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Error-Show-v0.5.0
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/ Error::Show

NAME

Error::Show - Locate and Diagnose Errors/Exceptions in Perl programs

SYNOPSIS

String EVAL

Consider the string/program to be evaluated that has a illegal divide by zero ( or could be a syntax error );

use Error::Show;

my $prog='

sub call_me {
  $a+1/0;
}

say "hello there";
call_me;

';

With normal string eval and exception checking, we get:

my $result =eval $prog; 
if($@){
  say $@;
}
#======== OUTPUT

hello there
Illegal division by zero at (eval 1) line 4.

Using normal string eval with context we get:

my $result =eval $prog; 
if($@){
  say "ERROR is $@".Error::Show::context $@;
}

#====== OUTPUT

(eval 2)
Illegal division by zero at (eval 2) 0
    examples/eval.pl
    30
    31   {
    32     say "================== Normal string with context =====================";
    33     my $result =eval $prog;
    34     if($@){
    35=>     say Error::Show::context $@;
    36     }
    37
    38   }
    39
    40

Using streval with context we get:

my $result =eval {streval $prog};
if($@){
  say Error::Show::context $@;
}

#======= OUTPUT

hello there
(eval 4)
1
2
3     sub call_me {
4=>     $a+1/0;
5     }
6
7     say "hello there";
8     call_me;
9
Illegal division by zero at (eval 4) 4
    examples/eval.pl
    41
    42   say "";
    43
    44   {
    45     say "================== eval streval =====================";
    46=>   my $result =eval {streval $prog};
    47     if($@){
    48       say Error::Show::context $@;
    49     }
    50
    51

Command Line Syntax Checking

Consider the following program (at examples/synopsis.pl in this distribution). It has a syntax error on line 13, and uses an experimental feature on line 7.

use strict;
use warnings;
use Time::HiRes;

use feature "refaliasing";

\my $a=\"hello";
my $time=time;
for(1..1000){
  print "$_\n";
}

my $crazy-var=2;

use Socket;

print "this will never work";

Attempting to run this program with Perl normally gives this error output:

->perl examples/synopsis.pl       
Aliasing via reference is experimental at examples/synopsis.pl line 7.
Can't modify subtraction (-) in scalar assignment at examples/synopsis.pl line 13, near "2;"
BEGIN not safe after errors--compilation aborted at examples/synopsis.pl line 15.

With Error::Show enabled with the -M switch, this instead looks like this:

->perl -MError::Show=warn  examples/synopsis.pl
examples/synopsis.pl
 3   use Time::HiRes;
 4
 5   use feature "refaliasing";
 6
 7=> \my $a=\"hello";
 8   my $time=time;
 9   for(1..1000){
10   	print "$_\n";
11   }
12
Aliasing via reference is experimental at examples/synopsis.pl line 7.
examples/synopsis.pl
 9   for(1..1000){
10   	print "$_\n";
11   }
12
13=> my $crazy-var=2;
14
15   use Socket;
16
17   print "this will never work";
Can't modify subtraction (-) in scalar assignment at examples/synopsis.pl line 13, near "2;"
examples/synopsis.pl
11   }
12
13   my $crazy-var=2;
14
15=> use Socket;
16
17   print "this will never work";
BEGIN not safe after errors--compilation aborted at examples/synopsis.pl line 15.

Command Line Exception Call Stack Dump (from v0.4.0)

Consider the following short program (examples/synopsis2.pl

use v5.36;

sub test {

  my $a=1/0;
}


test;

Running this program with Perl normally gives this error output:

perl examples/synopsis2.pl  
Illegal division by zero at examples/synopsis2.pl line 5.

With Error::Show enabled with the -M switch, this instead looks like this:

perl -MError::Show examples/synopsis2.pl
examples/synopsis2.pl
1   use v5.36;
2
3   sub test {
4
5=>   my $a=1/0;
6   }
7
8
9   test;
Illegal division by zero at examples/synopsis2.pl 5

    examples/synopsis2.pl
    4
    5     my $a=1/0;
    6   }
    7
    8
    9=> test;
Illegal division by zero at examples/synopsis2.pl 5

In Program

Use at runtime to supplement exception handling without signal handler modification:

use Error::Show;

#an die caught in a try/eval triggers an exception

# No argument uses $@ as error
#
eval { exceptional_code };
say STDERR context if $@;


# or a single exception argument of your choosing
#
use v5.36;
try { 
  exceptional_code
}
catch($e) {
  say STDERR context $e;
}

# Show context down a stack

try {

  Some_execption_class->throw("Bad things");

}
catch($e){
  say STDERR context $e;
}

DESCRIPTION

This module provides three tools/modes to help locate and diagnose errors in your Perl programs.

