Question

    -- Types
    type Title = String

type Actor = String
type Cast = [Actor]
type Year = Int
type Fan = String
type Fans = [Fan]
type Period = (Year, Year)
type Database = [Film]



 testDatabase :: Database
testDatabase = [("The Gunman", ["Idris Elba", "Sean Penn", " Javier Bardem"], 
               2015,["Garry", "Dave", "Zoe", "Kevin", "Emma"]),("The Shawshank Redemption", ["Tim Robbins", "Morgan Freeman", "Bob Gunton"],1994, ["Bill", "Jo", "Garry"]),
                ("The Dark Knight", ["Christian Bale", "Heath Ledger","Aaron Eckhart"], 2008, ["Zoe","Heidi", "Jo", "Emma", "Liz", "Sam", "Olga", "Kevin", "Tim"]),
                ("Inception", ["Leonardo DiCaprio", "Ellen Page"], 2010, ["Jo", "Emma", "Zack", "Olga", "Kevin"])]


displayAllFilms :: Database ->[(Title, Cast, Year, Fans)]
displayAllFilms [] = []
displayAllFilms ((i, j, k, l): xs)
    |l == [] = (i, j, k, []) : displayAllFilms xs
    |otherwise = (i, j, k, l) : displayAllFilms xs

我收到此错误＆gt;我不知道该怎么办。什么是错误？

*Main> displayAllFilms

<interactive>:13:1:
    No instance for (Show (Database -> [(Title, Cast, Year, Fans)]))
      arising from a use of `print'
    Possible fix:
      add an instance declaration for
      (Show (Database -> [(Title, Cast, Year, Fans)]))
    In a stmt of an interactive GHCi command: print it

Answer 1

查看错误消息; ghci告诉您，您正在尝试打印数据库 - ＆gt;类型的函数。 [（标题，演员，年份，粉丝）] 并且它不知道如何做到这一点。

据推测，您尝试打印displayAllFilms，而您可能意味着displayAllFilms testDatabase

在数据库haskell中显示所有电影

1 个答案: