Question

我只需要以下代码的帮助。我试图给出一个给定导演的电影的网站评级的平均值。

但是，运行以下代码时似乎出现错误。错误是：无效的类型签名：DirectorAverage :: String - ＆gt;浮动应该是::

形式

我似乎无法发现代码的错误，因为我相信我已正确编写代码，如果有人可以帮助解决此错误，我将不胜感激！

谢谢！

import Data.List 
import Text.Printf

type Rating = (String, Int)
type Title = String
type Director = String
type Year = Int
type Film = (Title, Director, Year,[Rating])


testDatabase :: [Film]
testDatabase = [("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)])]         


--Some functions needed later on:
averageFilmRating :: [(String,Int)] -> Float
averageFilmRating ratings
     = (fromIntegral(sum $ map snd ratings)) / (fromIntegral(length ratings))

--Formats the films for decimal, gives average rating of films instead of all users ratings.
formatFilm :: Film -> String
formatFilm (title, director, year, rating)
     = printf "%s by %s. Year: %d, Average Rating: %.1f" (title) (director) (year) (averageFilmRating rating)    


displayByDirector :: String -> [Film]
displayByDirector name
     =  filter(\(_,director,_,_) -> director == name) testDatabase

DirectorAverage :: String -> Float
DirectorAverage dir
     = averageFilmRating [ (user,rating) | (title, director, year, [(user, rating)]) <- displayByDirector dir]

Answer 1

在Haskell中使用Case来区分以大写字母开头的类型名称，以及以小写字母开头的值的名称。（注意，函数是Haskell中的值。）

因此DirectorAverage是函数的无效名称。按照惯例，您的函数应命名为directorAverage：

directorAverage :: String -> Float
directorAverage dir
     = averageFilmRating [ (user,rating) | (title, director, year, [(user, rating)]) <- displayByDirector dir]

Haskell平均误差

1 个答案: