use strict; use warnings; package Hook::LexWrap; # git description: v0.25-14-g33c34e7 # vi: noet sts=8 sw=8 ts=8 : # ABSTRACT: Lexically scoped subroutine wrappers # KEYWORDS: subroutine function modifier wrapper lexical scope our $VERSION = '0.26'; use Carp (); { no warnings 'redefine'; *CORE::GLOBAL::caller = sub (;$) { my ($height) = ($_[0]||0); my $i=1; my $name_cache; while (1) { my @caller = CORE::caller() eq 'DB' ? do { package # line break to foil [Git::Describe] DB; CORE::caller($i++) } : CORE::caller($i++); return if not @caller; $caller[3] = $name_cache if $name_cache; $name_cache = $caller[0] eq 'Hook::LexWrap' ? $caller[3] : ''; next if $name_cache || $height-- != 0; return wantarray ? @_ ? @caller : @caller[0..2] : $caller[0]; } }; } sub import { no strict 'refs'; *{caller()."::wrap"} = \&wrap } sub wrap (*@) { ## no critic Prototypes my ($typeglob, %wrapper) = @_; $typeglob = (ref $typeglob || $typeglob =~ /::/) ? $typeglob : caller()."::$typeglob"; my $original; { no strict 'refs'; $original = ref $typeglob eq 'CODE' && $typeglob || *$typeglob{CODE} || Carp::croak "Can't wrap non-existent subroutine ", $typeglob; } Carp::croak "'$_' value is not a subroutine reference" foreach grep {$wrapper{$_} && ref $wrapper{$_} ne 'CODE'} qw(pre post); no warnings 'redefine'; my ($caller, $unwrap) = *CORE::GLOBAL::caller{CODE}; my $imposter = sub { if ($unwrap) { goto &$original } my ($return, $prereturn); if (wantarray) { $prereturn = $return = []; () = $wrapper{pre}->(@_,$return) if $wrapper{pre}; if (ref $return eq 'ARRAY' && $return == $prereturn && !@$return) { $return = [ &$original ]; () = $wrapper{post}->(@_, $return) if $wrapper{post}; } return ref $return eq 'ARRAY' ? @$return : ($return); } elsif (defined wantarray) { $return = bless sub {$prereturn=1}, 'Hook::LexWrap::Cleanup'; my $dummy = $wrapper{pre}->(@_, $return) if $wrapper{pre}; unless ($prereturn) { $return = &$original; $dummy = scalar $wrapper{post}->(@_, $return) if $wrapper{post}; } return $return; } else { $return = bless sub {$prereturn=1}, 'Hook::LexWrap::Cleanup'; $wrapper{pre}->(@_, $return) if $wrapper{pre}; unless ($prereturn) { &$original; $wrapper{post}->(@_, $return) if $wrapper{post}; } return; } }; ref $typeglob eq 'CODE' and return defined wantarray ? $imposter : Carp::carp "Uselessly wrapped subroutine reference in void context"; { no strict 'refs'; *{$typeglob} = $imposter; } return unless defined wantarray; return bless sub{ $unwrap=1 }, 'Hook::LexWrap::Cleanup'; } package # hide from PAUSE Hook::LexWrap::Cleanup; sub DESTROY { $_[0]->() } use overload q{""} => sub { undef }, q{0+} => sub { undef }, q{bool} => sub { undef }, q{fallback}=>1; #fallback=1 - like no overloading for other operations 1; __END__ =pod =encoding UTF-8 =head1 NAME Hook::LexWrap - Lexically scoped subroutine wrappers =head1 VERSION version 0.26 =head1 SYNOPSIS use Hook::LexWrap; sub doit { print "[doit:", caller, "]"; return {my=>"data"} } SCOPED: { wrap doit => pre => sub { print "[pre1: @_]\n" }, post => sub { print "[post1:@_]\n"; $_[1]=9; }; my $temporarily = wrap doit => post => sub { print "[post2:@_]\n" }, pre => sub { print "[pre2: @_]\n "}; @args = (1,2,3); doit(@args); # pre2->pre1->doit->post1->post2 } @args = (4,5,6); doit(@args); # pre1->doit->post1 =head1 DESCRIPTION Hook::LexWrap allows you to install a pre- or post-wrapper (or both) around an existing subroutine. Unlike other modules that provide this capacity (e.g. Hook::PreAndPost and Hook::WrapSub), Hook::LexWrap implements wrappers in such a way that the standard C function works correctly within the wrapped subroutine. To install a prewrappers, you write: use Hook::LexWrap; wrap 'subroutine_name', pre => \&some_other_sub; #or: wrap *subroutine_name, pre => \&some_other_sub; The first argument to C is a string containing the name of the subroutine to be wrapped (or the typeglob containing it, or a reference to it). The subroutine name may be qualified, and the subroutine must already be defined. The second argument indicates the type of wrapper being applied and must be either C<'pre'> or C<'post'>. The third argument must be a reference to a subroutine that implements the wrapper. To install a post-wrapper, you write: wrap 'subroutine_name', post => \&yet_another_sub; #or: wrap *subroutine_name, post => \&yet_another_sub; To install both at once: wrap 'subroutine_name', pre => \&some_other_sub, post => \&yet_another_sub; or: wrap *subroutine_name, post => \&yet_another_sub, # order in which wrappers are pre => \&some_other_sub; # specified doesn't matter Once they are installed, the pre- and post-wrappers will be called before and after the subroutine itself, and will be passed the same argument list. The pre- and post-wrappers and the original subroutine also all see the same (correct!) values from C and C. =head2 Short-circuiting and long-circuiting return values The pre- and post-wrappers both receive an extra argument in their @_ arrays. That extra argument is appended to the original