99problems.hs:63:3: 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 five instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In a stmt of a 'do' block: print (flatten (List []))
      In the expression:
        do { print (flatten (Elem 5));
             print (flatten (List [Elem 1, List [...]]));
             print (flatten (List [])) }
      In an equation for ‘main’:
          main
            = do { print (flatten (Elem 5));
                   print (flatten (List [Elem 1, ....]));
                   print (flatten (List [])) }