Question

Following a haskell tutorial, the author provides the following implementation of the withFile method:

withFile' :: FilePath -> IOMode -> (Handle -> IO a) -> IO a  
withFile' path mode f = do  
    handle <- openFile path mode   
    result <- f handle  
    hClose handle  
    return result  

But why do we need to wrap the result in a return? Doesn’t the supplied function f already return an IO as can be seen by it’s type Handle -> IO a?

Answer 1

You’re right: f already returns an IO, so if the function were written like this:

withFile' path mode f = do  
    handle <- openFile path mode   
    f handle

there would be no need for a return. The problem is hClose handle comes in between, so we have to store the result first:

result <- f handle

and doing <- gets rid of the IO. So return puts it back.