putStrLn，类型Char不匹配[Char]

Question

I have an haskell problem. putStrLn is supposed to take a [Char], or a String, and even though it seems like I give that to mr Compiler, he still complains.

*** Expression     : putStrLn line
*** Term           : line
*** Type           : Char
*** Does not match : [Char]

The code that it refers to is this:

getV::[[(Char,Float)]] -> IO ()
getV [] = putStrLn ""
getV (x:xs) = do line <- getS x
   putStrLn line      <-- Complaining line
   getV xs

getS::[(Char,Float)] -> String
getS [] = ""
getS ((c,f):str) = do a <- "(L" ++ [c] ++")" ++ getS str
    return a

I did strip it a little bit, but it should be exactly the same behaviour. getS do return a String and that string is the argument for putStrLn. So what is the problem? :/

Answer 1

Since your getS returns a String , not an IO String , there's no need to "extract" the pure value with <- .

Just use

do
  let line = getS x
  putStrLn line
  getV xs

---

Also, there's a mapM function which you can

getV xs = mapM (putStrLn.getS) xs

and your getS is using monads unnecessarily.

getS [] = ""
getS ((c,_):str) = "(L" ++ [c] ++ ")" ++ getS str

Of course, it's possible to write it using built-in functions only.

getS ps = concatMap (\(c,_) -> "(L" ++ [c] ++ ")") ps

---

The reason your code doesn't fail on the line <- getS x line, and line becomes a Char is because List is also a monad. For instance, we can write the Cartesian product as

cartesian :: [a] -> [b] -> [(a,b)]
cartesian xs ys = do
    x <- xs    -- # Now x is of type 'a', representing any element in xs
    y <- ys    -- # Now y is of type 'b', representing any element in ys
    return (x, y) -- # Return the tuple composed of any elements in xs and ys.

In fact, list comprehension is based on this monadic property of list.

cartesian xs ys = [(x, y) | x <- xs, y <- ys]