Ross Paterson wrote:

On Fri, Aug 15, 2008 at 04:17:44PM +0200, Sean Leather wrote:

...

module A where class A t where a :: t

module B where import A instance A Int where a = 0 a0 :: Int a0 = a

module C where import A instance A Int where a = 1 a1 :: Int a1 = a

module Main where import A import B import C main = do putStrLn $ "a0=" ++ show a0 putStrLn $ "a1=" ++ show a1

This works, because of the way the instances are used. While overlapping instances are imported into Main, they are not used in Main.

Then that is a GHC bug. Haskell 98 Report 4.3.2: "A type may not be declared as an instance of a particular class more than once in the program."

That's interesting. So, maybe there should be some language extension or warning (with associated -fno-warn) for this in GHC. Personally, I prefer the way it's done now. (I guess that's obvious, considering I'm developing a library that will take advantage of it. ;) ) But it makes sense that instances are only looked up when needed, instead of globally tracked. Sean