Exception handling in parser implemented using Marpa::R2

Question

I have implemented a parser using Marpa::R2. Code appears like below:

I have a large number of test cases in a .t file, which i run to test my parser. So, if any exception arises in any of the input expression, testing shouldn't stop in mid and it should give proper error message for the one which has given an error (using exception handling) and rest of the test cases should run.

I want to do exception handling in this parser. If any sort of exception arrises even while tokenizing the input expression, I want to show appropriate message to the user, saying the position, string etc or any more details to show where the error came. Please help.

use strict;
use Marpa::R2;
use Data::Dumper;

my $grammar = Marpa::R2::Scanless::G->new({
    default_action => '[values]',
    source => \(<<'END_OF_SOURCE'),
    lexeme default = latm => 1

:start ::= expression

expression ::= expression OP expression
expression ::= expression COMMA expression
expression ::= func LPAREN PARAM RPAREN
expression ::= PARAM
PARAM ::= STRING | REGEX_STRING
REGEX_STRING ::= '"' QUOTED_STRING '"'

:discard    ~ sp
sp          ~ [\s]+

COMMA                      ~ [,]
STRING                     ~ [^ \/\(\),&:\"~]+
QUOTED_STRING              ~ [^ ,&:\"~]+
OP                         ~ ' - ' | '&'
LPAREN                     ~ '('
RPAREN                     ~ ')'
func                       ~ 'func'

END_OF_SOURCE
});

my $recce = Marpa::R2::Scanless::R->new({grammar => $grammar});
print "Trying to parse:\n$input\n\n";
$recce->read(\$input);
my $value_ref = ${$recce->value};
print "Output:\n".Dumper($value_ref);

my $input4 = "func(\"foo\")";

I want to do Proper error handling like :http://blogs.perl.org/users/jeffrey_kegler/2012/10/a-marpa-dsl-tutorial-error-reporting-made-easy.html

I dont know how to put all this stuff in place.

Have you tried to put an `eval` around the `$recce->read()` call? — Håkon Hægland, Apr 20 '18 at 08:14
@HåkonHægland can you please give an example, yes I have tried to do eval, but how do i tell the user that the error is this and this particular token at this position is causing that? — zubug55, Apr 20 '18 at 08:20
Can you use the exception string stored in `$@` ? It should contain the line and column number — Håkon Hægland, Apr 20 '18 at 08:28
alright, what if there is some error like this foo & ; problem here is there should be a string after & ; but this type of expression will not cause line and column no. sort of error; in this case the input expression is itself wrong. How do i project this sort of errror? — zubug55, Apr 20 '18 at 08:34
Maybe you could make the grammar fail if there is no string after a `&`? — Håkon Hægland, Apr 20 '18 at 08:39
Sorry, I have not used `Marpa::R2` for a while so I would need to read a little bit about the rules. But I would recommend looking into [Perl 6 Grammars](https://docs.perl6.org/language/grammar_tutorial) if you can. — Håkon Hægland, Apr 20 '18 at 08:49

score 2 · Answer 1 · answered Apr 20 '18 at 10:47

Wrap the lines that can fail in an exception handler:

use Try::Tiny;
⋮
try {
    $recce->read(\$input);
    my $value_ref = ${$recce->value};
    print "Output:\n".Dumper($value_ref);
} catch {
    warn $_;
};

The full error message from Marpa will be in $_, it is a single long string with newlines in it. I chose to print it to STDOUT with warn, and the program continues to run. As you can see in an example error message below, it contains the position where the parsing failed:

Error in SLIF parse: No lexeme found at line 1, column 5
* String before error: "fo\s
* The error was at line 1, column 5, and at character 0x006f 'o', ...
* here: o"
Marpa::R2 exception at so49932329.pl line 41.

If you need to, you could reformat it so it looks better to the user.

I want the proper way of showing: till where the string is parsed successfully parsed and all the stuff like that — zubug55, Apr 20 '18 at 21:41
I got a page which talks about error reporting : http://blogs.perl.org/users/jeffrey_kegler/2012/10/a-marpa-dsl-tutorial-error-reporting-made-easy.html ; but i dont know how to put this all stuff in place. — zubug55, Apr 20 '18 at 21:42

Exception handling in parser implemented using Marpa::R2

1 Answers1