[Haskell] Data.Generics.gzip3 anyone?

[David Fox]

On Mon, Jun 1, 2009 at 3:40 PM, Ralf Laemmel <rlaemmel at gmail.com> wrote:

> > Thank you!  What I have in mind is three way merging - you have two
> > revisions based on the same original value, and you need to decide
> whether
> > they can be merged automatically or they need to be merged by a user.
> You
> > only have a real conflict when both revisions differ from the original
> and
> > from each other.
>
> Here is the completed exercise.
> For comparison, the two args versions are shown up-front.
> There is gzipWithM3 needed for gzip3, and gzip3 itself.
> I also made it so that the top-level gzip functions have the
> appropriate polymorphism.
> Say same type for the args rather than independent polymorphism.
>
> {-# LANGUAGE RankNTypes #-}
>
> import Prelude hiding (GT)
> import Data.Generics
>
> -- As originally defined: Twin map for transformation
>
> gzipWithT2 :: GenericQ (GenericT) -> GenericQ (GenericT)
> gzipWithT2 f x y = case gmapAccumT perkid funs y of
>                    ([], c) -> c
>                    _       -> error "gzipWithT2"
>  where
>  perkid a d = (tail a, unGT (head a) d)
>  funs = gmapQ (\k -> GT (f k)) x
>
>
> -- As originally defined: Twin map for transformation
>
> gzipWithM2 :: Monad m => GenericQ (GenericM m) -> GenericQ (GenericM m)
> gzipWithM2 f x y = case gmapAccumM perkid funs y of
>                    ([], c) -> c
>                    _       -> error "gzipWithM"
>  where
>  perkid a d = (tail a, unGM (head a) d)
>  funs = gmapQ (\k -> GM (f k)) x
>
>
> -- As originally defined: generic zip
>
> gzip2 ::
>    (forall x. Data x => x -> x -> Maybe x)
>  -> (forall x. Data x => x -> x -> Maybe x)
>
> gzip2 f = gzip2' f'
>  where
>  f' :: GenericQ (GenericM Maybe)
>  f' x y = cast x >>= \x' -> f x' y
>  gzip2' :: GenericQ (GenericM Maybe) -> GenericQ (GenericM Maybe)
>  gzip2' f x y =
>    f x y
>    `orElse`
>    if toConstr x == toConstr y
>      then gzipWithM2 (gzip2' f) x y
>      else Nothing
>
>
> -- For three args now
>
> gzipWithT3 ::
>    GenericQ (GenericQ (GenericT))
>  -> GenericQ (GenericQ (GenericT))
> gzipWithT3 f x y z =
>    case gmapAccumT perkid funs z of
>      ([], c) -> c
>      _       -> error "gzipWithT3"
>  where
>  perkid a d = (tail a, unGT (head a) d)
>  funs = case gmapAccumQ perkid' funs' y of
>           ([], q) -> q
>           _       -> error "gzipWithT3"
>   where
>    perkid' a d = (tail a, unGQ (head a) d)
>    funs' = gmapQ (\k -> (GQ (\k' -> GT (f k k')))) x
>
> gzipWithM3 :: Monad m
>  => GenericQ (GenericQ (GenericM m))
>  -> GenericQ (GenericQ (GenericM m))
> gzipWithM3 f x y z =
>    case gmapAccumM perkid funs z of
>      ([], c) -> c
>      _       -> error "gzipWithM3"
>  where
>  perkid a d = (tail a, unGM (head a) d)
>   funs = case gmapAccumQ perkid' funs' y of
>           ([], q) -> q
>            _       -> error "gzipWithM3"
>    where
>    perkid' a d = (tail a, unGQ (head a) d)
>     funs' = gmapQ (\k -> (GQ (\k' -> GM (f k k')))) x
>
> gzip3 ::
>    (forall x. Data x => x -> x -> x -> Maybe x)
>  -> (forall x. Data x => x -> x -> x -> Maybe x)
>
> gzip3 f = gzip3' f'
>  where
>  f' :: GenericQ (GenericQ (GenericM Maybe))
>  f' x y z = cast x >>= \x' -> cast y >>= \y' -> f x' y' z
>  gzip3' ::
>       GenericQ (GenericQ (GenericM Maybe))
>    -> GenericQ (GenericQ (GenericM Maybe))
>  gzip3' f x y z =
>    f x y z
>    `orElse`
>    if and [toConstr x == toConstr y, toConstr y == toConstr z]
>      then gzipWithM3 (gzip3' f) x y z
>      else Nothing
>

Oh, thank goodness!

-david
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