[Haskell-cafe] trick to easily generate Eq/Ord instances

[Henning Thielemann]

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]