On Thursday 02 September 2010 23:10:29, Alec Benzer wrote:
Ah, ok, so the reason what I trying didn't work is because I used an actual type instead of a type variable?
Basically yes. There's a small additional problem because String is a type synonym (and type synonyms are forbidden in H98 instance declarations). Apparently, FlexibleInstances allows them in type variable positions, but if you want to put a type synonym in the type constructor position, you need TypeSynonymInstances. So for import Control.Monad.State type STI = StateT Int instance Foo (STI [] a) where you need FlexibleInstances ([] is not a type variable) and TypeSynonymInstances.
I got confused because of the emphasis you put on * distinct *.
Sorry for that. I wanted to prevent "Why can't I have instance Foo (Bar a a) where ... ?".
And so, if I want to make Maps applicative functors without dealing with FlexibleInstances, I'd have to do something like this?
import Control.Applicative import qualified Data.Map as M import Data.Monoid
instance (Monoid k, Ord k) => Applicative (M.Map k) where pure x = M.fromList [(mempty,x)] fs <*> xs = M.fromList [(k1 `mappend` k2,v1 v2) | (k1,v1) <- M.assocs fs, (k2,v2) <- M.assocs xs]
(sacrificing some functionality, since spaces won't get intercalated between keys if i use strings)
Yes, but why avoid FlexibleInstances?