一个类中的多个类型同义词
有没有办法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)
这是我收到的错误消息。
?> :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(Strand a, Strand a)任何地方使用，但使用Genome a.
回答
您可以与类分开定义类型同义词：
{-# 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，那么您可以像这样定义它：
{-# 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，但您可以将第一type Genome a行视为签名，将第二行视为默认实现。在这种情况下，签名是简单type Genome a :: *或type Genome a短期的，但它可以比变得更加复杂。