Brilliant, v interesring
Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage.
On 19 Jun 2015, at 19:37, Phil Ruffwind
class Wibble a where
visit :: (Foo -> b) -> (Bar -> b) -> a -> b
The type `forall b. (X -> b) -> (Y -> b) -> b` is isomorphic to `Either X Y`. If you rearrange the signature of `visit` a little bit,
visit_ :: a -> forall b . (Foo -> b) -> (Bar -> b) -> b
you can see that you're really just defining a function `a -> Either Foo Bar`. So the class can be simplified to:
class Wibble' a where visit' :: a -> Either Foo Bar
It follows that `WibblePackage'` is equivalent to
type WibblePackage' = forall a . (WibbleInstance a, a)
where
type WibbleInstance a = a -> Either Foo Bar
Because of the `forall`, there's nothing you can do with the second component of the tuple (`a`) except to apply it to the first (`a -> Either Foo Bar`), so it's just a long-winded way of saying
type WibblePackage'' = Either Foo Bar
In a non-strict language like Haskell, there's no reason to use such a complicated type signature to delay the application of `a -> Either Foo Bar` to `a`. CONFIDENTIALITY NOTICE
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