RE: Combinators for ReadP

On 27 July 2004 15:16, Martin Sjögren wrote:

On Tue, 27 Jul 2004 14:31:21 +0100, Simon Peyton-Jones wrote:

...

You could do this, I believe, but it'd make ReadP less covenient for most users, because they'd have to add these redundant (and inexplicable) 'r' parameters. By "inexplicable" I mean that given a value p::ReadP r a we can say that p is a parser that parses some input, returning a value of type 'a'... but what's this 'r' thing?

Wrapping up a polymorphic fn in a newtype is very standard, even though it is not H98.

I'd be pretty reluctant to make this change

Oh I agree. Having to pass around the extra parameter sucks. Unfortunately, nhc98 doesn't support forall. :)

Incedentally, universal quantification in constructor fields is one of the easier extensions to implement. Much easier than multi-parameter type classes, for example. You don't have to go the whole hog and implement rank-N polymorphism ala GHC in order to get most of the benefit. Cheers, Simon