- Haskell-Cafe - Haskell.org

How to convert DiffTime to a number?
by Magicloud Magiclouds 08 Jun '09

08 Jun '09

Hi, Documents said that DiffTime could treat as "how many seconds". So how to convert it to a number of seconds? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

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Take a look at my photos on Facebook
by Andrew Smith 08 Jun '09

08 Jun '09

Hi haskell-cafe(a)haskell.org, I set up a Facebook Profile where I can post my pictures, videos and events and I want to add you as a friend so you can see it. First, you need to join Facebook! Once you join, you can also create your own profile. Thanks, Andrew To join Facebook, please follow the link below: http://www.facebook.com/p.php?i=675326041&k=RZM5Y4QZR5WMZAEIS15UY&r haskell-cafe(a)haskell.org was invited to join Facebook by Andrew Smith. If you do not wish to receive this type of email from Facebook in the future, please click on the link below to unsubscribe. http://www.facebook.com/o.php?k=917e28&u=531922198&mid=967b97G1fb47d16G0G8 Facebook's offices are located at 1601 S. California Ave., Palo Alto, CA 94304.

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ghci: can't load .so/.DLL for: m (addDLL: could not load DLL) / is there a libm.dll for Windows
by Mark Wassell 08 Jun '09

08 Jun '09

Hello, I get this when using the logfloat package via ghci on Windows. For example *Main> :m + Data.Number.LogFloat *Main Data.Number.LogFloat> logFloat (1.0::Float) Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package logfloat-0.12.0.1 ... can't load .so/.DLL for: m (addDLL: could not load DLL) *Main Data.Number.LogFloat> When compiling the program using ghc, it works fine. This was previously reported as a ghc bug http://hackage.haskell.org/trac/ghc/ticket/3242 where the reporter was getting the message using hipmunk. The ticket was closed as the issue was deemed to be a bug in hipmunk, not GHC. As the ticket reporter asked, what is the correct approch to get around this? I have tried specifying -lm on ghci startup line but my system doesn't seem to have a libm.dll or anything similar. Is there such a thing? Thanks Mark

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Roman to Decimal Algorithms
by Andrew Savige 08 Jun '09

08 Jun '09

I recently played in a code golf Roman to Decimal challenge (in Perl, Python, Ruby and PHP). In playing this game, I found some interesting short algorithms for converting from Roman Numerals to Decimal. Because I think some of these are new, I'd like to present them here, in case they are not really new after all, or in case there are problems with these algorithms that I've overlooked. I'd like to eventually write up a "rosetta code" article comparing the implementation of these algorithms in various languages, including Haskell. Since I'm only a very occasional Haskell programmer, I thought it best to get feedback from Haskell experts before inflicting any of my Haskell code on a wider audience. Hence this post. To keep this post reasonably brief, note that these algorithms are for "modern" Roman Numerals only, limited to the range 1-3999, and with no error checking. That is, the code below assumes that the input is always a well formed Roman Numeral. I have tested each algorithm against every modern Roman Numeral in the range 1-3999. As a starting point, note this "HaskellWiki" function: romanToInt :: String -> Int romanToInt = fst . foldr (\p (t,s) -> if p >= s then (t+p,p) else (t-p,p)) (0,0) . map (fromJust . flip lookup (zip "IVXLCDM" [1,5,10,50,100,500,1000])) taken from http://haskell.cs.yale.edu/haskellwiki/Roman_numerals. I'm going to essentially duplicate the functionality of this code. {-# OPTIONS_GHC -fglasgow-exts -Wall #-} import Data.Char (toUpper) rtoa :: Char -> Int rtoa 'M' = 1000 rtoa 'D' = 500 rtoa 'C' = 100 rtoa 'L' = 50 rtoa 'X' = 10 rtoa 'V' = 5 rtoa 'I' = 1 rtoa r = error $ "Invalid rtoa char:" ++ show r urtoa :: Char -> Int urtoa = rtoa . toUpper romanToInt :: String -> Int romanToInt = foldl1 (\t n -> t+n-t`mod`n*2) . map urtoa The essential difference between this solution and the HaskellWiki one is the use of the "running total" for state rather than the "previous value". I see this as an improvement mainly because it is shorter -- though you might argue that it is less clear. An alternative way to express the romanToInt function is: romanToInt = foldl (\t c -> t+(urtoa c)-t`mod`(urtoa c)*2) 0 I'm open to persuasion as to which is better Haskell style. Since I'm not playing golf anymore, the rtoa function above, though hardly short, seemed to me to be the simplest and clearest way to express converting a single Roman Numeral to its corresponding arabic number. Again, suggestions for the "best"/most efficient way to do this in Haskell are most welcome. A common, and often winning technique, in golf is to perform these sorts of conversions by concocting a "magic formula". For fun, I rewrote rtoa using a magic formula I used in the golf game: rtoa c = 10^(205558`mod`(ord c)`mod`7)`mod`9995 I'm not suggesting that magic formulae are useful outside of golf and this second rtoa function, though shorter, is much less clear. I might add that this particular magic formula appears to be less useful in Haskell golf than the other languages because `mod` is five times longer than the % operator of the other languages. :) By way of explanation, notice that this formula: 205558`mod`(ord c)`mod`7 maps I->0, X->1, C->2, M->3, V->4, L->5, D->6 as shown below: Roman m 10^m 10^m`mod`9995 ----- - ----- ------------- M 3 1000 1000 D 6 1000000 500 C 2 100 100 L 5 100000 50 X 1 10 10 V 4 10000 5 I 0 1 1 Noticing this, you can replace the 205558`mod`(ord c)`mod`7 magic formula with a function that returns a string index (index() in Perl and Python). I am sometimes overwhelmed by the quantity and richness of all the functions in the GHC Haskell libraries. I eventually found a Haskell solution that seemed to work: {-# OPTIONS_GHC -fglasgow-exts -XOverloadedStrings -Wall #-} import Data.ByteString.Char8 (elemIndex) import Data.Maybe (fromJust) rtoa c = 10^(fromJust (elemIndex c "IXCMVLD"))`mod`9995 I got this to work by trial and error and have no clue what this "-XOverloadedStrings" and "Data.ByteString.Char8" business really means. If there is a better Haskell way of finding the numeric index of a particular character in a string, please let me know. Alternatively, you could write the rtoa function using an approach taken from the original HaskellWiki solution: rtoa = fromJust . flip lookup (zip "IVXLCDM" [1,5,10,50,100,500,1000]) What is your recommendation as to the "best"/most efficient way of writing the rtoa function in Haskell? Finally, here is an example complete test program. Suggestions for improving the style of this code are welcome. {-# OPTIONS_GHC -fglasgow-exts -Wall #-} import Data.Char (toUpper) import Data.List (concat, intersperse) rtoa :: Char -> Int rtoa 'M' = 1000 rtoa 'D' = 500 rtoa 'C' = 100 rtoa 'L' = 50 rtoa 'X' = 10 rtoa 'V' = 5 rtoa 'I' = 1 rtoa r = error $ "Invalid rtoa char:" ++ show r urtoa :: Char -> Int urtoa = rtoa . toUpper -- one is derived from http://haskell.cs.yale.edu/haskellwiki/Roman_numerals one :: String -> Int one = fst . foldr (\p (t,s) -> if p >= s then (t+p,p) else (t-p,p)) (0,0) . map urtoa two :: String -> Int two = foldl1 (\t n -> t+n-t`mod`n*2) . map urtoa three :: String -> Int three = foldl (\t c -> t+(urtoa c)-t`mod`(urtoa c)*2) 0 myshow :: String -> (String -> Int) -> String -> String myshow capt fn val = capt ++ val ++ " " ++ (show (fn val)) testdata :: [String] testdata = [ "I", "xiv", "DXVI", "CMLXIII" ] main :: IO () main = do putStrLn $ (concat $ intersperse "\n" (map (\c -> (myshow "one : " one c)) testdata)) putStrLn $ (concat $ intersperse "\n" (map (\c -> (myshow "two : " two c)) testdata)) putStrLn $ (concat $ intersperse "\n" (map (\c -> (myshow "three: " three c)) testdata)) Cheers, /-\ Need a Holiday? Win a $10,000 Holiday of your choice. Enter now.http://us.lrd.yahoo.com/_ylc=X3oDMTJxN2x2ZmNpBF9zAzIwMjM2MTY2MTMEdG1fZG…

