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[英]Proper refactor of a Haskell code when a non-monadic value becomes a monadic value
[英]Refactor Haskell monadic code to avoid copy-paste
我已經使用ST monad在Haskell中編寫了以下代碼,並且可以正常工作。 我唯一的問題是如何避免以下代碼中顯示的復制粘貼。 當我嘗試重構代碼時,出現編譯器錯誤,我無法完全理解。 有沒有一種方法可以避免以下代碼中的復制粘貼。 我想知道,是否可以將處理start_1和start_2(截至目前的復制/粘貼)的代碼重構到另一個輔助函數。
import qualified Control.Monad as CM
import qualified Control.Monad.ST as CMST
import qualified Data.Array as A
import qualified Data.Array.Unboxed as AU
import qualified Data.Array.ST as AST
prime_factors :: Int -> Int -> [(Int, Int)] -> A.Array Int [(Int, Int)]
prime_factors a_low a_high prms_sqrts = AST.runSTArray $ do
pfs <- AST.newArray (a_low, a_high) [] :: CMST.ST s (AST.STArray s Int [(Int, Int)])
as <- AST.newArray (a_low, a_high) 0 :: CMST.ST s (AST.STArray s Int Int)
CM.forM_ [a_low..a_high] $ \i -> do
AST.writeArray as i ((i * i) + 1)
CM.forM_ (takeWhile (\(prm, _) -> prm <= (a_high + 1)) prms_sqrts) $ \(prm, sqr_rt) -> do
let (q, r) = a_low `divMod` prm
let start_1 = a_low + sqr_rt - r
-- !!!! CODE TO DO SOME PROCESSING FOR start_1 !!!!!
CM.forM_ (takeWhile (<= a_high) [start_1 + (x * prm) | x <- [0..]]) $ \i -> do
a_i <- AST.readArray as i
let (a_i', mul) = remove_factor a_i prm 0
CM.when (mul > 0) $ do
AST.writeArray as i a_i'
pfs_i <- AST.readArray pfs i
AST.writeArray pfs i ((prm, mul) : pfs_i)
let start_2 = a_low + (prm - sqr_rt) - r
-- !!!! COPY-PASTE ABOVE CODE TO PROCESS start_2 !!!!
CM.forM_ (takeWhile (<= a_high) [start_2 + (x * prm) | x <- [0..]]) $ \i -> do
a_i <- AST.readArray as i
let (a_i', mul) = remove_factor a_i prm 0
CM.when (mul > 0) $ do
AST.writeArray as i a_i'
pfs_i <- AST.readArray pfs i
AST.writeArray pfs i ((prm, mul) : pfs_i)
CM.forM_ [a_low..a_high] $ \i -> do
a_i <- AST.readArray as i
CM.when (a_i > 1) $ do
pfs_i <- AST.readArray pfs i
AST.writeArray pfs i ((a_i, 1) : pfs_i)
return pfs
where remove_factor m p mul
| m `mod` p == 0 = remove_factor (m `div` p) p (mul + 1)
| otherwise = (m, mul)
當我嘗試將上面的復制粘貼代碼移動到使用“ let”綁定創建的本地函數(名為sieve_factors)時,收到的錯誤消息是:
vamsi@vamsi-laptop:~/learn/project_euler/129_to_256/problem_224$ ghc --make -O -i../.. problem_224.hs
[3 of 3] Compiling Main ( problem_224.hs, problem_224.o )
problem_224.hs:39:9: error:
• Non type-variable argument
in the constraint: AST.MArray (AST.STArray s) Int m
(Use FlexibleContexts to permit this)
• When checking the inferred type
sieve_factors :: forall (m :: * -> *).
(AST.MArray (AST.STArray s) Int m,
AST.MArray (AST.STArray s) [(Int, Int)] m) =>
Int -> m ()
In the expression:
do { let sieve_factors start = ...;
let (q, r) = a_low `divMod` prm;
let start_1 = a_low + sqr_rt - r;
sieve_factors start_1;
.... }
In the second argument of ‘($)’, namely
‘\ (prm, sqr_rt)
-> do { let ...;
let ...;
.... }’
編譯器告訴您
Use FlexibleContexts to permit this
這意味着啟用FlexibleContexts
語言擴展 。 您可以通過在源文件頂部添加LANGUAGE
編譯指示來完成此操作
{-# LANGUAGE FlexibleContexts #-}
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