Question

可能重复：
Pattern matching variables in a case statement in Haskell

我有以下简化代码（实际代码确实实现了一些有意义的东西）：

import Data.Char
import Data.Maybe
import Control.Monad
import Control.Applicative

testA :: [Int] -> Maybe Int -> Maybe Int
testA [] _       = Nothing
testA _  Nothing = Nothing
testA (x:xs) v = case x of
  1         -> (+) <$> v <*> return x
  otherwise -> testA xs v

testB :: (MonadPlus m, Applicative m) => [Int] -> m Int -> m Int
testB [] _     = mzero
testB _  mzero = mzero
testB (x:xs) v = case x of
  1         -> (+) <$> v <*> return x
  otherwise -> testB xs v

main = do
  let xpto = testA [1,2,3,1,5] (Just 5)
  print xpto
  let ypto = testB [1,2,3,1,5] (Just 5)
  print ypto

据我所知，当使用Maybe Monad进行feed时，testA和testB的行为应该相同，这意味着mzero应该转换为Nothing。

然而，在测试B中，ghc实际上发出警告：

Pattern match(es) are overlapped
In an equation for `testB': testB (x : xs) v = ..

输出结果不同：

Just 6 (correct)
Just 5 (incorrect)

我不确定我可能会遗漏什么，为什么mzero在模式匹配中不等同于什么？

关于模式匹配的mzero

0 个答案: