This ticket about "Quantified contexts" (forall a. ctx a => Show a) seems to be what you're looking for. https://ghc.haskell.org/trac/ghc/ticket/2893 On 06/07/2017 03:38 AM, Alexis King wrote:
Using ExistentialQuantification, it’s possible to create a rather generic type that packages some arbitrary dictionary up at runtime:
data Some ctx = forall a. ctx a => Some a
This is nice, but it’s not very interesting. It would be nice if we could define a Show instance for our type, and a naïve implementation is trivial:
instance Show (Some Show) where show (Some x) = show x
However, this isn’t very useful in practice, since it means we can only show values of precisely the type (Some Show), which are better represented as values of type String. However, we might reasonably have some class with Show as a superclass:
class Show a => Foo a instance Foo ()
...and we might want to show values of type (Some Foo). However, this won’t work with the naïve implementation, since Foo is not the same as Show.
Clearly, what we want is some instance like this:
instance (ctx ≈> Show) => Show (Some ctx)
...where (a ≈> b) can be read “a implies b”. Unfortunately, as far as I can tell, this isn’t actually possible in any general way.
The closest thing I was able to come up with was a less general solution using Edward Kmett’s constraints package. It’s possible to write (≈>) with some manual boilerplate:
import Data.Constraint import Data.Constraint.Forall
class Class (ctx a) (ctx' a) => Class1' ctx ctx' a instance Class (ctx a) (ctx' a) => Class1' ctx ctx' a
type Class1 ctx ctx' = Forall (Class1' ctx ctx') type a ≈> b = Class1 b a
...and then it’s possible to write that Show instance:
instance (ctx ≈> Show) => Show (Some ctx) where show (Some (x :: a)) = case inst @(Class1' Show ctx) @a of Sub Dict -> case (cls :: ctx a :- Show a) of Sub Dict -> show x
However, this means users have to manually implement Class instances for their classes. Using the above example of Foo, a user would have to write the following instance:
instance Class (Show a) (Foo a) where cls = Sub Dict
With that instance, we can properly show values of type (Some Foo):
ghci> Some () :: Some Foo ()
But this instance is both trivial and annoying, since it means the machinery leaks out of the implementation code and into the code that uses it. I’m wondering if there’s any way to do this more generally, and if not, if there’s any reason it could not or should not be supported by GHC itself.
Alexis
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