[Haskell-cafe] ANN: exists-0.1

Yves Parès yves.pares at gmail.com
Mon Feb 6 00:01:48 CET 2012

That is a great initiative.
I didn't know about those Kind extensions that enable you to pass a
typeclass as a type parameter...

However, have you considered putting the Data.Exists.Default module in a
separate package? That would reduce the dependencies for those who just
need Exists and Existential.

2012/2/5 Gábor Lehel <illissius at gmail.com>

> 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:
> - Did you know you can write useful existentials for Functor,
> Foldable, and Traversable? I sure didn't beforehand.
> - You can even write them for various Comonad classes, though in their
> case I don't think it's good for anything because you have no way to
> run them.
> - Surprisingly to me, the only * kinded class in the standardish
> libraries I found which is useful with existentials is Show, the * ->
> * kinded ones are more numerous.
> - I don't know if anyone's ever set out what the precise requirements
> are for a type class method to be useful with existentials. For
> example, any method which requires two arguments of the same type (the
> type in the class head) is clearly useless, because if you have two
> existentials there's no way to tell whether or not their contents were
> of the same type. I think this holds any time you have more than one
> value of the type among the method's parameters in any kind of way
> (even if it's e.g. a single parameter that's a list). If the
> type-from-the-class-head (is there a word for this?) is used in the
> method's parameters in a position where it's not the outermost type
> constructor of a type (i.e. it's a type argument), that's also no
> good, because there's no way to extract the type from the existential,
> you can only extract the value. On the other hand, in the method's
> return type it's fine if there are multiple values of the
> type-from-the-class-head (or if it's used as a type argument?),
> because (as long as the method also has an argument of the type) the
> type to put into the resulting existentials can be deduced to be the
> same as the one that was in the argument. But if the
> type-from-the-class-head is used *only* in the return type, then it's
> difficult to construct an existential out of the return value because
> the instance to use will be ambiguous.
> - There are a lot of ways you can write existentials, and the library
> only captures a small part of them. Multiparameter constraint? No go.
> More than one constraint? No go (though you can use
> Control.Constraint.Combine). More than one type/value stored? No go.
> Anything which doesn't exactly match the patterns data E where E :: C
> a => a -> E or data E a where E :: C f => f a -> E a? No go. I don't
> think there's any way to capture all of the possibilities in a finite
> amount of code.
> - ConstraintKinds lets you write class aliases as type synonyms, type
> Stringy a = (Show a, Eq a). The old way to do this is class (Show a,
> Eq a) => Stringy a; instance (Show a, Eq a) => Stringy a and requires
> UndecidableInstances. But if the alias has multiple parameters, the
> old way is still superior, because it can be partially applied where
> type synonyms can't. This is analogous to the situation with type
> synonyms versus newtype/data declarations, but interestingly, unlike
> data and newtypes, the class+instance method doesn't require you to do
> any manual wrapping and unwrapping, only the declaration itself is
> different.
> - 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.
> - PolyKinds only seems to be useful as long as there's no value-level
> representation of the polykinded type involved (it's only used as a
> phantom). As soon as you have to write 'a' for kind * and 'f a' for
> kind * -> *, you have to do the duplication manually. Is this right?
> - Writing this library really made me want to have a type-level "Ord
> instance" for constraints, more precisely a type-level is-implied-by
> operator. The typechecker clearly knows that Eq is-implied-by Ord, for
> example, and that Foo is-implied-by (Foo :&: Bar), but I have no way
> to ask it, I can only use (~). I tried implementing this with
> OverlappingInstances, but it seems to be fundamentally impossible
> because you really need a transitive case (instance (c :<=: d, d :<=:
> e) => c :<=: e) but the transitive case can't work. (My best
> understanding is that it's because the typechecker doesn't work
> forward, seeing "ah, c :<=: d and d :<=: e, therefore c :<=: e";
> rather it works backwards, and sees that "c might be :<=: e, if
> there's a suitable d", but then it has no idea what to choose for d
> and goes into a loop.) Filing a feature request is in the plans.
> Er... </ul>.
> Cheers,
> ~g
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