Profile of Try/Tiny.pm

Filename	/usr/share/perl5/Try/Tiny.pm
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1			package Try::Tiny;
2			BEGIN {
3			$Try::Tiny::AUTHORITY = 'cpan:NUFFIN';
4			}
5			$Try::Tiny::VERSION = '0.22';
6			use 5.006;
7			# ABSTRACT: minimal try/catch with proper preservation of $@
8
9			use strict;
10			use warnings;
11
12			use Exporter 5.57 'import';
13			our @EXPORT = our @EXPORT_OK = qw(try catch finally);
14
15			use Carp;
16			$Carp::Internal{+__PACKAGE__}++;
17
18			BEGIN { eval "use Sub::Name; 1" or *{subname} = sub {1} }
19
20			# Need to prototype as @ not $$ because of the way Perl evaluates the prototype.
21			# Keeping it at $$ means you only ever get 1 sub because we need to eval in a list
22			# context & not a scalar one
23
24			sub try (&;@) {
25			my ( $try, @code_refs ) = @_;
26
27			# we need to save this here, the eval block will be in scalar context due
28			# to $failed
29			my $wantarray = wantarray;
30
31			# work around perl bug by explicitly initializing these, due to the likelyhood
32			# this will be used in global destruction (perl rt#119311)
33			my ( $catch, @finally ) = ();
34
35			# find labeled blocks in the argument list.
36			# catch and finally tag the blocks by blessing a scalar reference to them.
37			foreach my $code_ref (@code_refs) {
38
39			if ( ref($code_ref) eq 'Try::Tiny::Catch' ) {
40			croak 'A try() may not be followed by multiple catch() blocks'
41			if $catch;
42			$catch = ${$code_ref};
43			} elsif ( ref($code_ref) eq 'Try::Tiny::Finally' ) {
44			push @finally, ${$code_ref};
45			} else {
46			croak(
47			'try() encountered an unexpected argument ('
48			. ( defined $code_ref ? $code_ref : 'undef' )
49			. ') - perhaps a missing semi-colon before or'
50			);
51			}
52			}
53
54			# FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's
55			# not perfect, but we could provide a list of additional errors for
56			# $catch->();
57
58			# name the blocks if we have Sub::Name installed
59			my $caller = caller;
60			subname("${caller}::try {...} " => $try);
61			subname("${caller}::catch {...} " => $catch) if $catch;
62			subname("${caller}::finally {...} " => $_) foreach @finally;
63
64			# save the value of $@ so we can set $@ back to it in the beginning of the eval
65			# and restore $@ after the eval finishes
66			my $prev_error = $@;
67
68			my ( @ret, $error );
69
70			# failed will be true if the eval dies, because 1 will not be returned
71			# from the eval body
72			my $failed = not eval {
73			$@ = $prev_error;
74
75			# evaluate the try block in the correct context
76	1	3.99ms	if ( $wantarray ) {
77			@ret = $try->();
78			} elsif ( defined $wantarray ) {
79			$ret[0] = $try->();
80			} else {
81	1	62µs	$try->();
82			};
83
84			return 1; # properly set $fail to false
85			};
86
87			# preserve the current error and reset the original value of $@
88			$error = $@;
89			$@ = $prev_error;
90
91			# set up a scope guard to invoke the finally block at the end
92			my @guards =
93			map { Try::Tiny::ScopeGuard->_new($_, $failed ? $error : ()) }
94			@finally;
95
96			# at this point $failed contains a true value if the eval died, even if some
97			# destructor overwrote $@ as the eval was unwinding.
98			if ( $failed ) {
99			# if we got an error, invoke the catch block.
100			if ( $catch ) {
101			# This works like given($error), but is backwards compatible and
102			# sets $_ in the dynamic scope for the body of C<$catch>
103			for ($error) {
104			return $catch->($error);
105			}
106
107			# in case when() was used without an explicit return, the C<for>
108			# loop will be aborted and there's no useful return value
109			}
110
111			return;
112			} else {
113			# no failure, $@ is back to what it was, everything is fine
114			return $wantarray ? @ret : $ret[0];
115			}
116			}
117
118			sub catch (&;@) {
119			my ( $block, @rest ) = @_;
120
121			croak 'Useless bare catch()' unless wantarray;
122
123			return (
124			bless(\$block, 'Try::Tiny::Catch'),
125			@rest,
126			);
127			}
128
129			sub finally (&;@) {
130			my ( $block, @rest ) = @_;
131
132			croak 'Useless bare finally()' unless wantarray;
133
134			return (
135			bless(\$block, 'Try::Tiny::Finally'),
136			@rest,
137			);
138			}
139
140			{
141			package # hide from PAUSE
142			Try::Tiny::ScopeGuard;
143
144			use constant UNSTABLE_DOLLARAT => ($] < '5.013002') ? 1 : 0;
145
146			sub _new {
147			shift;
148			bless [ @_ ];
149			}
150
151			sub DESTROY {
152			my ($code, @args) = @{ $_[0] };
153
154			local $@ if UNSTABLE_DOLLARAT;
155			eval {
156			$code->(@args);
157			1;
158			} or do {
159			warn
160			"Execution of finally() block $code resulted in an exception, which "
161			. 'CAN NOT BE PROPAGATED due to fundamental limitations of Perl. '
162			. 'Your program will continue as if this event never took place. '
163			. "Original exception text follows:\n\n"
164			. (defined $@ ? $@ : '$@ left undefined...')
