/extlib/man/man3/Error.3pm

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.\" ========================================================================
.\"
.IX Title "Error 3pm"
.TH Error 3pm "2010-05-08" "perl v5.10.0" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
Error \- Error/exception handling in an OO\-ish way
.SH "WARNING"
.IX Header "WARNING"
Using the \*(L"Error\*(R" module is \fBno longer recommended\fR due to the black-magical
nature of its syntactic sugar, which often tends to break. Its maintainers 
have stopped actively writing code that uses it, and discourage people
from doing so. See the \*(L"\s-1SEE\s0 \s-1ALSO\s0\*(R" section below for better recommendations.
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use Error qw(:try);
\&
\&    throw Error::Simple( "A simple error");
\&
\&    sub xyz {
\&        ...
\&        record Error::Simple("A simple error")
\&            and return;
\&    }
\& 
\&    unlink($file) or throw Error::Simple("$file: $!",$!);
\&
\&    try {
\&        do_some_stuff();
\&        die "error!" if $condition;
\&        throw Error::Simple "Oops!" if $other_condition;
\&    }
\&    catch Error::IO with {
\&        my $E = shift;
\&        print STDERR "File ", $E\->{\*(Aq\-file\*(Aq}, " had a problem\en";
\&    }
\&    except {
\&        my $E = shift;
\&        my $general_handler=sub {send_message $E\->{\-description}};
\&        return {
\&            UserException1 => $general_handler,
\&            UserException2 => $general_handler
\&        };
\&    }
\&    otherwise {
\&        print STDERR "Well I don\*(Aqt know what to say\en";
\&    }
\&    finally {
\&        close_the_garage_door_already(); # Should be reliable
\&    }; # Don\*(Aqt forget the trailing ; or you might be surprised
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \f(CW\*(C`Error\*(C'\fR package provides two interfaces. Firstly \f(CW\*(C`Error\*(C'\fR provides
a procedural interface to exception handling. Secondly \f(CW\*(C`Error\*(C'\fR is a
base class for errors/exceptions that can either be thrown, for
subsequent catch, or can simply be recorded.
.PP
Errors in the class \f(CW\*(C`Error\*(C'\fR should not be thrown directly, but the
user should throw errors from a sub-class of \f(CW\*(C`Error\*(C'\fR.
.SH "PROCEDURAL INTERFACE"
.IX Header "PROCEDURAL INTERFACE"
\&\f(CW\*(C`Error\*(C'\fR exports subroutines to perform exception handling. These will
be exported if the \f(CW\*(C`:try\*(C'\fR tag is used in the \f(CW\*(C`use\*(C'\fR line.
.IP "try \s-1BLOCK\s0 \s-1CLAUSES\s0" 4
.IX Item "try BLOCK CLAUSES"
\&\f(CW\*(C`try\*(C'\fR is the main subroutine called by the user. All other subroutines
exported are clauses to the try subroutine.
.Sp
The \s-1BLOCK\s0 will be evaluated and, if no error is throw, try will return
the result of the block.
.Sp
\&\f(CW\*(C`CLAUSES\*(C'\fR are the subroutines below, which describe what to do in the
event of an error being thrown within \s-1BLOCK\s0.
.IP "catch \s-1CLASS\s0 with \s-1BLOCK\s0" 4
.IX Item "catch CLASS with BLOCK"
This clauses will cause all errors that satisfy \f(CW\*(C`$err\->isa(CLASS)\*(C'\fR
to be caught and handled by evaluating \f(CW\*(C`BLOCK\*(C'\fR.
.Sp
\&\f(CW\*(C`BLOCK\*(C'\fR will be passed two arguments. The first will be the error
being thrown. The second is a reference to a scalar variable. If this
variable is set by the catch block then, on return from the catch
block, try will continue processing as if the catch block was never
found. The error will also be available in \f(CW$@\fR.
.Sp
To propagate the error the catch block may call \f(CW\*(C`$err\->throw\*(C'\fR
.Sp
If the scalar reference by the second argument is not set, and the
error is not thrown. Then the current try block will return with the
result from the catch block.
.IP "except \s-1BLOCK\s0" 4
.IX Item "except BLOCK"
When \f(CW\*(C`try\*(C'\fR is looking for a handler, if an except clause is found
\&\f(CW\*(C`BLOCK\*(C'\fR is evaluated. The return value from this block should be a
\&\s-1HASHREF\s0 or a list of key-value pairs, where the keys are class names
and the values are \s-1CODE\s0 references for the handler of errors of that
type.
