I know generic types, but sometimes I see let a (type v w) etc. in code, what's the difference?

CodePudding user response：

They are locally abstract types. See https://ocaml.org/manual/locallyabstract.html. They are useful for:

• creating local modules

let sort_and_deduplicate (type a) compare l =
  let module S = Set.Make(struct type t = a let compare = compare end) in
  S.elements (S.of_list l)

• refining types on pattern matching in presence of GADT

type _ monoid = Int: int monoid | Float: float monoid
let zero (type a) (m:a monoid): a =  match m with
| Int -> 0
| Float -> 0.

• debugging polymorphic functions

let wrong_id (type a) (x:a) = x   1

Error: This expression has type a but an expression was expected of type int