165			. "\n"
166			;
167			}
168			}
169			}
170
171			__PACKAGE__
172
173			__END__
174
175			=pod
176
177			=encoding UTF-8
178
179			=head1 NAME
180
181			Try::Tiny - minimal try/catch with proper preservation of $@
182
183			=head1 VERSION
184
185			version 0.22
186
187			=head1 SYNOPSIS
188
189			You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
190			conditions, avoiding quirks in Perl and common mistakes:
191
192			# handle errors with a catch handler
193			try {
194			die "foo";
195			} catch {
196			warn "caught error: $_"; # not $@
197			};
198
199			You can also use it like a standalone C<eval> to catch and ignore any error
200			conditions. Obviously, this is an extreme measure not to be undertaken
201			lightly:
202
203			# just silence errors
204			try {
205			die "foo";
206			};
207
208			=head1 DESCRIPTION
209
210			This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
211			minimize common mistakes with eval blocks, and NOTHING else.
212
213			This is unlike L<TryCatch> which provides a nice syntax and avoids adding
214			another call stack layer, and supports calling C<return> from the C<try> block to
215			return from the parent subroutine. These extra features come at a cost of a few
216			dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
217			occasionally problematic, and the additional catch filtering uses L<Moose>
218			type constraints which may not be desirable either.
219
220			The main focus of this module is to provide simple and reliable error handling
221			for those having a hard time installing L<TryCatch>, but who still want to
222			write correct C<eval> blocks without 5 lines of boilerplate each time.
223
224			It's designed to work as correctly as possible in light of the various
225			pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
226			of error values (simple strings, references, objects, overloaded objects, etc).
227
228			If the C<try> block dies, it returns the value of the last statement executed in
229			the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
230			context or the empty list in list context. The following examples all
231			assign C<"bar"> to C<$x>:
232
233			my $x = try { die "foo" } catch { "bar" };
234			my $x = try { die "foo" } \|\| { "bar" };
235			my $x = (try { die "foo" }) // { "bar" };
236
237			my $x = eval { die "foo" } \|\| "bar";
238
239			You can add C<finally> blocks, yielding the following:
240
241			my $x;
242			try { die 'foo' } finally { $x = 'bar' };
243			try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
244
245			C<finally> blocks are always executed making them suitable for cleanup code
246			which cannot be handled using local. You can add as many C<finally> blocks to a
247			given C<try> block as you like.
248
249			Note that adding a C<finally> block without a preceding C<catch> block
250			suppresses any errors. This behaviour is consistent with using a standalone
251			C<eval>, but it is not consistent with C<try>/C<finally> patterns found in
252			other programming languages, such as Java, Python, Javascript or C#. If you
253			learnt the C<try>/C<finally> pattern from one of these languages, watch out for
254			this.
255
256			=head1 EXPORTS
257
258			All functions are exported by default using L<Exporter>.
259
260			If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
261			L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
262
263			=over 4
264
265			=item try (&;@)
266
267			Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
268			subroutine.
269
270			The mandatory subroutine is evaluated in the context of an C<eval> block.
271
272			If no error occurred the value from the first block is returned, preserving
273			list/scalar context.
274
275			If there was an error and the second subroutine was given it will be invoked
276			with the error in C<$_> (localized) and as that block's first and only
277			argument.
278
279			C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
280			value it had before the C<try> block was executed.
281
282			Note that the error may be false, but if that happens the C<catch> block will
283			still be invoked.
284
285			Once all execution is finished then the C<finally> block, if given, will execute.
286
287			=item catch (&;@)
288
289			Intended to be used in the second argument position of C<try>.
290
291			Returns a reference to the subroutine it was given but blessed as
292			C<Try::Tiny::Catch> which allows try to decode correctly what to do
293			with this code reference.
