build with ghc-8.10.2

[jwaldmann]

for 0.7.7 branch, with ghc-8.10.2 and stackage nightly-2020-10-31

monadiccp       > [ 7 of 63] Compiling Control.CP.SearchTree
monadiccp       >      
monadiccp       > /tmp/stack-3aa44fdabb3e7b1a/monadiccp-0.7.7/src/Control/CP/SearchTree.hs:203:10: error:
monadiccp       >     • Illegal nested constraint ‘Solver (TreeSolver m)’
monadiccp       >       (Use UndecidableInstances to permit this)
monadiccp       >     • In the instance declaration for ‘MonadTree (ContT r m)’
monadiccp       >     |
monadiccp       > 203 | instance (MonadTree m, Solver (TreeSolver m)) => MonadTree (ContT r m) where
monadiccp       >     |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[jwaldmann]

and more 8.10.2 errors, e.g.,

src/Control/Monatron/AutoInstances.hs:12:5: error:
    ‘fail’ is not a (visible) method of class ‘Monad’

I'm gonna try to fix all of this and send PR.

The question is, how much backwards compatibility do we want?

• ghc-8.8? (probably, that's current stackage LTS).
• ghc-8.6 as well? (I don't need it)

[jwaldmann]

blocked on https://gitlab.haskell.org/ghc/ghc/-/issues/18914

[jwaldmann]

The work-around is to expand some type synonyms cfa45ca , see https://gitlab.haskell.org/ghc/ghc/-/issues/18914#note_310314

The code now works for ghc-8.{4,6,10} but not ghc-8.8 - which needs an additional kind signature cb43c61