From: Goswin von Brederlow <goswin-v-b@web.de>
To: guillaume.yziquel@citycable.ch
Cc: Andreas Rossberg <rossberg@mpi-sws.org>, OCaml List <caml-list@inria.fr>
Subject: Re: [Caml-list] Recursive subtyping issue
Date: Sat, 27 Feb 2010 12:49:49 +0100 [thread overview]
Message-ID: <87k4tyoq3m.fsf@frosties.localdomain> (raw)
In-Reply-To: <4B88F32C.3050701@citycable.ch> (Guillaume Yziquel's message of "Sat, 27 Feb 2010 11:25:48 +0100")
Guillaume Yziquel <guillaume.yziquel@citycable.ch> writes:
> Andreas Rossberg a écrit :
>> On Feb 27, 2010, at 02:52, Guillaume Yziquel wrote:
>>>
>>> I've been struggling to have a type system where I could do the
>>> following subtyping:
>>>
>>> 'a t1 :> t2 and t2 :> 'a t1
>>
>> Hm, what do you mean? Subtyping ought to be reflexive and
>> antisymmetric (although the latter is rarely proved for most type
>> systems), which means that your two inequations will hold if and
>> only if 'a t1 = t2, regardless of recursive types.
>>
>> /Andreas
...
> Not exactly, it seems.
>
> My goal is to implement a type inference barrier.
>
> You can do
>
>> type 'a q = private w
>
> and from the type inference point of view, int q and float q are two
> distinct types, that you can subtype to a common type.
>
> What I want is also to have the reverse, i.e.
>
>> type w = private 'a q
>
> But that doesn't work out this way because of the fact that 'a is unbound.
But then int q :> w :> float q and float q :> w :> int q. That would
make the whole thing somewhat pointless. Everyone could convert the type
to anything. I guess it would protect from accidentally passing the
wrong 'a q while allowing purposefully to pass any 'a q.
Why not supply conversion functions that do any additional checks to
ensure the conversion is a valid one? Consider the following:
module M : sig
type w = Int of int | Float of float
type 'a q = private w
val add : 'a q -> 'a q -> 'a q
val print : w -> unit
val as_int : w -> int q
val as_float : w -> float q
end = struct
type w = Int of int | Float of float
type 'a q = w
let add x y = match (x, y) with
(Int x, Int y) -> Int (x + y)
| (Float x, Float y) -> Float (x +. y)
| _ -> assert false(* typesystem failed us *)
let print = function
Int x -> print_int x
| Float x -> print_float x
let as_int x = match x with
Int _ -> x
| Float _ -> assert false
let as_float x = match x with
Int _ -> assert false
| Float _ -> x
end
# let i = M.as_int (M.Int 1);;
val i : int M.q = M.Int 1
# let f = M.as_float (M.Float 1.);;
val f : float M.q = M.Float 1.
# M.add i i;;
- : int M.q = M.Int 2
# M.add i f;;
Error: This expression has type float M.q
but an expression was expected of type int M.q
# M.print (i :> M.w);;
1- : unit = ()
MfG
Goswin
next prev parent reply other threads:[~2010-02-27 11:49 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-02-27 1:52 Guillaume Yziquel
2010-02-27 6:38 ` [Caml-list] " Andreas Rossberg
2010-02-27 10:25 ` Guillaume Yziquel
2010-02-27 11:49 ` Goswin von Brederlow [this message]
2010-02-27 13:11 ` Guillaume Yziquel
2010-02-27 16:52 ` Andreas Rossberg
2010-02-27 18:10 ` Guillaume Yziquel
2010-02-27 19:52 ` Guillaume Yziquel
2010-02-27 20:32 ` Guillaume Yziquel
2010-03-01 10:55 ` Stéphane Glondu
2010-03-01 11:21 ` Guillaume Yziquel
2010-03-01 12:28 ` Stéphane Glondu
2010-03-01 12:49 ` David Allsopp
2010-03-01 13:06 ` Guillaume Yziquel
2010-03-01 12:49 ` David Allsopp
2010-03-01 13:28 ` Goswin von Brederlow
2010-03-01 20:12 ` David Allsopp
2010-03-02 10:22 ` Goswin von Brederlow
2010-03-01 13:33 ` Guillaume Yziquel
2010-03-01 20:18 ` David Allsopp
2010-02-28 9:54 ` Goswin von Brederlow
2010-02-28 11:08 ` Guillaume Yziquel
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