Grammar::Message, a Raku module to make grammar and regex messages prettier

inspired by Grammar::PrettyErrors, and ripping off the code used there for printing messages, prints error messages highlighting the line and, if possible, the position where it last worked.

Installing

Do the usual

zef install Grammar::Message

Running

You will need to insert a variable assignment behind the match you are going to check

my $str = (('a'..'z').rotor(3).map: *.join("")).join("\n");
my $saved-match; # Saves in situ
$str ~~ / m { $saved-match = $/} /; # Code right behind match you need printed
say pretty-message( "Found", $saved-match);

Will print

Found
  3 │ ghi
  4 │ jkl
  5 │▶ mno
        ^
  6 │ pqr
  7 │ stu
  8 │ vwx

with highlights for the matching line.

grammar G {
    token TOP { <letters>+ % \v}
    token letters { { $saved-match = $/} \w ** 3 }
}
$str .= subst( "m", "mm" );
G.parse( $str );
say pretty-message( "Some failure around here", $saved-match);

will print

Some failure around here
  4 │ jkl
  5 │ mmno
  6 │▶ pqr
     ^
  7 │ stu
  8 │ vwx

Take into account that the marked point will always be the best bet, and in any case it will reflect the state of the pos attribute of the Match at the point you saved it.

See also

Code by Brian Duggan in Grammar::PrettyErrors is the origin of this code.

License

(c) Brian Duggan, JJ Merelo

Respecting the original code license, this is also licensed under the Artistic license (included).