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Re: [Haskell-cafe] Random Number
by michael rice 08 Jun '09

08 Jun '09

Good essay. Try this one for a laugh: http://www.mcs.vuw.ac.nz/comp/Publications/CS-TR-02-9.abs.html A good place to begin is PDF pg. 19. Michael --- On Sun, 6/7/09, Krzysztof Skrzętnicki <gtener(a)gmail.com> wrote: From: Krzysztof Skrzętnicki <gtener(a)gmail.com> Subject: Re: [Haskell-cafe] Random Number To: "ptrash" <ptrash(a)web.de> Cc: haskell-cafe(a)haskell.org Date: Sunday, June 7, 2009, 3:55 PM On Sun, Jun 7, 2009 at 21:33, ptrash<ptrash(a)web.de> wrote: > > Hi, > > is the are way (or a build in method) in haskell to get a random number from > a number bottom to a number top? > > Something like > > let randomNumber = random 1 30 > > to get a random number between 1 and 30. I don't mean to be rude, but did you even tried to read the documentation? The function you want is here: http://www.haskell.org/ghc/docs/latest/html/libraries/random/System-Random.… Before you ask any other questions please read this essay: http://mattgemmell.com/2008/12/08/what-have-you-tried Best regards Krzysztof Skrzętnicki _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Random Number
by ptrash 08 Jun '09

08 Jun '09

Hi, is the are way (or a build in method) in haskell to get a random number from a number bottom to a number top? Something like let randomNumber = random 1 30 to get a random number between 1 and 30. -- View this message in context: http://www.nabble.com/Random-Number-tp23914474p23914474.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Change value of a variable
by ptrash 08 Jun '09

08 Jun '09

Hi, how can I change the value of a variable. let x = 1 x = x + 2 First I set the value of x to 1. Then I want to increase it by 2. This way doesn't work, because I think it is a infinite expression. Is there a way to change the value? -- View this message in context: http://www.nabble.com/Change-value-of-a-variable-tp23913404p23913404.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Question on rank-N polymorphism
by Vladimir Reshetnikov 08 Jun '09

08 Jun '09

Hi, I have the following code: -------------------------------------------------------------------------------- fs g = (g fst, g snd) examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,"2")), fs ((,)id), fs (:[]), fs repeat) -------------------------------------------------------------------------------- The idea is that fs accepts a polymorphic function as its argument. What type signature can I specify for f in order to compile this code? If it is not possible in Haskell, is there another language with static typing which allows this? Thanks, Vladimir

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comprehension problem
by ptrash 08 Jun '09

08 Jun '09

Hi, what does this to code rows mean: newtype Probability = P Float newtype Dist a = D {unD :: [(a, Probability)]} newtype definies a new type called Probability. But what does P Float mean? And what is the a in Dist a? What does D {...} mean? Thanks for your help. -- View this message in context: http://www.nabble.com/comprehension-problem-tp23858359p23858359.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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totally confused about Haskell namespace issue conventions
by Vasili I. Galchin 07 Jun '09

07 Jun '09

Hello, Should namespace designation be specified in modules or in the .cabal file? Or to put it another way should a relative namespace be specified in a Haskell module and the remaining "top" part be specified in the associated .cabal file? Of course, yes/no answers are probably not sufficient ... i.e. please elaborate. Kind regards, Vasili

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Re: [Haskell-cafe] Question on rank-N polymorphism
by Ryan Ingram 07 Jun '09

07 Jun '09

Well, I don't really recommend programming in dependently typed languages right now :) But if you must, Agda has been getting a lot of attention recently. Also, the theorem prover Coq is based on the dependently-typed lambda calculus. In Haskell, giving a function an intersection type is generally done with typeclasses. You can write, for example: class Fs a where type FsResult a fs :: a -> FsResult a data Fmap = Fmap instance Fs Fmap where type FsResult a = forall f a b. Functor f => (f (a,b) -> f a, f (a,b) -> f b) fs Fmap = (fmap fst, fmap snd) (although this seems unusable to me!) You can also use Template Haskell to copy the argument: -- I may have the syntax wrong here fs :: a -> Q Exp fs a = [e| ($a fst, $a snd) ] test :: (Int, String) test = $(fs (`id` (1,"2")) -- ryan On Sun, Jun 7, 2009 at 2:28 AM, Vladimir Reshetnikov<v.reshetnikov(a)gmail.com> wrote: > Hi Ryan, > > Thanks for your explanation. What language with dependent types would > you recommend me to look at? > > Now I am studying rank-N polymorphism in Haskell and trying to > generalize some combinators in my libraries to multitypes. This is how > I came to this question. > > Thanks, > Vladimir > > On 6/7/09, Ryan Ingram <ryani.spam(a)gmail.com> wrote: >> This is a really interesting question. >> >> So, fs is well-typed in Haskell: >> fs :: (((a,a) -> a) -> t) -> (t,t) >> i.e. >> fs id :: ((a,a) -> a, (a,a) -> a) >> >> However, I believe what you are asking is for fs to be equivalent to >> the following: >>> fs2 f g = (f fst, g snd) >> >> which has the type >> fs2 :: (((a, b) -> a) -> t) -> (((a1, b1) -> b1) -> t1) -> (t, t1) >> >> except with the argument broadcast polymorphically to both positions. >> >> This means the argument must have the multitype >> >> g :: ((a,b) -> a) -> t /\ ((a1,b1) -> b1) -> t1 >> >> for some t and t1 which are functions of a,b and a1,b1. >> >> Unfortunately I don't believe it is possible to encode this type in >> System F or System F(c), the underlying lambda-calculus used by GHC, >> so Haskell isn't going to be able to solve this problem. But there >> are statically typed languages which can solve this problem. >> >> You can take the big hammer of dependent types, and write fs something >> like this (not Haskell; this is a dependently-typed language): >> >> typeof_g :: (Type -> Type -> Type -> Type) -> Type >> typeof_g res_type = (a :: Type) -> (b :: Type) -> (c :: Type) -> >> ((a,b) -> c) -> res_type a b c >> >> fs :: (res_type :: Type -> Type -> Type -> Type) -> (g :: typeof_g >> res_type) >> -> (a :: Type) -> (b :: Type) -> (res_type a b a, res_type a b b) >> fs _ g a b = (g a b a fst, g a b b snd) >> >> So, you'd write fs id like this: >>> fs (\a b c. (a,b) -> c) (\a b c. id ((a,b) -> c)) >> >> This is a fascinating problem, though. What put you on this path? >> >> -- ryan >> >> On Sat, Jun 6, 2009 at 3:06 PM, Vladimir >> Reshetnikov<v.reshetnikov(a)gmail.com> wrote: >>> Hi, >>> >>> I have the following code: >>> >>> -------------------------------------------------------------------------------- >>> fs g = (g fst, g snd) >>> examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,"2")), fs >>> ((,)id), fs (:[]), fs repeat) >>> -------------------------------------------------------------------------------- >>> >>> The idea is that fs accepts a polymorphic function as its argument. >>> What type signature can I specify for f in order to compile this code? >>> If it is not possible in Haskell, is there another language with >>> static typing which allows this? >>> >>> Thanks, >>> Vladimir >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> >

