[Haskell-beginners] Stymied by mutable arrays in the ST monad

[Matthew Moppett]

Brilliant. I think I've got it now. Thanks heaps, Tobias.

On Sat, May 26, 2012 at 4:01 AM, Tobias Brandt <tob.brandt at googlemail.com>wrote:

> On 25 May 2012 19:43, Matthew Moppett <matthewmoppett at gmail.com> wrote:
> > About the extra type info needed -- what part of the type "ST s (STArray
> s
> > Int Int)" is the compiler unable to infer?
>
> It doesn't know which array type to use, because 'newListArray' is
> polymorphic
> in the type of array it returns. This type is not mentioned in
> the signature of idST because the array is immediately consumed again by
> 'getElems'.
>
> You can try it out in ghci (':t expr' gives you the type of an expression):
>
> :m +Data.Array.ST
> let xs = [1,2,3 :: Int]
>
> :t xs
> [Int]
>
> :t newListArray (1, length xs) xs
> MArray a Int m => m (a Int Int)
>
> Applying getElems gets you this:
> :t (newListArray (1, length xs) xs) >>= getElems
> MArray a Int m => m [Int]
>
> From the 'runST' in your program, the compiler knows that m = ST s,
> for some s, but there is nothing constraining 'a' further.
> Any 'a' fulfilling the constraint 'MArray a Int (ST s)' will do.
> Therefore we have to fix 'a' to some type, e.g. 'STArray s', but
> 'STUArray s' would work too, in this case.
>
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