[Haskell-cafe] Question about ST arrays

[Jean-Marc Alliot]

Yes, thanks, it's exactly the same question, and the same trick works, 
using:
{-# LANGUAGE RankNTypes #-}
and
get2 :: (forall s.ST s (STArray s Int Int)) -> Int -> Int

In fact, Rank-2 types are enough here, we don't need Rank-N types.
I suppose the ST Array module uses the Rank-N extension, so using them 
requires also enabling Rank-N.

Thanks again.

Le 29/12/2017 à 16:11, Baa a écrit :
> Hello!
>
> I found this -
> https://mail.haskell.org/pipermail/haskell-cafe/2011-May/091622.html
>
> 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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