[... snip]
Of course instances above are overlapping. And when we add functional dependencies (since we really want type-functions rather type relations), they stop working at all. We had to employ work-arounds, which are described in detail in the HList paper (which is 8 years old already).
Yes, it's adding the FunDeps that puts the spanner in the works. Oleg, did you see this, and the discussion around that time? http://www.haskell.org/pipermail/haskell-prime/2012-May/003688.html I implemented hDeleteMany without FunDeps -- and it works in Hugs (using TypeCast -- but looks prettier in GHC with equality constraints). Essentially it's a FunDep-like mechanism without FunDeps (as SPJ calls it), to achieve what Ryan's talking about. But you are quite right that we still need overlapping instances for parts of the type-level logic. AntC