[Haskell-cafe] How does one delare a 2D STUArray in Haskell?

[Daniel Fischer]

Am Freitag 25 September 2009 09:22:25 schrieb Casey Hawthorne:
> Well that makes sense, but for a learner, how is he/she supposed to
> know that 'i' could be '(i,i)' or for that matter a tuple of n of
> those i's?

minh thu already explained this very well.

>
> "STUArray s i e"
>
> Could you also have a tuple of states?

You can't choose the state 's', the documentation says
"The strict state-transformer monad. A computation of type ST s a transforms an internal 
state indexed by s, and returns a value of type a. The s parameter is either

    * an uninstantiated type variable (inside invocations of runST), or
    * RealWorld (inside invocations of Control.Monad.ST.stToIO). "

Without evil hackery (or stToIO), you can only use ST actions/ST(U)Arrays via

runST :: (forall s. ST s a) -> a

or

runST(U)Array :: Ix i => (forall s. ST s (ST(U)Array s i e)) -> (U)Array i e

which have rank 2 types (universally qualified type as type of argument [result]),
the 'forall s' within the parentheses says it has to work whatever type the rts chooses 
(actually none), so if you write

myFancyArray :: forall s1, s2. ST (s1,s2) (STUArray (s1,s2) Int Int)

you can't use it.

>
> Obviosly, 'e' could be a tuple, for instance  (Int,Char)

Not for STUArrays, but for STArrays, there's no problem.

>
> --
> Regards,
> Casey