Haskell无法将类型'[Char]'与'Char'列表理解匹配

Question

So I'm trying to go through a list of films and output the title of each film, the compiler is saying that is couldn't match type [Char] with Char . The thing is I want [Char] (A string) and not Char ?? I'm confused as to why it wants a character and not a string?

fas:: [Film] -> String
fas database = [title x | x <- database]

test.hs:55:27: error:
    • Couldn't match type ‘[Char]’ with ‘Char’
      Expected type: Char
        Actual type: String
    • In the expression: title x
      In the expression: [title x | x <- database]
      In an equation for ‘fas’:

          fas database = [title x | x <- database]

Any help would be greatly appreciated, I've probably worded this poorly which hasn't helped me when trying to search other people have the same problem :/

regards, a novice

Answer 1

If I understand what you want to do correctly, the signature should be:

fas:: [Film] -> String

Indeed, a list of strings .

Why? Because you map every film to its title. Now since a title is a String , the result is a list of String s.

The compiler is actually complaining in the other way than you interpret it : since type String = [Char] , you wrote implicitly:

fas :: [Film] -> [Char] -- incorrect

whereas it should be:

fas :: [Film] -> [[Char]]