On 02/06/2012 03:32 AM, Gábor Lehel wrote:[snip]
There's a common pattern in Haskell of writing:
data E where E :: C a => a -> E
also written
data E = forall a. C a => E a
I recently uploaded a package to Hackage which uses the new
ConstraintKinds extension to factor this pattern out into an Exists
type parameterized on the constraint, and also for an Existential type
class which can encompass these kind of types:
http://hackage.haskell.org/package/exists
My motivation was mostly to play with my new toys, if it turns out to
be useful for anything that's a happy and unexpected bonus.
Some interesting things I stumbled upon while writing it:
Even better, you can write
- One of the advantages FunctionalDependencies has over TypeFamilies
is that type signatures using them tend to be more readable and
concise than ones which have to write out explicit equality
constraints. For example, foo :: MonadState s m => s -> m () is nicer
than foo :: (MonadState m, State m ~ s) => s -> m (). But with
equality superclass constraints (as of GHC 7.2), it's possible to
translate from TF-form to FD-form (but not the reverse, as far as I
know): class (MonadStateTF m, s ~ State m) => MonadStateFDish s m;
instance (MonadStateTF m, s ~ State m) => MonadStateFDish s m.
type ExistentialWith c e = (Existential e, c ~ ConstraintOf e)
instead of
class (Existential e, c ~ ConstraintOf e) => ExistentialWith c e
instance (Existential e, c ~ ConstraintOf e) => ExistentialWith c e
and drop UndecidableInstances.
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