Try::Tiny - Minimal try/catch with proper preservation of $@
- VERSION
- SYNOPSIS
- DESCRIPTION
- EXPORTS
- BACKGROUND
- ALTERNATE SYNTAX
- CAVEATS
- SEE ALSO
- LIGHTNING TALK
- SUPPORT
- AUTHORS
- CONTRIBUTORS
- COPYRIGHT AND LICENCE
VERSION
version 0.31
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 learned 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 optionalcatch
subroutine andfinally
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 thecatch
block it has the same value it had before thetry
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 thetry
, 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 $@ = $_;
- finally (&;@)
-
try { ... } catch { ... } finally { ... };
Or
try { ... } finally { ... };
Or even
try { ... } finally { ... } catch { ... };
Intended to be the second or third element of
try
.finally
blocks are always executed in the event of a successfultry
or ifcatch
is run. This allows you to locate cleanup code which cannot be done vialocal()
e.g. closing a file handle.When invoked, the
finally
block is passed the error that was caught. If no error was caught, it is passed nothing. (Note that thefinally
block does not localize$_
with the error, since unlike in acatch
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 ofDESTROY
-based scope guards, whichfinally
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 asTry::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, before Perl version 5.14.0 $@
was clobbered at the beginning of the eval
, which also made 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 (fixed in Perl 5.14.0) 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.
ALTERNATE SYNTAX
Using Perl 5.10 you can use "Switch statements" in perlsyn (but please don't, because that syntax has since been deprecated because there was too much unexpected magical behaviour).
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 $_ }
};
CAVEATS
@_
is not available within thetry
block, so you need to copy your argument list. In case you want to work with argument values directly via@_
aliasing (i.e. allow$_[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 thetry
block, not from the parent sub (note that this is also howeval
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 returnundef
for this to work, since if acatch
block exists, its return value is returned in place ofundef
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
$_
in thecatch
block is not guaranteed to be the value of the exception thrown ($@
) in thetry
block. There is no safe way to ensure this, sinceeval
may be used unhygienically in destructors. The only guarantee is that thecatch
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 thecatch
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
$SIG{__DIE__}
should be disabled inside ofeval
blocks, since it isn't people have grown to rely on it. Therefore in the interests of compatibility,try
does not disable$SIG{__DIE__}
for the scope of the error throwing code.Lexical
$_
may override the one set bycatch
.For example Perl 5.10's
given
form uses a lexical$_
, 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. However, since the entirety of lexical
$_
is now considered experimental , it is unclear whether the new version 18 behavior is final.
SEE ALSO
- Syntax::Keyword::Try
-
Only available on perls >= 5.14, with a slightly different syntax (e.g. no trailing
;
because it's actually a keyword, not a sub, but this means you canreturn
andnext
within it). Use Feature::Compat::Try to automatically switch to the nativetry
syntax in newer perls (when available). See also Try Catch Exception Handling. - TryCatch
-
Much more feature complete, more convenient semantics, but at the cost of implementation complexity.
- autodie
-
Automatic error throwing for builtin functions and more. Also designed to work well with
given
/when
. - Throwable
-
A lightweight role for rolling your own exception classes.
- Error
-
Exception object implementation with a
try
statement. Does not localize$@
. - Exception::Class::TryCatch
-
Provides a
catch
statement, but properly callingeval
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:
SUPPORT
Bugs may be submitted through the RT bug tracker (or [email protected]).
AUTHORS
יובל קוג'מן (Yuval Kogman) <[email protected]>
Jesse Luehrs <[email protected]>
CONTRIBUTORS
Karen Etheridge <[email protected]>
Peter Rabbitson <[email protected]>
Ricardo Signes <[email protected]>
Mark Fowler <[email protected]>
Graham Knop <[email protected]>
Aristotle Pagaltzis <[email protected]>
Dagfinn Ilmari Mannsåker <[email protected]>
Lukas Mai <[email protected]>
Alex <alex@koban.(none)>
anaxagoras <[email protected]>
Andrew Yates <[email protected]>
awalker <[email protected]>
chromatic <[email protected]>
cm-perl <[email protected]>
David Lowe <[email protected]>
Glenn Fowler <[email protected]>
Hans Dieter Pearcey <[email protected]>
Jens Berthold <[email protected]>
Jonathan Yu <[email protected]>
Marc Mims <[email protected]>
Mark Stosberg <[email protected]>
Pali <[email protected]>
Paul Howarth <[email protected]>
Rudolf Leermakers <[email protected]>
COPYRIGHT AND LICENCE
This software is Copyright (c) 2009 by יובל קוג'מן (Yuval Kogman).
This is free software, licensed under:
The MIT (X11) License