[Haskell-cafe] Types of partially instantiated polymorphic helper functions

[Henning Thielemann]

On Tue, 24 Nov 2020, Viktor Dukhovni wrote:

> On Tue, Nov 24, 2020 at 02:58:33PM -0800, Todd Wilson wrote:
>
>> I see, thanks! And I guess that if the type variable c in your type of g
>> were changed to b, it will still work, but now g would have a monomorphic
>> instead of polymorphic type.
>
> Yes, exactly.  I was not going to post my otherwise redundant
> answer, but since it in fact makes `g` monomorphic, here it is:
>
> The simplest thing (which you probably already know) is to not give `g`                                                                                                                      a type signature, and let GHC infer the type.  But since you asked,                                                                                                                          one solution is:
>
>    {-# LANGUAGE ScopedTypeVariables #-}
>
>    f :: forall a b. a -> [b] -> [(a, b)]
>    f a bs = g bs where
>      g :: [b] -> [(a, b)]
>      g [] = []
>      g (x:xs) = (a, x) : g xs

An alternative would be to let g be polymorphic but add 'a' as parameter:

   f a bs = g a bs where
      g :: a -> [b] -> [(a, b)]
      g = map ((,) a)