On Tue, Mar 17, 2015 at 2:55 PM, Mark Carter <alt.mcarter@gmail.com> wrote:

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

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