On Feb 5, 2008 4:58 PM, Chaddaï Fouché <chaddai.fouche@gmail.com> wrote:

2008/2/5, Jeff φ <jeff1.61803@gmail.com>:

> This is interesting. I've been programming in Concurrent Clean for a while.
> Instead of monads, Clean supports unique types for mutable arrays and IO.
> In Clean, I can write code that iterates through a mutable array by
> converting it to a lazy list. This is convenient because I can use all the
> nice list processing functions that are available.
>

You could absolutely write something like that in Haskell, but as some
have pointed out, this is probably _not a Good Idea_, even though it
works in your particular case, the array could be modified between the
time you get the lazy list and the time you actually read it... And
there's no way to insure it's not happening in Haskell, and I strongly
doubt there is in Concurrent Clean, could you confirm ?

Concurrent Clean can handle this in a safe way. Here's a code snippet for normalize_2D_ary from ArrayTest.icl:

uToList_2D :: *(a u:(b c)) -> (.[c],*(a u:(b c))) | Array a (b c) & Array b c
map_in_place_2d_arr :: (a -> a) *(b *(c a)) -> *(b *(c a)) | Array b (c a) & Array c a

normalize_2D_ary :: *(a *(b c)) -> *(a *(b c)) | Array a (b c) & Array b c & / c & Ord c
normalize_2D_ary ary =
    let (lst,ary2) = uToList_2D ary
        max_elem = foldl1 max lst
    in map_in_place_2d_arr (\ x -> x / max_elem) ary2

uToList_2D takes a unique array, ary, and returns a tuple containing a list of the array elements and a "new" array, ary2. uToList_2D does not modify ary, but Clean's type system forces any function that accesses a unique array to thread the array through and return a "new" array. Under the hood the "new" array actually uses the same memory storage as the original array. So, it is effecient. Threading the array serializes access insuring the array won't be modified until the list is complete. The type system will generate an error if I wrote code that breaks referential transparency.

Array and IO Monads can be implemented in Clean on top of the uniqueness type system. The do-notation could be added to the language.

However, the comments I've read so far lead me to believe the code I've shown cannot be implemented in Haskell using a lazy list without resorting to unsafe functions. I'm beginning to suspect the uniqueness type system of Clean is more general and flexible than Haskell's monads.