Question

我在Haskell的名单上打上了普通类。

class HasEmpty a where
  empty :: a
  isEmpty :: a -> Bool

class HasEmpty (l a) => List l where
  cons :: a -> l a -> l a
  uncons :: l a -> (a, l a)

下面是<代码>[:

instance HasEmpty [a] where
  empty = []
  isEmpty [] = True
  isEmpty _  = False

instance List [] where
  cons = (:)
  uncons (x:xs) = (x,xs)

然而,这产生了一个错误:

Not in scope: type variable  a

This is caused by the constraint HasEmpty (l a). I am not desperately interested in this particular example, but I am interested in the concept in general. HasEmpty is a class for types of kind *, while List is a class for types of kind * -> *. Is it possible for me to make a typeclass constraint of a different kind than the typeclass it is constraining?

Answer 1

不管怎样,你总是能够使用多参数类别表示基本逻辑(事实上,在):

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}

class HasEmpty a where
  empty :: a
  isEmpty :: a -> Bool

class HasEmpty (l a) => List l a where
  cons :: a -> l a -> l a
  uncons :: l a -> (a, l a)


instance HasEmpty [a] where
  empty = []
  isEmpty [] = True
  isEmpty _  = False

instance List [] a where
  cons = (:)
  uncons (x:xs) = (x,xs)

或者更多地通过家庭类型:

{-# LANGUAGE TypeFamilies #-}

class HasEmpty a where
  empty :: a
  isEmpty :: a -> Bool


class HasEmpty a => List a where
  type Elem a :: *
  cons :: Elem a -> a -> a
  uncons :: a -> (Elem a, a)


instance HasEmpty [a] where
  empty = []
  isEmpty [] = True
  isEmpty _  = False


instance List [a] where
  type Elem [a] = a
  cons = (:)
  uncons (x:xs) = (x,xs)

Answer 2

当然,你可以。比如,这两班将课以罚款:

class HasEmpty (l ()) => List l where
  cons :: a -> l a -> l a
  uncons :: l a -> (a, l a)

或(List1>* -> *-> *->*)

class HasEmpty1 (l a) => List1 l a where
  cons :: a -> l a -> l a
  uncons :: l a -> (a, l a)

你可以做的是,在制约因素中增加新的变量。

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