On Mon, Mar 8, 2010 at 11:28, Stephen Tetley
Hi Magnus
That's not quite equivalent to Joe Fox's original code, although it is an improvement as test in the original won't type check:
original...
test (DataConst1 x y) = x -- returns Int test (DataConst2 x y) = x -- returns Int test (DataConst3 x y) = y -- returns String test (DataConst4 x y) = y -- returns Int
This one does type check...
test2 :: TestData -> Either Int String test2 (DataConst1 x y) = Left x test2 (DataConst2 x y) = Left x test2 (DataConst3 x y) = Right y test2 (DataConst4 x y) = Left y
I don't see where it's different from my code (i.e. modulo the ordering of 'Either ...'). Now I may be missing something, but defining a conversion function and running smallcheck (or I guess quickcheck) shows that 'prop_test2EqTestM' holds (I took the liberty of swapping the Either around compared to your 'test2' above): data TestData = DataConst1 Int String | DataConst2 Int Int | DataConst3 String String | DataConst4 String Int test2 (DataConst1 x _) = Right x test2 (DataConst2 x _) = Right x test2 (DataConst3 _ y) = Left y test2 (DataConst4 _ y) = Right y type TestDataM = (Either String Int, Either String Int) testM :: TestDataM -> Either String Int testM (x@(Right _), _) = x testM (Left _, y) = y conv2Tuple :: TestDataM -> TestData conv2Tuple (Right x, Left y) = (DataConst1 x y) conv2Tuple (Right x, Right y) = (DataConst2 x y) conv2Tuple (Left x, Left y) = (DataConst3 x y) conv2Tuple (Left x, Right y) = (DataConst4 x y) prop_test2EqTestM tdm = let td = conv2Tuple tdm in test2 td == testM tdm What am I missing? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe