deniok

GHCi> :i ($)
($) ::
  forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r).
  (a -> b) -> a -> b
        -- Defined in `GHC.Base'
infixr 0 $
GHCi> :i undefined
undefined ::
  forall (r :: GHC.Types.RuntimeRep) (a :: TYPE r).
  GHC.Stack.Types.HasCallStack =>
  a
        -- Defined in `GHC.Err'

> :t ($)
($) :: (a -> b) -> a -> b
> :t undefined
undefined :: a