有一些类型Record
:范围模式匹配变量
type Day = Integer
type Description = String
type Name = String
type PhoneNumber = String
type Year = Integer
data Month = January | February | March | April | May | June | July
| August | September | October | November | December
deriving (Eq, Ord, Enum, Show)
data Birthday = Birthday Month Day
deriving (Eq, Show)
data DatingDate = DatingDate Year Month Day
deriving (Eq, Show)
data Record = BirthdayRecord Name Birthday
| PhoneRecord Name PhoneNumber
| DatingRecord DatingDate Description
deriving (Eq, Show)
和功能,其过滤这些记录日期:
getAssignment :: (Year, Month, Day) -> [Record] -> [Record]
getAssignment (year, month, day) = filter matchDate
where matchDate (BirthdayRecord _ (Birthday month day)) = True
matchDate (DatingRecord (DatingDate year month day) _) = True
matchDate _ = False
的getAssignment
这个定义是因为错误的不正确的是:
warning: Defined but not used: `year'
其实这对我来说是一种惊喜year
模式匹配部分getAssignment
和year
模式匹配部分matchDate
不一样。
那么,year
变量的范围界限在哪里开始和完成?这是因为where
部分?
顺便说一句,这个错误可以避免与一些冗余使用(year, month, day)
变量。
getAssignment' :: (Year, Month, Day) -> [Record] -> [Record]
getAssignment' date = filter (matchDate date)
where matchDate (_, m, d) (BirthdayRecord _ (Birthday month day)) =
month == m && day == d
matchDate (y, m, d) (DatingRecord (DatingDate year month day) _) =
year == y && month == m && day == d
matchDate _ _ = False
怎样才可以改写?
有什么办法可以抑制'month'和'month''变量吗? – 2012-04-07 20:02:29
对不起,我不明白你的评论。你能否提供你要求的更多细节? – dave4420 2012-04-07 20:07:02
当我看到'f x = x == y'的定义时,我想重写它,因为'f y == True',因为'x'变量的多余使用。在这种情况下,我想用同样的技巧来摆脱“年”,“月”和“日”变量。 – 2012-04-07 20:18:05