Try::Tiny - minimal try/catch with proper localization of $@
# handle errors with a catch handler
try {
die "foo";
} catch {
warn "caught error: $_";
};
# just silence errors
try {
die "foo";
};
This module provides bare bones try/catch statements that are designed to minimize common mistakes done with eval blocks (for instance assuming that $@ is set to a true value on error, or clobbering previous values of $@), 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 using Moose type constraints may not be desirable either.
The main focus of this module is to provide reliable but simple 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).
All are exported by default using Exporter.
In the future Sub::ExporteR may be used to allow the keywords to be renamed, but this technically does not satisfy Adam Kennedy's definition of "Tiny".
Takes one mandatory and one optional catch subroutine.
The mandatory subroutine is evaluated in the context of an eval block.
If no error occured the value from the first block is returned.
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.
Note that the error may be false
Just retuns the subroutine it was given.
catch { ... }
is the same as
sub { ... }
Intended to be used in the second argument position of try.
There are a number of issues with eval.
When you run an eval block and it succeeds, $@ will be cleared, potentially cloberring an error that is currently being caught.
$@ must be properly localized before invoking eval in order to avoid this issue.
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 while propagating your error.
The workaround is very ugly:
my $error = do {
local $@;
eval { ... };
$@;
};
...
die $error;
This code is wrong:
if ( $@ ) {
...
}
because due to the previous caveats it may have been unset. $@ could also 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 using eval it will set $@ to "".
The destructor is only fired after die sets $@ to "foo at Foo.pm line 42\n", so by the time if ( $@ ) is evaluated it has become false.
The workaround for this is even uglier. Even though we can't save the value of $@ from code that doesn't localize it but uses eval in destructors, we can at least be sure there was an error:
my $failed = not eval {
...
return 1;
};
This is because an eval that caught a die will always behave like return with no arguments.
Using Perl 5.10 you can enable the given/when construct. The catch block is invoked in a topicalizer context (like a given block).
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 (qr/^Can't locate .*?\.pm in \@INC/) { } # ignore
default { die $_ }
}
$_ in the catch block is not guaranteed to be preserved, there is no safe way to ensure this if eval is used unhygenically in destructors. It is guaranteed that catch will be called, though.Much more feature complete, more convenient semantics, but at the cost of implementation complexity.
Exception object implementation with a try statement. Does not localize $@.
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.
http://github.com/nothingmuch/try-tiny/
Yuval Kogman <nothingmuch@woobling.org>
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.
