Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
2009/04/21 Manlio Perillo manlio_peri...@libero.it: Luke Palmer ha scritto: And many other permutations, with differing degrees of laziness and parametericity. As long as you stricly read a string from the socket, this is ok. But you can not read a string lazily. Why not? It changes the way in which the program fails (if it does) but it does not change the result. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Luke Palmer ha scritto: [...] Note that it is not always possible to separate IO from pure code. As an example, consider an HTTP 1.1 server that read a request body containing a number for each line, and return a response body containing the sum of the numbers. What? sumRequest :: String - String -- strips header, sums numbers, returns response sumServer :: IO () -- reads from socket, writes sumRequest to socket And many other permutations, with differing degrees of laziness and parametericity. As long as you stricly read a string from the socket, this is ok. But you can not read a string lazily. Regards Manlio ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Sun, 19 Apr 2009, Manlio Perillo wrote: Note that it is not always possible to separate IO from pure code. As an example, consider an HTTP 1.1 server that read a request body containing a number for each line, and return a response body containing the sum of the numbers. Here, you can not really separate IO from pure computation. And you can not use lazy IO, if you want your server to support HTTP 1.1 pipelining. I have done exactly this, but I needed to define the LazyIO type: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/http-monad http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lazyio ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Henning Thielemann ha scritto: On Tue, 14 Apr 2009, rodrigo.bonifacio wrote: I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? The old Wiki had: http://www.haskell.org/wikisnapshot/ThatAnnoyingIoType.html Should be ported to the new Wiki since it is a FAQ ... Note that it is not always possible to separate IO from pure code. As an example, consider an HTTP 1.1 server that read a request body containing a number for each line, and return a response body containing the sum of the numbers. Here, you can not really separate IO from pure computation. And you can not use lazy IO, if you want your server to support HTTP 1.1 pipelining. Regards Manlio Perillo ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Sun, Apr 19, 2009 at 9:48 AM, Manlio Perillo manlio_peri...@libero.itwrote: Henning Thielemann ha scritto: On Tue, 14 Apr 2009, rodrigo.bonifacio wrote: I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? The old Wiki had: http://www.haskell.org/wikisnapshot/ThatAnnoyingIoType.html Should be ported to the new Wiki since it is a FAQ ... Note that it is not always possible to separate IO from pure code. As an example, consider an HTTP 1.1 server that read a request body containing a number for each line, and return a response body containing the sum of the numbers. What? sumRequest :: String - String -- strips header, sums numbers, returns response sumServer :: IO () -- reads from socket, writes sumRequest to socket And many other permutations, with differing degrees of laziness and parametericity. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, 14 Apr 2009, rodrigo.bonifacio wrote: I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? The old Wiki had: http://www.haskell.org/wikisnapshot/ThatAnnoyingIoType.html Should be ported to the new Wiki since it is a FAQ ... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, Apr 14, 2009 at 8:54 AM, rodrigo.bonifacio rodrigo.bonifa...@uol.com.br wrote: Dear Sirs, I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? This is very important: you cannot. But you can still get your hands on one inside a do block. As in, if you have tree :: IO [XmlTree], then you can say, eg. printTree = do t - tree print t Inside this block, t is an [XmlTree], and it is passed to print. Intutively, when you see x - y inside a do block, if y :: IO a, for some type a, then x :: a. But there is no function that will get you from IO [XmlTree] - [XmlTree], you have to use binding like this. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Hallo, On 4/14/09, rodrigo.bonifacio rodrigo.bonifa...@uol.com.br wrote: Dear Sirs, I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? The short answer is: You cannot. The longer answer is: Only put things in the IO monad when you need to interact with the outside, i. e., reading, displaying etc. Cheers, -- -alex http://www.ventonegro.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, Apr 14, 2009 at 4:54 PM, rodrigo.bonifacio rodrigo.bonifa...