Fix "Could not deduce: effs ~ (r : rs)" errors
Commit 42604669 added the "r has at
least two elements" constraint to the
Member' t r ('S n) instance but
Member t effs with a general (unknown) effs isn't enough to deduce
that effs is non-empty, so this doesn't typecheck:
f :: Eff (IO ': effs) () -> Eff effs () f = undefined g :: (Member Maybe effs) => Eff effs () g = undefined h :: (Member Maybe effs) => Eff effs () h = f g
GHC complains that it could not deduce:
effs ~ (r : rs) arising from
the use of
g. It can be worked around by using this instead:
h :: (Member Maybe (e ': es)) => Eff (e ': es) () h = f g
but I don't think this is a good user experience, and it's a regression from 0.2.3.0 that would normally require a major version bump to 0.3 as it breaks existing code. Therefore a fix should go in quickly and should get into lts-7 asap.
This commit adds the "effs is non-empty" constraint to
Member t effs
so the above typechecks again.