[Camllist] const equivalent for mutable types?
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Date:  20040823 (09:53) 
From:  Jon Harrop <jon@j...> 
Subject:  Re: [Camllist] Phantom types 
On Saturday 31 July 2004 17:31, Markus Mottl wrote: > module type PHANTOM_INT = sig > type 'p t > ... Right, I've had a bit more of a chance to play with phantom types now, and I'm quite confused. :) As far as I can tell, there were a few errors in Markus' original (I may well be wrong, of course), so here is my altered version: module PhantomInt : sig type 'p t val add : 'p t > 'p t > 'p t val add_even_even : [ `Even ] t > [ `Even ] t > [ `Even ] t val add_even_odd : [ `Even ] t > [ `Odd ] t > [ `Odd ] t val add_odd_even : [ `Odd ] t > [ `Even ] t > [ `Odd ] t val add_odd_odd : [ `Odd ] t > [ `Odd ] t > [ `Even ] t val neg : 'a t > 'a t val make_even : int > [ `Even ] t val make_odd : int > [ `Odd ] t end = struct type 'p t = int let add = (+) let add_even_even = add let add_even_odd = add let add_odd_even = add let add_odd_odd = add let neg n = n let make_even n = if n mod 2 = 0 then n else failwith "not even" let make_odd n = if n mod 2 <> 0 then n else failwith "not odd" end;; So I've changed the types to be [ `Even ] instead of [> `Even ] and the "make" functions to be "int > ...". This appear to work as desired: # let i = PhantomInt.make_even 2;; val i : [ `Even ] PhantomInt.t = <abstr> # let j = PhantomInt.make_odd 3;; val j : [ `Odd ] PhantomInt.t = <abstr> # PhantomInt.add_even_odd i j;;  : [ `Odd ] PhantomInt.t = <abstr> # PhantomInt.add_even_even i j;; This expression has type [ `Odd ] PhantomInt.t but is here used with type [ `Even ] PhantomInt.t These two variant types have no intersection Now, there are some subtle peculiarities of these which I don't understand. Firstly, the type checking of the phantom types only seems to work if the type is made abstract in the module signature. I can't think why this should make a difference though. For example, changing "type 'p t" to "type 'p t = int" in "PhantomInt : sig" then allows: # PhantomInt.add_even_even i j;;  : [ `Even ] PhantomInt.t = 5 which is clearly undesirable. Secondly, specifying the types as Markus did (e.g. [> `Even]), which I think should have been correct, leads to some kind of monomorphic type: # PhantomInt.add_even_odd i j;;  : _[> `Odd ] PhantomInt.t = <abstr> Note the "_" preceding the "[> `Odd]". I'm not sure what the implications of this are. Can someone explain these to me, please? Cheers, Jon. PS: I'm using 3.08.0, in case that makes a difference.  To unsubscribe, mail camllistrequest@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/camlbugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners