These operators are from Control.Applicative
. You need to import Control.Applicative
or say :m +Control.Applicative
in ghci
. You can find out where many standard operators come from using Hoogle.
As of GHC 7.10, with the Functor-Applicative-Monad Proposal implemented in base
4.8, <*>
is now present in the Prelude and does not require an import.