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Date: Fri, 03 Jun 2011 10:47:33 +0200
From: Dawid Toton <d0@wp.pl>
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Subject: [Caml-list] Forget a type without Obj.magic

I have the following situation:

______
type 'a consumer = 'a -> unit

type 'a inactive = ('a consumer -> unit) list
type active

val activate : 'a consumer -> 'a inactive -> active

type bottom = unit

val forget_type : 'a inactive -> bottom inactive
______
let activate consumer inactive =
   let act inact = inact consumer in
   List.map act inactive

let bottom = ()

let forget_type inactive =
   let forget inact =
     (fun simple_consumer ->
        inact (fun _ -> simple_consumer bottom)
     )
    in
   (List.map forget inactive
   : unit inactive
   )
______

I see that the function forget_type above is a waste of time: it 
repackages functions that take () to functions that ignore the argument. 
But the simple_consumer already ignores the () argument, since there is 
no much use of it anyway.

My question is: how to change the above example, so that the forget_type 
becomes just a typecast (or maybe a constant time operation)?

I think that the type and value called bottom in the above code are 
currently not what I really want. Maybe even it should be called "top" 
instead? The argument of simple_consumer should have a type of "values 
impossible to use". How to express this?

Dawid