NAME

Syntax::Feature::Try - try/catch/finally statement for exception handling

SYNOPSIS

    use syntax 'try';

    try {
        # run this code and handle errors
    }
    catch (My::Class::Err $e) {
        # handle exception based on class "My::Class::Err"
    }
    catch ($e) {
        # handle other exceptions
    }
    finally {
        # cleanup block
    }

DESCRIPTION

This module implements syntax for try/catch/finally statement with behaviour similar to other programming languages (like Java, Python, etc.).

It handles correctly return/wantarray inside try/catch/finally blocks.

It uses perl keyword/parser API. So it requires perl >= 5.14.

SYNTAX

initiliazation

To initialize this syntax feature call:

    use syntax 'try';

try

The try block is executed. If it throws an error, then first catch block (in order) that can handle thrown error will be executed. Other catch blocks will be skipped.

If none of catch blocks can handle the error, it is thrown out of whole statement. If try block does not throw an error, all catch blocks are skipped.

catch error class

    catch (My::Error $err) { ... }

This catch block can handle error that is instance of class My::Error or any of it's subclasses.

Caught error is accessible inside catch block via declared local variable $err.

catch all errors

To catch all errors use syntax:

    catch ($e) { ... }

Caught error is accessible inside catch block via declared local variable $e.

catch without variable

Variable name in catch block is not mandatory:

    try {
        ...
    }
    catch (MyError::FileNotFound) {
        print "file not found";
    }
    catch {
        print "operation failed";
    }

rethrow error

To rethrow caught error call "die $err".

For example (log any Connection::Error):

    try { ... }
    catch (Connection::Error $err) {
        log_error($err);
        die $err;
    }

finally

The finally block is executed at the end of statement. It is always executed (even if try or catch block throw an error).

    my $fh;
    try {
        $fh = IO::File->new("/etc/hosts");
        ...
    }
    finally {
        $fh->close if $fh;
    }

WARNING: If finally block throws an exception, originaly thrown exception (from try/catch block) is discarded. You can convert errors inside finally block to warnings:

    try {
        # try block
    }
    finally {
        try {
            # cleanup code
        }
        catch ($e) { warn $e }
    }

Exception::Class

This module is compatible with Exception::Class

    use Exception::Class (
        'My::Test::Error'
    );
    use syntax 'try';

    try {
        ...
        My::Test::Error->throw('invalid password');
    }
    catch (My::Test::Error $err) {
        # handle error here
    }

return from subroutine

This module supports calling "return" inside try/catch/finally blocks to return values from subroutine.

    sub read_config {
        my $file;
        try {
            $fh = IO::File->new(...);
            return $fh->getline; # it returns value from subroutine "read_config"
        }
        catch ($e) {
            # log error
        }
        finally {
            $fh->close() if $fh;
        }
    }

CAVEATS

@_

@_ is not accessible inside try/catch/finally blocks, because these blocks are internally called in different context.

next, last, redo

next, last and redo is not working inside try/catch/finally blocks, because these blocks are internally called in different context.

BUGS

None bugs known.

SEE ALSO

syntax - Active syntax extensions

Exception::Class - A module that allows you to declare real exception classes in Perl

Other similar packages

TryCatch

• It reports wrong line numbers from warn/die calls inside try/catch blocks.

• It does not support "finally" block.

• It works on perl < 5.14

Try

• It does not support catching errors by their ISA (i.e. it has only one catch block that takes all errors and you must write additinal if/else code to rethrow other exceptions).

Try::Tiny

• It does not support catching errors by their ISA (i.e. it has only one catch block that takes all errors and you must write additinal if/else code to rethrow other exceptions).

• It generates expression (instead of statement), i.e. it requires semicolon after last block. Missing semicolon before or after try/catch expression may be hard to debug (it is not always reported as syntax error).

• It works on perl < 5.14 (It is written in pure perl).

GIT REPOSITORY

http://github.com/tomas-zemres/syntax-feature-try

AUTHOR

Tomas Pokorny <tnt at cpan dot org>

COPYRIGHT AND LICENCE

Copyright 2013 - Tomas Pokorny.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

cpanm Syntax::Feature::Try

perl -MCPAN -e shell
install Syntax::Feature::Try