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using phantom types to validate html
by Mathijs Kwik 07 Jun '09

07 Jun '09

Hi all, Please have a look at http://moonpatio.com/fastcgi/hpaste.fcgi/view?id=2575#a2575 I wanted to use the typesystem to mandate businesslogic (in this case w3c validation rules). Thanks to some helpful people in #haskell I learned a bit about phantom types. Please let me know if I implemented them correctly. Also this little experiment raises some questions: The code becomes very verbose if there are more elements added or more rules to check. Since repetitive code can be a source of error, and hellish to maintain, I would like to know if there's some way to get this generated, or maybe there's some meta-programming stuff I don't know about. Another thing I can't figure out yet is how to do more advanced validation rules like "an html element cannot have 2 head sections", or (made up) "a span element isn't allowed to be a child(any level deep) of a p element". I think this would ask for an exponentially growing number of strange types and classes. Am I right? Just to be clear: this is just some practice to use the typesystem. I'm well aware that just using runtime validation checks will be a lot easier and clearer in most cases. Thanks, Mathijs

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xmobar plugin to show connectivity
by Deniz Dogan 07 Jun '09

07 Jun '09

Hi I'd like an xmobar plugin which in one way or another displays whether or not I'm connected to some network, i.e. if my network interface has an IP address. Has something liket his been done already? If not, what would be the best way to do this? -- Deniz Dogan

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Re: [Haskell-cafe] Question on rank-N polymorphism
by Vladimir Reshetnikov 07 Jun '09

07 Jun '09

Hi Zsolt, fs :: (((a, a) -> a) -> t) -> (t, t) fs g = (g fst, g snd) examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,"2")), fs ((,)id), fs (:[]), fs repeat) No instance for (Num [Char]) arising from the literal `1' at M.hs:6:54 Possible fix: add an instance declaration for (Num [Char]) In the expression: 1 In the second argument of `($)', namely `(1, "2")' In the first argument of `fs', namely `($ (1, "2"))' Anyways, this signature is not what intended. I want it to work for all tuples, regardless of their element types. Thanks Vladimir On 6/7/09, Zsolt Dollenstein <zsol.zsol(a)gmail.com> wrote: > On Sun, Jun 7, 2009 at 9:17 AM, Vladimir > Reshetnikov<v.reshetnikov(a)gmail.com> wrote: >> Hi Zsolt, >> >> It does not compiles with GHC without type annotations. > > It does with mine: The Glorious Glasgow Haskell Compilation System, version > 6.10.2 > > Anyway, try this: fs :: (((a, a) -> a) -> t) -> (t, t) > >> >> Thanks, >> Vladimir >> >> On 6/7/09, Zsolt Dollenstein <zsol.zsol(a)gmail.com> wrote: >>> Hi Vladimir, >>> >>> On Sun, Jun 7, 2009 at 12:06 AM, Vladimir >>> Reshetnikov<v.reshetnikov(a)gmail.com> wrote: >>>> Hi, >>>> >>>> I have the following code: >>>> >>>> -------------------------------------------------------------------------------- >>>> fs g = (g fst, g snd) >>>> examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,"2")), fs >>>> ((,)id), fs (:[]), fs repeat) >>>> -------------------------------------------------------------------------------- >>>> >>>> The idea is that fs accepts a polymorphic function as its argument. >>>> What type signature can I specify for f in order to compile this code? >>> >>> Have you tried putting the above into ghci for example, then asking for >>> ":t >>> fs"? >>> Or am I misunderstanding your point? >>> >>> Cheers, >>> Zsolt >>> >>>> If it is not possible in Haskell, is there another language with >>>> static typing which allows this? >>>> >>>> Thanks, >>>> Vladimir >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>> >> > >

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Still having problems building a very simple "Executable" ....
by Vasili I. Galchin 07 Jun '09

07 Jun '09

Hello, I picked an exceedingly case to build an "Executable": Executable QNameTest Hs-source-dirs: Swish/ Main-Is: HaskellUtils/QNameTest.hs Other-Modules: HaskellUtils.QName Here are the results of a "cabal build -v": Creating dist/build/QNameTest (and its parents) Creating dist/build/QNameTest/QNameTest-tmp (and its parents) /usr/local/bin/ghc -o dist/build/QNameTest/QNameTest --make -hide-all-packages -i -idist/build/QNameTest/QNameTest-tmp -iSwish/ -idist/build/autogen -Idist/build/QNameTest/QNameTest-tmp -odir dist/build/QNameTest/QNameTest-tmp -hidir dist/build/QNameTest/QNameTest-tmp -stubdir dist/build/QNameTest/QNameTest-tmp -package HUnit-1.2.0.0 -package array-0.1.0.0 -package base-3.0.1.0 -package binary-0.4.2 -package bytestring-0.9.1.0 -package containers-0.1.0.1 -package mtl-1.1.0.1 -package old-time-1.0.0.0 -package parallel-1.0.0.0 -package parsec-2.1.0.0 -package random-1.0.0.0 -O Swish/HaskellUtils/QNameTest.hs Swish/HaskellUtils/QNameTest.hs:36:7: Could not find module `Swish.HaskellUtils.QName': Use -v to see a list of the files searched for. ??? Thanks, Vasili

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Generic polyvariadic printf in Haskell98
by oleg＠okmij.org 07 Jun '09

07 Jun '09

Evan Klitzke wrote: > I'm writing code with hslogger, and I'm finding myself frequently > writing things like this: > > infoM $ printf "%s saw %s with %s" (show x) (show y) (show z) Indeed writing these `show' is tedious. Fortunately, we can get rid of them, still remaining in Haskell98. The following file defines the polyvariadic generic printf: http://okmij.org/ftp/typed-formatting/GenPrintF.hs It is like polyvariadic show with the printf formatting string. It handles values of any type for which there is a Show instance. Two instances of the class SPrintF are all we ever need. Here are the tests: t1 = unR $ printf "Hi there" -- "Hi there" t2 = unR $ printf "Hi %s!" "there" -- "Hi there!" t3 = unR $ printf "The value of %s is %s" "x" 3 -- "The value of x is 3" t4 = unR $ printf "The value of %s is %s" "x" [5] -- "The value of x is [5]" The appearance of unR is only because of Haskell98 (with flexible instances, we can get rid of that). On the other hand, your code post-processes the result of formatting with infoM, so the newtype unwrapping should not matter at all. Still, the code is a bit unsatisfactory because of the appearances of "error" in pr_aux functions. The errors like passing too many or too few arguments to printf (as demanded by the format specification) are caught only at run-time. We can certainly do better.

