**1 Α α alpha a:lf 阿尔法 2 Β β beta bet 贝塔 3 Γ γ gamma ga:m 伽马 4...
此时递归将大显身手,一旦确定了正确的基准条件和正确的子问题,Haskell就会根据子问题的求解结果组合成最终的结果。-- 递归计算最大值,注意什么时候用方括号,什么时候用圆括号mymax:: (Orda) => [a] -> amymax[] = error"empty list"-- 基准模式1,空列表抛出错误mymax[x] = | n<=0= [] | otherwise...
fromList [("foo",10000)] 函数名最后的钩号暗示我们insertWith'将对组合函数严格求值。这个设计帮你避免了内存泄漏。该函数同时存在一个惰性的变种(即没有最后钩号的insertWith),但你大概永远用不到它。 delete函数从map中删除指定键。如果键不存在的话,delete会将map原封不动返回。 ghci> :type M.delete M....
Data.Map - fromList, empty,insert,null,size,singleton,lookup, member,map,filter, toList,keys,elems,fromListWith..InsertWith.. Data.Set - fromList, intersection,difference, union,null,size,member,empty,singleton,insert,delete -- -- + moduels.hs -- module is way to organize the program, ...
From a style point of view, you should always replace a combination of concat and map with concatMap. Suggestion - for example x !! 0 suggests head x as a "suggestion" severity hint. Typically head is a simpler way of expressing the first element of a list, especially if you are ...
import Data.List(sort) -- | Initialize DB and return database Connection connect :: FilePath -> IO Connection connect fp = do dbh <- connectSqlite3 fp prepDB dbh return dbh {- | 对数据库进行设置,做好储存数据的准备。 这个程序会创建两个表,并要求数据库引擎为我们检查某些数据的一致性: ...
Delete [a] -> a -> [a] ✓ ✓ ✓ Returns the slice with the first occurance of the element deleted. Drop Int -> [a] -> [a] ✓ ✓ ✓ Returns the suffix of xs after the first n elements. DropWhile (a -> bool) -> [a] -> [a] ✓ ✓ ✓ Returns the suffix ...
unesc :: Element -> Element unesc = xmlUnEscape stdXmlEscaper 让我们好好看看这段代码。 它首先定义了两种类型: PodItem 和Feed。 程序会将 XML 文件转换为 Feed, 而每个 Feed 可以包含多个 PodItem。 此外, 程序还提供了一个函数, 它可以将 PodItem 转换为 PodTypes.hs 文件中定义的 Episode。
(Item a) An Item is just a pair of the first element of the Stream together with a Stream holding the rest of the data: data Item a = MkItem a (Stream a) A Stream can therefore be thought of as a list, consisting of alternating Items and full MVars, terminated with a "hole" ...
–Allow the customer to delete an item (or all items) from the shopping basket. – Be persistent within the session. •Another page, Checkout, should allow the user to enter their credit card payment details, and confirm the final price to be paid. As the user steps through the fields...