[Haskell-cafe] Re: Frisby grammars that have context

[Isaac Dupree]

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apfelmus wrote:
> Mark T.B. Carroll wrote:
>> I've been playing with Text.Parsers.Frisby to see how it stacks against
>> other options and, while it's been great so far, I am finding that I
>> can't encode a grammar where what's acceptable depends on what's already
>> been parsed in some nontrivial way.
>> [...]
>> Is this supposed to not be possible in Frisby, or (quite likely) am I
>> missing something that allows me to?
> 
> It's intentionally impossible. Frisby uses a dynamic programming
> approach that crucially depends on the fact that the grammar in question
> is context-free (actually something related, but the effect is the
> same).

Is that dependence crucial? What if it gained Monad operations that just
weren't intended to be used very often, and maybe locally harmed
performance a little where they are used?

BTW: (P s) should be an instance of Applicative (which is already
possible with Frisby's current code, just not there) (I prefer the
aesthetics of Frisby-applicative code to many of the combinators it
provides - I tried it a little sometime, not enough to send a darcs patch)

Isaac
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