Prelude> Data.Monoid.mempty
()
Prelude> default ()
Prelude> Data.Monoid.mempty

<interactive>:3:1: error:
    * Ambiguous type variable `a0' arising from a use of `print'
      prevents the constraint `(Show a0)' from being solved.
      Probable fix: use a type annotation to specify what `a0' should be.
      These potential instances exist:
        instance Show Ordering -- Defined in `GHC.Show'
        instance Show Integer -- Defined in `GHC.Show'
        instance Show a => Show (Maybe a) -- Defined in `GHC.Show'
        ...plus 22 others
        ...plus 15 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    * In a stmt of an interactive GHCi command: print it
Prelude> default (())
Prelude> Data.Monoid.mempty
()