May 2007 - Haskell-Cafe

Monad pronounced like gonad?
by Dan Weston 11 May '07

11 May '07

I've been pronouncing monad like gonad (moh-nad), but it occurs to me that it might be pronounced like monoid (mah-nad). Is there an official way to pronouce this word - maybe with a Scottish accent? :)

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Sudoku solving
by Antoine Latter 11 May '07

11 May '07

I'm a relatively new to Haskell, so I figured I needed to write a Sudoku solver. The key features of this solver are: * It's longer than other Haskell Sudoku solvers I've seen around * It fails to solve many solvable puzzles Fantastic! link: http://panicsonic.blogspot.com/2007/04/sudoku-solving.html The approach: Because I'm new to Haskell, the traditional guess-and-check method of Sudoku solving would involving things like backtracking down a search path, which is still in the realm of Haskell Black Magic. Instead, I cast the problem into successive function applications on data - A Sudoku puzzle is represented as an array of constraints on the values of cells. The constraints are reduced via analysis of the surrounding constraints until the puzzle is solved (or not). For those who like references: my approach is similar to "Solving Sudoku Puzzles with Rewritting Rules" by Gustavo Santos-Garcia and Miguel Palomino, 2005, Electronic Notes in Theoretical Computer Science. My main rules and techniques can be considered as generalizations on some of their rules and techniques - except their solver can solve all solvable puzzles (with the help of a guess-and-check rule). Thanks for taking a look, Antoine

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Re: [Haskell-cafe] generate Haskell code from model
by Brian Smith 11 May '07

11 May '07

On 4/14/07, Steffen Mazanek <steffen.mazanek(a)googlemail.com> wrote: > > Brian, but don't you think that you have to write a lot > of boilerplate code in Haskell? I have never felt I was writing a lot of boilerplate. There are a lot of abstraction mechanisms in Haskell to avoid boilerplate. Second, if Haskell should be more successful in the > real world there has to be a way of demonstrating > basic ideas of a big program to "customers". How > would you do this? Everybody knows UML class > diagrams, for example. In contrast, nobody knows > about termgraphs or lambda *g*. I've never had to show a UML or ER diagram to any business people--usually they want a slideshow that is far simpler and a little prettier. The fact that "nobody knows about termgraphs or lambda" in your group means that you probably shouldn't be considering Haskell (for the same reason my bosses always asked me to document everything--"in case you get hit by a bus"). Thank you very much for contributing to the discussion. > Please assume, that you have to generate the code from > a model. Further assume, that you have no choice and > are not allowed to discuss the sense of this approach :-) > How should the code look like? I am not sure if you are trying to solve a real problem or not. If you are solving a real problem, where you already happen to have an EMF model which you are required to generate code from, then I recommend to just do everything in Java using the existing tools built for EMF. If you decide to still keep working in Haskell, and it works out well, please share your solution because I think many people here will be very interested. wxHaskell, OOHaskell, and O'Haskell are all starting points for this type of project. - Brian

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Grabbing list of functions from current module
by Joel Reymont 11 May '07

11 May '07

Folks, Does anyone have code that can grab a list of functions named with a certain prefix from the current (or given) module? I want to find functions named, say, ast_* and produce a list of tuples like ("input1", ast_input1). Thanks, Joel -- http://wagerlabs.com/

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Co-arbitrary
by Joel Reymont 10 May '07

10 May '07

Would someone kindly explain why we need co-arbitrary in QuickCheck and how to define it? Detailed examples would be awesome! I would be willing to paste an in-depth explanation on my wall and keep it forever. Thanks in advance, Joel -- http://wagerlabs.com/

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Telling Cabal not to install the executable
by Joel Reymont 10 May '07

10 May '07

I tell Cabal to build library and a test harness. How can I tell Cabal that I only want the library installed? Thanks, Joel -- http://wagerlabs.com/

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Re: [Haskell-cafe] Debugging
by Ryan Dickie 10 May '07

