Wed Oct 26 16:33:32 EDT 2011

OverlappingInstances, IncoherentInstances


- GHC's default behaviour is that exactly one instance must match the
  constraint it is trying to resolve.

- It is fine for there to be a potential of overlap; an error is only
  reported if a particular constraint matches more than one.

- The -fallow-overlapping-instances flag instructs GHC to allow more
  than one instance to match, provided there is a most specific one

[1] http://www.haskell.org/ghc/docs/6.6/html/users_guide/type-extensions.html