Better type errors for a relatively simple expression

[meditans]

With reference to the issue #16, the code

c :: (Settable "pitch" Pitch old new, Default (Book' old)) => Book' new
c = set #pitch C def

which now errors with:

    • Could not deduce (Map.Submap
                          (Data.Type.Map.Nub (Data.Type.Set.Sort (old0 Map.:\ "pitch")))
                          old0,
                        (Map.Nubable
                           ((Data.Type.Set.Sort
                               (Data.Type.Set.Filter
                                  'Data.Type.Set.FMin
                                  ("pitch" ':-> Pitch)
                                  (Data.Type.Map.Nub (Data.Type.Set.Sort (old0 Map.:\ "pitch"))))
                             Data.Type.Set.:++ '["pitch" ':-> Pitch])
                            Data.Type.Set.:++ Data.Type.Set.Sort
                                                (Data.Type.Set.Filter
                                                   'Data.Type.Set.FMax
                                                   ("pitch" ':-> Pitch)
                                                   (Data.Type.Map.Nub
                                                      (Data.Type.Set.Sort (old0 Map.:\ "pitch"))))),
                         Data.Type.Map.Sortable
                           ("pitch" ':-> Pitch
                              : Data.Type.Map.Nub (Data.Type.Set.Sort (old0 Map.:\ "pitch")))),
                        new
                        ~
                        Data.Type.Map.Nub
                          ((Data.Type.Set.Sort
                              (Data.Type.Set.Filter
                                 'Data.Type.Set.FMin
                                 ("pitch" ':-> Pitch)
                                 (Data.Type.Map.Nub (Data.Type.Set.Sort (old0 Map.:\ "pitch"))))
                            Data.Type.Set.:++ '["pitch" ':-> Pitch])
                           Data.Type.Set.:++ Data.Type.Set.Sort
                                               (Data.Type.Set.Filter
                                                  'Data.Type.Set.FMax
                                                  ("pitch" ':-> Pitch)
                                                  (Data.Type.Map.Nub
                                                     (Data.Type.Set.Sort (old0 Map.:\ "pitch"))))))
      from the context: (Settable "pitch" Pitch old new,
                         Default (Book' old))
        bound by the type signature for:
                   c :: (Settable "pitch" Pitch old new, Default (Book' old)) =>
                        Book' new
        at /home/carlo/code/haskell/forks/bookkeeper/src/Bookkeeper/Internal.hs:256:6-71
      The type variable ‘old0’ is ambiguous
    • In the ambiguity check for ‘c’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      In the type signature:
        c :: (Settable "pitch" Pitch old new, Default (Book' old)) =>
             Book' new
Failed, modules loaded: Bookkeeper.Internal.Errors.

could expose a more friendly error to the user.