10 May '07

On 5/4/07, Monang Setyawan <monang(a)gmail.com> wrote: > > On 5/5/07, Stefan O'Rear <stefanor(a)cox.net> wrote: > > On Sat, May 05, 2007 at 11:36:16AM +0700, Monang Setyawan wrote: > > > Hi, I'm a beginner Haskell user. > > > > > > Is there any way to trace/debug the function application in GHC? > > > > Absolutely! > > > > stefan@stefans:/tmp$ ghci X.hs > > ___ ___ _ > > / _ \ /\ /\/ __(_) > > / /_\// /_/ / / | | GHC Interactive, version 6.7.20070502, for > Haskell 98. > > / /_\\/ __ / /___| | http://www.haskell.org/ghc/ > > \____/\/ /_/\____/|_| Type :? for help. > > > > Loading package base ... linking ... done. > > [1 of 1] Compiling Main ( X.hs, interpreted ) > > Ok, modules loaded: Main. > > *Main> :break fac > > Great!! Thanks, it really helps. > I should update my GHC to the newest version (I use the old 6.4.2 > with no break command) > > Is there any editor/IDE supporting this break command? It should be > cooler if we can debug functions just by placing mark in the line. > > > > > Stefan > > > > > -- > Demi masa.. > > > -- > Demi masa.. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > I've only written trivial applications and functions in haskell. But the title of this thread got me thinking. In an imperative language you have clear steps, states, variables to watch, etc. What techniques/strategies might one use for a functional language? --ryan

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built-in lists vs inductively defined list
by José Miguel Vilaça 10 May '07

10 May '07

Hi, Can someone tell me if would be any difference (beside syntax) between the built-in Haskell list with [] and : and lists given by a inductive datatype definition like data List a = Nil | Cons a (List a) This is, is any of the compilers doing some special optimizations for the built-in lists? Best Miguel Vilaça

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Vanishing polymorphism
by Matthew Sackman 10 May '07

10 May '07

Hi, To cut a long story short, could someone explain why: > :t negate negate :: forall a. (Num a) => a -> a > :t let f (fn :: forall a . Num a => a -> a) r s = (fn r, fn s) in f negate let f (fn :: forall a . Num a => a -> a) r s = (fn r, fn s) in f negate :: forall r s. (Num r, Num s) => r -> s -> (r, s) but > :t let f r s = (return negate) >>= (\fn -> return (fn r, fn s)) in f let f r s = (return negate) >>= (\fn -> return (fn r, fn s)) in f :: forall a (m :: * -> *). (Num a, Monad m) => a -> a -> m (a, a) and > :t let f r s = (return negate) >>= (\(fn::forall n . (Num n) => n -> n) -> return (fn r, fn s)) in f <interactive>:1:35: Couldn't match expected type `a -> a' against inferred type `forall n. (Num n) => n -> n' In the pattern: fn :: forall n. (Num n) => n -> n In a lambda abstraction: \ (fn :: forall n. (Num n) => n -> n) -> return (fn r, fn s) In the second argument of `(>>=)', namely `(\ (fn :: forall n. (Num n) => n -> n) -> return (fn r, fn s))' I.e. why does the polymorphism get destroyed? Cheers, Matthew -- Matthew Sackman http://www.wellquite.org/

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Object Files Become Downloader.Obfuskated
by SevenThunders 09 May '07

09 May '07

I discovered a curious thing after a compiling a medium sized Haskell project using the 64 bit linux Haskell GHC 6.6.1 compiler. Several of the generated object files (.o) show up as containing the virus Downloader.Obfuskated by my AVG anti-virus program when viewing the files from windows xp 64. Since the files were generated by GHC on linux I find it hard to believe that they actually contain this virus. A google search reveals one other mention of something like this happening with no further explanation. The virus detection appears no where else except in object files generated by the ghc linux compiles. -- View this message in context: http://www.nabble.com/Object-Files-Become-Downloader.Obfuskated-tf3718136.h… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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