[MLton] Monadic MLton.Vector.create

[Daniel C. Wang]

Oh, never mind. I just figured out a counter example. :( You can use a 
closure to encapsulate the key

val upd = ref (fn(x:int,y:int) => ())
    fun evil {key,sub,update} = let
      fun f(x,y) = update(key,x,y)
    in
      upd := f
    end

now if we could only bad higher-order functions we'd be set :)


Daniel C. Wang wrote:
> Just a thought, but can you use the value restriction to your 
> advantage here? I mean you can't store a polymorphic value into a ref 
> cell. Can sub and update be given types too polymorphic to stash away?
>
> |val create: int * 'a * ({key: 'b,
>                         sub: 'b * int -> 'a,
>                      update: 'b * int * 'a -> unit} -> unit) -> 'a 
> vector |
>
> Unless, I'm missing something there's no way for sub,update, or key to 
> escape the scope of  create. I think a similar argument has been made 
> for certain monads in Haskell, but my memory is flakey.
>
> Vesa Karvonen wrote:
>> Quoting Henry Cejtin <henry.cejtin@sbcglobal.net>:
>> [...]
>>  
>>> The only ugliness to me in the create proposal is the fact  that  
>>> update  and
>>> sub might be squirreled away, so you have to make them raise Fail if 
>>> they are
>>> called after create returns.  It would be nice if one could make it  
>>> so  that
>>> they  couldn't  be  in scope or saved any where, but I don't see any 
>>> way that
>>> isn't REALLY convoluted and ugly.
>>>     
>> [...]
>>
>> Were you thinking about monads?  Here is a suitable signature for 
>> create:
>>
>> signature CREATE =
>>    sig
>>       type 'a m
>>
>>       val create : int -> (int -> 'a m) -> 'a vector
>>
>>       val >>= : 'a m * ('a -> 'b m) -> 'b m
>>       val return : 'a -> 'a m
>>       val sub  : int -> 'a m
>>       val update : int * 'b -> unit m
>>    end
>>
>> The below functor implements the same fib function as in Stephen's
>> earlier post.
>>
>> functor Fib (C : CREATE) =
>>    struct
>>       local
>>          open C
>>          infix >>=
>>       in
>>       fun fib i =
>>           create i (fn i =>
>>                        if i <= 1 then
>>                           return i
>>                        else
>>                           sub (i-1) >>= (fn x =>
>>                           sub (i-2) >>= (fn y =>
>>                           return (x+y))))
>>       end
>>    end
>>
>> I wouldn't call it "ugly", but YMMV.
>>
>> -Vesa Karvonen
>>
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>
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