On Mon, 12 Dec 2005, Bulat Ziganshin wrote:

Hello

sometimes, Eq/Ord classes can't be derived automatically because we need to comare only part of fields. in such situations i use the following trick to easify generation of class instances:

data ArchiveBlock = ArchiveBlock { blArchive :: Archive , blType :: BlockType , blCompressor :: Compressor , blPos :: Integer , blOrigSize :: Integer , blCompSize :: Integer , blCRC :: CRC , blFiles :: Int }

instance Eq ArchiveBlock where (==) = map2eq $ map3 (blArchive,blPos,blCRC)

instance Ord ArchiveBlock where compare = map2cmp $ map2 (blArchive,blPos)

{- instance Ord ArchiveBlock where compare = map2cmp blPos -- for comparision on just one field -}

I solved that problem with two generic functions: Compare the same item of two records.

compareField :: Ord b => (a -> b) -> a -> a -> Ordering compareField f x y = compare (f x) (f y)

Lexicographically compare a list of attributes of two records.

compareRecord :: [a -> a -> Ordering] -> a -> a -> Ordering compareRecord cs x y = head (dropWhile (EQ==) (map (\c -> c x y) cs) ++ [EQ])

Use it this way:

instance Ord ArchiveBlock where compare = compareRecord [compareField blArchive, compareField blPos, compareField blCRC]