[Haskell-cafe] Question about ST arrays

Baa aquagnu at gmail.com
Fri Dec 29 15:11:17 UTC 2017


I found this -

I'm not sure is it helpful.

PS. As I understand, `get2` signature has own `forall s`, but `runST`
is `(forall s. ST s a) -> a` which "escapes" top `s`.

Somebody else? :)

Best regards, Paul

> Hi,
> This is my first post to this list so I apologize in advance if I
> don't use it properly, or if my question is too simple or
> inapropriate.
> I come from the Caml world and I am quite new to Haskell (but not to 
> functional programming). I am currently trying to get the hang of 
> Haskell arrays. I have gone through regular arrays, IO Arrays and I
> am now working with ST Arrays.
> This is the problem I am currently stuck with. I write the following
> code:
> arr = newArray (-1, 1) 0 :: ST s (STArray s Int Int)
> get :: Int -> Int
> get i = runST (arr >>= (\b -> readArray b i))
> Here everything is perfectly OK.
> Now I want a more general version that could deal with any array like 
> arr. So I write:
> get2 :: ST s (STArray s Int Int) -> Int -> Int
> get2 tab i = runST (tab >>= (\b -> readArray b i))
> And the compiler is clearly very upset by my code:
> Couldn't match type ‘s’ with ‘s1’
>        ‘s’ is a rigid type variable bound by
>          the type signature for:
>            get2 :: forall s. ST s (STArray s Int Int) -> Int -> Int
>          at testst.hs:17:9
>        ‘s1’ is a rigid type variable bound by
>          a type expected by the context:
>            forall s1. ST s1 Int
>          at testst.hs:18:14
>        Expected type: ST s1 Int
>          Actual type: ST s Int
> I am pretty sure that the compiler is right and I am wrong, but I
> don't get why... Anyone could help?
> Thanks
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