在这种情况下真的需要 FlexibleContexts 吗？

我发布的问题没有意义。

类型签名由 ghci 提供。

我误读了：不是(MonadReader (Map k (Set a -> Set a)) m ;

而是：MonadReader (Map k (Set a -> Set a) m)。

这只是一个疏忽。很抱歉浪费您的时间。

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在这种情况下真的需要 FlexibleContexts 吗？

“m”代表哪个单子？

我尝试用 [] 替换“m”，但没有用。

{-# LANGUAGE FlexibleContexts #-}

module LeftCensorsList where

import Data.Set as S
import Data.Map as M
import Data.List as L
import Control.Monad.Reader

-- ============================== FUNCTIONS ==============================

censorsList :: Ord a => Map a (Set a -> Set a) -> [Set a] -> ([Set a], [Set a])
censorsList _ [] = ([],[])
censorsList cnsrmap (xs:xss) = runReader (go xs xss [xs]) cnsrmap
    where
    go _ [] invacc = pure (reverse invacc,[])
    go cnsrs (ys:yss) invacc = do
        ys' <- applyCensors cnsrs ys
        if S.null ys'
          then pure (reverse invacc,ys:yss)
          else go ys' yss (ys': invacc)        

applyCensors :: (MonadReader (Map k (Set a -> Set a)) m, Ord k) => Set k -> Set a -> m (Set a)  -- <<<<<
applyCensors cnsrs xs = go (S.toList cnsrs) xs
    where
    go [] ys = pure ys
    go (cnsr:rest) ys = do
        ys' <- applyCensor cnsr ys    
        if S.null ys'
          then pure S.empty
          else go rest ys'        

applyCensor :: (MonadReader (Map k (t -> t)) m, Ord k) => k -> t -> m t -- <<<<<
applyCensor cnsr xs = do
    cnsrmap <- ask
    case M.lookup cnsr cnsrmap of
        Nothing -> pure xs
        Just f  -> pure $ f xs

-- ============================== TEST ==============================
 
t1,t2,t3, t4 :: ([Set Int],[Set Int])
t1 = censorsList M.empty $ L.map S.fromList [[1,3,13],[2,4,6]]  -- ok
t2 = censorsList exCensorsMap $ L.map S.fromList [[1,3,13],[2,4,6]]  -- ok
t3 = censorsList exCensorsMap []  -- ok
t4 = censorsList exCensorsMap $ L.map S.fromList [[1,4,13],[2,4,5]]  -- ok

-- ============================== EXAMPLES ==============================

-- -------------------- MAPS --------------------

exCensorsMap :: Map Int (Set Int -> Set Int)
exCensorsMap = M.fromList $
    [(1,censor1)
    ,(2,censor2)
    ,(3,censor3)
    ,(4,censor4)
    ]
 
-- -------------------- CENSORS --------------------

censor1, censor2, censor3, censor4 :: Set Int -> Set Int
censor1 = \xs -> xs S.\\ (S.fromList [2,4])
censor2 = \xs -> xs S.\\ (S.fromList [3])
censor3 = \xs -> xs S.\\ (S.fromList [1,2])
censor4 = \xs -> xs S.\\ (S.fromList [2,5])