On Tue, Mar 17, 2015 at 2:55 PM, Mark Carter
Persuant to my previous question, I have made some progress, but there is something odd going on.
Suppose we want to implement the following algorithm, which is a little simpler than the original problem:
The user types the name of a file as input. Haskell then puts the text "it works" to that file. I tried this:
import System.IO let openFileWrite name = openFile name WriteMode h :: IO Handle h <- fmap openFileWrite getLine -- you need to type in a file name
This looks wrong, because h presumably needs to be a fixed Handle. Being an IO Handle seems to imply it is mutable. Haskell doesn't complain, though.
This is wrong because the type of `h` should be Handle, not IO Handle. `fmap` is not the correct function to use here, you use `fmap` to lift a pure function into some Functor. What you need to do is sequence these two IO actions, like this: name <- getLine h <- openFileWrite name Which is equivalent to this: h <- getLine >>= openFileWrite or this (`=<<` is just a flipped `>>=` operator): h <- openFileWrite =<< getLine -bob