CODEWITHSUNDEEP
listhaskelllist-comprehension
ThatGuyOverThereIsMe
ThatGuyOverThereIsMe

Reputation: 79

Haskell Couldn't match type ‘[Char]’ with ‘Char’ list comprehension

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

Upvotes: 1

Views: 354

Answers (1)

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476709

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 Strings.

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]]

Upvotes: 3

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