On Mon, 26 Jul 2004 11:00:31 +0100, Malcolm Wallace
While we're discussing changes, is there an overwhelmingly good reason for the use of local quantification beyond making the type signatures simpler? Writing newtype ReadP r a = R ((a -> P r) -> P r) (like in ContT) would make ReadP completely Haskell98 as far as I can tell.
I think you'll find that without the local universal quantifier, you cannot make ReadP a (useful) instance of Monad, because your 'r' parameter would be fixed across >>=, whereas it really needs to be variable.
Uhh. But it works for ContT, so why not for ReadP? The only point time you have to fix the r is when you want to "run" the parser, all other parsers will be polymorphic in r: parseFoo :: ReadP r Foo parseBar :: ReadP r Bar readP_to_S :: ReadP a a -> ReadS a I do exactly this in the package description parsing code of Cabal (using a locally hacked ReadP, Cabal really must be H98...), and it works just fine. /Martin