Monadic way to read bits from Get monad
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01-07-2021 - |
题
I need to read some amount of bits from Get monad. Now my code looks like
readBits :: Int -> Int -> Get (Word32, Int)
readBits count state = ...
readValue :: Get (Word32, Word32)
readValue = do
-- read fst bit count
(bits1, s0) <- readBits 5 0
-- read bits1 bits as fst
(fst, s1) <- readBits bits1 s0
-- read snd bit count
(bits2, s2) <- readBits 5 s1
-- read bits2 bits as snd
(snd, s3) <- readBits bits2 s2
-- flush incomplete byte
when (s3 /= 0) $ skip 1
return (fst, snd)
I want to wrap this into some kind of state monad, to have code like
readBits :: Int -> BitReader Word32
readBits count = ...
runBitReader :: BitReader a -> Get a
readValue :: Get (Word32, Word32)
readValue = runBitReader $ do
bits1 <- readBits 5
fst <- readBits bits1
bits2 <- readBits 5
snd <- readBits bits2
return (fst, snd)
What functions should I implement? How should they be implemented?
I have looked into Get and BitGet source code, but not fully understand what's going on.
解决方案
This is the most typical usecase for Monad Transformers.
You have defined most of the structure correctly. To answer your questions
What functions should I implement?
- Well you first need to wrap
Get
monad into theStateT
Transformer to getBitReader
. - You need to implement proper definition for
readBits
usingget
to get the current state andput
to save the state back. - You need to run your code wrapped in
BitReader
to get back the output inGet
Monad. So you need to definerunBitReader
usingrunStateT
.
To answer your next question.
How should they be implemented?
I have given the possible implementation. You still need to define some functions to make it work.
import Control.Monad.State
import qualified Control.Monad.State as ST
import Data.Binary
type BitReader = StateT Int Get
readBits' :: Int -> Int -> Get (Word32, Int)
readBits' = undefined
readBits :: Int -> BitReader Word32
readBits n = do
s0 <- ST.get
(a,s1) <- lift $ readBits' n s0
ST.put s1
return a
runBitReader :: BitReader a -> Get a
runBitReader w = do
(a,s) <- runStateT w 0
return a
readValue = do
fst <- readBits 5
snd <- readBits 10
return (fst, snd)
I don't know how looking into code of Get
was going to help you. You were looking in the wrong house. You need to read about State Monads
and Monad Transformers
.
You can read more about monad transformers here.