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[Caml-list] inference and phantom types
• Julien Cohen
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 Date: 2002-10-21 (15:23) From: Julien Cohen Subject: [Caml-list] inference and phantom types
```I read in the thread
http://caml.inria.fr/archives/200109/msg00097.html
that phantom types could be used in ocaml but I have some
troubles understanding the way they really work.

In the following session I use a phantom type ('a,'b)t to
simulate a list with an additionnal type 'b. I create a special
cons preserving the type. I create an (int,bool)t value and when
I make a cons on it and the phantom type seems not to be well
infered:

Objective Caml version 3.04

# type ('a,'b) t = 'a list;;
type ('a, 'b) t = 'a list

# let f (x:'a) (y:('a,'b) t) = (x::y : ('a,'b) t);;
val f : 'a -> ('a, 'b) t -> ('a, 'b) t = <fun>

# let (v: (int,bool) t) = [1];;
val v : (int, bool) t = [1]

# f 1 v;;
- : (int, '_a) t = [1; 1]

Is there a fundamental reason for the bool type not to be inferred?
(no response in the ocaml-beginner list)

Thanks
Julien Cohen

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```