[Haskell-cafe] Existential question

oleg at okmij.org oleg at okmij.org
Thu Aug 18 07:27:43 CEST 2011

> -- Is there a way to make this one work also?
> data Kl i o = forall s. Kl (i -> s -> (s, o))
> iso :: (i -> Kl () o) -> Kl i o
> iso f = Kl $ \i s -> (\(Kl kl) -> kl () s) (f i)

Yes, if you move the quantifier:

type Kl i o = i -> Kl1 o
data Kl1 o = forall s. Kl1 (s -> (s,o))
iso :: (i -> Kl () o) -> Kl i o
iso f = \i -> f i ()

iso1 :: Kl i o -> (i -> Kl () o)
iso1 f = \i -> (\() -> f i)

I'm not sure if it helps in the long run: the original Kl and mine Kl1
are useless. Suppose we have the value kl1 :: Kl Int Bool 
with the original declaration of Kl:

data Kl i o = forall s. Kl (i -> s -> (s, o))

Now, what we can do with kl1? We can feed it an integer, say 1, and
obtain function f of the type s -> (s,Bool) for an _unknown_ type s.
Informally, that type is different from any concrete type. We can
never find the Bool result produced by that function since we can
never have any concrete value s. The only applications of f that will
type check are
	\s -> f s
	f undefined
both of which are useless to obtain f's result.

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