> In Haskell or ML or whatever `emptyList` would be given a polymorphic type, `List a` or `'a list`.
That alone would not work. Think about it: `List a` means "A list that contains values of type `a` and `a` can be any type whatsoever". Now imagine you combine that list with a list of integers. That obviously cannot work, since `(++) :: [a] -> [a] -> [a]` as you see, the types must align.
To me that feels like hacky way to exactly resolve the problem that I described, and if you turn it off then that code would stop working and fail to compile as expected.
You are misreading the quantification, a value l of type List a means that for all type a, the element of the list has type a. In other words, this is an universal quantification whereas your interpretation is an existential quantification.
This is obviously only possible if the list itself has no elements, and indeed a simple proof is that the statement above is valid for the empty type: all elements of a list of type List a have type empty (among other types). Thus there are no elements in this list.
And both Haskell or OCaml can prove it:
type empty = | (* this is defining a never type *)
type polymorphic_list = { l: 'a. 'a list }
(* OCaml require to explicit construct polymorphic type *)
let polymorphic_lists_are_empty ({l} : polymorphic_list ) =
match (l:empty list) with
| [] -> () (* this is the empty list *)
| _ -> .
(* this clause requires to the OCaml typechecker to prove that the remaining cases are unreachable *)
That alone would not work. Think about it: `List a` means "A list that contains values of type `a` and `a` can be any type whatsoever". Now imagine you combine that list with a list of integers. That obviously cannot work, since `(++) :: [a] -> [a] -> [a]` as you see, the types must align.
The way Haskell fixes that is (apparently) by doing something called `Let-generalisation` (https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/let_...)
To me that feels like hacky way to exactly resolve the problem that I described, and if you turn it off then that code would stop working and fail to compile as expected.