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當我在學習Haskell的過程中，碰到這個問題就來了：「無法推斷（MARRAY（STUArray S）INT（ST S））從上下文（）」 將runST
使用Glasgow Haskell Compiler, Version 6.10.4, for Haskell 98, stage 2 booted by GHC version 6.10.1
文件
{-# LANGUAGE FlexibleContexts #-}
module UPSO where
import Control.Monad(forM,forM_)
import Control.Monad.ST.Lazy (ST,runST)
import Data.Array.MArray (MArray, Ix, getBounds, newArray, readArray, writeArray)
import Data.Array.ST (STArray,STUArray)
minmax [email protected](x:_) = foldr (\x (l,u) -> (min x l,max x u)) (x,x) xs
modify a i f = do
x <- readArray a i
writeArray a i (f x)
increment a i = modify a i (+1)
decrement a i = modify a i (\x -> x - 1)
uniquePermutationsM t 0 = return $! [[]]
uniquePermutationsM t pos = do
(l,u) <- getBounds t
perms <- forM [l..u] (\d -> do
count <- readArray t d -- t[d]
if count == 0
then return $! []
else do
decrement t d
pss <- uniquePermutationsM t (pos-1)
increment t d
return $! (map (d:) pss)
)
return $! (concat perms)
使用STArray（作品）
mkArray :: (Int,Int) -> (ST s) (STArray s Int Int)
mkArray bounds = newArray bounds 0
uniquePermutationsST :: [Int] -> ST s [[Int]]
uniquePermutationsST xs = do
let [email protected](l,u) = (minmax xs)
t <- mkArray bounds
forM_ xs (increment t)
pos <- sum `fmap` mapM (readArray t) [l..u]
uniquePermutationsM t pos
uniquePermutations xs = runST (uniquePermutationsST xs)
USI的常見開始ng STUArray（不起作用）
但是，當我嘗試切換到unboxed數組時，我收到一條錯誤消息。
mkArray :: (Int,Int) -> (ST s) (STUArray s Int Int)
mkArray bounds = newArray bounds 0
uniquePermutationsST :: [Int] -> ST s [[Int]]
uniquePermutationsST xs = do
let [email protected](l,u) = (minmax xs)
t <- mkArray bounds
forM_ xs (increment t)
pos <- sum `fmap` mapM (readArray t) [l..u]
uniquePermutationsM t pos
uniquePermutations xs = runST (uniquePermutationsST xs)
錯誤消息
Could not deduce (MArray (STUArray s) Int (ST s))
from the context()
arising from a use of 'newArray' at UPSO.hs:35:17-33
Possible fix:
add (MArray (STUArray s) Int (ST s)) to the context of
the type signature for 'mkArray'
or add an instance declaration for (MArray (STUArray s) Int (ST s))
In the expression: newArray bounds 0
In the definition of 'mkArray': mkArray bounds = newArray bounds 0
也：
Could not deduce (MArray (STUArray s) Int (ST s))
from the context()
arising from a use of 'increment' at UPSO.hs:41:14-24
近兩個小時與類型註釋擺弄之後，我希望有人能指出我在正確的方向。到底什麼地方出了問題？
謝謝你的時間。
似乎爲我工作。也許發佈完整的源代碼來重現問題（理想情況下縮短它，同時還會產生問題）和GHC版本？ – yairchu 2009-07-21 19:10:48