[Haskell-cafe] Re: Go Haskell! -> array libraries

[Dan Doel]

On Sunday 30 November 2008 6:28:29 am Roman Leshchinskiy wrote:
> On 30/11/2008, at 11:36, Don Stewart wrote:
> > Should mutable arrays have list-like APIs? All the usual operations,
> > just in-place and destructive where appropriate?
>
> I don't know. To be honest, I don't think that the term "mutable
> array" describes a single data structure. For instance, one of the
> central questions which unveils a whole bunch of design possibilities
> is: can mutable arrays be concatenated and how does that work if yes?

I don't know about concatenation, but it is useful for them to be able to be 
zipped. For instance:

  schwartz f arr = do keys <- cloneWith f arr
                      pairs <- zip keys arr
                      sortBy (comparing fst) pairs

(cloneWith should construct a new array where new[i] = f (old[i])) is a 
mutable sort of arr using the comparison

  compare e e' = compare (f e) (f e')

caching the results of f for each element. This, obviously, takes advantage 
of the fact that sorting the zipped array mutably updates the underlying 
(original) arrays.

On the other hand, I could see such an operation easily leading to bugs if it 
slips your mind that changing one array can effect another (maybe it should 
be called unsafeZip :)).

-- Dan