从文件中读取，然后使用数据列表

Question

所以我有一个名为TESTDATA一个文本文件，其中包含了以下数据：

[("Blade Runner","Ridley Scott",1982,[("Amy",6),("Bill",9),("Ian",7),("Kevin",9), 
("Emma",4),("Sam",5),("Megan",4)]), 
("The Fly","David Cronenberg",1986,[("Megan",4),("Fred",7),("Chris",5),("Ian",0),("Amy",5)]), 
("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",2),("Wally",8),("Megan",9),("Tim",5),("Zoe",8),("Emma",3)]), 
("Titanic","James Cameron",1997,[("Zoe",7), ("Amy",2), ("Emma",5), ("Heidi",3), ("Jo",8), ("Megan",5), ("Olga",7), ("Tim",10)]), 
("The Departed","Martin Scorsese",2006,[("Heidi",2), ("Jo",8), ("Megan",5), ("Tim",2), ("Fred",5)]), 
("Aliens","Ridley Scott",1986,[("Fred",8), ("Dave",6), ("Amy",10), ("Bill",7), ("Wally",2), ("Zoe",5)]), 
("Prometheus","Ridley Scott",2012,[("Garry",3), ("Chris",4), ("Emma",5), ("Bill",1), ("Dave",3)]), 
("E.T. The Extra-Terrestrial","Steven Spielberg",1982,[("Ian",7), ("Amy",2), ("Emma",7), ("Sam",8), ("Wally",5), ("Zoe",6)]), 
("The Birds","Alfred Hitchcock",1963,[("Garry",7), ("Kevin",9), ("Olga",4), ("Tim",7), ("Wally",3)]), 
("Goodfellas","Martin Scorsese",1990,[("Emma",7), ("Sam",9), ("Wally",5), ("Dave",3)]), 
("The Shawshank Redemption","Frank Darabont",1994,[("Jo",8), ("Sam",10), ("Zoe",3), ("Dave",7), ("Emma",3), ("Garry",10), ("Kevin",7)]), 
("Gladiator","Ridley Scott",2000,[("Garry",7), ("Ian",4), ("Neal",6), ("Wally",3), ("Emma",4)]), 
("The Green Mile","Frank Darabont",1999,[("Sam",3), ("Zoe",4), ("Dave",8), ("Wally",5), ("Jo",5)]), 
("True Lies","James Cameron",1994,[("Dave",3), ("Kevin",4), ("Jo",0)]), 
("Minority Report","Steven Spielberg",2002,[("Dave",5), ("Garry",6), ("Megan",2), ("Sam",7), ("Wally",8)]), 
("The Wolf of Wall Street","Martin Scorsese",2013,[("Dave",6), ("Garry",6), ("Megan",0), ("Sam",4)]), 
("War Horse","Steven Spielberg",2011,[("Dave",6), ("Garry",6), ("Megan",3), ("Sam",7), ("Wally",8), ("Zoe",8)]), 
("Lincoln","Steven Spielberg",2012,[("Ian",3), ("Sam",7), ("Wally",3), ("Zoe",4), ("Liz",7), ("Megan",4)]), 
("Vertigo","Alfred Hitchcock",1958,[("Bill",7), ("Emma",5), ("Zoe",9), ("Olga",6), ("Tim",10)]), 
("The Terminal","Steven Spielberg",2004,[("Olga",3), ("Heidi",8), ("Bill",2), ("Sam",6), ("Garry",8)]), 
("Jaws","Steven Spielberg",1975,[("Fred",3), ("Garry",0), ("Jo",3), ("Neal",9), ("Emma",7)]), 
("Hugo","Martin Scorsese",2011,[("Sam",4), ("Wally",3), ("Zoe",4), ("Liz",7)])] 

而且我读这个文件是这样的：

input2 <- readFile "C:/Users/Alex/Dropbox/Uni Work Year 2/Year 2/MATHFUN/testData.txt" 
let testDatabase = input2 

现在，我有另一个被调用的IO函数，名为addFilms，它具有参数String，String，Int和[Film]。 [电影]是我的数据库的定义。

但是当我尝试致电温控功能我得到这个错误：

HaskellCW.hs:155:47: 
Couldn't match type `Char' with `(String, String, Int, [Rating])' 
Expected type: [Film] 
    Actual type: String 
In the fourth argument of `addFilm', namely `testDatabase' 
In the first argument of `putStrLn', namely 
    `(addFilm title director year testDatabase)' 
In a stmt of a 'do' block: 
    putStrLn (addFilm title director year testDatabase) 

谁能帮助我呢？我需要保持数据库类型[电影]。找到下面的代码我有。

type Film = (String, String, Int, [Rating]) 

testDatabase :: [Film] 

addFilm :: String -> String -> Int -> [Film] -> String 
addFilm nTitle nDirector nYear films | [nTitle] == title = show films 
            | otherwise   = show (films ++ [newfilm]) 
           where newfilm  = (nTitle, nDirector, nYear, ratings) 
             title   = [title | (title,director,year,ratings) <- films] 
             ratings  = [] 

main :: IO() 
main = do 
putStrLn "Please Enter Your Name: " 
input <- getLine 
let userName = input 
putStrLn ("Your name is: " ++ userName) 
input2 <- readFile "C:/Users/Alex/Dropbox/Uni Work Year 2/Year 2/MATHFUN/testData.txt" 
let testDatabase = input2 
putStrLn testDatabase 

putStrLn "Type a function to perform. List of Functions: " 
putStrLn "addFilm, showFilms, getDirector, getHighRatings, getAverageDirector, getUserRatings" 

str <- getLine 
if str == "addFilm" then do 
    putStrLn "Function addFilm" 
    putStrLn "Enter film title: " 
    input1 <- getLine 
    let title = input1 
    putStrLn "Enter film director: " 
    input2 <- getLine 
    let director = input2 
    putStrLn "Enter film year: " 
    input3 <- getLine 
    let year = (read input3 :: Int) 
    putStrLn (addFilm title director year testDatabase) 
else if str == "showFilms" then do 
    putStrLn "Function showFilms" 
else if str == "getDirector" then do 
    putStrLn "Function getDirector" 
else if str == "getHighRatings" then do 
    putStrLn "Function getHighRatings" 
else if str == "getAverageDirector" then do 
    putStrLn "Function getAverageDirector" 
else if str == "getUserRatings" then do 
    putStrLn "Function getUserRatings" 
else do 
    putStrLn "Thankyou for using the system, saving the database." 
    writeFile "C:/Users/Alex/Dropbox/Uni Work Year 2/Year 2/MATHFUN/testData.txt" testDatabase 

Answer 1

你的字符串包含实际的Haskell信息的字符串表示：1是Int，但"1"是String。您无法将String传递给接受[Film]的函数，出于同样的原因5 + 4 + "6"不是有效的表达式。

使用read：

input2 <- readFile "C:/Users/Alex/Dropbox/Uni Work Year 2/Year 2/MATHFUN/testData.txt" 
let testDatabase = read input2 :: [Film] 

它会分析输入并将其转换为[Film]。 read基本上与show相反：它获取Haskell值的字符串表示形式并返回实际值。

请注意，类型签名在这里可能至关重要。如果你写1 + read "2"哈斯克尔可以从上下文推断"2"表示Int，但仅靠read "2"会导致编译器错误，因为"2"可能意味着几件事情 - 数2和字符串"2"，例如 - 和Haskell不会能够推断出它的类型。

您可以阅读更多关于read和showhere。