[Haskell-beginners] Getting Stuff out of IO Handles

[Kyle Murphy]

I had started to say something about the circumstances under which you
actually can use unsafePerformIO, but it sort of bogged down the reply and
muddled the point I was trying to make. You seem to have done a better job
explaining the circumstances under which you can actually use unsafePerfomIO
safely than I was able to at the time.


-R. Kyle Murphy
--
Curiosity was framed, Ignorance killed the cat.


On Thu, Jun 23, 2011 at 19:46, Jared Hance <jaredhance at gmail.com> wrote:

> > If you want to know if you can do something like:
> >
> > someFunction :: Int -> Int
> > someFunction n = escapeIO $ someIOAction n
> >
> > then the answer is you shouldn't. It is technically possible, by using a
> > function that every experienced haskell developer will tell you to never
> > ever ever ever ever ever ever ever (get the picture) use, which is
> > unsafePerformIO, but if you use unsafePerformIO it's likely that your
> code
> > won't do what you actually expect it to do, even if the type signatures
> all
> > check correctly. The IO Monad serves a very important purpose and implies
> > certain things about a computation, likewise the absence of the IO Monad
> > implies certain guarantees, and when you break those guarantees bad
> things
> > happen.
>
> I'd like to clarify a bit here. The unsafePerformIO function has a use,
> and should be used, but only in the right circumstances. The problem is
> that often a function is referentially transparent, but haskell cannot
> prove that it is because it uses the IO Monad.
>
> The circumstance in which this crops up is usually when you use the FFI.
> The function you call in C might return a pointer to something, rather
> than the data itself. The pointer will usually be different every time,
> so the result isn't referentially transparent. However, if you extract
> the data from the pointer, that data _is_ referentially transparent and
> is, as such, a good use case for unsafePerformIO.
>
> So I would say an experienced programmer would say to never use it if
> you don't know what you are doing, but if you do, use it when you need
> it.
>
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