当 ghc 无法确定具体类型类实例时，您将收到如下消息:
No instance for ...
arising from a use of `it'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
(lists a few instances)
...plus 13 others
Possible fix: ...
有没有办法显示类型类的所有已定义实例？
最佳答案
您可以使用 :info 命令(缩写为 :i)来执行此操作:
> :i Num
class Num a where
(+) :: a -> a -> a
(*) :: a -> a -> a
(-) :: a -> a -> a
negate :: a -> a
abs :: a -> a
signum :: a -> a
fromInteger :: Integer -> a
-- Defined in ‘GHC.Num’
instance Num Integer -- Defined in ‘GHC.Num’
instance Num Int -- Defined in ‘GHC.Num’
instance Num Float -- Defined in ‘GHC.Float’
instance Num Double -- Defined in ‘GHC.Float’
关于haskell - ghci 列出所有可能的类型类实例吗？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/25395091/