Try::Tiny - Minimal try/catch with proper preservation of $@

1. VERSION
2. SYNOPSIS
3. DESCRIPTION
4. EXPORTS
5. BACKGROUND
   1. Clobbering $@
   2. Localizing $@ silently masks errors
   3. $@ might not be a true value
6. ALTERNATE SYNTAX
7. CAVEATS
8. SEE ALSO
9. LIGHTNING TALK
10. SUPPORT
11. AUTHORS
12. CONTRIBUTORS
13. 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 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, 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 the try 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 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 unhygienically 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. 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 can return and next within it). Use Feature::Compat::Try to automatically switch to the native try 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 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

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