Try::Tiny(3) User Contributed Perl Documentation Try::Tiny(3)NAME
Try::Tiny - minimal try/catch with proper localization of $@
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 stanalone "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 two examples both assign "bar" to $x.
my $x = try { die "foo" } catch { "bar" };
my $x = eval { die "foo" } || "bar";
You can add finally blocks making the following true.
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.
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 &
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.
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
(before the localization) 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!";
}
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
}
}
}
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://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_asia_2009/try_tiny.txt>
Or read the source:
<http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
VERSION CONTROL
http://github.com/nothingmuch/try-tiny/
<http://github.com/nothingmuch/try-tiny/>
AUTHOR
Yuval Kogman <nothingmuch@woobling.org>
COPYRIGHT
Copyright (c) 2009 Yuval Kogman. All rights reserved.
This program is free software; you can redistribute
it and/or modify it under the terms of the MIT license.
perl v5.16.2 2011-08-30 Try::Tiny(3)