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Announce: haskeem 0.7.0 uploaded to hackage
by Uwe Hollerbach 07 Jun '09

07 Jun '09

Hi, all, a little while ago I uploaded haskeem 0.7.0 to hackage: this is my small scheme interpreter. I had been busy with other important stuff for a while, and hadn't worked on it for a while; but I've now updated it to build ok with ghc 6.10.3 + haskeline, plus I added a simple macro system; that had been one of the two big items left on my list for it. It's not yet full R6RS Scheme hygienic macros, but I'll get there, too... I had kind of a "functional programming moment" while doing this macro system... I had been thinking and reading about it for, oh, a couple of weeks now, and today I sat down to start that, with the hope of maybe finishing it in a week or two. Nope! All in all, it took about 45 minutes, with the really important changes to the code spanning, oh, call it a generous three lines of code. There were changes in 50ish other lines, but those were all pretty trivial, just adding another data type. Such a let-down! :-) :-) :-) There is one small regression in all of this, namely that with haskeline I have lost the ability to interrupt the interpreter and land back at the "lisp>" prompt. I can interrupt it all right, but I end up at the shell prompt... not so good! I know all that stuff can be set up to interrupt a little less thoroughly than that, I just haven't got around to it yet... soon, I hope. Uwe

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ANNOUNCE: numtype 1.0 -- Type-level (low cardinality) integers
by Bjorn Buckwalter 07 Jun '09

07 Jun '09

Dear all, Since its inception my dimensional library has been built around a unary type-level representation of integers (NumTypes) defined in the Numeric.NumType module. This module has proven itself useful outside the context of dimensional and after dragging my feet for a long time I've finally gotten around packaging it up in its own library: numtype[1]. The Numeric.NumType module is completely self-contained (only imports Prelude) and is heavily commented in a narrative manner inspired by Oleg Kiselyov's expositions. I believe it provides a good case study for type-level programming with multi-parameter type classes and functional dependencies. Addition, subtraction, division, and multiplication of NumTypes is supported. NumTypes have no value-level representation but can be converted to any Num instance with the 'toNum' function. The numtype library has two significant short-comings: * Minimal haddocks -- as with my dimensional library the literate Haskell source code is the documentation. The flip-side is that the code is very well-commented. * Due to the unary implementation the practical size of the NumTypes is severely limited, making them unsuitable for large-cardinality applications. If you will be working with integers beyond (-20, 20) this package probably isn't for you. (If the second bullet is a show-stopper Edward Kmett's type-int[2] library may be a better choice. Peter Gavin's tfp[3] library also provides type-level integers but uses type families instead of MPTCs and fundeps. I cannot vouch for either of these libraries as I haven't used them.) Numtype version 1.0 can be downloaded from Hackage or the dimensional project page[4]. I've also updated dimensional to version 0.8 with the Numeric.NumType module removed and Julian 'year' and 'century' units added. Enjoy! Thanks, Bjorn Buckwalter [1]: http://code.google.com/p/dimensional/wiki/numtype [2]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/type-int [3]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tfp [4]: http://dimensional.googlecode.com

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Haskell Weekly News: Issue 120 - June 6, 2009
by Brent Yorgey 07 Jun '09