.IP "otherwise \s-1BLOCK\s0" 4
.IX Item "otherwise BLOCK"
Catch any error by executing the code in \f(CW\*(C`BLOCK\*(C'\fR
.Sp
When evaluated \f(CW\*(C`BLOCK\*(C'\fR will be passed one argument, which will be the
error being processed. The error will also be available in \f(CW$@\fR.
.Sp
Only one otherwise block may be specified per try block
.IP "finally \s-1BLOCK\s0" 4
.IX Item "finally BLOCK"
Execute the code in \f(CW\*(C`BLOCK\*(C'\fR either after the code in the try block has
successfully completed, or if the try block throws an error then
\&\f(CW\*(C`BLOCK\*(C'\fR will be executed after the handler has completed.
.Sp
If the handler throws an error then the error will be caught, the
finally block will be executed and the error will be re-thrown.
.Sp
Only one finally block may be specified per try block
.SH "COMPATIBILITY"
.IX Header "COMPATIBILITY"
Moose exports a keyword called \f(CW\*(C`with\*(C'\fR which clashes with Error's. This
example returns a prototype mismatch error:
.PP
.Vb 1
\&    package MyTest;
\&
\&    use warnings;
\&    use Moose;
\&    use Error qw(:try);
.Ve
.PP
(Thanks to \f(CW\*(C`maik.hentsche@amd.com\*(C'\fR for the report.).
.SH "CLASS INTERFACE"
.IX Header "CLASS INTERFACE"
.SS "\s-1CONSTRUCTORS\s0"
.IX Subsection "CONSTRUCTORS"
The \f(CW\*(C`Error\*(C'\fR object is implemented as a \s-1HASH\s0. This \s-1HASH\s0 is initialized
with the arguments that are passed to it's constructor. The elements
that are used by, or are retrievable by the \f(CW\*(C`Error\*(C'\fR class are listed
below, other classes may add to these.
.PP
.Vb 5
\&        \-file
\&        \-line
\&        \-text
\&        \-value
\&        \-object
.Ve
.PP
If \f(CW\*(C`\-file\*(C'\fR or \f(CW\*(C`\-line\*(C'\fR are not specified in the constructor arguments
then these will be initialized with the file name and line number where
the constructor was called from.
.PP
If the error is associated with an object then the object should be
passed as the \f(CW\*(C`\-object\*(C'\fR argument. This will allow the \f(CW\*(C`Error\*(C'\fR package
to associate the error with the object.
.PP
The \f(CW\*(C`Error\*(C'\fR package remembers the last error created, and also the
last error associated with a package. This could either be the last
error created by a sub in that package, or the last error which passed
an object blessed into that package as the \f(CW\*(C`\-object\*(C'\fR argument.
.IP "Error\->\fInew()\fR" 4
.IX Item "Error->new()"
See the Error::Simple documentation.
.IP "throw ( [ \s-1ARGS\s0 ] )" 4
.IX Item "throw ( [ ARGS ] )"
Create a new \f(CW\*(C`Error\*(C'\fR object and throw an error, which will be caught
by a surrounding \f(CW\*(C`try\*(C'\fR block, if there is one. Otherwise it will cause
the program to exit.
.Sp
\&\f(CW\*(C`throw\*(C'\fR may also be called on an existing error to re-throw it.
.IP "with ( [ \s-1ARGS\s0 ] )" 4
.IX Item "with ( [ ARGS ] )"
Create a new \f(CW\*(C`Error\*(C'\fR object and returns it. This is defined for
syntactic sugar, eg
.Sp
.Vb 1
\&    die with Some::Error ( ... );
.Ve
.IP "record ( [ \s-1ARGS\s0 ] )" 4
.IX Item "record ( [ ARGS ] )"
Create a new \f(CW\*(C`Error\*(C'\fR object and returns it. This is defined for
syntactic sugar, eg
.Sp
.Vb 2
\&    record Some::Error ( ... )
\&        and return;
.Ve
.SS "\s-1STATIC\s0 \s-1METHODS\s0"
.IX Subsection "STATIC METHODS"
.IP "prior ( [ \s-1PACKAGE\s0 ] )" 4
.IX Item "prior ( [ PACKAGE ] )"
Return the last error created, or the last error associated with
\&\f(CW\*(C`PACKAGE\*(C'\fR
.IP "flush ( [ \s-1PACKAGE\s0 ] )" 4
.IX Item "flush ( [ PACKAGE ] )"
Flush the last error created, or the last error associated with
\&\f(CW\*(C`PACKAGE\*(C'\fR.It is necessary to clear the error stack before exiting the
package or uncaught errors generated using \f(CW\*(C`record\*(C'\fR will be reported.
