问题
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: ...
Is there way to display all of the defined instances of a type class?
回答1:
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’
来源：https://stackoverflow.com/questions/25395091/have-ghci-list-all-possible-type-class-instances