[Haskell-cafe] Splitting HList

[Gautier DI FOLCO]

Hi adam,

Thanks for your answer.

2014-06-13 21:44 GMT+02:00 adam vogt <vogt.adam at gmail.com>:

> 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.
>

ok, I was confused, thanks.


>  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.
>

ok, that's clear, thanks.

Regards.
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