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
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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