07 Jun '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090606 Issue 120 - June 06, 2009 --------------------------------------------------------------------------- Welcome to issue 120 of HWN, a newsletter covering developments in the [1]Haskell community. Sorry for the massive HWN, I missed last week so you're getting two for the price of one! Registration for [2]Hac phi is now open, be sure to register soon (register by June 15 to get a special hotel rate). Announcements Reminder: Haskell Implementers' Workshop CFT deadline in 2 weeks. Simon Marlow [3]reminded everyone to consider submitting a talk proposal for the Haskell Implementers' Workshop, to be held in conjunction with ICFP in Edinburgh, Scotland on 5 September. The deadline for submissions is a couple of weeks away (15 June); all that is needed is an abstract. storable-record. Henning Thielemann [4]announced [5]storable-record, a small package for simplified declaration of Storable instances for records. It may be used as an alternative to the [6]c2hs preprocessor. It was made possible by advanced applicative technology, a cutting edge LCM monoid and an incredible constructor power tower. Haskell Communities and Activities Report (16th ed., May 2009). Janis Voigtlaender [7]announced the availability of the [8]16th Haskell Communities and Activities Report. hledger 0.5 released. Simon Michael [9]announced the release of version 0.5 of [10]hledger, a (mostly) text-mode double-entry accounting tool that generates precise activity and balance reports from a plain text journal file. New repository and trac for haskell-src-exts. Niklas Broberg [11]announced some new infrastructure for the haskell-src-exts package, set up in preparation for his GSoC project. with the HSP packages, it's now old enough to be allowed to [12]live on its own. There is also a [13]bug tracker. Please help by reporting any bugs you come across, or by requesting new and cool features. bsd-sysctl 1.0.3. Maxime Henrion [14]announced the release of [15]bsd-sysctl 1.0.3, a package that provides a System.BSD.Sysctl module allowing access to the C sysctl(3) API. It should fully work on FreeBSD, NetBSD and Mac OS X platforms. multirec-binary. Sebastiaan Visser [16]announced the release of [17]multirec-binary, which allows generic derivation of Data.Binary instances using the [18]MultiRec library. notice for package authors. Duncan Coutts [19]announced that Hackage uploads will soon require an upper bound on the version of the base package and reject packages that omit it. This will hopefully result in less breakage the next time a new version of the base package is released. (Pre-) Announce: Data.GDS 0.1.0. Uwe Hollerbach [20](pre-) announced Data.GDS, a small module to write and (eventually) read GDS files, a classic format of the semiconductor industry. The module can currently generate GDS files with a fairly low-level interface; planned future versions (which will be uploaded to Hackage) will have a higher-level interface and be able to parse GDS files as well. new version of uu-parsinglib. S. Doaitse Swierstra [21]announced that a new version of the [22]uu-parsinglib library has been uploaded to hackage. It is now based on Control.Applicative where possible. Be warned that functions like some and many will be redefined in the future. Hac phi: Haskell hackathon in Philadelphia, July 24-26. Brent Yorgey [23]announced Hac phi, a Haskell hackathon/get-together to be held July 24-26 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Everyone is welcome---you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please [24]register. There is a block of hotel rooms available at a special rate only until June 15, so register early! More details can be found on the [25]Hac phi wiki. Job for someone: make a VM image for GHC development. Simon Marlow [26]suggested a useful project for someone looking for something to do: create a VM image of a Linux system with a complete GHC development environment set up and ready to go. My attempt at Haskell USB. Mauricio [27]announced some [28]Haskell bindings to libusb, and gave another plug for his [29]bindings-common package, which makes it easier to generate Haskell bindings to low-level libraries. second alpha release of OSX haskell platform installer. Gregory Collins [30]announced a [31]second candidate release for the OSX Haskell Platform installer. Please try it out! Release Schedule for 2009.2.0.2. Don Stewart [32]announced the [33]release schedule for the next minor release of the 2009.2.0 branch of the Haskell Platform. The freeze for package changes will be Wednesday 1 July, and the release is scheduled for Monday 13th July. hscamwire, for IIDC1394 cameras. Frederick Ross [34]announced the release of [35]hscamwire 0.1, which provides a nice Haskellized layer over Camwire, a library to connect to IIDC1394 cameras (most scientific and industrial Firewire cameras) on Linux. Safe and generic printf with C-like format string. oleg [36]announced some code to implement a type-safe polyvariadic version of printf, which is also integrated with Show so that any showable type can be printed. A library for serial ports. Frederick Ross [37]announced the release of [38]serial-0.1, a library for line-oriented interaction with serial ports on POSIX compatible systems. HaL4: Haskell-Meeting in Germany, 12th June 2009. Janis Voigtlaender [39]reminded everyone of [40]Hal4, a German-language Haskell gathering to be held in Halle/Saale on June 12. There are already close to 50 registered participants, so expect a very lively meeting! Late registration still possible. wp-archivebot 0.1 - archive Wikipedia's external links in WebCite. Gwern Branwen [41]announced [42]wp-archivebot, a relatively simple little script which follows all the links in a RSS feed, combs the destination for http:// links, and submits them to [43]WebCite. memscript-0.0.0.2. Ki Yung Ahn [44]announced [45]memscript, a command line utility for memorizing scriptures or any other text. HSH 2.0.0. John Goerzen [46]announced the release of [47]version 2.0.0 of HSH, the Haskell shell scripting library. This version features a complete rewrite of the core using System.Process, a drastic reduction in code size and complexity, cross-platform support, and a simpler and more flexible API. atom-0.0.5. Tom Hawkins [48]announced version 0.5 of the [49]atom library, a DSL for embedded hard realtime applications. This version includes a few bug fixes and doc improvements. heap-1.0.0. Stephan Friedrichs [50]announced a rewrite of the heap package, [51]heap-1.0.0. It is not 100% compatible with version 0.6.0, but provides major improvements, including a better mechanism for instantiating min-, max-, min-prio- and max-prio-heaps, and faster {from,to}{Asc,Desc}List conversions. The Haskell Platform 2009.2.0.1. Don Stewart [52]announced the second release (2009.2.0.1) of the [53]Haskell Platform, a single, standard Haskell distribution for everyone. The specification, along with installers (including Windows and Unix installers for a full Haskell environment) are available. Anglohaskell 2009. Philippa Cowderoy [54]announced [55]Anglohaskell 2009, to be held at MSR Cambridge on the 7th and 8th of August. code reviewers wanted for hashed-storage (darcs). Eric Kow [56]solicited anyone with a few spare hours this summer willing to help the Darcs project as a code reviewer for the standalone hashed-storage module, which will be used by Darcs in the future. No Darcs experience is needed! Google Summer of Code Progress updates from participants in the 2008 [57]Google Summer of Code. Haddock improvements. Isaac Dupree has begun looking at the Haddock code, and has a [58]question about which of two options he should pursue. EclipseFP. Thomas Ten Cate has posted an [59]explanation of how the Scion client/server model works. Space profiling. Gergely Patai has [60]uploaded a preliminary version of the [61]hp2any core library which handles heap profiles both during and after execution. He has also [62]posted some pretty graphs generated by a simple utility built on top of the core library. haskell-src-exts. Niklas Broberg has begun work by making a [63]list of all language extensions and the ways in which they affect lexing and parsing, since haskell-src-exts will need to be parameterized over these extensions. Fast Darcs. Petr Rockai has posted two detailed [64]progress [65]reports already, with many changes to both the standalone [66]hashed-storage library and a [67]fork of darcs which uses it. Discussion Error message reform (was: Strange type error with associated type synonyms). Max Rabkin began an interesting [68]discussion about error messages. Do you have an intuitive sense of which is the 'expected' and which the 'inferred' type? time library dependencies. Ashley Yakeley [69]asked what dependencies are acceptable for the time library, leading to a discussion of what dependencies are acceptable for base packages. Bool as type class to serve EDSLs. Sebastiaan Visser started a [70]discussion on the possibility of a type class for representing Boolean values, much like the current Num class for numeric values. Jobs 10 jobs in declarative programming. Oege de Moor [71]announced the availability of positions with Semmle and LogicBlox for ten declarative programming consultants, who will work with clients to write custom queries in Datalog, and to create user interfaces in a declarative framework. Semmle and LogicBlox are creating a platform for declarative programming in Datalog, a pure logic programming language. Semmle is based in Oxford, headed by Oege de Moor; LogicBlox is based in Atlanta, headed by Molham Aref. See the announcement for more information and how to apply. Blog noise [72]Haskell news from the [73]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * David Amos: [74]Welcome to Haskell for Maths. David's Haskell library for mathematics exploration is under development again! * Joachim Breitner: [75]Third place in AI programming contest. * Bryan O'Sullivan: [76]Dealing with encoding errors in Data.Text. * Remco Niemeijer: [77]Programming Praxis - Ternary Search Tries. * beelsebob: [78]Collecting Non-Memory Resources. * Luke Palmer: [79]It is never safe to cheat. Ceiling cat is watching you. * Alex McLean: [80]More hackery. More cool livecoding with Haskell. * Thomas ten Cate: [81]Client/server communication. * Gergely Patai: [82]The first graphs. * Alson Kemp: [83]Turbinado V0.6.5. * Don Stewart (dons): [84]The Haskell Platform 2009.2.0.1. The first minor update release of the Haskell Platform is here. * Alson Kemp: [85]Turbinado V0.6.5. * Michael Snoyman: [86]Functors and Monads (containers). * Well-Typed.Com: [87]Come talk at the Haskell Implementers' Workshop!. * GSoC Fast Darcs: [88]soc progress 2. * Shin-Cheng Mu: [89]On a Basic Property for the Longest Prefix Problem. * >>> Ben Hutchison: [90]OO/Imperative programmers: 'Study Functional Programming or Be Ignorant'. * Michael Snoyman: [91]Run a MonadCGI as a CGI application!. * Michael Snoyman: [92]Wordify: RESTful Haskell web apps. * Marco Tulio Gontijo e Silva: [93]xmlGetWidget without castTo*. * >>> slawekk: [94]Probability monad. * Brandon Simmons: [95]Huffman Coding. * Niklas Broberg: [96]Parametrising haskell-src-exts on extensions. A list of language extensions and how they affect parsing. * Manuel M T Chakravarty: [97]Instant Generics now has a website!. * GHC / OpenSPARC Project: [98]The CAS experiment. * Brent Yorgey: [99]Hac phi!. Registration is now open. * Jeff Heard: [100]Buster 2.2 - Application Orchestration redux. Example code showing off Buster. * Bryan O'Sullivan: [101]I put a pidgit in your widget so you can fidget while you calculate pi. GHC and the language shootout. * Niklas Broberg: [102]Haskell Platform, I'm in love. * Bjorn Buckwalter: [103]Benchmarking Amazon EC2 with GHC. * Bjorn Buckwalter: [104]Blogging with Pandoc, literate Haskell, and a bug. * >>> Chris Moos: [105]Haskell AIM Client - a cool proof of concept. * Marco Tulio Gontijo e Silva: [106]Generating code with Haskell-src and TH. Quotes of the Week * pumpkin: we should throw it [CReal] in with Foreign.C.Types to confuse people * MyCatVerbs: The *real* best way to optimize a program is to tell dons that it's been added to the Shootout. * SimonFrankau: The points-free approach, while elegant, can make code unreadable, especially if it is written by quantitative analysts moonlighting as functional programmers. * ValarQ: l33t_h4x0r: could you help me port GHC to the AVR architecture? <-- l33t_h4x0r has left #haskell * gwern: drat. what *do* all you people talk about? only one bacon and one zombie quote * quicksilver: well if you can get proggit to help with your interview, then perhaps you can get proggit to help with the job when you get it. So it's not cheating, it's just an indication of one of your skill sets. * shapr: I haven't tried F#, everytime I get the urge to do something fun with .NET I have SharePoint flashbacks and buy more hardware instead. * gwern: bleh. haskell is messing me up. I wondered what operator =) is, before I realized it was a syntax error, before I realized it was an emoticon About the Haskell Weekly News New editions are posted to [107]the Haskell mailing list as well as to [108]the Haskell Sequence and [109]Planet Haskell. [110]RSS is also available, and headlines appear on [111]haskell.org. To help create new editions of this newsletter, please see the information on [112]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [113]http://code.haskell.org/~byorgey/code/hwn/ . 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78. http://noordering.wordpress.com/2009/06/05/collecting-non-memory-resources/ 79. http://lukepalmer.wordpress.com/2009/06/04/it-is-never-safe-to-cheat/ 80. http://yaxu.org/more-hackery/ 81. http://eclipsefp.wordpress.com/2009/06/03/clientserver-communication/ 82. http://just-bottom.blogspot.com/2009/06/first-graphs.html 83. http://www.alsonkemp.com/haskell/turbinado-v065/ 84. http://donsbot.wordpress.com/2009/06/02/the-haskell-platform-2009-2-0-1/ 85. http://www.alsonkemp.com/haskell/turbinado-v065/ 86. http://blog.snoyman.com/2009/06/02/functors-and-monads-containers/ 87. http://blog.well-typed.com/2009/06/come-talk-at-the-haskell-implementers-wo… 88. http://web.mornfall.net/blog/soc_progress_2.html 89. http://www.iis.sinica.edu.tw/~scm/2009/on-a-basic-property-for-the-longest-… 90. http://benhutchison.wordpress.com/2009/06/02/study-functional-programming-o… 91. http://blog.snoyman.com/2009/05/20/run-a-monadcgi-as-a-cgi-application/ 92. 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Why are these record accesses ambiguous
by John Ky 06 Jun '09