294
295			catch { ... }
296
297			Inside the C<catch> block the caught error is stored in C<$_>, while previous
298			value of C<$@> is still available for use. This value may or may not be
299			meaningful depending on what happened before the C<try>, but it might be a good
300			idea to preserve it in an error stack.
301
302			For code that captures C<$@> when throwing new errors (i.e.
303			L<Class::Throwable>), you'll need to do:
304
305			local $@ = $_;
306
307			=item finally (&;@)
308
309			try { ... }
310			catch { ... }
311			finally { ... };
312
313			Or
314
315			try { ... }
316			finally { ... };
317
318			Or even
319
320			try { ... }
321			finally { ... }
322			catch { ... };
323
324			Intended to be the second or third element of C<try>. C<finally> blocks are always
325			executed in the event of a successful C<try> or if C<catch> is run. This allows
326			you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
327			handle.
328
329			When invoked, the C<finally> block is passed the error that was caught. If no
330			error was caught, it is passed nothing. (Note that the C<finally> block does not
331			localize C<$_> with the error, since unlike in a C<catch> block, there is no way
332			to know if C<$_ == undef> implies that there were no errors.) In other words,
333			the following code does just what you would expect:
334
335			try {
336			die_sometimes();
337			} catch {
338			# ...code run in case of error
339			} finally {
340			if (@_) {
341			print "The try block died with: @_\n";
342			} else {
343			print "The try block ran without error.\n";
344			}
345			};
346
347			B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
348			not do anything about handling possible errors coming from code located in these
349			blocks.
350
351			Furthermore B<exceptions in C<finally> blocks are not trappable and are unable
352			to influence the execution of your program>. This is due to limitation of
353			C<DESTROY>-based scope guards, which C<finally> is implemented on top of. This
354			may change in a future version of Try::Tiny.
355
356			In the same way C<catch()> blesses the code reference this subroutine does the same
357			except it bless them as C<Try::Tiny::Finally>.
358
359			=back
360
361			=head1 BACKGROUND
362
363			There are a number of issues with C<eval>.
364
365			=head2 Clobbering $@
366
367			When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
368			clobbering an error that is currently being caught.
369
370			This causes action at a distance, clearing previous errors your caller may have
371			not yet handled.
372
373			C<$@> must be properly localized before invoking C<eval> in order to avoid this
374			issue.
375
376			More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
377			also makes it impossible to capture the previous error before you die (for
378			instance when making exception objects with error stacks).
379
380			For this reason C<try> will actually set C<$@> to its previous value (the one
381			available before entering the C<try> block) in the beginning of the C<eval>
382			block.
383
384			=head2 Localizing $@ silently masks errors
385
386			Inside an C<eval> block, C<die> behaves sort of like:
387
388			sub die {
389			$@ = $_[0];
390			return_undef_from_eval();
391			}
392
393			This means that if you were polite and localized C<$@> you can't die in that
394			scope, or your error will be discarded (printing "Something's wrong" instead).
395
396			The workaround is very ugly:
397
398			my $error = do {
399			local $@;
400			eval { ... };
401			$@;
402			};
403
404			...
405			die $error;
406
407			=head2 $@ might not be a true value
408
409			This code is wrong:
410
411			if ( $@ ) {
412			...
413			}
414
415			because due to the previous caveats it may have been unset.
416
417			C<$@> could also be an overloaded error object that evaluates to false, but
418			that's asking for trouble anyway.
419
420			The classic failure mode is:
421
422			sub Object::DESTROY {
423			eval { ... }
424			}
425
426			eval {
427			my $obj = Object->new;
428
429			die "foo";
430			};
431
432			if ( $@ ) {
433
434			}
435
436			In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
437			C<eval>, it will set C<$@> to C<"">.
438
439			The destructor is called when the stack is unwound, after C<die> sets C<$@> to
440			C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
441			been cleared by C<eval> in the destructor.
442
443			The workaround for this is even uglier than the previous ones. Even though we
444			can't save the value of C<$@> from code that doesn't localize, we can at least
445			be sure the C<eval> was aborted due to an error:
446
447			my $failed = not eval {
448			...
449
450			return 1;
451			};
452
453			This is because an C<eval> that caught a C<die> will always return a false
454			value.
455
456			=head1 SHINY SYNTAX
457
458			Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
459
460			The C<catch> block is invoked in a topicalizer context (like a C<given> block),
461			but note that you can't return a useful value from C<catch> using the C<when>
462			blocks without an explicit C<return>.
