ghci列出了所有可能的类型类实例吗? [英] have ghci list all possible type class instances?
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问题描述
当ghc无法确定具体的类型类实例时,您会收到类似以下消息:
When ghc can't determine a concrete type class instance, you'll get a message like:
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: ...
是否可以显示类型类的所有已定义实例?
Is there way to display all of the defined instances of a type class?
推荐答案
您可以使用:info
命令(缩写为:i
)来执行此操作:
You can use the :info
command (shortened to :i
) to do this:
> :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’
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