[Haskell-beginners] Have I just broken Haskell?

[Anurag Ohri]

I recently encountered the same Exception. It occurs when you open a file
but don't close it using "hClose".
On Mar 18, 2015 3:46 AM, "Bob Ippolito" <bob at redivi.com> wrote:

> On Tue, Mar 17, 2015 at 2:55 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.
>>
>
> 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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