[Haskell-cafe] data kinds and exhaustiveness of typeclass instances

Richard Eisenberg rae at richarde.dev
Tue Mar 2 15:00:55 UTC 2021

> Is there a way to somehow convince GHC of that fact so that g typechecks?


First, it would actually be unsound to do so, because of the possibility of exotic types, built with pathological combinations of type and data families: https://gitlab.haskell.org/ghc/ghc/-/issues/14420 <https://gitlab.haskell.org/ghc/ghc/-/issues/14420>

But, actually, the bigger problem is that we need a class constraint in order to allow a function to compute at runtime. The function f actually takes two arguments at runtime: a representation of the instance which carries f's implementation (this is sometimes called a dictionary), and the normal argument of type a. `g`, on the other hand, has no access to the dictionary needed at runtime, and so it's unclear how it should compute.

Put another way: a value of type Foo carries no information (beyond the fact that it terminates), because Foo has only one data constructor. So there's no way that g :: Foo a -> Int could be anything but a constant function. You need the class constraint to change this fact.

Hope this helps!

> On Mar 2, 2021, at 9:16 AM, Paul Brauner <polux2001 at gmail.com> wrote:
> Hello,
> the following program doesn't typecheck in GHC 9:
> data Tag = A | B
> data Foo (a :: Tag) = Foo
> class C a where
>   f :: a -> Int
> instance C (Foo A) where
>   f x = 1
> instance C (Foo B) where
>   f x = 2
> g :: Foo a -> Int
> g = f
> Yet one could argue that for all a :: Tag, C (Foo a) holds because a :: Tag can only take two values: A or B, and C (Foo A) and C (Foo B) hold. Is there a way to somehow convince GHC of that fact so that g typechecks?
> Cheers,
> Paul
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