[Haskell-cafe] Getting my mind around UArray -> STUArray conversion

[Don Stewart]

scooter.phd:
> On Fri, Jun 19, 2009 at 7:08 PM, Dan Doel<dan.doel at gmail.com> wrote:
> > Oops, I replied too hastily.
> >
> > What I wrote in my first mail is a problem, as witnessed by the "ix" and "ix1"
> > in the error message. However, it isn't the main error. The main error is that
> > you have a monadic expression, with type something like:
> >
> >    ST s (UArray ix e)
> >
> > but the return type of your function is:
> >
> >    UArray ix e
> >
> > To make a no-op you need to add a runST, something like:
> >
> >    runST (unsafeThaw mem >>= unsafeFreeze)
> 
> Actually, I probably wanted runSTUArray. :-)
> 
> But even then, I can't manage to get wombat to compile correctly. I'm
> starting to think that MArray is itself a WOMBAT (waste of money,
> brains and time), for three reasons:
> 
> a) Overly restrictive Monads in which implementation is supported
> (i.e., ST and IO)
> b) The triviality of the examples gives no insight as to how they
> could be used, other than a create array, do something completely
> trivial and freeze.
> c) They are evidently very hard to use in a general sense.
> 
> Even google-ing for examples just comes up with trivial examples of
> MArray usage.


Can you just use STUArrays directly without worrying about MArray
overloading? writeArray/readArray on ST is just type safe raw memory
access, so should be easy.

> wombat :: forall e ix s. (IArray UArray e, Ix ix, MArray (STUArray s)
> e (ST s)) => e -> ix -> UArray ix e -> UArray ix e
> wombat val idx mem = runSTUArray (unsafeThaw mem >>= return)

Argh. That type is scary overloaded. 

-- Don