'foo' => HERE;
will set $x to a reference to the string 'foo'. Other sigils ('@',
'%', '&' and '*') require $value to be a reference of the
corresponding type.
When the symbol is given by a string, it is resolved when the actual
- localization takes place and not when "localize" is called. This
- means that
+ localization takes place and not when "localize" is called. Thus, if
+ the symbol name is not qualified, it will refer to the variable in
+ the package where the localization actually takes place and not in
+ the one where the "localize" call was compiled. For example,
+
+ {
+ package Scope;
+ sub new { localize '$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.
+
+ 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.
+
+ 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".
+
+ * 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}".
+
+ * 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.
+
+ "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.
+
+ 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.
+
+ "want_at $context"
+ Like "wantarray", but for the subroutine/eval/format at or just above
+ $context.
+
+ 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.
+
+CONSTANTS
+ "SU_THREADSAFE"
+ True iff the module could have been built when thread-safety features.
+
+WORDS
+ Constants
+ "TOP"
+ Returns the context that currently represents the highest scope.
+
+ "HERE"
+ The context of the current scope.
+
+ Getting a context from a context
+ For any of those functions, $from is expected to be a context. When
+ omitted, it defaults to the the current context.
+
+ "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".
+
+ "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".
+
+ Getting a context from a level
+ 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".
+
+ "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.
+
+ Examples
+ Where "reap" fires depending on the $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)
+ ...
+ }->(); # $cxt = SCOPE(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
+ ...
- sub tag { localize '$x', $_[0] => 1; }
+ 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
+ ...
- will localize in the caller's namespace.
+ 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)
+ ...
- "localize_elem $what, $key, $value, $level"
- Similar to "localize" but for array and hash elements. If $what is a
- glob, the slot to fill is determined from which type of reference $value
- is ; otherwise it's inferred from the sigil. $key is either an array
- index or a hash key, depending of which kind of variable you localize.
+EXPORT
+ 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'.
- "TOPLEVEL"
- Returns the level that currently represents the highest scope.
+ The constant "SU_THREADSAFE" is also only exported on request,
+ individually or by the tags ':consts' and ':all'.
-EXPORT
- The functions "reap", "localize", "localize_elem" and "TOPLEVEL" are
- only exported on request, either individually or by the tags ':funcs'
- and ':all'.
+ 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'.
CAVEATS
Be careful that local variables are restored in the reverse order in
local $x = 0;
{
- reap sub { print $x } => 0;
+ reap sub { print $x } => HERE;
local $x = 1;
...
}
...
{
local $x = 1;
- reap sub { $x = 2 } => 0;
+ 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", "localize" and "localize_elem" effects 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
+ 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.
+
+ 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.
DEPENDENCIES
XSLoader (standard since perl 5.006).
SEE ALSO
+ "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.
+
AUTHOR
Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
ACKNOWLEDGEMENTS
Inspired by Ricardo Signes.
+ Thanks to Shawn M. Moore for motivation.
+
COPYRIGHT & LICENSE
- Copyright 2008 Vincent Pit, all rights reserved.
+ 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.
projects / perl / modules / Scope-Upper.git / blobdiff