[Haskell-cafe] fishing for ST mutable Vector examples

[Daniel Fischer]

On Friday 22 April 2011 20:14:38, Stephen Tetley wrote:
> Hi Brad
> 
> I think all you can do with an ST array is covered by the MArray class
> and its derived operations - note the class is exported opaquely from
> Data.Array.MArray - it has these two members that aren't exported so
> aren't documented:
> 
> 
> unsafeRead  :: Ix i => a i e -> Int -> m e
> unsafeWrite :: Ix i => a i e -> Int -> e -> m ()

Those are available from Data.Array.Base. I use them a lot, because there's 
no point in letting the runtime check array bounds after I just did to 
determine whether the loop is finished.

> 
> To actually read and write you have to use the safe derived operations
> which wrap the unsafe versions:
> 
> readArray :: (MArray a e m, Ix i) => a i e -> i -> m e
> writeArray :: (MArray a e m, Ix i) => a i e -> i -> e -> m ()
> 
> For practical purposes I've found STArray's a bit of a white elephant

I on the other hand use 'STUArray's very much. When you fill an array with 
an algorithm which works best with mutation (a sieve for example) and 
afterwards use it only for querying, runSTUArray (or runSTArray) is a great 
friend. ST guarantees that no other thread can mess with your array while 
you build it, when you're done it's immutable. IO doesn't give you these 
guarantees, you have to ascertain yourself that no other thread can mutate 
your array.

It's the same for 'Vector's, ST's phantom type parameter isolates you from 
the outside world, with IOVectors, you have to do the protection yourself. 
I think vector doesn't provide an analogue to runST(U)Array, but if you 
need it, you can write

runST (do
   vec <- stuff
   frz <- unsafeFreeze vec
   return frz

yourself.

> - I always use IOArray instead, as I've either needed to initially
> read an array from file or write one to file at the end.

On the other hand, if you're doing IO, an IOArray is a fairly natural 
choice.