[Haskell-cafe] Question about ST arrays

[Brandon Allbery]

Rank2Types has been an alias for RankNTypes for several years.

In theory, rank-2 types allow some things that aren't possible for general
rank-N types (e.g. decidable typechecking). In practice, ghc does not and
probably never will implement those as special cases for rank-2 types, so
it no longer distinguishes them.

On Fri, Dec 29, 2017 at 10:31 AM, Jean-Marc Alliot <jm at alliot.org> wrote:

> 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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>
>
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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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