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[Caml-list] Odd Type Checking Problem
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 Date: 2002-02-07 (11:21) From: Alain Frisch Subject: [Caml-list] Type variables (was: Odd Type Checking Problem)
```Hello,

On Wed, 6 Feb 2002, I wrote:

> I guess this is a problem of understanding type variable scoping rules.

Actually, I feel myself somewhat confused with implicit introduction and
scoping of type variables.

These one are refused:

let f (x : 'a) = let module M = struct exception X of 'a end in ();;
let f (x : 'a) = let module M = struct type t = 'a end in ();;

This is accepted:

let f (x : 'a) =
let module M =
struct
type t constraint t = 'a;;
exception X of t;;
end in ();;

but is quite useless, since both:

let f (x : 'a) =
let module M =
struct
type t constraint t = 'a;;
exception X of t;;
raise (X x);;
end in ();;

and

let f (x : 'a) =
let module M =
struct
type t constraint t = 'a;;
exception X of t;;
end in raise (M.X x);;

are rejected by the type checker.

(in "constraint ...", no new variable is introduced)

Another example:

let f (x : 'a) =
let module M =
struct
type u = { a : 'a }
end in ();;

=> rejected (Unbound type parameter 'a)

(and accepted by
"Objective Caml version 3.04+1 with explicit polymorphism (2002-01-07)",
but with a different meaning that one could expect)

Is there a way to use a type variable such as the 'a above to define
types in a local structure ?

-- Alain

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```