- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Getting the x out
by michael rice 22 Apr '09

22 Apr '09

There's a lot of well thought out stuff in Haskell, but getting familiar with it all seems like a huge task. Thanks for the help. Michael --- On Tue, 4/21/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Getting the x out To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Tuesday, April 21, 2009, 8:53 PM If you want to just get the value out, meaning you'll get a program error if it happens to be Nothing, then you can use Data.Maybe.fromJust. But usually, you'd want to preserve the Nothing. Applicative or Monad is pretty good for this: import Control.Applicative (3+) <$> safeDivision 10 5 the result will be Just 5.0 in this case, but if the division was incorrect it would be nothing. If you want to do something else, you can either pattern match on it: case safeDivision 10 5 of Just x -> -- do something with x Nothing -> -- do something else or use some functions from Data.Maybe. Say you want to evaluate to 1 instead of Nothing: import Data.Maybe fromMaybe 1 (safeDivision 10 5) -Ross On Apr 21, 2009, at 8:49 PM, michael rice wrote: How do I get the x out of Just x? Michael ============= safeDivision :: Float -> Float -> Maybe Float safeDivision x y = if y == 0 then Nothing else Just (x/y) *Main Data.List> safeDivision 10 5 Just 2.0 *Main Data.List> 3 + (safeDivision 10 5) <interactive>:1:0: No instance for (Num (Maybe Float)) arising from a use of `+' at <interactive>:1:0-22 Possible fix: add an instance declaration for (Num (Maybe Float)) In the expression: 3 + (safeDivision 10 5) In the definition of `it': it = 3 + (safeDivision 10 5) *Main Data.List> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Comments from two weeks of using Leksah
by Jeff Heard 21 Apr '09

21 Apr '09

I've been using the latest and greatest version of leksah for the last couple of weeks and I wanted to give a short report on the things I've found. First of all, it's crashed only once, and the error was an actual segfault, so I'm not sure what went wrong there. All in all, I like the eyecandy and have left it on for everything I do, but I've noticed that arrows have an extra space after them, no matter which arrow. Also, the candy for =<< and >>= is the same. Only one "missing feature" that I've noticed: regex find and replace. I'd like very much to be able to find and replace using regexes, as this is pretty much why I keep coming back to vi and emacs. I really really like the modules pane which gives me quick views into the doc of everything that's installed and the signatures. I wonder, though, could it be limited to the packages made available by the dependencies in configuring the current working package? That way, it would be easy to tell whether a dependency was missing. I wonder if there's a way to setup the preferences so that autocomplete can be accelerator based (like Ctrl+Space) instead of constantly on, because I've noticed that occasionally I'll hit enter at the end of a line and have it autocorrect my last identifier incorrectly. It'd be kind of neat if the error lines from the continuous background build showed up highlighted in the editor in some way, like they do in oh say Eclipse, but that's just a nice feature. Only other feature i'd like to see added is the ability to have multiple packages open at once, allow them to depend on each other, and handle configuring, building, and installing each of them be a one-step process. That's about it. For large projects, very neat and very useful! I like it. -- Jeff

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A haskell error message
by siso dagbovie 21 Apr '09

21 Apr '09

Hi, I have been trying to specify the depth-first numbering algorithm as follow: >_dfNum :: Int -> [Tree a] -> [Tree Int] >_dfNum i ((Node n ts):us) = (Node i vs):(_dfNum (i+1+(length ts)) us) where > vs=_dfNum (i+1) ts But when I test it, I receive the message: Non-exhaustive patterns in function _bfNum Please, could you give me an idea of how I could fix this? Thanks Kind regards, Ayawoa Sitsopé Dagbovie African Institute for Mathematical Sciences #6 Melrose Road Muizenberg 7945 South Africa. Phone: +27 782 960453

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UPDATE: haskellmode for Vim now at projects.haskell.org (+screencast; -)
by Claus Reinke 21 Apr '09

21 Apr '09

I have become aware that many Haskellers are not aware of the Haskell mode plugins for Vim, in spite of the >100 downloads per month I saw when I last checked. Since the plugins have just completed their move to their new home at http://projects.haskell.org/haskellmode-vim/ this seems to be a good opportunity to mention them again (now with 100% more screencasts!-). They do much (though certainly not all) of the stuff people often wish for here, such as showing or adding types, looking up source where available locally or looking up documentation where source isn't available. Mostly, they collect my attempts to automate tasks when I have to do them often enough. Here is a section of the quick reference (:help haskellmode-quickref): |:make| load into GHCi, show errors (|quickfix| |:copen|) |_ct| create |tags| file |_si| show info for id under cursor |_t| show type for id under cursor |_T| insert type declaration for id under cursor |balloon| show type for id under mouse pointer |_?| browse Haddock entry for id under cursor |:IDoc| {identifier} browse Haddock entry for unqualified {identifier} |:MDoc| {module} browse Haddock entry for {module} |:FlagReference| {s} browse Users Guide Flag Reference for section {s} |_.| qualify unqualified id under cursor |_i| add 'import <module>(<identifier>)' for id under cursor |_im| add 'import <module>' for id under cursor |_iq| add 'import qualified <module>(<identifier>)' for id under cursor |_iqm| add 'import qualified <module>' for id under cursor |_ie| make imports explit for import statement under cursor |_opt| add OPTIONS_GHC pragma |_lang| add LANGUAGE pragma |i_CTRL-X_CTRL-O| insert-mode completion based on imported ids (|haskellmode-XO|) |i_CTRL-X_CTRL-U| insert-mode completion based on documented ids (|haskellmode-XU|) |i_CTRL-N| insert-mode completion based on imported sources |:GHCi|{command/expr} run GHCi command/expr in current module For those who have never used these plugins, or haven't used Vim at all, it has often been difficult to imagine what editing in Vim can be like. The old quick tour of features available has now been updated from screenshots to screencasts (my first venture into this area - please let me know whether that is useful or a waste of time!-), so you can watch them on Youtube before deciding to invest time into learning Vim. For those who are using Vim, the only reason not to use my haskellmode plugins would be if you had your own (not uncommon among Vim users;-), in which case I hope you make yours available as well (feel free to adopt features from my plugins, and let me know if you have some useful features to contribute), here: http://www.haskell.org/haskellwiki/Libraries_and_tools/Program_development#… For those who have happily been using these plugins: in the process of moving the site, I noticed that I hadn't updated the published version for a quite some time - apparently, noone had missed the fixes, but in case you want to check, the relevant section of my update log is appended below. Happy Vimming!-) Claus ------------- updates since last published version on old site 04/04/2009 haskell_doc.vim: when narrowing choices by qualifier for _?, take lookup index from un-narrowed list (else we could end up in the docs for the wrong module) 02/04/2009 ghc.vim: actually, we can try to make a reasonable guess at the parent type for constructors in _ie, from their type signature 01/04/2009 ghc.vim: try a bit harder to escape " and ' in :GHCi eliminate duplicates in _ie and mark data constructor imports as ???(Cons) - we can't reliably figure out the parent type for the constructor:-( handle Prelude as special case in _ie (can't just comment out import, need to import [qualified] Prelude [as X]() ) haskell_doc.vim: fix keys (no namespace tags) and urls (modules) for :MDoc 31/03/2009 all files: new home page at projects.haskell.org 28/03/2009 haskell_doc.vim: in ProcessHaddockIndexes2, fix a case where making new entries could lose old ones (eg, zipWith's base package locations got lost when adding its vector package locations) 07/12/2008 haskell_doc.vim: since we're now reading from multiple haddock indices in DocIndex, we need to extend, not overwrite entries.. 03/12/2008 ghc.vim: do not reset b:ghc_static_options on every reload

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ANNOUNCE: Runge-Kutta library -- solve ODEs
by Uwe Hollerbach 21 Apr '09

21 Apr '09

Hello, all, I'm pleased to announce a small Runge-Kutta library for numerically solving ordinary differential equations, which I'm hereby unleashing upon an unsuspecting world. The README is as follows: This is a small module collecting about a dozen Runge-Kutta methods of different orders, along with a couple of programs to exercise them. Build and run testrk, volterra, volterra2, and arenstorf: o testrk exercises all of the methods in a non-adaptive way, solving a test problem with a known analytic solution, to check convergence. (This was what first indicated that there was a problem with the Fehlberg 7(8) listing in HNW.) o volterra uses a non-adaptive method to solve the Lotka-Volterra equations from t=0 to t=40: either from a built-in starting point, or from a starting point specified on the command line. o volterra2 does the same, except it uses an adaptive solver o arenstorf solves the restricted 3-body problem (earth+moon+satellite) using an adaptive solver with some specific initial conditions which yield periodic orbits The volterra2 and arenstorf examples use an "oracle" function to decide what is a good step size. Right now that oracle function is in each test file; arguably it should be in the RungeKutta module. Eventually it will be, but I haven't spent much time yet on making that oracle especially good. I have so far only tested it with ghc 6.8.3 on MacOS 10.3.9 (powerPC), but I know of no reason why it wouldn't work with other versions and OSs. It's BSD licensed, in fact I stole the LICENSE file from ghc (and filed off the serial numbers). I'm afraid I haven't messed with cabal much yet, so it's not cabalized; neither is it uploaded to hackage, as I have no account there. If someone wants to do either of those, please feel entirely free to do so. In addition to attaching a tarball to this message, I'm also putting this onto my (sadly neglected) web site: it will live at http://www.korgwal.com/software/rk-1.0.0.tar.gz. Enjoy! Uwe

