On Thu, 2013-01-10 at 22:17 +0000, Simon Peyton-Jones wrote:
Is http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extension... insufficiently clear? If so, let's clarify it.
Thanks for getting back to me Simon. The document says "For example, consider these declarations: instance context1 => C Int a where ... -- (A) instance context2 => C a Bool where ... -- (B) instance context3 => C Int [a] where ... -- (C) instance context4 => C Int [Int] where ... -- (D) ... The -XOverlappingInstances flag instructs GHC to allow more than one instance to match, provided there is a most specific one. For example, the constraint C Int [Int] matches instances (A), (C) and (D), but the last is more specific, and hence is chosen. If there is no most-specific match, the program is rejected." What it doesn't says is how "most-specific match" is computed. While it is pretty clear in the given example (one matches exactly), not all cases are so clear. For example, which of instance context1 => C Int b instance context2 => C a [[b]] does C Int [[Int]] match best against? If there isn't currently a good way to resolve this, I would like to suggest the type-shape measure I proposed in that paper I wrote up awhile back could be used. Thanks! -Tyson