@uol.com.br wrote: Dear Sirs, I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? Quick and dirty answer: unsafePerformIO. That's an easy finding on Hoogle: http://www.haskell.org/hoogle/?hoogle=IO+a+-%3E+a Anyhow, as the name suggest, the function is unsafe, so you better know what you're doing. Bye, Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, Apr 14, 2009 at 5:01 PM, Luke Palmer lrpal...@gmail.com wrote: ... This is very important: you cannot. I'd answer You shouldn't, unless you know what you are doing. In some cases, not only is unsafePerformIO desirable but also necessary (I'm thinking of Debug.Trace). Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, Apr 14, 2009 at 9:01 AM, Cristiano Paris fr...@theshire.org wrote: On Tue, Apr 14, 2009 at 4:54 PM, rodrigo.bonifacio rodrigo.bonifa...@uol.com.br wrote: Dear Sirs, I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? Quick and dirty answer: unsafePerformIO. Please don't say that. He's a beginner. You realize that the path of least resistance will be to use it, right? You see why that's not a good thing? Even experts don't use this function. (To the O.P.: don't use it) Luke That's an easy finding on Hoogle: http://www.haskell.org/hoogle/?hoogle=IO+a+-%3E+a Anyhow, as the name suggest, the function is unsafe, so you better know what you're doing. Bye, Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Here's another way of looking at what others have already said. The only way you can do that is within the scope of another IO action. For example: outputXmlTrees :: IO () outputXmlTrees = do trees - inputXmlTrees; let newTrees = transform trees; print . show $ newTrees Notice a few things: - First, the line trees - inputXmlTrees effectively takes an IO [XmlTree] and turns it into a [XmlTrees]. That is, it runs the IO action inputXmlTrees, and gives you the resulting list of XmlTrees to work with. - You can then pass these off to a pure function which will monkey around with them in a pure (non-IO) context - You must do this in an IO action, however, and any monadic action gets its type from the last line. Thus, the last line must be of type IO something. In this case, it is simply the action that would print out the trees. - Thus, this gives you a way to glue together different IO actions and pure actions and combine them into larger IO actions Hope this clarifies On Tue, Apr 14, 2009 at 10:54 AM, rodrigo.bonifacio rodrigo.bonifa...@uol.com.br wrote: Dear Sirs, I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? Regards, Rodrigo. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Quick and dirty answer: unsafePerformIO. You can do a lot of cool things with a table saw if you take the blade guard off. On Tue, Apr 14, 2009 at 11:01 AM, Cristiano Paris fr...@theshire.orgwrote: On Tue, Apr 14, 2009 at 4:54 PM, rodrigo.bonifacio rodrigo.bonifa...@uol.com.br wrote: Dear Sirs, I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? Quick and dirty answer: unsafePerformIO. That's an easy finding on Hoogle: http://www.haskell.org/hoogle/?hoogle=IO+a+-%3E+a Anyhow, as the name suggest, the function is unsafe, so you better know what you're doing. Bye, Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- /jve ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Hello rodrigo.bonifacio, Tuesday, April 14, 2009, 6:54:07 PM, you wrote: I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? IO [XmlTree] is an action returning [XmlTree]. so to convert it to [XmlTree] you just need to execute it in IO monad: value - action i suggest you to read http://haskell.org/haskellwiki/IO_inside in order to manage IO monad -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, Apr 14, 2009 at 5:09 PM, Luke Palmer lrpal...@gmail.com wrote: ... Please don't say that. He's a beginner. You realize that the path of least resistance will be to use it, right? You see why that's not a good thing? Even experts don't use this function. (To the O.P.: don't use it) Mmmh, sorry Luke but I don't understand this ostracism. unsafePerformIO is not evil by itself, it's there for a purpose and, as for anything else in the language, it's better to understand when to use it and when not rather than just knowing that is something that MUST not be used, without any further explanation. More, from my personal experience, knowing unsafePerformIO helped me understand better what Monads are and how they should be used. I wounder what so-called experts have to say about this. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, Apr 14, 2009 at 10:54 AM, rodrigo.bonifacio
rodrigo.bonifa...@uol.com.br wrote:
Dear Sirs,
I guess this is a very simple question. How can I convert IO [XmlTree] to
just a list of XmlTree?
Regards,
Rodrigo.
One good link on this topic is http://haskell.org/haskellwiki/Avoiding_IO
Maybe there's no need for it be be IO [XmlTree], if you can write
functions that instead produce/consume just [XmlTree]. They can always
be turned later with liftM and whatnot into a IO [XmlTree] (but not
the reverse).
