[Haskell] Data.Generics.gzip3 anyone?

[David Fox]

On Tue, Jun 2, 2009 at 1:13 PM, David Fox <ddssff at gmail.com> wrote:

> 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
>>
>
> Ok, what I initially thought would work is not.  I tried to do the three
> way merge as follows:
>
> combine3 :: (Data a) => a -> a -> a -> Maybe a
> combine3 original left right =
>     gzip3 f original left right
>     where
>       f :: forall a. (Data a) => a -> a -> a -> Maybe a
>       f original left right
>           | geq original left = Just right
>           | geq original right = Just left
>           | geq left right = Just left
>           | otherwise = Nothing
>
> However, what happens is that we usually reach the otherwise clause when
> processing the top level of the data structure, so you get nothing.  What
> really needs to happen is that it traverses down into the data structure and
> finds out that f is able to merge all the more primitive pieces of the data
> structure, in which case it combines those merged parts to yield a merged
> whole.  I'm not quite sure how to fit this operation into the generic
> framework.
>

Oh, I got it.  I have to remove the "f x y z `orElse`" from the definition
of gzip3.
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