This is a basic implementation of a CKY parser
that supports grammars with and without weights.
The sample grammar in test.lhs is particularly small,
but it may serve as a useful example.
For best results in probabilistic grammars,
consider importing Data.Ratio and using (Weighted Rational)
to ensure there is no loss of precision.
Future work may include implementing a (lossy) alternative
based on negative logs in order to replace multiplication with addition.