haskell - 一个类中的多个类型同义词

Question

有没有一种方法可以根据关联的类型同义词来定义类型同义词？ (不确定我的术语是否正确。)

{-# LANGUAGE TypeFamilies #-}

class Reproductive a where

  -- | A sequence of genetic information for an agent.
  type Strand a

  -- | Full set (both strands) of genetic information for an organism.
  type Genome a = (Strand a, Strand a)

这是我收到的错误消息。

λ> :l stackOverflow.hs 
[1 of 1] Compiling Main             ( stackOverflow.hs, interpreted )

stackOverflow.hs:9:8: error:
    ‘Genome’ is not a (visible) associated type of class ‘Reproductive’
  |
9 |   type Genome a = (Strand a, Strand a)
  |        ^^^^^^
Failed, no modules loaded.

我可以到处使用(Strand a, Strand a)，但是使用Genome a会更好。

Answer 1

您可以从类中单独定义类型同义词:

{-# LANGUAGE TypeFamilies #-}
  
-- | Full set (both strands) of genetic information for an organism.
type Genome a = (Strand a, Strand a)

class Reproductive a where

  -- | A sequence of genetic information for an agent.
  type Strand a

或者，如果您希望能够为某些实例覆盖它，那么您可以这样定义它:

{-# LANGUAGE TypeFamilies #-}

class Reproductive a where

  -- | A sequence of genetic information for an agent.
  type Strand a

  -- | Full set (both strands) of genetic information for an organism.
  type Genome a 
  type Genome a = (Strand a, Strand a)

看似多余，但你可以将第一行type Genome a视为签名，将第二行视为默认实现。在这种情况下，签名只是type Genome a::* 或简称type Genome a，但它可能比这更复杂。