I wrote small program in haskell to count all ocurences of Int values in Tree using State Monad with Vector:
import Data.Vector
import Control.Monad.State
import Control.Monad.Identity
data Tree a = Null | Node (Tree a) a (Tree a) deriving Show
main :: IO ()
main = do
print $ runTraverse (Node Null 5 Null)
type MyMon a = StateT (Vector Int) Identity a
runTraverse :: Tree Int -> ((),Vector Int)
runTraverse t = runIdentity (runStateT (traverse t) (Data.Vector.replicate 7 0))
traverse :: Tree Int -> MyMon ()
traverse Null = return ()
traverse (Node l v r) = do
s <- get
put (s // [(v, (s ! v) + 1)]) -- s[v] := s[v] + 1
traverse l
traverse r
return ()
But 'update' of immutable Vectors is done in O(n) complexity. And I am looking for update in O(1) and access in O(1). As I understand Mutable Vectors do what I want. To use them I need to use ST or IO. Because I would like to do some UnitTests I prefer ST monad, but I don't want to have to pass that vector around in function calls. I need to keep using Monad Transformers, because I will be adding transformers like ErrorT and WriterT.
Question: How to put Mutable Vector into State Monad using Monad Transformers ?
I came up with following code that does not compile:
import Data.Vector
import Control.Monad.State
import Control.Monad.Identity
import qualified Data.Vector.Mutable as VM
import Control.Monad.ST
import Control.Monad.ST.Trans
type MyMon2 s a = StateT (VM.MVector s Int) (STT s Identity) a
data Tree a = Null | Node (Tree a) a (Tree a) deriving Show
main :: IO ()
main = do
print $ runTraverse (Node Null 5 Null)
runTraverse :: Tree Int -> ((),Vector Int)
runTraverse t = runIdentity (Control.Monad.ST.Trans.runST $ do
emp <- VM.replicate 7 0
(_,x) <- (runStateT (traverse t) emp)
v <- Data.Vector.freeze x
return ((), v)
)
traverse :: Tree Int -> MyMon2 s ()
traverse Null = return ()
traverse (Node l v r) = do
d <- get
a <- (VM.read d v)
VM.write d v (a + 1)
put d
return ()
Compile errors are:
TranformersExample: line 16, column 16:
Couldn't match type `s'
with `primitive-0.5.2.1:Control.Monad.Primitive.PrimState
(STT s Identity)'
`s' is a rigid type variable bound by
a type expected by the context: STT s Identity ((), Vector Int)
at test/ExecutingTest.hs:15:30
Expected type: STT s Identity (MVector s Int)
Actual type: STT
s
Identity
(MVector
(primitive-0.5.2.1:Control.Monad.Primitive.PrimState
(STT s Identity))
Int)
In the return type of a call of `VM.new'
In a stmt of a 'do' block: emp <- VM.new 7
In the second argument of `($)', namely
`do { emp <- VM.new 7;
(_, x) <- (runStateT (traverse t) emp);
v <- freeze x;
return ((), v) }'
TranformersExample: line 26, column 14:
Couldn't match type `s'
with `primitive-0.5.2.1:Control.Monad.Primitive.PrimState
(StateT (MVector s Int) (STT s Identity))'
`s' is a rigid type variable bound by
the type signature for traverse :: Tree Int -> MyMon2 s ()
at test/ExecutingTest.hs:21:13
Expected type: MVector
(primitive-0.5.2.1:Control.Monad.Primitive.PrimState
(StateT (MVector s Int) (STT s Identity)))
Int
Actual type: MVector s Int
In the first argument of `VM.write', namely `d'
In a stmt of a 'do' block: VM.write d v (a + 1)
In the expression:
do { d <- get;
a <- (VM.read d v);
VM.write d v (a + 1);
put d;
.... }
Note: I am aware of not checking bounds.
When using ST
state you're never explicitly passing the vector around (that's always in hidden in the s
argument), but a reference to it. That reference is immutable and not copied, so you need not State
but simply a reader to pass it implicitly.
import Data.Vector
import Control.Monad.Reader
import qualified Data.Vector.Mutable as VM
import Control.Monad.ST
type MyMon3 s = ReaderT (VM.MVector s Int) (ST s)
data Tree a = Null | Node (Tree a) a (Tree a) deriving Show
main :: IO ()
main = do
print $ runTraverse (Node Null 5 Null)
runTraverse :: Tree Int -> Vector Int
runTraverse t = runST $ do
emp <- VM.replicate 7 0
runReaderT (traverse t) emp
Data.Vector.freeze emp
traverse :: Tree Int -> MyMon3 s ()
traverse Null = return ()
traverse (Node l v r) = do
d <- ask
a <- lift $ VM.read d v
lift $ VM.write d v (a + 1)
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