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

[Todd Wilson]

Thanks for the additional responses. For the record, in response to

On Tue, Nov 24, 2020 at 3:51 PM Henning Thielemann
<lemming at henning-thielemann.de> wrote:
> 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)

this is what I had been doing before -- adding variables that were in
scope as additional arguments to recursive helper functions, even
though they don't change during recursive calls -- just so that I
could give type annotations to helper functions for the purposes of
documentation, since I didn't see any other way. (Of course, my helper
function here is a simple application of `map`, but I was choosing
this minimal example to illustrate something that came up repeatedly
in more complicated situations.) The use of `ScopedTypeVariables`,
though, is a better solution for me, since it allows me to continue my
annotation practice without having to change the code I would
ordinarily write. FWIW, my audience is college students using Haskell
to supplement non-PL classes, and the annotations, perhaps more than
usual, help them understand the code. Since I've already discussed
polymorphism and type variables by that point, the explicit `forall`s
(and language pragmas) are a satisfyingly minimal and easily
explainable intrusion.

Todd Wilson