Tweak some comments in the uplevel() code

[perl/modules/Scope-Upper.git] / README

diff --git a/README b/README
index 16f88ca56cd9895c60ad6903efac19530457fa7e..3a5d76782328366d48bed86937fcd8cb843cd0d9 100644 (file)
--- a/README
+++ b/README
@@ -2,53 +2,81 @@ NAME
     Scope::Upper - Act on upper scopes.
 
 VERSION
     Scope::Upper - Act on upper scopes.
 
 VERSION

-    Version 0.07
+    Version 0.16

 
 SYNOPSIS
 
 SYNOPSIS

-        package X;
+    "reap", "localize", "localize_elem", "localize_delete" and "WORDS" :

 
 

-        use Scope::Upper qw/reap localize localize_elem localize_delete :words/;
+        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 => $desc }, __PACKAGE__) => UP; # one scope up
+         localize '$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";
-         } => SCOPE 1; # same as UP here
+         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 {
          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
+          print Scope->tag->name, ': ', @_;
+         } => UP;
+        }

 
 

-         # delete last @ARGV element
-         localize_delete '@ARGV', -1 => UP SUB HERE; # same as UP here
+        # 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 Y;
+        ...
+
+        package UserLand;

 
         {
 
         {

-         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
+         Scope->new("top");      # initializes $UserLand::tag
+
+         {
+          Scope->catch;
+          my $one = 1 + undef;   # prints "top: Use of uninitialized value..."

 
 

-        package Z;
+          {
+           Scope->private;
+           eval { require Cwd };
+           print $@;             # prints "Can't locate Cwd.pm in @INC (@INC contains:) at..."
+          }

 
 

-        use Scope::Upper qw/unwind want_at :words/;
+          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->();
 
         sub try (&) {
          my @result = shift->();

-         my $cx = SUB UP SUB;
+         my $cx = SUB UP; # Point to the sub above this one

          unwind +(want_at($cx) ? @result : scalar @result) => $cx;
         }
 
          unwind +(want_at($cx) ? @result : scalar @result) => $cx;
         }
 


@@ -56,11 +84,34 @@ SYNOPSIS
 
         sub zap {
          try {
 
         sub zap {
          try {

+          my @things = qw<a b c>;

           return @things; # returns to try() and then outside zap()
           return @things; # returns to try() and then outside zap()

-         }
+          # not reached
+         };
+         # not reached

         }
 
         }
 

-        my @what = zap(); # @what contains @things
+        my @stuff = zap(); # @stuff contains qw<a b c>
+        my $stuff = zap(); # $stuff contains 3
+
+    "uplevel" :
+
+        package Uplevel;
+
+        use Scope::Upper qw<uplevel CALLER>;
+
+        sub target {
+         faker(@_);
+        }
+
+        sub faker {
+         uplevel {
+          my $sub = (caller 0)[3];
+          print "$_[0] from $sub()";
+         } @_ => CALLER(1);
+        }
+
+        target('hello'); # "hello from Uplevel::target()"

 
 DESCRIPTION
     This module lets you defer actions *at run-time* that will take place
 
 DESCRIPTION
     This module lets you defer actions *at run-time* that will take place


@@ -73,7 +124,10 @@ DESCRIPTION
         "localize_delete" ;
 
     *   return values immediately to an upper level with "unwind", and know
         "localize_delete" ;
 
     *   return values immediately to an upper level with "unwind", and know

-        which context was in use then with "want_at".
+        which context was in use then with "want_at" ;
+
+    *   execute a subroutine in the context of an upper subroutine stack
+        frame with "uplevel".

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


@@ -86,12 +140,12 @@ FUNCTIONS
     needed, and it will still denote the original scope.
 
   "reap $callback, $context"
     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.
+    Adds a destructor that calls $callback (in void context) when the upper
+    scope represented by $context ends.

 
   "localize $what, $value, $context"
 
   "localize $what, $value, $context"

-    A "local" delayed to the time of first return into the upper scope
-    denoted by $context. $what can be :
+    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
 
     *   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


@@ -111,22 +165,54 @@ FUNCTIONS
         corresponding type.
 
         When the symbol is given by a string, it is resolved when the actual
         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', $_[0] => UP }
-
-        will localize in the caller's namespace.
+        localization takes place and not when "localize" is called. Thus, if
+        the symbol name is not qualified, it will refer to the variable in
+        the package where the localization actually takes place and not in
+        the one where the "localize" call was compiled. For example,
+
+            {
+             package Scope;
+             sub new { localize '$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"
 
   "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.
+    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"
 
   "localize_delete $what, $key, $context"

-    Similiar to "localize", but for deleting variables or array/hash
-    elements. $what can be:
+    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 glob, in which case $key is ignored and the call is equivalent to
         "local *x".


@@ -141,9 +227,9 @@ FUNCTIONS
 
   "unwind @values, $context"
     Returns @values *from* the context pointed by $context, i.e. from the
 
   "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.
+    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
 
     The upper context isn't coerced onto @values, which is hence always
     evaluated in list context. This means that


@@ -151,12 +237,13 @@ FUNCTIONS
         my $num = sub {
          my @a = ('a' .. 'z');
          unwind @a => HERE;
         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"
         }->();
 
     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
+    Like "wantarray", but for the subroutine/eval/format at or just above

     $context.
 