06 Jun '09

Hi Haskell Cafe, In the following code, I get an error saying Ambiguous occurrence `x'. Why can't Haskell work out which x to call based on the type of getA? Thanks -John #!/usr/bin/env runhaskell > {-# LANGUAGE DisambiguateRecordFields #-} > import A > import B > > main = do > let xx = getA > putStrLn $ show x xx ---------------------- module A where data TypeA = TypeA { a :: Int , x :: Int } getA = TypeA { a = 1, x = 2 } ------------------------- module B where data TypeB = TypeB { b :: Int , x :: Int } getB = TypeB { b = 1, x = 3 } -------------------------- ./test.lhs:8:21: Ambiguous occurrence `x' It could refer to either `A.x', imported from A at ./test.lhs:3:2-9 (defined at A.hs:5:5) or `B.x', imported from B at ./test.lhs:4:2-9 (defined at B.hs:5:5)

5 5

Pike's Newsqueak talk
by Tim Newsham 06 Jun '09

06 Jun '09

I just watched http://video.google.com/videoplay?docid=810232012617965344 It's a great talk that is suprisingly relevant to Haskell programming (although at first blush it looks a bit unrelated). (It refs a lot of older work that actually led me to Haskell in the first place by way of McIlroy's haskell power-series paper). Anyway, I thought it would be of general interest. A lot of the progrms he discusses are a lot more elegant in pure Haskell code (ie. prime number sieve, power series), but his language also supports an interesting imperative primitive that lets you pick the first available value from a set of channels which isn't available in pure Haskell expressions. Has anyone implemented a primitive like this for Haskell? Tim Newsham http://www.thenewsh.com/~newsham/

4 5

Conversion from string to array
by John Ky 06 Jun '09

06 Jun '09

Hi Haskell Cafe, I'm trying to send stuff over UDP. To do that, I've written a test program that sends strings across. That was fine, but I wanted to send binary data too. So I tried that only to find I am having difficulty putting together some binary data. For examples take the fromHex function in the following code. It is supposed to convert from a Hexadecimal string to a list of bytes, but I am having trouble coercing the integer types to the size I want. Is this the right way to do it? Cheers, -John > import Data.Bits > import Data.Char > import Data.Word > import System.Environment > import XStream.Tsmr.Client > data CmdLineOptions = CmdLineOptions > { optionHelp :: Bool > , optionVersion :: Bool > , optionPort :: String > , optionMessage :: String > , optionInvalids :: [String] > } > deriving (Eq, Show) > initCmdLineOptions = CmdLineOptions > { optionHelp = False > , optionVersion = False > , optionPort = "1234" > , optionMessage = "" > } > parseArgs :: [String] -> CmdLineOptions > parseArgs [] = initCmdLineOptions > parseArgs ("--port":port:xs) = (parseArgs xs) { optionPort = port } > parseArgs ("--help":xs) = (parseArgs xs) { optionHelp = True } > parseArgs ("--version":xs) = (parseArgs xs) { optionVersion = True } > parseArgs (('-':opt):xs) = let option = (parseArgs xs) in > option { optionInvalids = ('-':opt):optionInvalids option } > parseArgs (message:xs) = (parseArgs xs) { optionMessage = message } > printUsage = do > putStrLn "Usage: udp-server.lhs [options] <message>" > putStrLn "" > putStrLn "Options:" > putStrLn " --help Get help information." > putStrLn " --vesion Get version information." > putStrLn " --port n The port number to listen on." > putStrLn "" > putStrLn "Message:" > putStrLn " The message to send." > putStrLn "" > printVersion = do > putStrLn "Version." > fromHex :: String -> [Word8] > fromHex [] = [] > fromHex (u:l:xs) = (hexU .|. hexL):fromHex xs > where > hexU = (fromInteger $ hexValue u) :: Word8 > hexL = (fromInteger $ hexValue l) :: Int > hexValue c > | '0' <= c && c <= '9' = ord c - ord '0' > | 'a' <= c && c <= 'z' = ord c - ord 'a' + 10 > run port message = do > h <- openlog "localhost" port "udp-client.lhs" > syslog h (fromHex message) > main = do > args <- getArgs > let options = parseArgs args > let port = optionPort options > let message = optionMessage options > if optionHelp options > then printUsage > else if optionVersion options > then printVersion > else do > putStrLn ("Starting UDP listener on port: " ++ port) > run port message

