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lopezrican304
lopezrican304

Reputation: 175

Haskell- IO String get multiple Lines

I am trying to write a function that gets multiple string inputs and terminates on an empty line ('\n') I have following

getLines :: IO [String]
getLines = do x <- getLine
           if x == ""
           return ()
           else do xs <- getLines
                return (x:xs)

it fails to compile saying there is something wrong with the if statement

Ideally I want to to work like this

getLines

Apple

Bird

Cat

(enter is pressed)

Output: ["Apple","Bird","Cat"]

Upvotes: 2

Views: 2353

Answers (2)

Jon Purdy
Jon Purdy

Reputation: 55029

You need then after if, and () (empty tuple) should be [] (empty list). In if, the then and else need to be indented as well if they’re on separate lines, because if then else is an operator, and the compiler needs to know that the expression continues. Your indentation needs to be adjusted so that everything is indented more than the leading do:

getLines :: IO [String]
getLines = do x <- getLine
              if x == ""
                then return []
                else do xs <- getLines
                        return (x:xs)

Or everything is indented more than the line containing the leading do:

getLines :: IO [String]
getLines = do
  x <- getLine
  if x == ""
    then return []
    else do
      xs <- getLines
      return (x:xs)

As a matter of style, you can also use null x instead of x == "".

Upvotes: 7

MathematicalOrchid
MathematicalOrchid

Reputation: 62848

This is nearly right.

You need a then after your if. You may also need to change the indentation slightly, so that everything is indented as far as the x rather than the do. I think that should just about do it.

Upvotes: 1

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