However, it seems that your particular problem can be solved with
simpler means:
instance (HList a) = HListAppendArbitrary a HNil a where
hAppendArbitrary a _ = a
instance (HList a, HList b, HList c)
= HListAppendArbitrary a (HCons b d) c where
hAppendArbitrary a b = hAppend
The problem you report can be fixed with some trickery and local
functional dependencies. I'd like to show a different solution, which
follows a useful general pattern, of isolating overlapping instances
to one small part of the program that analyzes the type. The rest of
the type program just
Wow.
That said, it is quite possible in Haskell to achieve genuine
class-based dispatch, with backtracking if necessary:
http://pobox.com/~oleg/ftp/Haskell/poly2.txt
Thanks for digging this up.
I'll have to reread it tomorrow.
I wasn't able to find the definition of AllOf(But):
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