463
464			This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
465			concisely match errors:
466
467			try {
468			require Foo;
469			} catch {
470			when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
471			default { die $_ }
472			};
473
474			=head1 CAVEATS
475
476			=over 4
477
478			=item *
479
480			C<@_> is not available within the C<try> block, so you need to copy your
481			arglist. In case you want to work with argument values directly via C<@_>
482			aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
483
484			sub foo {
485			my ( $self, @args ) = @_;
486			try { $self->bar(@args) }
487			}
488
489			or
490
491			sub bar_in_place {
492			my $self = shift;
493			my $args = \@_;
494			try { $_ = $self->bar($_) for @$args }
495			}
496
497			=item *
498
499			C<return> returns from the C<try> block, not from the parent sub (note that
500			this is also how C<eval> works, but not how L<TryCatch> works):
501
502			sub parent_sub {
503			try {
504			die;
505			}
506			catch {
507			return;
508			};
509
510			say "this text WILL be displayed, even though an exception is thrown";
511			}
512
513			Instead, you should capture the return value:
514
515			sub parent_sub {
516			my $success = try {
517			die;
518			1;
519			};
520			return unless $success;
521
522			say "This text WILL NEVER appear!";
523			}
524			# OR
525			sub parent_sub_with_catch {
526			my $success = try {
527			die;
528			1;
529			}
530			catch {
531			# do something with $_
532			return undef; #see note
533			};
534			return unless $success;
535
536			say "This text WILL NEVER appear!";
537			}
538
539			Note that if you have a C<catch> block, it must return C<undef> for this to work,
540			since if a C<catch> block exists, its return value is returned in place of C<undef>
541			when an exception is thrown.
542
543			=item *
544
545			C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
546			will not report this when using full stack traces, though, because
547			C<%Carp::Internal> is used. This lack of magic is considered a feature.
548
549			=item *
550
551			The value of C<$_> in the C<catch> block is not guaranteed to be the value of
552			the exception thrown (C<$@>) in the C<try> block. There is no safe way to
553			ensure this, since C<eval> may be used unhygenically in destructors. The only
554			guarantee is that the C<catch> will be called if an exception is thrown.
555
556			=item *
557
558			The return value of the C<catch> block is not ignored, so if testing the result
559			of the expression for truth on success, be sure to return a false value from
560			the C<catch> block:
561
562			my $obj = try {
563			MightFail->new;
564			} catch {
565			...
566
567			return; # avoid returning a true value;
568			};
569
570			return unless $obj;
571
572			=item *
573
574			C<$SIG{__DIE__}> is still in effect.
575
576			Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
577			C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
578			the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
579			the scope of the error throwing code.
580
581			=item *
582
583			Lexical C<$_> may override the one set by C<catch>.
584
585			For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
586			confusing behavior:
587
588			given ($foo) {
589			when (...) {
590			try {
591			...
592			} catch {
593			warn $_; # will print $foo, not the error
594			warn $_[0]; # instead, get the error like this
595			}
596			}
597			}
598
599			Note that this behavior was changed once again in L<Perl5 version 18
600			\|https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
601			However, since the entirety of lexical C<$_> is now L<considired experimental
602			\|https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
603			is unclear whether the new version 18 behavior is final.
604
605			=back
606
607			=head1 SEE ALSO
608
609			=over 4
610
611			=item L<TryCatch>
612
613			Much more feature complete, more convenient semantics, but at the cost of
614			implementation complexity.
615
616			=item L<autodie>
617
618			Automatic error throwing for builtin functions and more. Also designed to
619			work well with C<given>/C<when>.
620
621			=item L<Throwable>
622
623			A lightweight role for rolling your own exception classes.
624
625			=item L<Error>
626
627			Exception object implementation with a C<try> statement. Does not localize
628			C<$@>.
629
630			=item L<Exception::Class::TryCatch>
631
632			Provides a C<catch> statement, but properly calling C<eval> is your
633			responsibility.
634
635			The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
636			issues with C<$@>, but you still need to localize to prevent clobbering.
637
638			=back
639
640			=head1 LIGHTNING TALK
641
642			I gave a lightning talk about this module, you can see the slides (Firefox
643			only):
644
645			L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
646
647			Or read the source:
648
649			L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
650
651			=head1 VERSION CONTROL
652
653			L<http://github.com/doy/try-tiny/>
654
655			=head1 AUTHORS
656
657			=over 4
658
659			=item *
660
661			Yuval Kogman <nothingmuch@woobling.org>
662
663			=item *
664
665			Jesse Luehrs <doy@tozt.net>
666
667			=back
668
669			=head1 COPYRIGHT AND LICENSE
670
671			This software is Copyright (c) 2014 by Yuval Kogman.
672
673			This is free software, licensed under:
674
675			The MIT (X11) License
676
677			=cut