Haskell在元组中打印列表的所有元素

Question

I'm trying to print all elements of the Fans list for a given film title for example all fans of "Avatar" not quite sure how to filter this and display the correct elements as a string. Here is the type definition of the program with some of the test data.

import Data.Char
import Data.List

--types
type Title = String
type Director = String
type Year = Int
type Fans = [String]

type Film = (Title, Director, Year, Fans)

type Database = [Film]

testDatabase :: [Film]
testDatabase = [
 ("Blade Runner", "Ridley Scott", 1982, ["Zoe", "Heidi", "Jo", "Kate", "Emma", "Liz", "Sam", "Olga", "Tim"]),
 ("The Fly", "David Cronenberg", 1986, ["Garry", "Dave", "Zoe", "Kevin", "Emma"]),
 ("Body Of Lies", "Ridley Scott", 2008, ["Bill", "Olga", "Tim", "Zoe", "Paula"]),
 ("Avatar", "James Cameron", 2009, ["Dave", "Amy", "Liz"]),
 ("Titanic", "James Cameron", 1997, ["Zoe", "Emma", "Paula", "Liz", "Olga", "Dave"])]

Answer 1

You're really shooting yourself in the foot by using type synonyms for everything. data Film = Film Title Director Year Fans is much better than using a tuple in this case. Defining a separate Film data type brings the semantic difference between a Film and a tuple of Title , Director , Year , and Fans into the type system. This means that you can define instances for it (like Show or Eq ) that will behave in a specific way that is different from what a tuple would give you. It also causes the compiler to reject code that (potentially accidentally) mixes a tuple that happens to have the type (String,String,Int,[String]) with an actual Film . This adds safety, which is one of the main reasons to use Haskell in the first place.

That said, listing the fans only is pretty simple either way:

listFans :: Film -> String
-- data way
listFans (Film _ _ _ fans) = intersperse ',' fans
-- tuple way
listFans (_,_,_,fans) = intersperse ',' fans

Next, we need a lookupFilm :: Database -> Title -> Maybe Film to find the film we want. We need the Maybe because the Film might not be in the Database .

lookupFilm :: Database -> Title -> Maybe Film
lookupFilm db t = find (\(Film t' _ _ _) -> t == t') db

Note that this doesn't "print" the strings, it just returns them, so you would need putStrLn . listFans :: Film -> IO () putStrLn . listFans :: Film -> IO () to actually output them.

If you always lookup films by their title, you might as well use a Map Title Film from Data.Map .

Lastly: be sure you understand how find and intersperse work; Data.List is like being given a fish, but you need to learn how to fish for yourself in order to tackle more complex problems as they arise.