Question

我可以看到数据构造函数和实例，比如ghci中的Maybe：

Prelude Control.Applicative> :i Maybe
data Maybe a = Nothing | Just a     -- Defined in `Data.Maybe'
instance Eq a => Eq (Maybe a) -- Defined in `Data.Maybe'
instance Monad Maybe -- Defined in `Data.Maybe'
instance Functor Maybe -- Defined in `Data.Maybe'
instance Ord a => Ord (Maybe a) -- Defined in `Data.Maybe'
instance Read a => Read (Maybe a) -- Defined in `GHC.Read'
instance Show a => Show (Maybe a) -- Defined in `GHC.Show'
instance Applicative Maybe -- Defined in `Control.Applicative'
instance Alternative Maybe -- Defined in `Control.Applicative'

我也可以看到ghci中如何定义Applicative类型类：

Prelude Control.Applicative> :i Applicative
class Functor f => Applicative f where
  pure :: a -> f a
  (<*>) :: f (a -> b) -> f a -> f b
  (*>) :: f a -> f b -> f b
  (<*) :: f a -> f b -> f a
    -- Defined in `Control.Applicative'
instance Applicative [] -- Defined in `Control.Applicative'
instance Applicative ZipList -- Defined in `Control.Applicative'
instance Monad m => Applicative (WrappedMonad m)
  -- Defined in `Control.Applicative'
instance Applicative Maybe -- Defined in `Control.Applicative'
instance Applicative IO -- Defined in `Control.Applicative'
instance Applicative (Either e) -- Defined in `Control.Applicative'
instance Applicative ((->) a) -- Defined in `Control.Applicative'

但如何查找某个类型的特定实例的信息，例如instance Alternative Maybe？

Answer 1

ghci没有任何命令可以显示函数和实例的基础源，但如果确实如此，它会很简洁。

我找到这些东西的来源的方法是找到它们的hackage（对于base中的东西，hoogle是要走的路）。然后是黑线鳕include links to source from the documentation。

如何在ghci中找到实例的信息？

1 个答案: