类中的多个类型同义词

Question

是否有一种方法可以根据关联的类型同义词定义类型同义词?(不确定我是否有正确的术语)

{-# 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)

这是我得到的错误信息。

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.

我可以在任何地方使用(Strand a, Strand a)，但使用Genome a会更好。

Answer 1

可以将类型同义词与类分开定义:

{-# 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，如果你想在某些实例中覆盖它，那么你可以这样定义它:

{-# 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)

这似乎是多余的，但是您可以将第一个type Genome a视为签名，第二行视为默认实现。在本例中，签名仅为type Genome a :: *或type Genome a，但它可能比这更复杂。