homework13.hs:3:19: error:
    • Expecting one more argument to ‘FstPair’
      Expected kind ‘* -> *’, but ‘FstPair’ has kind ‘* -> * -> *’
    • In the first argument of ‘Foldable’, namely ‘FstPair’
      In the instance declaration for ‘Foldable FstPair’
Failed, modules loaded: none.