2 1

Re: [Haskell-cafe] Record initialise question (RESOLVED)
by John Ky 06 Jun '09

06 Jun '09

On Fri, Jun 5, 2009 at 8:05 PM, Martijn van Steenbergen < martijn(a)van.steenbergen.nl> wrote: > Hi John, > > John Ky wrote: > >> > full = do >> > let myOrder = init -- [1] >> > { item = Just init >> > { itemId = "Something" >> > } >> > , operation = Just Buy >> > } >> > putStrLn $ show myOrder >> > return () >> >> Where initOrder and initItem are both replaced with just 'init' and the >> compiler works out from the context (ie. the type of the field) what the >> types are supposed to be and therefore the actual init function to call? >> > > Sure! This is one of the simplest forms of overloading. Here's a small > example: > > data Circle = Circle { center :: (Float, Float), radius :: Float } >> data WrapCircle = WrapCircle { circle :: Circle } >> >> class Ini a where >> ini :: a >> >> instance Ini Circle where >> ini = Circle { center = (0, 0), radius = 1 } >> >> exampleCircle :: WrapCircle >> exampleCircle = WrapCircle { circle = ini { radius = 2 } } >> > > Hope this helps! > > Martijn. >

1 0

A small puzzle: inTwain as function of foldr
by Martijn van Steenbergen 06 Jun '09

06 Jun '09

Bonjour café, A small puzzle: Consider the function inTwain that splits a list of even length evenly into two sublists: > inTwain "Hello world!" ("Hello ","world!") Is it possible to implement inTwain such that the recursion is done by one of the standard list folds? Is there a general way to tell if a problem can be expressed as a fold? Thank you, Martijn.

9 13

Re: Non Empty List?
by Tillmann Rendel 06 Jun '09

06 Jun '09

Hi, please write to the whole list, not just me. There are a lot of people around who can help you. MH wrote: > Rendel do you mind to explain to me how Container a = Many a > (Container [a]) prevents user from creating an empty list? > I did try the following: > let a = Many "string" > a :: Container [Char] -> Container [Char] > let b = Many [] > b :: forall a. Container [a] -> Container [a] > > but I can not understand what is the difference. You cannot prevent users from creating an empty list, because lists are already defined in the Haskell prelude, and you cannot change that definition. But you can define your own datatype Container which prevents users from creating empty containers. Your datatype data Container a = Many a (Container [a]) is quite complicated, because you have (Container a) on the left hand side of the equation, but (Container [a]) on the right hand side. Is that on purpose? Anyway, your Container has the property that a (Container a) is never empty, because it always contains at least one value of type a, namely the first parameter of Many. To see that, we can write a function which extracts that element: getContainerElement :: Container a -> a getContainerElement (Many element something) = element It is not possible to write such a function for ordinary lists without calling error or entering a nonterminating loop: getListElement :: [a] -> a getListElement (element : something) = element getListElement [] = error "no element around :(" So ordinary lists can be empty, but your Containers can't. Tillmann

5 4

Is it possible to do type-level arithmetic without UndeciableInstances?
by Ryan Ingram 06 Jun '09

06 Jun '09

> {-# LANGUAGE TypeFamilies #-} > module PeanoArith > data Z = Z > data S n = S n So, this type family works just fine: > type family Plus a b > type instance Plus Z b = b > type instance Plus (S a) b = S (Plus a b) As does this one: > type family Minus a b > type instance Minus a Z = Z > type instance Minus Z (S b) = Z > type instance Minus (S a) (S b) = Minus a b But this one doesn't work without UndecidableInstances: > type family Times a b > type instance Times Z b = Z > type instance Times (S a) b = Plus b (Times a b) I tried several different implementations for Times but I was unable to come up with one that passed the type family termination checker. Is there a way to do so? Some examples of attempts: > -- (a+1)*(b+1) = ab + a + b + 1 > type family Times1 a b > type instance Times1 Z Z = Z > type instance Times1 (S n) Z = Z > type instance Times1 Z (S n) = Z > type instance Times1 (S a) (S b) = S (Plus (Times1 a b) (Plus a b)) > -- inline Plus > type family Times2_H a b acc > type Times2 a b = Times2_H a b Z > type instance Times2_H a b (S acc) = S (Times2_H a b acc) > type instance Times2_H Z b Z = Z > type instance Times2_H (S a) b Z = Times2_H a b b Do you have any other ideas? Is (a*b) too big for the termination checker to ever accept? -- ryan

3 2

Fast code question
by Bartosz Wójcik 06 Jun '09

06 Jun '09

Hi Folks, I had to transform Packed Decimal file into csv format (does anybody here know what this Mainframe format is?). Because of the file size I could not do this on mainframe directly. So I've created simply program using ByteString. Generally I'm happy with my solution: pgm processes arroud 2MB/s on my pc, so my 3GB file was transformed in reasonable 30 min time. My question is: how to do this faster? {code} module Main where import qualified Data.ByteString.Lazy as B main = B.getContents >>= myPrint . myConcat . B.unpack myConcat = myConcat' 0 myConcat' :: (Integral a) => Integer -> [a] -> [Integer] myConcat' _ [] = [] myConcat' acc (x:xs) = case x `mod` 16 of 12 -> (10*acc + (restOf . fromIntegral) x) : myConcat' 0 xs 13 -> ((-10)*acc + (restOf . fromIntegral) x) : myConcat' 0 xs 15 -> (10*acc + (restOf . fromIntegral) x) : myConcat' 0 xs 10 -> fail $ show acc 11 -> fail $ show acc 14 -> fail $ show acc v -> myConcat' (100*acc + (numberOf . fromIntegral) x) xs where restOf n = (n - 12) `div` 16 numberOf n = n - 6 * (n `div` 16) myPrint [] = return () myPrint xs = mapM_ myShow (take 14 xs) >> putStrLn "" >> myPrint (drop 14 xs) myShow x = (putStr . show) x >> putStr ";" {code} I knew that csv output had to be 14 fields per line. Best, Bartek

4 4

Re: [Haskell-cafe] Fast code question
by Bartosz Wójcik 06 Jun '09

06 Jun '09

Packed Decimal downloaded on pc is just a stream of bytes without any comma. I was supposed to reformat data. If I undersdand bytestring-csv library, it parses csv format data. Thanks for the hint. I'll investigate next time when I have to deal with huge files. Bartek On Thursday 04 June 2009 23:21:21 you wrote: > Can you use the bytestring csv parser (or convert it into a pretty > printer?) > > bartek: > > Hi Folks, > > > > I had to transform Packed Decimal file into csv format (does anybody here > > know what this Mainframe format is?). Because of the file size I could > > not do this on mainframe directly. So I've created simply program using > > ByteString. Generally I'm happy with my solution: pgm processes arroud > > 2MB/s on my pc, so my 3GB file was transformed in reasonable 30 min time. > > > > My question is: how to do this faster? > > > > {code} > > module Main > > where > > > > import qualified Data.ByteString.Lazy as B > > > > main = B.getContents >>= myPrint . myConcat . B.unpack > > ^^^^^^^^^^^^^^^^^^^^^ > That looks bad. -------------------------------------------------------

