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Try::Tiny - phpMan
Try::Tiny(3pm) User Contributed Perl Documentation Try::Tiny(3pm)
NAME
Try::Tiny - minimal try/catch with proper preservation of $@
VERSION
version 0.22
SYNOPSIS
You can use Try::Tiny's "try" and "catch" to expect and handle exceptional conditions,
avoiding quirks in Perl and common mistakes:
# handle errors with a catch handler
try {
die "foo";
} catch {
warn "caught error: $_"; # not $@
};
You can also use it like a standalone "eval" to catch and ignore any error conditions.
Obviously, this is an extreme measure not to be undertaken lightly:
# just silence errors
try {
die "foo";
};
DESCRIPTION
This module provides bare bones "try"/"catch"/"finally" statements that are designed to
minimize common mistakes with eval blocks, and NOTHING else.
This is unlike TryCatch which provides a nice syntax and avoids adding another call stack
layer, and supports calling "return" from the "try" block to return from the parent
subroutine. These extra features come at a cost of a few dependencies, namely
Devel::Declare and Scope::Upper which are occasionally problematic, and the additional
catch filtering uses Moose type constraints which may not be desirable either.
The main focus of this module is to provide simple and reliable error handling for those
having a hard time installing TryCatch, but who still want to write correct "eval" blocks
without 5 lines of boilerplate each time.
It's designed to work as correctly as possible in light of the various pathological edge
cases (see "BACKGROUND") and to be compatible with any style of error values (simple
strings, references, objects, overloaded objects, etc).
If the "try" block dies, it returns the value of the last statement executed in the
"catch" block, if there is one. Otherwise, it returns "undef" in scalar context or the
empty list in list context. The following examples all assign "bar" to $x:
my $x = try { die "foo" } catch { "bar" };
my $x = try { die "foo" } || { "bar" };
my $x = (try { die "foo" }) // { "bar" };
my $x = eval { die "foo" } || "bar";
You can add "finally" blocks, yielding the following:
my $x;
try { die 'foo' } finally { $x = 'bar' };
try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
"finally" blocks are always executed making them suitable for cleanup code which cannot be
handled using local. You can add as many "finally" blocks to a given "try" block as you
like.
Note that adding a "finally" block without a preceding "catch" block suppresses any
errors. This behaviour is consistent with using a standalone "eval", but it is not
consistent with "try"/"finally" patterns found in other programming languages, such as
Java, Python, Javascript or C#. If you learnt the "try"/"finally" pattern from one of
these languages, watch out for this.
EXPORTS
All functions are exported by default using Exporter.
If you need to rename the "try", "catch" or "finally" keyword consider using Sub::Import
to get Sub::Exporter's flexibility.
try (&;@)
Takes one mandatory "try" subroutine, an optional "catch" subroutine and "finally"
subroutine.
The mandatory subroutine is evaluated in the context of an "eval" block.
If no error occurred the value from the first block is returned, preserving
list/scalar context.
If there was an error and the second subroutine was given it will be invoked with the
error in $_ (localized) and as that block's first and only argument.
$@ does not contain the error. Inside the "catch" block it has the same value it had
before the "try" block was executed.
Note that the error may be false, but if that happens the "catch" block will still be
invoked.
Once all execution is finished then the "finally" block, if given, will execute.
catch (&;@)
Intended to be used in the second argument position of "try".
Returns a reference to the subroutine it was given but blessed as "Try::Tiny::Catch"
which allows try to decode correctly what to do with this code reference.
catch { ... }
Inside the "catch" block the caught error is stored in $_, while previous value of $@
is still available for use. This value may or may not be meaningful depending on what
happened before the "try", but it might be a good idea to preserve it in an error
stack.
For code that captures $@ when throwing new errors (i.e. Class::Throwable), you'll
need to do:
local $@ = $_;
finally (&;@)
try { ... }
catch { ... }
finally { ... };
Or
try { ... }
finally { ... };
Or even
try { ... }
finally { ... }
catch { ... };
Intended to be the second or third element of "try". "finally" blocks are always
executed in the event of a successful "try" or if "catch" is run. This allows you to
locate cleanup code which cannot be done via "local()" e.g. closing a file handle.
When invoked, the "finally" block is passed the error that was caught. If no error
was caught, it is passed nothing. (Note that the "finally" block does not localize $_
with the error, since unlike in a "catch" block, there is no way to know if "$_ ==
undef" implies that there were no errors.) In other words, the following code does
just what you would expect:
try {
die_sometimes();
} catch {
# ...code run in case of error
} finally {
if (@_) {
print "The try block died with: @_\n";
} else {
print "The try block ran without error.\n";
}
};
You must always do your own error handling in the "finally" block. "Try::Tiny" will
not do anything about handling possible errors coming from code located in these
blocks.
Furthermore exceptions in "finally" blocks are not trappable and are unable to
influence the execution of your program. This is due to limitation of "DESTROY"-based
scope guards, which "finally" is implemented on top of. This may change in a future
version of Try::Tiny.
In the same way "catch()" blesses the code reference this subroutine does the same
except it bless them as "Try::Tiny::Finally".
BACKGROUND
There are a number of issues with "eval".
Clobbering $@
When you run an "eval" block and it succeeds, $@ will be cleared, potentially clobbering
an error that is currently being caught.
This causes action at a distance, clearing previous errors your caller may have not yet
handled.
$@ must be properly localized before invoking "eval" in order to avoid this issue.
More specifically, $@ is clobbered at the beginning of the "eval", which also makes it
impossible to capture the previous error before you die (for instance when making
exception objects with error stacks).
