Sun, 07 Aug 2011
Changes to The Way Marpa Orders Parse Results
This post is for users of Marpa's Constant Ranking Method.
You are using the Constant Ranking Method if you specify
the "ranking_method" named argument of a Marpa recognizer,
with the value "constant".
If you're not using the Constant Ranking Method,
you can stop reading here.
Marpa::XS 0.008000 is the last release that will support the
Constant Ranking Method.
In future releases of Marpa, Marpa::PP, and Marpa::XS,
the Constant Ranking Method may be removed.
At a minimum, it will behave differently
at the interface level.
Marpa is alpha but previously, whenever I've changed the
documented behavior of an
interface, I have kept backward compatibility.
As alpha development ends and I approach a beta release,
I am forced to be more ruthless.
I will be changing or eliminating the Constant Ranking
duplicating its exact semantics
for backward compatibility
is simply too difficult.
By setting the ranking method to "constant", users have
been able to control the order in which the parse results
of an ambiguous parse are returned.
The Constant Ranking Method never offered
more than limited capabilities
for ordering parses.
Marpa applications which needed a general capability for ranking parses
have always needed to produce all the parses,
with ranking information included,
and to sort them at the application level,
outside of Marpa.
Many applications using ambiguous grammars are quite happy
to get the parse results in arbitrary order.
That was, and remains, Marpa's default behavior.
Embedding a fully general parse-ranking logic in the evaluator
would be high cost and have no added value ---
ordering the parses inside Marpa's evaluation logic would
have no advantage over sorting the parses at
the application level.
The Constant Ranking Method was an attempt at a compromise.
The intention was that it would provide a set of features
- Was flexible enough for many applications;
- Added value, in the sense that
it was more efficient than the user doing
the same thing herself
at the application level;
- Had little cost for users who were not
But the Constant Ranking Method,
as currently implemented,
It is reasonably flexible,
but the code for it is grotty,
and its efficiency advantages are debatable.
Facing a C conversion of the code,
I realize that
the right place for much or all of the current
Constant Ranking Method logic is the bit bucket.
posted at: 08:28 |
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