parse() — first impression and suggestions

[Ruslan Shvedov]

I've been using parse when working on commonmark parser and here is the how it turned out. 

First, parse() does a great job when testing with unambiguous grammar and input — it throws an exception on ambiguity and that's what you want.

However, when thing get more complex and I needed, e.g. just dump multiple parses, the recognizer is hidden behind the otherwise very useful parse() interface, and to do that _while still keeping the behaviour in unambiguous case_, I had to mimic parse()'s functionality, like this. 

I understand that it is by design, hence my suggestion is that in list context parse(), rather than throwing an exception, return its recognizer (probably after series_restart() so that it's ready to use), like this. 

my ($value, $recognizer) = $slg->parse( { input => \$input } );

Another, arguably better, option would be to return the abstract syntax forest parse() produces, e.g.

my ($value, $asf) = $slg->parse( { input => \$input } );

This would require adding recognizer() method to Marpa::R2::ASF (grammar() is already there) .

The full use case I have in mind:

my ($value, $recognizer_or_asf) = $slg->parse( { input => \$input } );

# check for ambiguity

if (not defined $value){

# use the recognizer or the asf to handle ambiguity
# and find the required value 
# e.g. sort the parses or prune the ASF or the ASTs
# ...

}

# process $value if needed

# ...

The case where exception is thrown on ambiguity continues as is:

my $value = $slg->parse( { input => \$input } );

[Jeffrey Kegler]

Marpa::R2 is nearly frozen and the new $slg->parse() is squeezed in past the freeze as an exception, because it will be very good for those just starting -- it simplifies the initial tutorial and example a good deal.  These suggestions may have more merit as a design, but they get *way* beyond that intent.  My intent is that those who want to do anything fancy simply ditch $slg->parse() and use the lower level methods, as currently, and with that idea I kept the features to a minimum, and avoided access to the fancier features, like events or ASF's.

-- jeffrey

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