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我正在重写我的一些库以使用类型族而不是功能依赖项。但是，似乎我必须添加到函数中以使它们编译的一些约束是不必要的。这让我怀疑我没有以最好的方式做事。

在下面的示例中，有没有办法改进和的定义，Grid以便GridMap和的签名diff更classify简单？特别是，约束BaseGrid (Container gm k) ~ BaseGrid gm、Container (Container gm k) ~ Container gm和GridMap (Container gm k)onclassify似乎不优雅。

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}

import Prelude hiding (map)
import Data.List (minimumBy)
import qualified Data.Map as M
import Data.Ord (comparing)

class Grid g where
  type Index g
  indices :: g -> [Index g]
  -- plus other functions

class (Grid (BaseGrid gm)) => GridMap gm where
  type BaseGrid gm
  type Value gm
  type Container gm :: * -> *

  toMap :: gm -> M.Map (Index (BaseGrid gm)) (Value gm)
  toList :: gm -> [(Index (BaseGrid gm), Value gm)]
  toList = M.toList . toMap

  map
    :: (GridMap gm2, gm2 ~ Container gm (Value gm2)) => 
      (Value gm -> Value gm2) -> gm -> gm2

  mapWithKey 
    :: (GridMap gm2, gm2 ~ Container gm (Value gm2)) => 
      (Index gm -> Value gm -> Value gm2) -> gm -> gm2
  -- plus other functions

class Pattern p where
  type Metric p
  difference :: p -> p -> Metric p
  makeSimilar :: p -> Metric p -> p -> p

diff 
  :: (GridMap gm1, p ~ Value gm1, GridMap gm2, Pattern p, 
      Metric p ~ Value gm2, Container gm1 ~ Container gm2, 
      BaseGrid gm1 ~ BaseGrid gm2,
      gm2 ~ Container gm2 (Value gm2)) => 
    gm1 -> p -> gm2
diff c pattern = map (pattern `difference`) c

classify
  :: (GridMap gm, p ~ Value gm, Pattern p, Ord k, k ~ Metric p, 
      k ~ Index (BaseGrid gm), k ~ Value (Container gm k),
      BaseGrid (Container gm k) ~ BaseGrid gm,
      Container (Container gm k) ~ Container gm,
      GridMap (Container gm k)) =>
    gm -> p -> k
classify c pattern = 
  fst $ minimumBy (comparing snd) $ toList $ diff c pattern

编辑：我喜欢 leventov 的解决方案，但是当我尝试实现它时，我得到一个我不理解的编译错误。

{-# LANGUAGE TypeFamilies, FlexibleContexts #-}

import Prelude hiding (map)
import Data.List (minimumBy)
import qualified Data.Map as M
import Data.Ord (comparing)

class Grid g where
  type Index g
  indices :: g -> [Index g]
  -- plus other functions

class (Grid (BaseGrid gm a)) => GridMap gm a where
  type BaseGrid gm a

  toMap :: gm -> M.Map (Index (BaseGrid gm a)) a
  toList :: gm -> [(Index (BaseGrid gm a), a)]
  toList = M.toList . toMap

  map :: GridMap gm b => (a -> b) -> gm a -> gm b -- <<<<<LINE 20>>>>>

  mapWithKey 
    :: GridMap gm b => 
      (Index (BaseGrid gm a) -> a -> b) -> gm a -> gm b
  -- plus other functions

class Pattern p where
  type Metric p
  difference :: p -> p -> Metric p
  makeSimilar :: p -> Metric p -> p -> p

diff 
  :: (GridMap gm p, Pattern p, GridMap gm m,
      Metric p ~ m, BaseGrid gm p ~ BaseGrid gm m) => 
    gm p -> p -> gm m
diff c pattern = map (pattern `difference`) c

classify
  :: (GridMap gm p, Pattern p, Ord k, k ~ Metric p, 
      k ~ Index (BaseGrid gm p),
      BaseGrid gm k ~ BaseGrid gm p) =>
    gm p -> p -> k
classify c pattern = 
  fst $ minimumBy (comparing snd) $ toList $ diff c pattern

我得到的错误是：

../Amy5.hs:20:42:
    `gm' is applied to too many type arguments
    In the type `GridMap gm b => (a -> b) -> gm a -> gm b'
    In the class declaration for `GridMap'
Failed, modules loaded: none.

如果我离开约束，我也会收到此错误GridMap gm b =>。

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1 回答 1

2

我会GridMap使用 2 个参数：容器本身和值类型。

就像是

{-# LANGUAGE TypeFamilies, FlexibleContexts, MultiParamTypeClasses #-}

import Prelude hiding (map)
import Data.List (minimumBy)
import qualified Data.Map as M
import Data.Ord (comparing)

class Grid g where
  type Index g
  indices :: g -> [Index g]
  -- plus other functions

class (Grid (BaseGrid gm a)) => GridMap (gm :: * -> *) a where
  type BaseGrid gm a

  toMap :: gm a -> M.Map (Index (BaseGrid gm a)) a
  toList :: gm a -> [(Index (BaseGrid gm a), a)]
  toList = M.toList . toMap

  map :: GridMap gm b => (a -> b) -> gm a -> gm b -- <<<<<LINE 20>>>>>

  mapWithKey 
    :: GridMap gm b => 
      (Index (BaseGrid gm a) -> a -> b) -> gm a -> gm b
  -- plus other functions

class Pattern p where
  type Metric p
  difference :: p -> p -> Metric p
  makeSimilar :: p -> Metric p -> p -> p

diff 
  :: (GridMap gm p, Pattern p, GridMap gm m,
      Metric p ~ m, BaseGrid gm p ~ BaseGrid gm m) => 
    gm p -> p -> gm m
diff c pattern = map (pattern `difference`) c

classify
  :: (GridMap gm p, Pattern p, Ord k, k ~ Metric p, 
      k ~ Index (BaseGrid gm p),
      GridMap gm k,
      BaseGrid gm k ~ BaseGrid gm p) =>
    gm p -> p -> k
classify c pattern = 
  fst $ minimumBy (comparing snd) $ toList $ diff c pattern
于 2013-03-04T12:44:30.583 回答