.Sp
.Vb 1
\&     $Error\->flush;
.Ve
.SS "\s-1OBJECT\s0 \s-1METHODS\s0"
.IX Subsection "OBJECT METHODS"
.IP "stacktrace" 4
.IX Item "stacktrace"
If the variable \f(CW$Error::Debug\fR was non-zero when the error was
created, then \f(CW\*(C`stacktrace\*(C'\fR returns a string created by calling
\&\f(CW\*(C`Carp::longmess\*(C'\fR. If the variable was zero the \f(CW\*(C`stacktrace\*(C'\fR returns
the text of the error appended with the filename and line number of
where the error was created, providing the text does not end with a
newline.
.IP "object" 4
.IX Item "object"
The object this error was associated with
.IP "file" 4
.IX Item "file"
The file where the constructor of this error was called from
.IP "line" 4
.IX Item "line"
The line where the constructor of this error was called from
.IP "text" 4
.IX Item "text"
The text of the error
.ie n .IP "$err\->associate($obj)" 4
.el .IP "\f(CW$err\fR\->associate($obj)" 4
.IX Item "$err->associate($obj)"
Associates an error with an object to allow error propagation. I.e:
.Sp
.Vb 2
\&    $ber\->encode(...) or
\&        return Error\->prior($ber)\->associate($ldap);
.Ve
.SS "\s-1OVERLOAD\s0 \s-1METHODS\s0"
.IX Subsection "OVERLOAD METHODS"
.IP "stringify" 4
.IX Item "stringify"
A method that converts the object into a string. This method may simply
return the same as the \f(CW\*(C`text\*(C'\fR method, or it may append more
information. For example the file name and line number.
.Sp
By default this method returns the \f(CW\*(C`\-text\*(C'\fR argument that was passed to
the constructor, or the string \f(CW"Died"\fR if none was given.
.IP "value" 4
.IX Item "value"
A method that will return a value that can be associated with the
error. For example if an error was created due to the failure of a
system call, then this may return the numeric value of \f(CW$!\fR at the
time.
.Sp
By default this method returns the \f(CW\*(C`\-value\*(C'\fR argument that was passed
to the constructor.
.SH "PRE-DEFINED ERROR CLASSES"
.IX Header "PRE-DEFINED ERROR CLASSES"
.SS "Error::Simple"
.IX Subsection "Error::Simple"
This class can be used to hold simple error strings and values. It's
constructor takes two arguments. The first is a text value, the second
is a numeric value. These values are what will be returned by the
overload methods.
.PP
If the text value ends with \f(CW\*(C`at file line 1\*(C'\fR as $@ strings do, then
this infomation will be used to set the \f(CW\*(C`\-file\*(C'\fR and \f(CW\*(C`\-line\*(C'\fR arguments
of the error object.
.PP
This class is used internally if an eval'd block die's with an error
that is a plain string. (Unless \f(CW$Error::ObjectifyCallback\fR is modified)
.ie n .SH "$Error::ObjectifyCallback"
.el .SH "\f(CW$Error::ObjectifyCallback\fP"
.IX Header "$Error::ObjectifyCallback"
This variable holds a reference to a subroutine that converts errors that
are plain strings to objects. It is used by Error.pm to convert textual
errors to objects, and can be overrided by the user.
.PP
It accepts a single argument which is a hash reference to named parameters. 
Currently the only named parameter passed is \f(CW\*(Aqtext\*(Aq\fR which is the text
of the error, but others may be available in the future.
.PP
For example the following code will cause Error.pm to throw objects of the
class MyError::Bar by default:
.PP
.Vb 7
\&    sub throw_MyError_Bar
\&    {
\&        my $args = shift;
\&        my $err = MyError::Bar\->new();
\&        $err\->{\*(AqMyBarText\*(Aq} = $args\->{\*(Aqtext\*(Aq};
\&        return $err;
\&    }
\&
\&    {
\&        local $Error::ObjectifyCallback = \e&throw_MyError_Bar;
\&
\&        # Error handling here.
