Monad 与顺序函数调用
Monad vs sequential function calls
Wikipedia article on monads 说：
Purely functional programs can use monads to structure procedures that include sequenced operations like those found in structured programming.
不是在这里寻找（另一个）monad 教程。请举一个 monad 的例子，当仅仅 运行ning 一个函数然后另一个是不够的时候使顺序操作成为可能。根据功能语言规范，它是否与那些懒惰的函数调用有某种联系？为什么顺序 运行 的不互换功能需要任何 "wrapper"?
Haskell Monad Tutorial 清楚地显示了以单子方式对函数调用进行排序的示例：
type Sheep = ...
father :: Sheep -> Maybe Sheep
father = ...
mother :: Sheep -> Maybe Sheep
mother = ...
-- comb is a combinator for sequencing operations that return Maybe
comb :: Maybe a -> (a -> Maybe b) -> Maybe b
comb Nothing _ = Nothing
comb (Just x) f = f x
-- now we can use `comb` to build complicated sequences
mothersPaternalGrandfather :: Sheep -> Maybe Sheep
mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father
Wikipedia article on monads 说：
Purely functional programs can use monads to structure procedures that include sequenced operations like those found in structured programming.
不是在这里寻找（另一个）monad 教程。请举一个 monad 的例子，当仅仅 运行ning 一个函数然后另一个是不够的时候使顺序操作成为可能。根据功能语言规范，它是否与那些懒惰的函数调用有某种联系？为什么顺序 运行 的不互换功能需要任何 "wrapper"?
Haskell Monad Tutorial 清楚地显示了以单子方式对函数调用进行排序的示例：
type Sheep = ...
father :: Sheep -> Maybe Sheep
father = ...
mother :: Sheep -> Maybe Sheep
mother = ...
-- comb is a combinator for sequencing operations that return Maybe
comb :: Maybe a -> (a -> Maybe b) -> Maybe b
comb Nothing _ = Nothing
comb (Just x) f = f x
-- now we can use `comb` to build complicated sequences
mothersPaternalGrandfather :: Sheep -> Maybe Sheep
mothersPaternalGrandfather s = (Just s) `comb` mother `comb` father `comb` father