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regex-tdfa: mkRegex and matchRegex in ghci
by j.romildo＠gmail.com 21 Apr '09

21 Apr '09

Hello. How mkRegex and matchRegex can be used with the regex-tdfa backend in ghci? The following fails: $ ghci GHCi, version 6.10.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> import Text.Regex Prelude Text.Regex> import Text.Regex.TDFA Prelude Text.Regex Text.Regex.TDFA> let re = mkRegex "jos. romildo" Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package regex-base-0.93.1 ... linking ... done. Loading package regex-posix-0.94.1 ... linking ... done. Loading package regex-compat-0.92 ... linking ... done. Prelude Text.Regex Text.Regex.TDFA> let text = "josé romildo" Prelude Text.Regex Text.Regex.TDFA> matchRegex re text Nothing Clearly regex-posix is being used here. The standalone version of the program works as expected: $ cat t.hs module Main (main) where import Text.Regex import Text.Regex.TDFA re = mkRegex "jos. romildo" text = "josé romildo" result = matchRegex re text main = print result $ ghc --make t [1 of 1] Compiling Main ( t.hs, t.o ) Linking t ... $ ./t Just [] Any clues? Romildo

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Building regex-tdfa-1.1.1 documentation
by j.romildo＠gmail.com 21 Apr '09

21 Apr '09

Hello. I want to build regex-tdfa-1.1.1 haddock documentation locally, but the procedure fails: $ ./setup haddock Preprocessing library regex-tdfa-1.1.1... Running Haddock for regex-tdfa-1.1.1... Warning: The documentation for the following packages are not installed. No links will be generated to these packages: rts-1.0 Text/Regex/TDFA/Common.hs:106:25: parse error on input `-- ^ identity of Position tag to set during a transition -- | Internal use to indicate type of tag and preference for larger or smaller Positions' $ haddock --version Haddock version 2.4.2, (c) Simon Marlow 2006 Ported to use the GHC API by David Waern 2006-2008 Any clues? Romildo

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ANN: curl-1.3.5
by Sigbjorn Finne 21 Apr '09

21 Apr '09

Hi, a refresh release of the Haskell (lib)curl package has been uploaded to Hackage, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/curl It enables 6.10.2 use, taking into account the updated story on how to register Haskell-based finalizers. Thanks to Carl Howells for reporting this problem. Bug reports / feature requests etc. most welcome, btw. enjoy --sigbjorn

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Petr Ročkai's darcs project accepted for Haskell GSoC
by Eric Kow 20 Apr '09

20 Apr '09

Hi everybody, I'm pleased to announce that Petr Ročkai's project on optimising darcs has been accepted for Google Summer of Code. More details can be found here: http://socghop.appspot.com/org/home/google/gsoc2009/haskell Petr's project fits into an ongoing effort by the darcs community to improve our performance so that one day we will able to recommend darcs for medium sized and larger projects. Until early 2008, we have been focused on darcs 2: which provides the darcs 2 theory (to solve most of the exponential merge problems), and also the "hashed" format repositories (including darcs 2). The project for this summer will allow us to complete the work on hashed repositories. In their current state, hashed format repositories make darcs a lot more robust and also allow us to fetch repositories faster (with darcs get --lazy and with a global patch cache). But in their current form, hashed repositories can can also slow down day-to-day operations like darcs whatsnew and darcs record. Petr has identified the cause of these delays has provided a work plan which we think will get hashed storage right. If his project is successful, future versions of darcs should make operations like darcs record and whatsnew much more comfortable in practice. Congratulations and good luck to Petr! And many thanks to the darcs and Haskell communities for their support. -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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RE: [Announce] primes
by Niemeijer, R.A. 20 Apr '09

20 Apr '09

Sebastian Fischer wrote: > I am pleased to announce the package 'primes' that implements lazy > wheel sieves for efficient, purely functional generation of prime > numbers in Haskell. > > The implementation is reasonably efficient. The query > > > primes !! 1000000 > 15485867 > > answers after a few seconds. > > Feel free to contribute more functionality to this package. The > sources are on Github: > > http://github.com/sebfisch/primes > > If you fork my project, I'll be happy to merge your changes. I have just finished benchmarking all the implementations provided in http://www.cs.hmc.edu/~oneill/code/haskell-primes.zip (the zip file linked to from the Haskell wiki article on primes). NaurPrimes.hs is by far the fastest version, and at least 2 or 3 times faster than your current implementation. I'm pretty sure it also uses less memory. I want to find efficient algorithms for the other proposed functions before forking, but I figured I'd let you know in the meantime.

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