I wonder, what were the sales numbers like for the earlier iteration of the Haskell IDE? Seems like a very niche market

Yes. I would also consider every generic value a function that takes a type argument. For example
nothing :: forall a. Maybe a

Uhh

Uhh

An intuitive way of seeing this that also generalizes to arbitrary categories not just Set/Types is -

Currying means that for any arbitrary object A, functions of the form -
(A,()) -> X
are the same as functions of the form -
A -> (() -> X)

1/2

Oh and X->() is actually isomorphic to (). Because by the definition of the terminal object, there can only be one unique (X->()) for any X, which makes it isomorphic to () (another set with only one member)

But also, we know that (A,()) is the same as A, because () is the identity for products.

Combining we can see that functions of the form -
A->X are the same as A->(()->X)

This means (by yoneda lemma) that X is the same as ()->X

QED

2/2

An intuitive way of seeing this that also generalizes to arbitrary categories not just Set/Types is -

Currying means that for any arbitrary object A, functions of the form -
(A,()) -> X
are the same as functions of the form -
A -> (() -> X)

1/2