On 13-03-12 04:06 PM, Jeremy Shaw wrote:
It would be pretty damn cool if you could create a data type for generically describing a monadic parser, and then use template haskell to generate a concrete parser from that data type. [...] I would like to suggest that while it would be cool, it is impossible.
Impossibility proofs are notoriously difficult. You showed that this approach:
data ParserSpec a where AnyChar :: ParserSpec Char Return :: a -> ParserSpec a Join :: ParserSpec (ParserSpec a) -> ParserSpec a FMap :: (a -> b) -> ParserSpec a -> ParserSpec b
does not work. The flaw is indeed in FMap. It should not take a function as first argument, but rather a *description* of a function (the same way ParserSpec gives you a description of a parser). Then you can make it work, if your 'description' language is adequate. For some strange reason, I am biased towards 'finally tagless' descriptions, but YMMV. Jacques