Doing as much as possible in pure code, and wrapping around it/calling
it from a few short IO functions is the Haskell Way. Sometimes it
looks hard to see, but XMonad is probably the best example here of how
it can both be done where it looks infeasible ('What's a window
manager but a bunch of IO?) and is rewarding (testability, etc. as
exemplified by http://cgi.cse.unsw.edu.au/~dons/blog/2007/05/17) The
best writeup on this seems to be
http://www.cse.unsw.edu.au/~dons/talks/xmonad-hw07.pdf
--
gwern
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Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, Apr 14, 2009 at 9:24 AM, Cristiano Paris fr...@theshire.org wrote: On Tue, Apr 14, 2009 at 5:09 PM, Luke Palmer lrpal...@gmail.com wrote: ... Please don't say that. He's a beginner. You realize that the path of least resistance will be to use it, right? You see why that's not a good thing? Even experts don't use this function. (To the O.P.: don't use it) Mmmh, sorry Luke but I don't understand this ostracism. unsafePerformIO is not evil by itself, it's there for a purpose and, as for anything else in the language, it's better to understand when to use it and when not rather than just knowing that is something that MUST not be used, without any further explanation. You have a point. I would like to avoid introducing unfounded authoritarian stigmas whenever possible. However, the way I see it is that unsafePerformIO *is* evil by itself, and it is only by the addition of Holy Water that it is benign to use. Ryan Ingram described it as a way to achieve RTS extensions, which I think is a fine way to put it I consider Debug.Trace to be an instance of this: we are extending the RTS to provide execution traces. I guess it's a teaching style thing. Mostly, if someone sees I have an IO [XmlTree] and I need an [XmlTree], I want the I'm asking the wrong question synapse path to fire, rather than the just use unsafePerformIO one. Luke More, from my personal experience, knowing unsafePerformIO helped me understand better what Monads are and how they should be used. I wounder what so-called experts have to say about this. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, Apr 14, 2009 at 5:42 PM, Luke Palmer lrpal...@gmail.com wrote: ... However, the way I see it is that unsafePerformIO *is* evil by itself, and it is only by the addition of Holy Water that it is benign to use. That's what I meant but your words are indeed more effective :) Ryan Ingram described it as a way to achieve RTS extensions, which I think is a fine way to put it I consider Debug.Trace to be an instance of this: we are extending the RTS to provide execution traces. I guess it's a teaching style thing. Mostly, if someone sees I have an IO [XmlTree] and I need an [XmlTree], I want the I'm asking the wrong question synapse path to fire, rather than the just use unsafePerformIO one. Yes, I think your analysis is correct. Perhaps I was a bit too dry in my original answer. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Cristiano Paris wrote: On Tue, Apr 14, 2009 at 5:09 PM, Luke Palmer lrpal...@gmail.com wrote: ... Please don't say that. He's a beginner. You realize that the path of least resistance will be to use it, right? You see why that's not a good thing? Even experts don't use this function. (To the O.P.: don't use it) Mmmh, sorry Luke but I don't understand this ostracism. unsafePerformIO is not evil by itself, it's there for a purpose and, as for anything else in the language, it's better to understand when to use it and when not rather than just knowing that is something that MUST not be used, without any further explanation. Sure, the explanation is there if people are interested in it. However, in context, your answer was wrong. It is like someone asking: How do I get hold of a new phone and the answer Pull a gun on someone walking down the street and demand they give you their phone ...that is, the answer was solving the wrong problem, or solving it in the wrong context. If you have IO [XmlTree], then you don't have an [XmlTree] at all - rather you have a description of an (IO-involving) action which you need to run to get one. You can run it many times, or once, or never. It will (in general) give different results depending exactly when you run it. Therefore you need to carefully decide when to run it - i.e. attach it indirectly or directly into your main action, as the various other answers have shown. unsafePerformIO is not part of the haskell language - it does not respect the type system. It is an extension mechanism which allows us to add hooks into the RTS; effectively a way to extend the language. This is a useful and powerful thing, but nothing in the questioner's question suggested that language extension was what they wanted. Jules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
