haskell - 模式和视图模式：我这样做对吗

Question

我想实现

{-# LANGUAGE PatternSynonyms, ViewPatterns    #-}

pattern Just2 :: (Num a, Eq a) => a -> a -> Maybe (a, a)

-- Just2 0 0 ≡ Nothing
-- Just2 x y ≡ Just (x, y)

我想我是通过 View 模式到达那里的，但代码似乎很冗长。有没有更优雅的方式？

pattern Just2 x y <- (fromMaybe2 -> (x, y))  where
  Just2 0 0 = Nothing
  Just2 x y = Just (x, y)

fromMaybe2 Nothing       = (0, 0)
fromMaybe2 (Just (x, y)) = (x, y)

-- examples

foo (Just2 x y) = x + y

z0  = Nothing
z2  = Just (7, 8)
z00 = Just (0, 0)

*Main> foo z0
0
*Main> foo z2
15
*Main> foo z00
0

-- here's an oddity

*Main> (\ z@(Just2 x y) -> z) Nothing                -- round trip - as expected
Nothing

*Main> (\ z@(Just2 x y) -> Just2 x y) $ Just (0, 0)  -- not a round trip - as expected
Nothing
*Main> (\ z@(Just2 x y) -> z) $ Just (0, 0)          -- round trip - not as expected
Just (0,0)

补充：谢谢@chi。我很想写

pattern { Just2 0 0 = Nothing;
          Just2 x y = Just (x, y)
        }

有了这个想法，编译器将这些方程视为定义一对函数，根据需要从 l 到 r 或 r 到 l 读取，并从上到下读取以进行模式匹配。