[Haskell-cafe] ANN: exists-0.1

[Yves Parès]

Are there documentation on constraints being types, how they can be
declared/handled and what are the interests?

2012/2/7 Mikhail Vorozhtsov <mikhail.vorozhtsov at gmail.com>

> 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/exists<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:
>>
>>  [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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