Haskell 编程答案

【问题标题】：Haskell ProgrammingHaskell 编程
【发布时间】：2017-04-29 14:46:34
【问题描述】：

我需要实现一个函数，它接受Dists 的列表并返回Dists 的列表。我需要该函数仅返回带有标签"pass" 的Dists，但不知何故这不起作用。有什么帮助吗？

data Ex = Ex Float Float String String deriving Show 
data NewSt = NewSt Float Float String deriving Show 
data Dist = Dist Float NewSt Ex deriving Show 

helper1 [] = []
helper1 (Dist x (NewSt midterm1 quiz1 name1) (Ex midterm quiz name label) : xs) = if (label == "pass")
  then Dist x (NewSt midterm1 quiz1 name1) (Ex midterm quiz name label) : (helper1 xs) 
  else helper1 xs

【问题讨论】：

标签： list haskell types

【解决方案1】：

使用更多的模式匹配比使用if 表达式更容易编写。

helper1 :: [Dist] -> [Dist]
helper1 [] = []
helper1 (Dist x newst (Ex midterm quiz name "pass") : xs) = Dist x newst (Ex midterm quiz name "pass") : (helper1 xs)
helper (_:xs) = helper1 xs

但是，一旦您认识到您的递归已经由 filter 函数实现，就更简单了。

helper1 :: [Dist] -> [Dist]
helper1 = filter passed
          where passed (Dist _ _ (Ex _ _ _ "pass")) = True
                passed _ = False

【讨论】：

【解决方案2】：

我猜你可以这样做；

data Ex    = Ex Float Float String String deriving Show 
data NewSt = NewSt Float Float String deriving Show 
data Dist  = Dist Float NewSt Ex deriving Show 

myData :: [Dist]
myData = [Dist 1 (NewSt 66 100 "John") (Ex 55 90 "John" "pass"),
          Dist 2 (NewSt 20 45 "Tom") (Ex 33 50 "Tom" "fail"),
          Dist 3 (NewSt 75 75 "Mary") (Ex 90 100 "Mary" "pass")]

helper1 [] = []
helper1 d@(Dist _ _ (Ex _ _ _ label):ds) | label == "pass" = (head d):(helper1 ds)
                                         | otherwise       = helper1 ds

【讨论】：