[Haskell-beginners] question on typeclasses and applicatives

Daniel Fischer daniel.is.fischer at web.de
Thu Sep 2 17:50:39 EDT 2010

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 

> 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?

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