> From: Jan-Willem Maessen <jmaes...@alum.mit.edu> > Sent: Wed, March 23, 2011 8:43:14 PM > > Hi all - > > I've been trying to construct a class declaration with an associated > type synonym, but I'd like to constrain that type to belong to a > particular class. > > Consider the following class: > > class Monoid m => Constructs c m | c -> m where > construct :: m -> c > > This captures the idea that the collection c ought to be constructed > using the monoid m (say if we're doing the construction using the > Writer monad)--the functional dependency indicates the desire for the > type c to injectively determine the choice of monoid m. For example: > > newtype ListBuilder a = Builder ([a] -> [a]) deriving (Monoid) > > instance Constructs [a] (ListBuilder a) where > construct (Builder f) = f [] > > instance (Ord a) => Constructs (Set a) (Set a) where > construct = id > > Now I'd like to be able to do the same thing using an associated type > synonym, something like this: > > type GeneratorOf a :: * -> * > construct :: GeneratorOf a -> a > > Now, it seems I need FlexibleInstances to do this when I'm using an > associated type synonym, but I don't need the flexibility when using a > multiparameter type class.
The conditions in the report are quite restrictive - in particular, the context must consist only of classes applied to type variables. When you used a multiparameter type class and an FD, the type you wanted to mention was just a type variable. http://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-750004.3 The GHC user's guide suggests FlexibleContexts should be enough to allow you to declare that class: http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/type-class-extensions.html FlexibleInstances seems only to affect what is allowed in an instance head. I don't see how it helps at all, unless it implies some other extensions. You might still run into termination issues - as a an associated type synonym rather than an associated data type, GeneratorOf a might very well be something large, and the conditions (disabled by UndecidableInstances) don't take advantage of the acyclic superclass relationship. http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/type-class-extensions.html#instance-rules > In both cases the instance constraint > involves types that can be injectively inferred (if I have my > terminology straight; work with me here) from a single type mentioned > in the class head. In particular, I can imagine storing the > dictionary for Monoid (GeneratorOf a) in the dictionary for Generable > a, and thus allowing context reduction of (Monoid (GeneratorOf tyvar)) > to (Generable tyvar). Meanwhile, I think there are things that are > permitted by FlexibleInstances that I'd rather *not* have creeping > into my programs. Do you have any examples? I've always found FlexibleInstances alone unproblematic - it's only when you add OverlappingInstances or worse that things can get messy. Brandon _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
