[Haskell-cafe] Do people use visitor pattern in Haskell...

Nicholls, Mark nicholls.mark at vimn.com
Mon Jun 22 07:28:32 UTC 2015

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 <rf at rufflewind.com> wrote:

>>    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`.

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