[Haskell] Data.Generics.gzip3 anyone?

[Ralf Laemmel]

> 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