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

[Tom Ellis]

On Tue, Nov 24, 2020 at 02:26:03PM -0800, Todd Wilson wrote:
> This has got to be a trivial question, but I don't see a solution, nor
> could I find anything through searching: If I want to write a type
> declaration for the helper function g here, what do I write?
> 
> f :: a -> [b] -> [(a,b)]
> f a bs = g bs where
>   g :: ????
>   g [] = []
>   g (x:xs) = (a,x) : g xs

You probably want this:


{-# 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