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.\" ========================================================================
.\"
.IX Title "private-Error 3pm"
.TH private-Error 3pm "2017-12-11" "perl v5.26.1" "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 "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 \-text => "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 => $general_handler
\&        };
\&    }
\&    otherwise {
\&        print STDERR "Well I don\*(Aqt know what to say\en";
\&    }
\&    finally {
\&        close_the_garage_door_already(); # Should be reliable
\&    }; # Don\*(Aqt forget the trailing ; or you might be surprised
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
The \f(CW\*(C`Error\*(C'\fR package provides two interfaces. Firstly \f(CW\*(C`Error\*(C'\fR provides
a procedural interface to exception handling. Secondly \f(CW\*(C`Error\*(C'\fR is a
base class for errors/exceptions that can either be thrown, for
subsequent catch, or can simply be recorded.
.PP
Errors in the class \f(CW\*(C`Error\*(C'\fR should not be thrown directly, but the
user should throw errors from a sub-class of \f(CW\*(C`Error\*(C'\fR.
.SH "PROCEDURAL INTERFACE"
.IX Header "PROCEDURAL INTERFACE"
\&\f(CW\*(C`Error\*(C'\fR exports subroutines to perform exception handling. These will
be exported if the \f(CW\*(C`:try\*(C'\fR tag is used in the \f(CW\*(C`use\*(C'\fR line.
.IP "try \s-1BLOCK CLAUSES\s0" 4
.IX Item "try BLOCK CLAUSES"
\&\f(CW\*(C`try\*(C'\fR is the main subroutine called by the user. All other subroutines
exported are clauses to the try subroutine.
.Sp
The \s-1BLOCK\s0 will be evaluated and, if no error is throw, try will return
the result of the block.
.Sp
\&\f(CW\*(C`CLAUSES\*(C'\fR are the subroutines below, which describe what to do in the
event of an error being thrown within \s-1BLOCK.\s0
.IP "catch \s-1CLASS\s0 with \s-1BLOCK\s0" 4
.IX Item "catch CLASS with BLOCK"
This clauses will cause all errors that satisfy \f(CW\*(C`$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.
.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.
.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 "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 its 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 "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 METHODS\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
\&     $Error\->flush;
.Ve
.SS "\s-1OBJECT METHODS\s0"
.IX Subsection "OBJECT METHODS"
.IP "stacktrace" 4
.IX Item "stacktrace"
If the variable \f(CW$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
.SS "\s-1OVERLOAD METHODS\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"
.IP "Error::Simple" 4
.IX Item "Error::Simple"
This class can be used to hold simple error strings and values. Its
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.
.Sp
If the text value ends with \f(CW\*(C`at file line 1\*(C'\fR as $@ strings do, then
this information will be used to set the \f(CW\*(C`\-file\*(C'\fR and \f(CW\*(C`\-line\*(C'\fR arguments
of the error object.
.Sp
This class is used internally if an eval'd block die's with an error
that is a plain string. (Unless \f(CW$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 overridden 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 "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>.
.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>