have ghci list all possible type class instances?

Question

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         


        

Answer 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’