/ 
Try-Tiny-0.08
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/ Try::Tiny

NAME

Try::Tiny - minimal try/catch with proper localization of $@

SYNOPSIS

# handle errors with a catch handler
try {
	die "foo";
} catch {
	warn "caught error: $_"; # not $@
};

# 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. 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 bar {
	try { return "foo" };
	return "baz";
}

say bar(); # "baz"

  • 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/

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.