NAME

Scope::Upper - Act on upper scopes.

VERSION

Version 0.09

SYNOPSIS

    package X;

    use Scope::Upper qw/reap localize localize_elem localize_delete :words/;

    sub desc { shift->{desc} }

    sub set_tag {
     my ($desc) = @_;

     # First localize $x so that it gets destroyed last
     localize '$x' => bless({ desc => $desc }, __PACKAGE__) => UP; # one scope 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";
     } => SCOPE 1; # same as UP here

     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('', @_));
     } => UP CALLER 0; # same as UP here

     # delete last @ARGV element
     localize_delete '@ARGV', -1 => UP SUB HERE; # same as UP here
    }

    package Y;

    {
     X::set_tag('pie');
     # $x is now a X object, and @ARGV has one element less
     warn 'what'; # warns "pie: what at ..."
     ...
    } # "pie: done" is printed

    package Z;

    use Scope::Upper qw/unwind want_at :words/;

    sub try (&) {
     my @result = shift->();
     my $cx = SUB UP SUB;
     unwind +(want_at($cx) ? @result : scalar @result) => $cx;
    }

    ...

    sub zap {
     try {
      return @things; # returns to try() and then outside zap()
      # not reached
     }
     # not reached
    }

    my @what = zap(); # @what contains @things

DESCRIPTION

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

In all those functions, $context refers to the target scope.

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`

Add a destructor that calls $callback (in void context) when the upper scope represented by $context ends.

`localize $what, $value, $context`

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 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 $x to 1.
A string beginning with a sigil, representing the symbol to localize and to assign to. If the sigil is '$', "localize" follows the same syntax as local $x = $value, i.e. $value isn't dereferenced. For example,
```
    localize '$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
```
    sub tag { localize '$x', $_[0] => UP }
```
will localize in the caller's namespace.

`localize_elem $what, $key, $value, $context`

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.

`localize_delete $what, $key, $context`

Similiar to "localize", but for deleting variables or array/hash elements. $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 just above $context, and immediately restart the program flow at this point - thus effectively returning to (or from, depending on how you see it) an upper context.

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 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 righteously 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)
    ...

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)
    ...

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'.

The constant "SU_THREADSAFE" is also only exported on request, individually or by the tags ':consts' 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 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".

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).

AUTHOR

Vincent Pit, <perl at profvince.com>, http://www.profvince.com.

You can contact me by mail or on irc.perl.org (vincent).

BUGS

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.

SUPPORT

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.

ACKNOWLEDGEMENTS

Inspired by Ricardo Signes.

Thanks to Shawn M. Moore for motivation.

COPYRIGHT & LICENSE

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.