On Wed, Feb 16, 2011 at 7:01 AM, Ross Paterson <r...@soi.city.ac.uk> wrote: > On Tue, Feb 15, 2011 at 07:46:29PM -0800, Evan Laforge wrote: >> Do I really have to add (Functor m) to the 300 or so functions with >> (Monad m) on them? Or just not use fmap or applicative? > > If you're using Monad m to get Functor or Applicative instances for a > functor built from m, then I'm afraid you will need to add (Functor m) > or (Applicative m) to the constraints in most cases.
I've noticed another solution, which is to make a class M with Applicative and Monad as superclasses, and put the lifted basic operations into that, then instead of '(Functor m, Monad m) => MyM m a', write '(M m) => m a'. I already do this same thing in a different place so I can get unlifted access from another transformer stack. It makes nicer looking signatures, but it's more complicated and less direct. Whenever it comes time to play games like that I have to do a fair amount of trial and error because I don't understand typeclasses or the type errors produced to get it right the first time. And I suppose if I can barely write it, I shouldn't, but I know other people understand this stuff a lot better than I do. Are other people using tricks like this? Or will there just be massive signature rewriting in the wake of mtl2? Or maybe others don't use monads so extensively? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
