在CLEAN中使用Maybe类型时出错

Question

我是功能编程和CLEAN的新手.我有一些功能,我在一个中得到错误,我无法弄清楚为什么.(我用Haskell标记了它,因为它与CLEAN非常相似.)

我的模块:

module Prac

combine :: (Maybe a) (Maybe [a]) -> Maybe [a]
combine Nothing _ = Nothing
combine _ Nothing = Nothing
combine (Just d) (Just [x:xs]) = Just [d, x:xs]

sequence :: [Maybe a] -> Maybe [a]
sequence [Just x:xs] =  combine  (Just x)  Just[xs]

它在序列定义中失败:

 Type error [Prac.icl,32,sequence]: near combine : cannot unify types:
 [v8] -> Maybe [v4]
 Maybe [v5]

非常感谢!!

Answer 1

这将在Haskell中起作用:

combine :: Maybe a -> Maybe [a] -> Maybe [a]
combine Nothing _ = Nothing
combine _ Nothing = Nothing
combine (Just d) (Just xs) = Just (d:xs)

sequence :: [Maybe a] -> Maybe [a]
sequence [] = Just []
sequence (a:xs) =  combine  a (sequence xs)

但我不确定它是否符合你的要求:

?> sequence [Just 1, Just 3, Just 4]
Just [1,3,4]

?> sequence [Just 1, Nothing, Just 4]
Nothing

?> sequence []
Just []

好吧,我找到了一个翻译计划 - 但没有保证,因为我现在没办法测试它

sequence :: [Maybe a] -> Maybe [a]
sequence [] = Just []
sequence [x:xs] =  combine x (sequence xs)

虽然不确定空列表和签名 - 抱歉

无论如何,如果你可以重用你只是将给定列表的头部与尾部的递归计算序列结合起来的想法你应该没问题

这适合我使用干净

所以我刚刚下载了IDE,做了一个项目,添加了一个模块:

module seq

:: MayBe a = Just a | Nothing

Start = sequence [Just 3, Just 4, Just 5]

combine Nothing _ = Nothing
combine _ Nothing = Nothing
combine (Just d) (Just xs) = Just [d:xs]

sequence [] = Just []
sequence [x:xs] =  combine x (sequence xs)

编译了这个,更新了项目并运行了 - 这就行了