[MLton] Re: [MLton-commit] r6352

[Matthew Fluet]

On Thu, 24 Jan 2008, Matthew Fluet wrote:
> On Thu, 24 Jan 2008, Vesa Karvonen wrote:
>>  I've
>>  been thinking about redefining all array types in my extended basis
>>  library to include a generated hash value with the array.
>>  Unfortunately, refs
>>  can't be redefined in a library, so a language/basis library solution
>>  would be better.
>
> The only aspects of refs that can't be redefined is the ref constructor for 
> use in patterns.  It is a fairly little used feature (and, arguably, a 
> misfeature), so I suspect few clients would notice the difference.

Oops, I also forgot that the language disallows you to redefine the 'ref' 
variable.

Nonetheless, I should point out that if one were to do:

structure MyRef :>
   sig
     eqtype 'a t
     val mref : 'a -> 'a t
     val ! : 'a t -> 'a
     val := : 'a t * 'a -> unit
   end =
   struct
     datatype 'a t = T of 'a ref * Word32.word

     val w : Word32.word ref = ref 0wx0

     fun mref v = T (ref v, !w) before w := !w + 0wx1
     fun ! (T (r, _)) = General.! r
     fun (T (r, _)) := v = General.:= (r, v)
   end

then a structural hash (and also a structural equality) on an 'a MyRef.t 
value would "do the right thing", because it would hash an integer along 
with the constant hash of ref type.  It is o.k. if the counter wraps, 
because we're only using it for a hash.  Equality will still check the 
references.