To declare "ReadP" an instance of MonadFix; you'll first have to make the P monad into a MonadFix instance. That can be done using existing techniques in the literature.
ReadP is essentially the continuation monad transformer wrapped around P. It's well known in the value-recursion literature that continuation monad is too "strong" to have a value-recursion operator. I am not aware of any "simple" solutions in that space. Hence, ReadP is beyond the realm of current theories of value recursion.
Having said that, I'd also like to point out that Amr Sabry and Eugenio Moggi, and independently Magnus Carlsson has done some interesting work to extend value recursion to the world of continuations; which might help with your particular problem. Essentially, you end up adding some extra infrastructure to your monad, and then forgo some of the basic axioms of value recursion. But you can get running examples!
Maybe all you'll need is a MonadFix instance of P; which is definitely doable with the current techniques. Anything further would actually make a nice research paper...
-Levent. (I could provide references to above work if needed; all is available on the net freely, anyhow.)
On 6/23/06, Gracjan Polak <[EMAIL PROTECTED]> wrote:
Hi all,
A question for hot summer day: Text.ParserCombinators.ReadP.ReadP is
an instance of Monad. Could it be an instance of MonadFix too?
I'm not that sharp in Haskell to write it myself, but it seems I could
make use of such a beast. :) Anybody willing to share?
This will also present the advantage of Lazy over Eager Parser
Combinators, mentioned in some other thread.
--
Gracjan
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