Command Line

From the command line this module transparently executes your syntactically correct program. No code changes are required. However in the case of syntax errors (or warnings if desired), it extracts context (lines of code) surrounding them. The lines are prefixed with numbers and the nicely formatted context is dumped on STDERR for you to see the error or your ways.

The resulting output is optionally filtered seamlessly through the splain program (see diagnostics), giving more information on why the reported syntax errors and warnings might have occurred.

In Program Exception Context

From within a program, this module can be used to give formatted code context around the source of an exception and the context of each of the stack frames captured when the exception was raised.

It supports Perl string exceptions and warnings directly and also provides the ability to integrate third party CPAN exception objects and traces with minimal effort. Please see examples in this document or in the examples directory of the distribution showing use with Mojo::Exception, Exception::Base, Exception::Class::Base and Class::Throwable.

From v0.5.0 a throw routine is exported which, generates a very basic exception object, with stack frame capture, in case using an larger exception object/class is unneeded

String evals with Execption/Syntax Error Context

From v0.5.0 the streval subroutine has been added to allow context information to be generated for errors originating from code dynamically created from string evaluations.

Changes and Options

A handful of options are available for basic configuration of how many lines of code to print before and after the error line, indenting of stack trace context, etc.

v0.5.0 Has alot of changes which might break compatibility. THe up side is the codes is simpler, has less errors, and easier to use. This is especial the case for third party exceptions objects. It also gives the intended purpose of handling errors and exceptions from string evals like any other.

That being said, an earlier version might need to be used if you want the use the $@ variable implicitly in a call to context. This subroutine now requires an explicated error argument

Sub routines context, throw and streval are exported by default.

From v0.3.0: context subroutine is now exported by default. To prevent this, import with an empty list, ie use Error::Show ().

From v0.2.0:, Added 'advanced string eval' support has been added for better context reporting of dynamically generated code.

USAGE

Command Line Usage (Syntax check and Exception Catching)

perl -MError::Show  [options] file.pl

When included in a command line switch to Perl, -MError::Show syntax checks the input program. If the syntax is OK, normal execution continues in a transparent fashion. Otherwise, detailed code context surrounding the source of the error is generated and printed on STDERR.

From v0.4.0 a global __DIE__ handler is also installed, which will catch any stray exceptions during execution and present a line numbered summary stack trace. Programs a free to overload this handler. However the features of this module will be lost.

NOTE: It is important that it's the first -M switch for this module to operate correctly and to prevent any incompatibilities withe global signal handlers.

If the -c flag is specified, only a syntax check will be performed, mimicking normal Perl behaviour.

Additional @INC directories using the -I switch are supported as are additional modules via the -M switch.

CLI Usage Options

The following options can be used in isolation or together:

clean

If you prefer just the code context without the Perl error, add the clean option:

perl -MError::Show=clean file.pl

warn

This options enables processing of warnings as well as errors.

perl -MError::Show=warn file.pl

splain

Runs the output through the splain program (see diagnostics), giving probable reasons behind the error or warning

perl -MError::Show=splain file.pl

no_handler (from v0.4.0)

perl -MError::Show=no_handler file.pl

Prevents the global DIE handler from being installed.

Return code

When in check only mode (-c), the main process is exited, just has Perl normally would have done. The return code is a replica of what Perl would have reported for success/failure of a syntax check.

In Program (Exception) Usage

Simply bring Error::Show into your program with a use statement:

use Error::Show;

It provides a single subroutine for processing errors and exceptions.

Error::Show::context

my $context=Error::Show::context $error , options_pairs, ...;

Takes an error string, or exception object $error and extracts the code surrounding the source of the error. The code lines are prefixed with line numbers and the error line marked with a fat arrow.

The expected types of error are

Error string, as per die and warn

This is expected to contain file and line number. These are extracted from the string error to locate context.

Supported Execption Object

Several common exceptions classes on CPAN are supported:

Error::Show::Exception
Exception::Class::Base
Exception::Base
Class::Throwable
Mojo::Exception

Other classes will likely work as long as the stringify to resemble a perl error string (with the filename and line number formatted correctly). The stack frame capture would work however)

The return value is the formatted context, followed by the original Perl error strings, or stringified exception objects/messages:

filename.pl 
10  #code before 
11  #code before 
12=>#this line caused the error
13  #code after
14  #code after

... error... at filename.pl line 12 ...

The context subroutine (5) is exported by default, so does not need a fully qualified name

context Can also be called with package arrow notation (6) ie Error::Show->context(...) if prefered, with the same argument handling as the other forms.

