October 2012 - Beginners

Re: [Haskell-beginners] Beginners Digest, Vol 52, Issue 16
by Gregory Guthrie 12 Oct '12

12 Oct '12

I am trying to install yesod, But it fails and one of the failures is to install http-conduit. So I tried to install that, and it fails, with this message: Building http-conduit-1.6.1.1... Preprocessing library http-conduit-1.6.1.1... <command line>: cannot satisfy -package-id asn1-data-0.7.1-25c9f2b3799c26151af97c0d9c79754c: asn1-data-0.7.1-25c9f2b3799c26151af97c0d9c79754c is unusable due to missing or recursive dependencies: base-4.5.0.0-597748f6f53a7442bcae283373264bb6 bytestring-0.9.2.1-df82064cddbf74693df4e042927e015e cereal-0.3.5.2-b01d76d961ef4facbe3e52a7b6ccef63 mtl-2.1.1-87121149c5eb4fc03f06f5894ea4ac0c text-0.11.2.0-b2986f5478f0efa626f8ba0494ed9670 (use -v for more information) Failed to install http-conduit-1.6.1.1 cabal.exe: Error: some packages failed to install: http-conduit-1.6.1.1 failed during the building phase. The exception was: ExitFailure 1 Any hints? I find that with each usage of cabal-install, my success rate and appreciation for it falls! :-) Each time it fails, and says to -force-reinstalls, that breaks more, etc... Not sure how to break out of this maze of "twisty turny passages that all look alike".. Haskell-platform 2.0.0, GHCi, version 7.4.1 cabal-install version 0.14.0 using version 1.14.0 of the Cabal library -------------------------------------------

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conduit and happstack dependence problem
by Никитин Лев 12 Oct '12

12 Oct '12

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calling inpure functions from pure code
by Emmanuel Touzery 12 Oct '12

12 Oct '12

Hello, I'm trying to write my first real program in Haskell, a web page scraper; at the top level, which is impure anyway, i fetch the top-level pages (which is IO), then call the pure functions parsing the structure. So far so good. That was the first part of the program and up to this point I'm pretty sure I did it right (for the structure anyway). Most of the demo programs and Haskell books cover this: you do the IO at the top level, then you process and you come back to the top level to print your results for instance. But here comes the problem: these pure functions that parse the structure, sometimes they find links and must open another page on the site... And opening that new page, well it's IO and can't be pure.. Now if I call from those pure methods IO methods, then they're not pure anymore, and in fact since those are leaf calls basically my entire program becomes impure... I had this idea, that I would make some sort of input data structure, which would be like a lazy String reading from a file: doing IO while the caller doesn't even realize and the caller can be pure. So some sort of fake webserver or website or htmlpage data structure... which is lazy. And then I give this data structure to my pure methods which parse the data, they call functions on that structure, and they can stay pure. But it sounds really contrived and probably completely the wrong solution. I can see that this is a very basic question which was probably answered hundreds of time, but I could not find the answer so far... Thank you! Emmanuel

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TextMate with Haskell GHC
by Patrick Lynch 12 Oct '12

12 Oct '12

Good morning, Can anyone tell me how to configure TextMate with the :edit command in ghci when used with a Mac [I'm new to the Mac]? [BTW: TextMate has a "bundle" that works well with Haskell.] Also, can anyone recommend an online course on Category Theory? Good weekend

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Errors and the compiler
by Christopher Howard 12 Oct '12

12 Oct '12

Point of curiosity: Does an error in a Haskell program have a negative impact on the compilers reasoning about or optimization of the code, even if said error is never reached in the control flow? For example, both of the following would have the same result: code: -------- case mExp of Nothing -> expr Just x -> f x -- or... if isNothing mExp then expr else f (fromJust mExp) -------- In my naive reasoning, I would think that in the latter case the use of fromJust introduces, from the compilers perspective, an additional possible outcome (an error being thrown). This presumably would complicate the compilers reasoning about the code, unless of course the compiler is smart enough to figure out that the error will never be reached. -- frigidcode.com indicium.us

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Memory leak after going from type Integer to Int
by Andreas Lemke 11 Oct '12

11 Oct '12

Hi everyone, I wrote a simple program that returns the n-th prime number when given n as its command-line argument: import System.Environment isPrime n | n < 2 = False | n == 2 = True | otherwise = not $ any (`divides` n) dividerList where a `divides` b = b `mod` a == 0 dividerList = 2:[3, 5 .. floor $ sqrt $ fromIntegral n] primes = filter isPrime [1 ..] main = do [arg] <- getArgs print $ primes !! (read arg) I compiled it with "ghc -O2". As expected, it runs in constant space, i. e. space complexity O(1). Then, I added the declaration: "primes :: [Int]", expecting a performance improvement. Runtime was indeed reduced by about 50%. However, it no longer runs in constant space. Memory usage rises significantly for arguments > 1e5. I tried all sorts of strictness annotations, which did not help. Turning on profiling with "ghc -O2 -prof -auto-all" made the problem disappear, so I could not analyze the memory leak using the profiler. Only if I remove the cases "n < 2" and "n == 2" from the definition of isPrime, does the program run in constant space with type Int. If I replace the guards by an if statement or pattern matching, the problem remains. I have no idea how to continue from here. Can anyone explain why going from type Integer to Int causes a different space complexity? How would I have to change the program to get it to run in constant space with type Int? I used ghc 7.4.1. Regards, Andreas

