Haskell IO (String) and String

Question

I want to write functions and put result to string.

I want function:

read' :: FilePath -> String

I use:

:t readFile
readFile :: FilePath -> IO String

I make:

read' :: IO ()
read' = do
     str <- readFile "/home/shk/workspace/src/test.txt" 
     putStrLn str

I want to ask str is string or not?

We know that:

:t putStrLn
putStrLn :: String -> IO ()

Then why i can't:

read' :: String
read' = do
     str <- readFile "/home/shk/workspace/lxmpp/src/test.txt" 
     str

I get error that:

 Couldn't match expected type `[t0]' with actual type `IO String'
    In the return type of a call of `readFile'
    In a stmt of a 'do' expression:
        str <- readFile "/home/shk/workspace/lxmpp/src/test.txt"
    In the expression:
      do { str <- readFile "/home/shk/workspace/src/test.txt";
           str }

Thank you.

Answer 1

Just to quibble a bit more, while the other answers are perfectly correct, I want to emphasize something: Something with the type IO String isn't just a string that the type system won't let you get at directly. It's a computation that performs I/O to get a string for you. Applying readFile to a file path doesn't return a String value any more than putting a steak next to a meat grinder magically turns them into a hamburger.

When you have some code like this:

foo = do let getStr = readFile "input.txt"
         s1 <- getStr
         s2 <- getStr
         -- etc.

That doesn't mean you're "taking the string out of getStr twice". It means you're performing the computation twice and can easily get different results between the two.