Scope::Upper - Act on upper scopes.
VERSION
- Version 0.06
+ Version 0.10
SYNOPSIS
package X;
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 that will take place when the control
- flow returns into an upper scope. Currently, you can hook an upper scope
- end, or localize variables, array/hash values or deletions of elements
- in higher contexts. You can also return to an upper level and know which
- context was in use then.
+ 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 to
- pass to these functions. This is needed in order to ensure that the
- module still works when your program is ran in the debugger. Don't try
- to use a raw value or things will get messy.
-
- The only thing you can assume is that it is an *absolute* indicator of
- the frame. This means that you can safely store it at some point and use
- it when needed, and it will still denote the original 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 when the upper scope represented
- by $context ends.
+ 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
"unwind @values, $context"
Returns @values *from* the context pointed by $context, i.e. from the
- subroutine, eval or format just above $context.
+ 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.
my $num = sub {
my @a = ('a' .. 'z');
unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
+ # not reached
}->();
- will righteously set $num to 26.
+ will rightfully set $num to 26.
+
+CONSTANTS
+ "SU_THREADSAFE"
+ True iff the module could have been built when thread-safety features.
WORDS
Constants
The context of the scope just above $from.
"SUB $from"
- The context of the closest subroutine above $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.
+ 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.
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'.
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 fixes.
+ context-related fixes.
DEPENDENCIES
XSLoader (standard since perl 5.006).
SEE ALSO
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.
+
AUTHOR
Vincent Pit, "<perl at profvince.com>", <http://www.profvince.com>.
Thanks to Shawn M. Moore for motivation.
COPYRIGHT & LICENSE
- Copyright 2008-2009 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