argument list (i.e. is can always be accessed as $_[-1]) and acts as a place-holder for the original subroutine's return value. In a pre-wrapper, $_[-1] is -- for obvious reasons -- C. However, $_[-1] may be assigned to in a pre-wrapper, in which case Hook::LexWrap assumes that the original subroutine has been "pre-empted", and that neither it, nor the corresponding post-wrapper, nor any wrappers that were applied I the pre-empting pre-wrapper was installed, need be run. Note that any post-wrappers that were installed after the pre-empting pre-wrapper was installed I still be called before the original subroutine call returns. In a post-wrapper, $_[-1] contains the return value produced by the wrapped subroutine. In a scalar return context, this value is the scalar return value. In an list return context, this value is a reference to the array of return values. $_[-1] may be assigned to in a post-wrapper, and this changes the return value accordingly. Access to the arguments and return value is useful for implementing techniques such as memoization: my %cache; wrap fibonacci => pre => sub { $_[-1] = $cache{$_[0]} if $cache{$_[0]} }, post => sub { $cache{$_[0]} = $_[-1] }; or for converting arguments and return values in a consistent manner: # set_temp expects and returns degrees Fahrenheit, # but we want to use Celsius wrap set_temp => pre => sub { splice @_, 0, 1, $_[0] * 1.8 + 32 }, post => sub { $_[-1] = ($_[0] - 32) / 1.8 }; =head2 Lexically scoped wrappers Normally, any wrappers installed by C remain attached to the subroutine until it is undefined. However, it is possible to make specific wrappers lexically bound, so that they operate only until the end of the scope in which they're created (or until some other specific point in the code). If C is called in a I context: my $lexical = wrap 'sub_name', pre => \&wrapper; it returns a special object corresponding to the particular wrapper being placed around the original subroutine. When that object is destroyed -- when its container variable goes out of scope, or when its reference count otherwise falls to zero (e.g. C), or when it is explicitly destroyed (C<$lexical-EDESTROY>) -- the corresponding wrapper is removed from around the original subroutine. Note, however, that all other wrappers around the subroutine are preserved. =head2 Anonymous wrappers If the subroutine to be wrapped is passed as a reference (rather than by name or by typeglob), C does not install the wrappers around the original subroutine. Instead it generates a new subroutine which acts as if it were the original with those wrappers around it. It then returns a reference to that new subroutine. Only calls to the original through that wrapped reference invoke the wrappers. Direct by-name calls to the original, or calls through another reference, do not. If the original is subsequently wrapped by name, the anonymously wrapped subroutine reference does not see those wrappers. In other words, wrappers installed via a subroutine reference are completely independent of those installed via the subroutine's name (or typeglob). For example: sub original { print "ray" } # Wrap anonymously... my $anon_wrapped = wrap \&original, pre => sub { print "do..." }; # Show effects... original(); # prints "ray" $anon_wrapped->(); # prints "do..ray" # Wrap nonymously... wrap *original, pre => sub { print "fa.." }, post => sub { print "..mi" }; # Show effects... original(); # now prints "fa..ray..mi" $anon_wrapped->(); # still prints "do...ray" =head1 DIAGNOSTICS =over =item C An attempt was made to wrap a subroutine that was not defined at the point of wrapping. =item C<'pre' value is not a subroutine reference> The value passed to C after the C<'pre'> flag was not a subroutine reference. Typically, someone forgot the C on the anonymous subroutine: wrap 'subname', pre => { your_code_here() }; and Perl interpreted the last argument as a hash constructor. =item C<'post' value is not a subroutine reference> The value passed to C after the C<'post'> flag was not a subroutine reference. =item C (warning only) When the subroutine to be wrapped is passed as a subroutine reference, C does not install the wrapper around the original, but instead returns a reference to a subroutine which wraps the original (see L). However, there's no point in doing this if you don't catch the resulting subroutine reference. =back =head1 BLAME Schwern made me do this (by implying it wasn't possible ;-) =head1 SEE ALSO Sub::Prepend =head1 BUGS There are undoubtedly serious bugs lurking somewhere in code this funky :-) Bug reports and other feedback are most welcome. Bugs may be submitted through L (or L). =head1 AUTHOR Damian Conway =head1 CONTRIBUTORS =for stopwords Karen Etheridge Alexandr Ciornii Father Chrysostomos =over 4 =item * Karen Etheridge =item * Alexandr Ciornii =item * Father Chrysostomos =back =head1 COPYRIGHT AND LICENSE This software is copyright (c) 2001 by Damian Conway. This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself. =cut