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NAME

Error::Show - Show context around syntax errors and exceptions

SYNOPSIS

Command Line

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.

In Program

Use at runtime to supplement exception handling:

  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($e){
    say STDERR context message=>$e, frames=>$e->frames
  }
  

DESCRIPTION

From the command line this module transparently executes your syntactically correct program. 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.

From within a program at runtime, this module can be used to give the same formatted code context around the source of an exception and iterate through any associated stack frames if provided.

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.

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.

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 run)

        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.

NOTE: It is important that it's the first -M switch.

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 Syntax Checking 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

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;                     (1)
  my $context=Error::Show::context $error;              (2)
  my $context=Error::Show::context option_pairs, message=>$error_as_string, frames=>$stack frames (3)

  my $context=Error::Show::context undef;               (4)
  my $context=context undef;                            (5)

  my $context=Error::Show->context(...);                (6)

        

Takes an error string, or exception object 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 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 ...

In the first form (1), the $@ variable is implicitly used as the error. No processing options can be supplied in this form. This is stringified for processing.

In the second form (2), a single argument is supplied, which becomes the error to process. No processing options can be supplied in this form. This is stringified for processing.

In the third for (3), all options are provided as key value pairs.

From v0.3.0: In the forth form (4), an explicit single argument of undef, will generate call stack/frames internally and generate context information from it.

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 arguemtn handling as the other forms.

The expected types of data are as follows:

1. String Errors (perl errors)

Error string, as per die and warn, containing file and line number. These are extracted from the string error to locate context.

The output message after the context is this string,

2. An reference to an array containing results from caller

The filename, and line elements are used to process. No error message is output unless the message option is also specified.

3. An Devel::StackTrace::Frame object

This is converted internally to a array of caller() output. As above.

4. or, an array of 2. or 3.

An reference to an array of call frames in caller() or Devel::StackTrace::Frame format can also be supplied as the error or frames. Again the message option needs to be provided if error string is required in the output.

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.

EXAMPLES

Integrating with Exception classes

The following are a cheat sheet / example code to interoperate this module with exception objects.

The most reliable way is usually to explicitly set the message and frames options explicitly. This works with a single frame (for the latest exception) or ref to array, for a complete trace

Mojo::Exception

FYI: Mojo::Exception does provide it's own facility to show the code context around an exception.

  use v5.36;
  use feature "try";
  use Error::Show;
  use Mojo::Exception qw<check raise>;

  try{
    raise 'MyApp::X::Name', 'The name Minion is already taken';
  }
  catch($e){

    # Message is the stringified $e object. Print the first frame
    #
    say Error::Show::context message=>$e, error=> $e->frames->[0];


    # Message is the stringified $e object. Print all frames
    #
    say Error::Show::context message=>$e, error=>$e->frames;

  }

Class::Throwable

  use v5.36;
  my @a=qw<a,b,c>;
  use Class::Throwable;# VERBOSE=>1;
  Class::Throwable->setVerbosity(2);
  #use Exception::Class;
  use Error::Show;
  use feature "try";
  sub my_func {
    try{
      Class::Throwable->throw("Something has gone wrong");

    }
    catch($e){
      
      #Show the top of the stack, the latest exception
      say Error::Show::context message=>$e, frames=>$e->getStackTrace->[0];

      #Show the whole stack
      say Error::Show::context message=>"$e", frames=>[$e->getStackTrace];
    }
  }

  sub my_func2{
    my_func;
  }
  warn "some warning";
  my_func2;

Exception::Base

  use v5.36;
  use feature qw<try say>;

  use Exception::Base verbosity=>4;
  use Error::Show;

  sub my_func {
    try{
      my $e= Exception::Base->new();
      #$e->verbosity(10);
      $e->throw(message=>"Bad things");

    }
    catch($e){
      
      # Set verbosity to stop duplicate outputs, but provide a file and line number
      # in the stringified version of the error
      #
      $e->verbosity=2;

      # Message normally contatins the file and line numbers. So stringified
      # process will work
      #
      say Error::Show::context $e;

      # Access the frames in the caller stack
      #
      say Error::Show::context message=>$e->message, frames=>$e->caller_stack;
    }
  }

  sub my_func2{
    my_func;
  }

  my_func2;

Exception::Class::Base

  use v5.36;
  use Exception::Class;
  use Error::Show;
  use feature "try";
  try{
    Exception::Class::Base->throw("An error occured");
  }
  catch($e){

    my @frames=$e->trace->frames;
    
    # Message is the stringified $e object. Print the first frame
    #
    say Error::Show::context message=>$e, frames=>$frames[0];

    # Message is the stringified $e object. Print all frames
    #
    say Error::Show::context message=>$e, frames=>\@frames;

  }

Syntax Checking String eval, Without Execution

A block eval will have it's syntax checked during normal compilation time. A string eval is checked at run time and it uses the same variable $@ to report syntax errors and run time errors.

Together these limitation make it impossible to distinguish between syntax errors and runtime errors (without some kind of heavy error lookup). The code is also executed immediately.

To work around these limitations, and have Error::Show still provide context information, start by wrapping your eval string with "sub { ... }. The eval result will be code ref if syntactically correct. Otherwise the error in $@ is the syntax error string, which can be feed directly into Error::Show. The code ref is executed in a block eval, which if dies from an exception will place the runtime error in $@, which again can be used in Error::Show. As an example:

  Example: Separate compiling/syntax checking from eval execution
  =======

  use strict;
  use warnings;
  use feature "say";

  use Error::Show;


  # Orginal string to eval
  my $prog='say "hello there";
  $a+1/0;
  ';

  # Wrap string in a sub

  $prog="sub { $prog }";


  # Do an eval to retun an actual code ref

  my $code =eval $prog; 


  # Check for syntax errors here. No run time error as the code ref has not been executed.
  # Use the program option in Error::Show to specifiy the text of the program

  if($@){
    say "ERROR is".Error::Show::context error=>$@, program=>$prog;
  }

  # Execute the code reference. Any errors here are run time

  eval {$code->()};

  # Again use the program option in Error::Show to specifiy the text of the program

  if($@){
    say "ERROR is".Error::Show::context error=>$@, program=>$prog;
  }

Please see the examples directory in this distribution

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

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: http://github.com/drclaw1394/perl-error-show

COPYRIGHT AND LICENSE

Copyright (C) 2023 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.