Are there documentation on constraints being types, how they can be
declared/handled and what are the interests?
2012/2/7 Mikhail Vorozhtsov
On 02/06/2012 03:32 AM, Gábor Lehel wrote:
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/existshttp://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:
[snip]
- 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.
Even better, you can write
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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