On 13 August 2010 00:13, John Lask
I have wondered and perhaps someone can explain: what are the issues in explicit control of instance export and import? (apart from defining an appropriate syntax)
IMHO main problem with this (and related feature requests like local instance definitions) is a sort of incoherency: """ module A where data Foo = Yes | No deriving (Eq) """ """ module B ( mySet, .. don't export Ord instance .. ) where import A instance Ord Foo where Yes `compare` Yes = EQ No `compare` No = EQ Yes `compare _ = LT _ `compare` _ = GT mySet = Data.Set.fromList [Yes, No] """ """ module Main where import A import B instance Ord Foo where Yes `compare` Yes = EQ No `compare` No = EQ Yes `compare` _ = GT _ `compare` _ = LT main = do print $ valid mySet print $ S.fromList [Yes, No] == mySet """ This program will likely print "False" twice. It is also highly likely that trying to use other Set operations on mySet will result in an error, because the ordering that is being used on Foo in calls from Main is different from that used to construct the set. This is a very subtle error, and could be tricky to debug. Kiselyov and Shan have proposed a sort of local instance definition that appears not to give rise to these problems (because its restricted to only instances that mention a skolem variable, so they can't "leak"). See http://okmij.org/ftp/Haskell/types.html#Prepose for details. This may capture at least some of the use cases that people want instance export control for. Cheers, Max