For this reason "try" will actually set $@ to its previous value (the one available before
entering the "try" block) in the beginning of the "eval" block.
Localizing $@ silently masks errors
Inside an "eval" block, "die" behaves sort of like:
sub die {
$@ = $_[0];
return_undef_from_eval();
}
This means that if you were polite and localized $@ you can't die in that scope, or your
error will be discarded (printing "Something's wrong" instead).
The workaround is very ugly:
my $error = do {
local $@;
eval { ... };
$@;
};
...
die $error;
$@ might not be a true value
This code is wrong:
if ( $@ ) {
...
}
because due to the previous caveats it may have been unset.
$@ could also be an overloaded error object that evaluates to false, but that's asking for
trouble anyway.
The classic failure mode is:
sub Object::DESTROY {
eval { ... }
}
eval {
my $obj = Object->new;
die "foo";
};
if ( $@ ) {
}
In this case since "Object::DESTROY" is not localizing $@ but still uses "eval", it will
set $@ to "".
The destructor is called when the stack is unwound, after "die" sets $@ to "foo at Foo.pm
line 42\n", so by the time "if ( $@ )" is evaluated it has been cleared by "eval" in the
destructor.
The workaround for this is even uglier than the previous ones. Even though we can't save
the value of $@ from code that doesn't localize, we can at least be sure the "eval" was
aborted due to an error:
my $failed = not eval {
...
return 1;
};
This is because an "eval" that caught a "die" will always return a false value.
SHINY SYNTAX
Using Perl 5.10 you can use "Switch statements" in perlsyn.
The "catch" block is invoked in a topicalizer context (like a "given" block), but note
that you can't return a useful value from "catch" using the "when" blocks without an
explicit "return".
This is somewhat similar to Perl 6's "CATCH" blocks. You can use it to concisely match
errors:
try {
require Foo;
} catch {
when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
default { die $_ }
};
CAVEATS
· @_ is not available within the "try" block, so you need to copy your arglist. In case
you want to work with argument values directly via @_ aliasing (i.e. allow "$_[1] =
"foo""), you need to pass @_ by reference:
sub foo {
my ( $self, @args ) = @_;
try { $self->bar(@args) }
}
or
sub bar_in_place {
my $self = shift;
my $args = \@_;
try { $_ = $self->bar($_) for @$args }
}
· "return" returns from the "try" block, not from the parent sub (note that this is also
how "eval" works, but not how TryCatch works):
sub parent_sub {
try {
die;
}
catch {
return;
};
say "this text WILL be displayed, even though an exception is thrown";
}
Instead, you should capture the return value:
sub parent_sub {
my $success = try {
die;
1;
};
return unless $success;
say "This text WILL NEVER appear!";
}
# OR
sub parent_sub_with_catch {
my $success = try {
die;
1;
}
catch {
# do something with $_
return undef; #see note
};
return unless $success;
say "This text WILL NEVER appear!";
}
Note that if you have a "catch" block, it must return "undef" for this to work, since
if a "catch" block exists, its return value is returned in place of "undef" when an
exception is thrown.
· "try" introduces another caller stack frame. Sub::Uplevel is not used. Carp will not
report this when using full stack traces, though, because %Carp::Internal is used.
This lack of magic is considered a feature.
· The value of $_ in the "catch" block is not guaranteed to be the value of the
exception thrown ($@) in the "try" block. There is no safe way to ensure this, since
"eval" may be used unhygenically in destructors. The only guarantee is that the
"catch" will be called if an exception is thrown.
· The return value of the "catch" block is not ignored, so if testing the result of the
expression for truth on success, be sure to return a false value from the "catch"
block:
my $obj = try {
MightFail->new;
} catch {
...
return; # avoid returning a true value;
};
return unless $obj;
· $SIG{__DIE__} is still in effect.
Though it can be argued that $SIG{__DIE__} should be disabled inside of "eval" blocks,
since it isn't people have grown to rely on it. Therefore in the interests of
compatibility, "try" does not disable $SIG{__DIE__} for the scope of the error
throwing code.
· Lexical $_ may override the one set by "catch".
For example Perl 5.10's "given" form uses a lexical $_, creating some confusing
behavior:
given ($foo) {
when (...) {
try {
...
} catch {
warn $_; # will print $foo, not the error
warn $_[0]; # instead, get the error like this
}
}
}
Note that this behavior was changed once again in Perl5 version 18
<https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>. However,
since the entirety of lexical $_ is now considired experimental
<https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it is unclear
whether the new version 18 behavior is final.
SEE ALSO
TryCatch
Much more feature complete, more convenient semantics, but at the cost of
implementation complexity.
autodie
Automatic error throwing for builtin functions and more. Also designed to work well
with "given"/"when".
Throwable
A lightweight role for rolling your own exception classes.
Error
Exception object implementation with a "try" statement. Does not localize $@.
Exception::Class::TryCatch
Provides a "catch" statement, but properly calling "eval" is your responsibility.
The "try" keyword pushes $@ onto an error stack, avoiding some of the issues with $@,
but you still need to localize to prevent clobbering.
LIGHTNING TALK
I gave a lightning talk about this module, you can see the slides (Firefox only):
<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
Or read the source:
<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
VERSION CONTROL
<http://github.com/doy/try-tiny/>
AUTHORS
· Yuval Kogman <nothingmuch AT woobling.org>
· Jesse Luehrs <doy AT tozt.net>
COPYRIGHT AND LICENSE
This software is Copyright (c) 2014 by Yuval Kogman.
This is free software, licensed under:
The MIT (X11) License
perl v5.18.2 2014-04-29 Try::Tiny(3pm)
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