Options include:

limit

limit=>$int

From v0.2.0: Limits the number of errors to extract and generate context for. Default is 100. If <=0, no limiting is applied all all errors are processed.

reverse

reverse=>$bool

From v0.2.0: Reverses the order of error processing.

Perl type string errors are sorted and processing in ascending line number order. When this option is used, the lines are processed by descending line number first. Does not change order of files processed.

If frames are used instead, they are processed in reverse order to how they where supplied when this option is in effect.

pre_lines

pre_lines=>value

Specific the maximum lines of code to display before the error line. Default is 5.

post_lines

post_lines=>value

Specific the maximum lines of code to display after the error line. Default is 5.

clean

clean=>bool

When true, the normal Perl error string is not included in the context information, for a cleaner look.

indent

indent=>string

The string to use for each level of indent when printing multiple stack frames. Defaults to 4 spaces.

splain

splain=>1

The resulting output will be filtered through the splain program.

program

program=>$prog

The program option is used to specify the program text to process when there is no actual file. This is needed when to show syntax errors in a string eval:

my $prog='my $a="This will Fail"+b';
eval $prog;
if($@){
  say Error::Show::context error=>$@, program=>prog;
}

For advanced string eval processing options please see the ADVANCED STRING EVAL section in this document.

streval

local $@;

my $result=eval { streval "program";

if($@){
  print STDERR context $@;
}


##### or ####


try {
  my $result=strval "program";
}
catch($e){
  print STDERR context $e;
}

streval makes it possible to debug code generated from string eval. Internally it caches program code, to allow a context to be generated in the case of an exception.

NOTE: It also throws an exception on syntax error, which also then can have a context generated for it.

throw

throw "My error message";

Raises an exception with a basic exception object with the provided message. Captures the call stack frames at the point of call.

Use this if you don't want to use other exception classes/modules

ADVANCED STRING EVAL

From v0.2.0 features to support advanced string evaluation are available. This was added to support the error reporting needs of the Template::Plex module.

Consider the following example. The 'meat' is the sub in the middle of the string. Any errors/context reported should be relative to this, not the start of the overall eval string. With some help from comment markers "##_PREAMBLE" and "##_POSTAMBLE", when can search for the middle and rebase the error line numbering.

eval {
  "my $some_prep_work=1;
  call_somthing();
  #comments...
  
  ##_PREAMBLE

  sub {
     #code generated from user input...
     say "My really cool dynamically created code"
  }

  ##_POSTAMBLE

  #More code 
  #Cleanup stuff.
  "
}

Additional configuration options can be provided to search for the relevant code lines and offset the error line numbers.

NOTE if these options are used, the message field is modified with updated line numbers if its in the form of a normal Perl errors ie 'error line 123 at file'.

start_mark

start_mark=>$regexp

If specified, is a used as a regexp to match against source code lines. The line after a successful match is now the first line.

This allows inserting a special marker to indicate the start of 'code of interest' with out knowing the exact line number in the resulting code.

This is undefined and unused by default.

end_mark

end_mark=>$regexp

If specified, is used as a regexp to match against source lines, in reverse order. The line after a successful match is now the last line

This allows inserting a special marker to indicate the end of 'code of interest'.

This is undefined unused by default.

start_offset

start_offset=>$int

A static offset to add to the start line (which may have been modified by the start_mark option). The result will be classed as the minimum line number of the file/string.

This is useful to prevent any preamble after the start_mark line in your string eval showing up in the user program context.

end_offset

A static offset to subtract to the end line (which may have been modified by the end_mark option). The result will be classed as the maximum line number of the file.

This is useful to prevent any postamble before the end_mark in your string eval showing up in the user program context.

FUTURE WORK/TODO

Possible just use the DIE and WARN signal handler instead of forking processes?
Make usable from a Language Server?
Colour terminal output?
JSON output?

KNOWN ISSUES/GOTCHAS

Checking/running programs via STDIN, -e and -E switches is not supported and will die with an error message.

More data then needed is pushed through the splain program when splain option is used, which isn't ideal.

SEE ALSO

Perl::Syntax provides syntax checking from the command line. However it doesn't show any errors by design (only interested in process return code)

Syntax::Check provides programmatic syntax checking of files.

Perl::Critic gives actual Perl linting, but not great for syntax errors.

diagnostics and the splain program give some very useful explanations about the otherwise terse error strings normally output. It is part of the Perl distribution

AUTHOR

Ruben Westerberg, <drclaw@mac.com>

REPOSITORTY and BUGS

Please report any bugs via git hub: https://github.com/drclaw1394/perl-error-show

COPYRIGHT AND LICENSE

Copyright (C) 2025 by Ruben Westerberg

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

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.