[Haskell-cafe] runST readSTRef type error
I run into the following type error:
foo :: ST s (STRef s Int) - Int
foo p = (runST (p = readSTRef))
with ghc 6.12.1
st.hs:8:16:
Couldn't match expected type `s1' against inferred type `s'
`s1' is a rigid type variable bound by
the polymorphic type `forall s1. ST s1 a' at st.hs:8:9
`s' is a rigid type variable bound by
the type signature for `foo' at st.hs:7:10
Expected type: ST s1 (STRef s Int)
Inferred type: ST s (STRef s Int)
In the first argument of `(=)', namely `p'
In the first argument of `runST', namely `(p = readSTRef)'
However, if I add
{-# LANGUAGE RankNTypes #-}
and change the type signature to
foo :: (forall s.ST s (STRef s Int)) - Int
it works. I don't fully understand what's going on here.
Is this the right way to fix the problem? Are there other options?
My gut feeling is, for such a simple use case of the ST monad, I
shouldn't need such a big hammer as RankNTypes.
--ken
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] runST readSTRef type error
Hello Ken,
Strictly speaking, you only need Rank-2 types. This indeed the right
way to fix the problem.
Cheers,
Edward
Excerpts from Ken Takusagawa II's message of Wed May 04 02:00:49 -0400 2011:
I run into the following type error:
foo :: ST s (STRef s Int) - Int
foo p = (runST (p = readSTRef))
with ghc 6.12.1
st.hs:8:16:
Couldn't match expected type `s1' against inferred type `s'
`s1' is a rigid type variable bound by
the polymorphic type `forall s1. ST s1 a' at st.hs:8:9
`s' is a rigid type variable bound by
the type signature for `foo' at st.hs:7:10
Expected type: ST s1 (STRef s Int)
Inferred type: ST s (STRef s Int)
In the first argument of `(=)', namely `p'
In the first argument of `runST', namely `(p = readSTRef)'
However, if I add
{-# LANGUAGE RankNTypes #-}
and change the type signature to
foo :: (forall s.ST s (STRef s Int)) - Int
it works. I don't fully understand what's going on here.
Is this the right way to fix the problem? Are there other options?
My gut feeling is, for such a simple use case of the ST monad, I
shouldn't need such a big hammer as RankNTypes.
--ken
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] runST readSTRef type error
On Wed, 2011-05-04 at 02:00 -0400, Ken Takusagawa II wrote:
I run into the following type error:
foo :: ST s (STRef s Int) - Int
foo p = (runST (p = readSTRef))
with ghc 6.12.1
st.hs:8:16:
Couldn't match expected type `s1' against inferred type `s'
`s1' is a rigid type variable bound by
the polymorphic type `forall s1. ST s1 a' at st.hs:8:9
`s' is a rigid type variable bound by
the type signature for `foo' at st.hs:7:10
Expected type: ST s1 (STRef s Int)
Inferred type: ST s (STRef s Int)
In the first argument of `(=)', namely `p'
In the first argument of `runST', namely `(p = readSTRef)'
However, if I add
{-# LANGUAGE RankNTypes #-}
and change the type signature to
foo :: (forall s.ST s (STRef s Int)) - Int
it works. I don't fully understand what's going on here.
Is this the right way to fix the problem? Are there other options?
My gut feeling is, for such a simple use case of the ST monad, I
shouldn't need such a big hammer as RankNTypes.
--ken
To make the interface of ST works - i.e. to keeps it pure the signature
of runST is:
runST :: (forall s. ST s a) - a
Otherwise consider following code:
incST :: Num a = STRef s a - ST s a
incST r = readSTRef r = \v - writeSTRef r (v + 1) return v
add :: STRef s Int - Int - Int
add r x = runST (incST r = \v - return (v + x))
test :: [Int]
test = runST (newSTRef 0) = \r - map (add r) [1,2,3]
What is the result?
And what is the result of:
test2 :: [Int]
test2 = runST (newSTRef 0) = \r - map (add r) (map (add r) [1,2,3])
Regards
signature.asc
Description: This is a digitally signed message part
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
