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typing problem with sexplib and mutually recursive polymorphic types
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Date: -- (:)
From: Damien Doligez <damien.doligez@i...>
Subject: Re: [Caml-list] typing problem with sexplib and mutually recursive polymorphic types

On 2009-03-11, at 15:44, Markus Mottl wrote:

> That's true, but unlike Haskell OCaml doesn't have mandatory types.
> This means the user can't force the compiler to start out with
> user-provided type declarations.  The OCaml compiler will always run
> type inference first and only try to unify the result with the
> user-provided type declaration, i.e. when it's too late.

That is not quite true any more.  For example, I changed the
type-checker a few years ago to start with the user-provided type
when typing a let rec, in order to be able to debug my large
recursive definitions.  Note that I didn't do that from scrach,
I used an infrastructure that was already present for seeding the
type inference in some cases.  IIRC, it is there for some object-
oriented reason.

Next time you have a type error on the wrong recursive call, try
annotating the function at its definition point.

For example, compare the error messages for:

   let rec f x = g x []
   and g x l =
     match l with
     | [] -> f "a"
     | [a] -> f 1
     | [a; b] -> f 2
     | _ -> f 3
   ;;

versus:

   let rec f (x : int) = g x []
   and g x l =
     match l with
     | [] -> f "a"
     | [a] -> f 1
     | [a; b] -> f 2
     | _ -> f 3
   ;;

Note that your second sentence is still right, because type annotations
are only used in this way in a limited number of cases (let rec is one
example).

-- Damien