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Grammar::PrettyErrors cpan:BDUGGAN last updated on 2019-03-27

Grammar-PrettyErrors-0.0.3/

Grammar::PrettyErrors

Build Status

Make grammars fail parsing with a pretty error instead of returning nil.

SYNOPSIS

Input:

use Grammar::PrettyErrors;

grammar G does Grammar::PrettyErrors {
  rule TOP {
    'orange'+ % ' '
  }
}

# Handle the failure.
G.parse('orange orange orange banana') orelse
    say "parse failed at line {.exception.line}";

# Don't handle it, an exception will be thrown:
G.parse('orange orange orange banana');

Output:

failed at line 1

--errors--
  1 │▶orange orange orange banana
                           ^

Uh oh, something went wrong around line 1.
Unable to parse TOP.

DESCRIPTION

When the Grammar::PrettyErrors role is applied to a Grammar, it changes the behavior of a parse failure. Instead of returning nil, a failure is thrown. The exception object has information about the context of the failure, and a pretty error message that includes some context around the input text.

It works by wrapping the <ws> token and keeping track of a highwater mark (the position), and the name of the most recent rule that was encountered.

This technique is described by moritz in his excellent book [0] (see below).

CLASSES, ROLES

Grammar::PrettyErrors (Role)

ATTRIBUTES

METHODS

X::Grammar::PrettyError (class)

METHODS

EXAMPLES

grammar G does Grammar::PrettyErrors { ... }

# Throw an exception with a message when a parse fails.
G.parse('orange orange orange banana');

# Same thing
G.new.parse('orange orange orange banana');

# Pass options to the constructor.  Don't throw a failure.
my $g = G.new(:quiet, :!colors);
$g.parse('orange orange orange banana');
# Just print it ourselves.
say .report with $g.error;

# Use the failure to handle it without throwing it.
G.parse('orange orange orange banana') orelse
    say "parse failed at line {.exception.line}";

SEE ALSO

AUTHOR

Brian Duggan