Re: [Haskell-beginners] Getting Stuff out of IO Handles

Thanks for your replies! A short example of what I'm doing: handle <- connectTo ... --opening the handle on a tcp connection hPut handle message -- sending a ByteString answer <- hGet handle n -- getting the answer back from the server hClose handle I have now several things I would like to do with the answer and my question is, if I should put the functions into the handle or if I can make the function with the handle of a type that returns answer, not of IO () as it is currently. Does this make sense? I hope so. Raf On 6/24/11, Kyle Murphy wrote:

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 wrote:

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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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