Haskell - 在数据库中对影片进行评分或重制

Question

我的编码非常健壮，但由于某种原因在swapRate函数中，它需要一些修复才能产生正确的结果。

import Prelude 
import Data.Char 
import Data.List 
import Text.Printf 
import Data.Ord 

-- Types 
type Title = String 
type Director = String 
type Year = Int 
type UserRatings = (String,Int) 

-- Define Film type here 
type Film = (Title,Director,Year,[UserRatings]) 

-- Database Type 
type Database = [Film] 
testDatabase = [  
("Blade Runner", "Ridley Scott", 1982, [("Amy",5), ("Bill",8), ("Ian",7), ("Kevin",9), ("Emma",4), ("Sam",7), ("Megan",4)]), 
("The Fly", "David Cronenberg", 1986, [("Megan",4), ("Fred",7), ("Chris",5), ("Ian",0), ("Amy",6)]), 
("Psycho", "Alfred Hitchcock", 1960, [("Bill",4), ("Jo",4), ("Garry",8), ("Kevin",7), ("Olga",8), ("Liz",10), ("Ian",9)]), 
("Body Of Lies", "Ridley Scott", 2008, [("Sam",3), ("Neal",7), ("Kevin",2), ("Chris",5), ("Olga",6)]), 
("Avatar", "James Cameron", 2009, [("Olga",1), ("Wally",8), ("Megan",9), ("Tim",5), ("Zoe",8), ("Emma",3)]), 
("Hugo", "Martin Scorsese", 2011, [("Sam",9), ("Wally",3), ("Zoe",5), ("Liz",7)]) 
       ] 

swapRate :: String -> Int -> String -> Title -> [UserRatings] -> [UserRatings] 
swapRate _ _ _ _ [] = [] 
swapRate user newRate film title ((name, rate):xs) 
    | user==name && film==title = (user,newRate):xs -- Does demo 11 
    -- | user/=name && film==title = (user,newRate):xs -- Does demo 1 
    | otherwise = (name,rate) : (swapRate user newRate film title xs) 
-- * Outputs an updated list 

printSwappedFilmRating :: String -> Int -> String -> Film -> String 
printSwappedFilmRating user newRate film (t,d,y,u) = "Title: " ++ t ++ "\nDirector: " ++ d ++ "\nYear: " ++ show y ++ "\nUser's Rating: " ++ (show (swapRate user newRate film t u)) ++ "\n" ++ "\n" 
-- * Formatting the output of Title, Director, Year and the tuple UserRatings 

printSwappedFilmRate :: String -> Int -> String -> Database -> String 
printSwappedFilmRate user newRate film database = concat (map (printSwappedFilmRating user newRate film) database) 
-- * Applies the printSwappedFilmRating funtion to the database and the list is concatenated as String 


----- Main Function ----- 

rateOrRerate :: String -> Int -> String -> Database -> String 
rateOrRerate user newRate film database = printSwappedFilmRate user newRate film database 

-- * Receives two Strings and an Int and with the use of printSwappedFilmRate, 
-- it prints all the movies and the updated rating of a particular user with nice formatting  


----- DEMO FUNCTIONS ----- 
demo :: Int -> IO() 

----- Demo 1 ----- 
demo 1 = putStrLn (rateOrRerate "Emma" 10 "Hugo" testDatabase) 
-- * All films after Emma rates "Hugo" 10 

----- Demo 11 ----- 
demo 11 = putStrLn (rateOrRerate "Emma" 10 "Avatar" testDatabase) 
-- * All films after Emma rates "Avatar" 10 

怎样做：
- 第一届后卫 “阿凡达” 的电影改写的速率 “艾玛”，3]至[ “艾玛”，10]
- 第二届后卫补充道[”艾玛“，10]到”雨果“电影

所以在swapRate函数，我不能同时使两个警卫工作。如果我评论第一名后卫，第二名后卫则适用，反之亦然。寻找一种方法，使都工作。

Answer 1

为了摆脱的警告，改变你的图案空字符串匹配""只是_的伎俩两个checkRateRerate和swapRate（这也提醒我的机器上）。

您的具体问题是您缺少的情况是“如果列表为空，但字符串不全是空字符串，会发生什么？”在这种情况下，这两种功能都会失效。看起来就是这样的情况，你真的只关心处理列表为空时发生的情况。

至于如何处理您的输出，我建议您使用map而不是filter。基本上你正在做的是改变testDatabase的元素，而不是去除元素。要做到这一点，你必须改变checkRateRerating输出Film而不是Bool（也许更改checkRateRerate输出[UserRatings]而不是Bool）。