     The previous example can then be "corrected" :
     $context.
 
     The previous example can then be "corrected" :


@@ -164,9 +251,83 @@ FUNCTIONS
         my $num = sub {
          my @a = ('a' .. 'z');
          unwind +(want_at(HERE) ? @a : scalar @a) => HERE;
         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.
+
+  "uplevel $code, @args, $context"
+    Executes the code reference $code with arguments @args as if it were
+    located at the subroutine stack frame pointed by $context, effectively
+    fooling "caller" and "die" into believing that the call actually
+    happened higher in the stack. The code is executed in the context of the
+    "uplevel" call, and what it returns is returned as-is by "uplevel".
+
+        sub target {
+         faker(@_);
+        }
+
+        sub faker {
+         uplevel {
+          map { 1 / $_ } @_;
+         } @_ => CALLER(1);
+        }
+
+        my @inverses = target(1, 2, 4); # @inverses contains (0, 0.5, 0.25)
+        my $count    = target(1, 2, 4); # $target is 3
+
+    Sub::Uplevel also implements a pure-Perl version of "uplevel". Both are
+    identical, with the following caveats :
+
+    *   The Sub::Uplevel implementation of "uplevel" may execute a code
+        reference in the context of any upper stack frame. The Scope::Upper
+        version only allows to uplevel to a subroutine stack frame, and will
+        croak if you try to target an "eval" or a format.
+
+    *   Exceptions thrown from the code called by this version of "uplevel"
+        will not be caught by "eval" blocks between the target frame and the
+        uplevel call, while they will for Sub::Uplevel's version. This means
+        that :
+
+            eval {
+             sub {
+              local $@;
+              eval {
+               sub {
+                uplevel { die 'wut' } CALLER(2); # for Scope::Upper
+                # uplevel(3, sub { die 'wut' })  # for Sub::Uplevel
+               }->();
+              };
+              print "inner block: $@";
+              $@ and exit;
+             }->();
+            };
+            print "outer block: $@";
+
+        will print "inner block: wut..." with Sub::Uplevel and "outer block:
+        wut..." with Scope::Upper.
+
+    *   Sub::Uplevel globally overrides "CORE::GLOBAL::caller", while
+        Scope::Upper does not.
+
+    A simple wrapper lets you mimic the interface of "uplevel" in
+    Sub::Uplevel :
+
+        use Scope::Upper;
+
+        sub uplevel {
+         my $frame = shift;
+         my $code  = shift;
+         my $cxt   = Scope::Upper::CALLER($frame);
+         &Scope::Upper::uplevel($code => @_ => $cxt);
+        }
+
+    Albeit the three exceptions listed above, it passes all the tests of
+    Sub::Uplevel.
+
+CONSTANTS
+  "SU_THREADSAFE"
+    True iff the module could have been built when thread-safety features.

 
 WORDS
   Constants
 
 WORDS
   Constants


@@ -246,27 +407,33 @@ WORDS
         # $cxt = SCOPE(4), UP SUB UP SUB, or UP SUB EVAL, or UP CALLER(2), or TOP
         ...
 
         # $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:
+    Where "unwind", "want_at" and "uplevel" point to depending on the $cxt:

 
         sub {
          eval {
           sub {
            {
 
         sub {
          eval {
           sub {
            {

-            unwind @things => $cxt;
+            unwind @things => $cxt;     # or uplevel { ... } $cxt;

             ...
            }
            ...
           }->(); # $cxt = SCOPE(0 .. 1), or HERE, or UP, or SUB, or CALLER(0)
           ...
             ...
            }
            ...
           }->(); # $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(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(3), or SUB UP SUB, or SUB EVAL, or CALLER(2)
         ...
 

+        # (*) Note that uplevel() will croak if you pass that scope frame,
+        #     because it can't target eval scopes.
+

 EXPORT
     The functions "reap", "localize", "localize_elem", "localize_delete",
 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'.
+    "unwind", "want_at" and "uplevel" 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
 
     Same goes for the words "TOP", "HERE", "UP", "SUB", "EVAL", "SCOPE" and
     "CALLER" that are only exported on request, individually or by the tags


@@ -309,8 +476,19 @@ DEPENDENCIES
     XSLoader (standard since perl 5.006).
 
 SEE ALSO
     XSLoader (standard since perl 5.006).
 
 SEE ALSO

+    "local" in perlfunc, "Temporary Values via local()" in perlsub.
+

     Alias, Hook::Scope, Scope::Guard, Guard.
 
     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.
+
+    Sub::Uplevel provides a pure-Perl implementation of "uplevel".
+

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


@@ -337,7 +515,7 @@ ACKNOWLEDGEMENTS
     Thanks to Shawn M. Moore for motivation.
 
 COPYRIGHT & LICENSE
     Thanks to Shawn M. Moore for motivation.
 
 COPYRIGHT & LICENSE

-    Copyright 2008-2009 Vincent Pit, all rights reserved.
+    Copyright 2008,2009,2010,2011 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.
 
     This program is free software; you can redistribute it and/or modify it
     under the same terms as Perl itself.