On Tue, Apr 14, 2009 at 5:54 PM, Jules Bean ju...@jellybean.co.uk wrote: ... I'm convinced about what you say and perhaps I answered the way I did just because I'm convinced that, for a newbie, knowing about the existence of unsafePerformIO can't cause any harm. I was a bit surprised by the strong reaction about my citation of unsafePerformIO. Maybe it'd useful, for the future, to write a document explaining how to help newbies properly, maybe putting it in the mailing list charter. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
rodrigo.bonifacio wrote: I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? You can't, unless you use `unsafePeformIO`, as others have already pointed out. Yet others have, more correctly, suggested that you use do notation to bind a variable with the type you expect. I want to go into a little more detail .I have a spontaneous Monad tutorial to throw out, I guess. How you can convert a value of type `IO [XmlTree]` is probably the wrong question. The right question is how you can convert a function of type `[XmlTree] - A` into a function of type `IO [XmlTree] - IO A`. The correct answer has many names: fmap, liftA, liftM, ($) :: Monad m = (a - b) - (m a - m b) I will use `fmap` to mean any of the above names. In your case, applying fmap to a function foo: foo ::[XmlTree] -A fmap foo :: IO [XmlTree] - IO A So any time you need to pass an IO value to a pure function, this is one way to do it. Suppose that the function actually returns an IO value, though. Here, we will call the function `bar`: bar ::[XmlTree] - IO A fmap bar :: IO [XmlTree] - IO (IO A) Now we seem to be in a similar situation as before. We have an extra IO that we don't want. There is a function for this: join :: Monad m = m (m a) - m a So, we can use `join` to transform an expression of type `IO (IO a)` to an expression of type `IO a`. Putting it all together: bar ::[XmlTree] - IO A fmap bar:: IO [XmlTree] - IO (IO A) join . fmap bar :: IO [XmlTree] - IO A And we now have a sensible function again. Of course, this is a common pattern, using `join` and `fmap` together, so we have yet another function: (=) :: Monad m = (a - m b) - (m a - m b) (Note that this has a different argument order than (=). I prefer this form since it emphasizes that it actually transforms a function.) So, now we have bar ::[XmlTree] - IO A (bar =) :: IO [XmlTree] - IO A Putting it all together, with a function that gives us the `IO [XmlTree]`: xmlTree :: IO [XmlTree] bar ::[XmlTree] - IO A bar = XmlTree :: IO A And that last line is equivalent to this in do notation: do tree - xmlTree bar tree If you have any questions, please do ask. I understand that it can appear quite dense to a beginner. I'm thinking about using this approach in a blog article, which would have more detail and examples, but I would like to be aware of potential stumbling blocks. - Jake ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Quoth Cristiano Paris fr...@theshire.org, I was a bit surprised by the strong reaction about my citation of unsafePerformIO. Well, there might be a couple of things going on here. Part of it is how to guess the unstated context of a question - I'm fairly sure that given a more thorough presentation of the question, there would have been no controversy about the answer. The general problem is that people who are comfortable with extremely esoteric parts of Haskell and used to discussing such things here, fail to recognize when they're dealing with people who are at a point where their needs are much more basic. (And who knows, which one was the present case? Not really enough information to know absolutely for sure.) But as you have found, unsafePerformIO is not just an esoteric topic, it's an uncomfortable one. We read that it isn't even part of the language, one should never really have any use for it in computation, only as a sort of meta-programming RTS thing. Yet, you might never guess this from reading the GHC documentation, which only urges you to be careful. Or from encountering it in fairly widespread use as a way to implement top level program state with IORefs. This sort of unresolved tension between practice and theory rightly makes people uneasy, and in my opinion you shouldn't take it personally. It's a good thing to occasionally probe those sore spots, and maybe if it bothers us enough it will lead to improvements. Donn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Converting IO [XmlTree] to [XmlTree]
Never answer such newbie questions with unsafePerformIO. That's the wrong answer 99.99% of the time, but giving it to a newbie they might follow your advice. On Tue, Apr 14, 2009 at 5:01 PM, Cristiano Paris fr...@theshire.org wrote: On Tue, Apr 14, 2009 at 4:54 PM, rodrigo.bonifacio rodrigo.bonifa...@uol.com.br wrote: Dear Sirs, I guess this is a very simple question. How can I convert IO [XmlTree] to just a list of XmlTree? Quick and dirty answer: unsafePerformIO. That's an easy finding on Hoogle: http://www.haskell.org/hoogle/?hoogle=IO+a+-%3E+a Anyhow, as the name suggest, the function is unsafe, so you better know what you're doing. Bye, Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