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What is a closer to functional (haskell) ideology
by Никитин Лев 10 Oct '12

10 Oct '12

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How Interactive can Haskell Ultimately be?
by Michael Carpenter 08 Oct '12

08 Oct '12

Yep, I'm verbose in my questions. I put the core question(s) in bold. Everything else is elaboration for clarity. *- How interactive and "live" can GHCI make Haskell? What are the limits?**By "live", I mean on-the-fly coding. **- Would it be possible to make a Haskell plugin for something like the Light Table IDE even if it required some behind-the-scenes trickery?** **- How dependent is Haskell's interactivity on it's structure as a language and how much of it is dependent on the given compiler?**Is it that GHC could never provide on-the-fly coding or is it that the idea of highly interactive coding is completely incompatible with the core principles that make Haskell work? (static type system vs. dynamic type system) **- Could GHC/GHCi be used or extended to provide a more functional reactive Haskell similar to Elm such that you could play with your code the same way you can with SuperCollider or Chuck?* By interactive, I mean that the language allows for the programmer to edit the program's source as it's running and see the results of the edit play out in the current running session without having to restart the program. A lot of languages use this for debugging (GHCi), but I know that this feature is typically very limited unless you have a dynamic, reactive, and/or interpreted language. There seems to be many tricks out there for at least giving the illusion of "live coding" such as interpreting the files being edited and leaving everything that's not dependent on it compiled. I've also read about having the compiler and interpreter actively dance around each other in the source, with the interpreter managing the files that were recently edited, providing the slow but interactive functionality with only the code local to the programmer's current activity, and the compiler recompiling files after they've been left alone for a while. I totally don't know what I'm talking about here with regard to compiler behavior, but is this interactive behavior even possible with GHC/GHCI given how much optimization takes place at compile time? Can you cherry pick like that? Can you even distinguish between files anymore once it's been melted down into C--? I'm really taking to Haskell for general purpose programming and I'm finding little to complain about, but there is one thing that Haskell lacks in that other expressive high-level languages like Lisp, Python, and Ruby seem to have. Interactivity. I like that Haskell has both a good compiler and a nice interpreter, but I don't know whether (in the future) the two could work together to provide GHCI's human-centric interactivity with GHC's computer-centric optimization. Can I have good performance and eat my cake too? I was incredibly inspired by the prototype of the Light Table IDE project that got funding recently. I, like many, really liked how it gave very quick feedback in multiple ways to what you are doing. That helps me tremendously and its probably the most powerful feature a language could ever include imo. I have no doubt that such a project can become a successful reality, but the language that Light Table was performing this interactivity with was Clojure and it seems like only dynamic interpreted languages will work. Is Haskell, despite its differences from Lisp and the fact that it is a mostly compiled language, disqualified from granting the coder a true interactive environment? GHCi is REALLY powerful and helpful in this regard, but it seems limited to a debugger and a useful tool for tutorials. I'm asking this because Haskell seems to have everything I would want in a language that is not C. The only competitor to it as a functional language, in my opinion, is Lisp because of this interactivity. Haskell seems to be doing everything right as a modern, high-level, functional language, but it would be perfect (for me) if it further reduced the barrier between the coder and the code by supporting this interactive paradigm. Interactivity is a killer feature for me and I love things like the friendly interactive shell for their focus on it. I've used Vim plugins before for compiled languages where my changes were automatically compiled as I made them and a live preview presented after each compilation. In this sense, I know that virtually any language can act like you are directly affecting your software during run-time, but I'm talking about the sweet spot between that fake method and something like SuperCollider where musicians can modify their code in real-time as their code runs in real-time and not suffer from any interruptions or sudden delays - the source language is the same language that you use to interface with it. I think this is the ideal way to program and I believe it is an important feature for any language that aims to serve the human and not the machine. I want to use Haskell's high-level, expressive, and modular language as the way to play with the code I've written. Whether it's for debugging, experimentation, scripting, or learning and exploring. If this is possible (even if it's just theory and speculation for now), Haskell will be that language that lets me code the way I want to. Sorry for the long message, I didn't want any ambiguity to my question. - Michael

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IO action on a list of [IO a]
by Manfred Lotz 07 Oct '12

07 Oct '12

Hi all, I want to do an IO action on a list of [IO a] like this: myfor :: (a -> IO () ) -> [IO a] -> IO () myfor _ [] = return () myfor f (x:xs) = do x' <- x f x' myfor f xs Is there a library function doing just this? -- Thanks, Manfred

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Questions on type declarations
by thorsopia＠lavabit.com 06 Oct '12

06 Oct '12

Hi, Could you comment on the following type declarations? - getArgs :: IO [String] It can get several params, but its type declaration looks like it gets none. - dispatch :: [(String, [String] -> IO ())] [1] I don't know how to read this one. It looks bizarre. [1] http://learnyouahaskell.com/input-and-output#files-and-streams

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