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
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