\&    }
.Ve
.SH "MESSAGE HANDLERS"
.IX Header "MESSAGE HANDLERS"
\&\f(CW\*(C`Error\*(C'\fR also provides handlers to extend the output of the \f(CW\*(C`warn()\*(C'\fR perl
function, and to handle the printing of a thrown \f(CW\*(C`Error\*(C'\fR that is not caught
or otherwise handled. These are not installed by default, but are requested
using the \f(CW\*(C`:warndie\*(C'\fR tag in the \f(CW\*(C`use\*(C'\fR line.
.PP
.Vb 1
\& use Error qw( :warndie );
.Ve
.PP
These new error handlers are installed in \f(CW$SIG{_\|_WARN_\|_}\fR and
\&\f(CW$SIG{_\|_DIE_\|_}\fR. If these handlers are already defined when the tag is
imported, the old values are stored, and used during the new code. Thus, to
arrange for custom handling of warnings and errors, you will need to perform
something like the following:
.PP
.Vb 5
\& BEGIN {
\&   $SIG{_\|_WARN_\|_} = sub {
\&     print STDERR "My special warning handler: $_[0]"
\&   };
\& }
\&
\& use Error qw( :warndie );
.Ve
.PP
Note that setting \f(CW$SIG{_\|_WARN_\|_}\fR after the \f(CW\*(C`:warndie\*(C'\fR tag has been
imported will overwrite the handler that \f(CW\*(C`Error\*(C'\fR provides. If this cannot be
avoided, then the tag can be explicitly \f(CW\*(C`import\*(C'\fRed later
.PP
.Vb 1
\& use Error;
\&
\& $SIG{_\|_WARN_\|_} = ...;
\&
\& import Error qw( :warndie );
.Ve
.SS "\s-1EXAMPLE\s0"
.IX Subsection "EXAMPLE"
The \f(CW\*(C`_\|_DIE_\|_\*(C'\fR handler turns messages such as
.PP
.Vb 1
\& Can\*(Aqt call method "foo" on an undefined value at examples/warndie.pl line 16.
.Ve
.PP
into
.PP
.Vb 1
\& Unhandled perl error caught at toplevel:
\&
\&   Can\*(Aqt call method "foo" on an undefined value
\&
\& Thrown from: examples/warndie.pl:16
\&
\& Full stack trace:
\&
\&         main::inner(\*(Aqundef\*(Aq) called at examples/warndie.pl line 20
\&         main::outer(\*(Aqundef\*(Aq) called at examples/warndie.pl line 23
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
See Exception::Class for a different module providing Object-Oriented
exception handling, along with a convenient syntax for declaring hierarchies
for them. It doesn't provide Error's syntactic sugar of \f(CW\*(C`try { ... }\*(C'\fR,
\&\f(CW\*(C`catch { ... }\*(C'\fR, etc. which may be a good thing or a bad thing based
on what you want. (Because Error's syntactic sugar tends to break.)
.PP
Error::Exception aims to combine Error and Exception::Class
\&\*(L"with correct stringification\*(R".
.PP
TryCatch and Try::Tiny are similar in concept to Error.pm only providing 
a syntax that hopefully breaks less.
.SH "KNOWN BUGS"
.IX Header "KNOWN BUGS"
None, but that does not mean there are not any.
.SH "AUTHORS"
.IX Header "AUTHORS"
Graham Barr <gbarr@pobox.com>
.PP
The code that inspired me to write this was originally written by
Peter Seibel <peter@weblogic.com> and adapted by Jesse Glick
<jglick@sig.bsh.com>.
.PP
\&\f(CW\*(C`:warndie\*(C'\fR handlers added by Paul Evans <leonerd@leonerd.org.uk>
.SH "MAINTAINER"
.IX Header "MAINTAINER"
Shlomi Fish <shlomif@iglu.org.il>
.SH "PAST MAINTAINERS"
.IX Header "PAST MAINTAINERS"
Arun Kumar U <u_arunkumar@yahoo.com>
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 1997\-8  Graham Barr. All rights reserved.
This program is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.