[Haskell-beginners] Have I just broken Haskell?

[Chaddaï Fouché]

On Tue, Mar 17, 2015 at 10:55 PM, Mark Carter <alt.mcarter at 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.
>

No it doesn't, nothing is "mutable" in Haskell, you can have IORef or STRef
whose content can be mutated in the IO or ST monad but the IORef or STRef
themselves won't be mutable. IO Handle is not "a mutable Handle", it's an
IO action that returns an Handle... And that's exactly the type of h in
your code !

That's because this code doesn't open anything, it read a line and thanks
to fmap, it applies openFileWrite to this line, giving you an "IO Handle"
in h, an action that would give you an Handle if it was executed... but it
is not executed, it is just stocked in h.
The type of "fmap openFileWrite getLine" is IO (IO Handle), not as you
probably wanted IO Handle.

As Bob said, if you want an Handle, you shouldn't use fmap but rather
(>>=). Or you could do :

act :: IO handle <- fmap openFileWrite getLine
h :: Handle <- act

but that's really not the usual way to do it (on the other hand it's
interesting because it shows you that "IO a" are just values, you can
manipulate them just like normal values.

Basically your code was "correct", it just didn't give you an Handle (that
would have been clear as soon as you tried to use h as an Handle) !
-- 
Jedaï
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/beginners/attachments/20150318/955826ad/attachment.html>