[Haskell-cafe] Splitting HList

[adam vogt]

Hi Gautier,

> So, you mean that there is no way to fit a (* -> * -> *) or so when a (* ->
> *) is required?

No.If you decide on a kind signature ([k], [k]), you have to use '(,)
not (,) when making types for that kind. Similarly if you have a type
[xs], the kind of that is * which doesn't match the kind [k] you
specify.


> For example, this week I worked on a zipWith ($) version for HList, I have
> this code:
> data HApL :: [*] -> [*] -> * where
>   Nil2  :: HApL '[] '[]
>   (:**) :: (a -> b) -> HApL as bs -> HApL (a ': as) (b ': bs)
>
> infixr 5 :**
>
> zipWithHApL :: HApL xs ys -> HList xs -> HList ys
> zipWithHApL Nil2       HNil      = HNil
> zipWithHApL (x :** xs) (HCons y ys) = x y `HCons` zipWithHApL xs ys
>
> It doesn't satisfy me because:
>   1. I had to create an ad hoc data-type to manage it, instead of [] deal
> with it out-of-the-box
>   2. There no easy way to convert HList to HApL
>
> There is no way to get HList closer than List?

I'm not sure. With the HList library on hackage, you can do the equivalent of

zipWithDollar xs ys = map ($) (zip xs ys)

by defining:

data Dollar = Dollar
instance (fx ~ (x -> y, x)) => ApplyAB Dollar fx y where
  applyAB _ (f,x) = f x

hZipWithDollar xs ys = hMap Dollar (hZip xs ys)

It works like:

>>> hZipWithDollar (hBuild (+1) tail) (hBuild 3 "ab")
H[4, "b"]

But in more complicated cases there is a tendency to need
-XAllowAmbiguousTypes, which means writing type signatures (with
-XScopedTypeVariables) that should be unnecessary.

Regards,
Adam