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

[Daniel Fischer]

Am Samstag 20 Juni 2009 03:51:08 schrieb Dan Doel:
> On Friday 19 June 2009 9:43:29 pm Scott Michel wrote:
> > wombat :: (IArray UArray e, Ix ix, MArray (STUArray s) e (ST s)) => e
> > -> ix -> UArray ix e -> UArray ix e
> > wombat val idx mem = (unsafeThaw mem :: ST s (STUArray s ix e)) >>=
> > (\mmem -> unsafeFreeze mmem)
>
> Based on the error message and dealing with this sort of thing before, your
> problem is that when you say:
>
>   ":: ST s (STUArray s ix e)"
>
> the s, ix and e there aren't the same as they are in the signature of
> wombat. To make them the same, you need the ScopedTypeVariables extension,
> and to make wombat's signature:
>
>   wombat :: forall e ix s. ...
>
> where the dots are your current signature.
>
> It's possible you'll still have errors, but that will solve the one in your
> mail.
>
> -- Dan

No, only part of it. Another part is

unsafeFreeze :: (Ix i, MArray a e m, IArray b e) => a i e -> m (b i e)

so 
unsafeThaw arr >>= unsafeFreeze
lives in a monad, here (ST s) and to get the type he wants, he has to wrap it in runST.