1 0

web musing
by Conor McBride 06 Jun '09

06 Jun '09

Comrades I'm in a perplexing situation and I'd like to appeal to the sages. I've never written anything other than static HTML in my life, and I'd like to make a wee web service: I've heard some abbreviations, but I don't really know what they mean. I've got a function (possibly the identity, possibly const "", who knows?) assistant :: String -> String and I want to make a webpage with an edit box and a submit button. If I press the submit button with the edit box containing string s, I'd like the page to reload with the edit box reset to (assistant s). Will I need to ask systems support to let me install some haskelly sort of web server? Looks likely, I suppose. In general, what's an easy way to put a web front end on functionality implemented in Haskell? Hoping this isn't a hard question Conor

6 5

specialization in type classes
by Cetin Sert 06 Jun '09

06 Jun '09

module IOStream where import System.IO import System.IO.Unsafe class Out a where out :: a → String instance Show a ⇒ Out a where out = show instance Out String where {-# SPECIALISE out :: String → String #-} out = id instance Out Char where {-# SPECIALISE out :: Char → String #-} out = \x → [x] infixl 0 <<, ≪ (≪), (<<) :: Out a ⇒ IO Handle → a → IO Handle (<<)= (≪) h ≪ a = do s ← h hPutStr s $ out a return s cout, cin, cerr :: IO Handle cout = return stdout cin = return stdin cerr = return stderr endl :: String endl = "\n" --- cetin@unique:~/lab/c/linking/demo$ ghci -fglasgow-exts iostream.hs 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. Ok, modules loaded: IOStream. Prelude IOStream> cout << 22 << False 22False{handle: <stdout>} Prelude IOStream> cout << 22 << False << endl <interactive>:1:0: Overlapping instances for Out String arising from a use of `<<' at <interactive>:1:0-26 Matching instances: instance (Show a) => Out a -- Defined in IOStream instance Out String -- Defined in IOStream In the expression: cout << 22 << False << endl In the definition of `it': it = cout << 22 << False << endl o________________O how can I specialise a type class function?

2 2

Non Empty List?
by Günther Schmidt 05 Jun '09

05 Jun '09

Hi, I need to design a container data structure that by design cannot be empty and can hold n elements. Something like a non-empty list. I started with: data Container a = Single a | Many a [a] but the problem above is that the data structure would allow to construct a Many 5 [] :: Container Int. I can't figure out how to get this right. :( Please help. Günther

11 17

Record initialise question
by John Ky 05 Jun '09

05 Jun '09

Hi all, I have some sample code: > full = do > let myOrder = initOrder > { item = Just initItem > { itemId = "Something" > } > , operation = Just Buy > } > putStrLn $ show myOrder > return () This is just a test project, but in a real project, I would have a more elaborate structure. Notice initOrder and initItem. I actually need to know the type of field I am initialising or the name of the corresponding function that provides default values. Is there any kind of polymorphic magic that lets me do this instead? > full = do > let myOrder = init -- [1] > { item = Just init > { itemId = "Something" > } > , operation = Just Buy > } > putStrLn $ show myOrder > return () Where initOrder and initItem are both replaced with just 'init' and the compiler works out from the context (ie. the type of the field) what the types are supposed to be and therefore the actual init function to call? Thanks, -John [1] The second example is silly because there isn't actually any context at this point, but let's pretend there is.

2 1

Umlauts in command line arguments
by Günther Schmidt 05 Jun '09

05 Jun '09

Hi all, When a command line argument contains an umlaut that argument gets garbled. I'm using ghc 6.10.2 on Win XP. Are there any known solutions for this problem? Günther

4 4

Functors and the Visitor Pattern
by Tom.Amundsen 05 Jun '09

05 Jun '09

So, last night, I was having this problem with my Java code where I couldn't figure out for the life of me how to write a piece of code without a big if {} else if {} else if {} ... else {} structure. I was Googling "Java Reflection" to try to determine how to "cast to the most concerete subclass at runtime." Then it dawned on me that what I was trying to do has already been solved by using the Visitor design pattern. Then, after reading the Visitor design pattern page on Wiki, it said that the visitor pattern is essentially an implementation of a functor. Aha! It totally clicked. The Visitor pattern allows you to collect code for similar operations, while spreading apart code for similar objects. Now that really sounds like a functor! Although, now I'm second guessing myself, because I can't figure out how we could create some design pattern that simulates an applicative functor. I'm pretty sure the Visitor pattern doesn't take you this far (but I am willing to be corrected). So, is there a way to create applicative functors in non-functional languages? What would that pattern look like? - Tom -- View this message in context: http://www.nabble.com/Functors-and-the-Visitor-Pattern-tp23851113p23851113.… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

6 7

GHCI & Curl under Windows
by Martin Huschenbett 05 Jun '09

05 Jun '09

Hi Haskellers, I've installed the newest Haskell Platform under Vista and downloaded a pre compiled version of curl-7.19.4 for MinGW. After changing the build type in curl.cabal to Simple and supplying the needed paths I could even build and install curl for haskell. If I write a program using curl and compile it with GHC everything works fine. But if I try to execute the main function of my program in GHCI I always get the following error message: Loading package curl-1.3.5 ... linking ... <interactive>: C:\Devel\Haskell\curl-1.3.5\ghc-6.10.3\HScurl-1.3.5.o: unknown symbol `__imp__curl_easy_getinfo': unable to load package `curl-1.3.5' Did anybody have the same problem and knows how to fix it? Thanks in advance, Martin.

4 3

RE: Nested Lists
by Paul Keir 05 Jun '09

05 Jun '09

Emil, Felipe, Thanks. I don't know Type Families, but take the point that the input can be parameterised with something something other than a list. i.e. (8 :+: 4 :+: 2 :+: ()) presumably has the same type as (4 :+: 2 :+: ()). My intention was to use common list functions on the sublists, but always then a concat for each level, to return to a flat list. With that in mind I made the following oddity, which in any case doesn't compile due to its use of infinite types. app (f:fs) es = appUp (f:fs) es where len = genericLength (f:fs) appUp [] es = appDown es len appUp (f:fs) es = appUp (map map fs) (f es) appDown es len = appDown (concat es) (len - 1) appDown es 0 = es Henning, I agree with you, a tree would be much better for this. Thanks. -Paul

2 1

Nested Lists
by VoidEx 05 Jun '09

05 Jun '09

You can also try this: bunch n f = takeWhile (not . null) . unfoldr (Just . (f *** id) . splitAt n) > (bunch 8 . bunch 4 . bunch 2) id [1..16] Any way, it's not possible to take list [8, 4, 2], because it's length need to be known at compile time.

1 0

Static link to packages on Hackage?
by John Goerzen 05 Jun '09

05 Jun '09

Hi, I'd like to be able to put a static link to the Haddock docs for the current version of various packages on my homepage. Right now, I can't find any such URL; they all look like: http://hackage.haskell.org/packages/archive/HSH/2.0.0/doc/html/HSH.html I'd like if there could be something like: http://hackage.haskell.org/packages/archive/HSH/latest/doc/html/HSH.html Incidentally, you can click on this "latest" URL but it doesn't go to the correct version. I can't just link to the package page either, because sometimes it won't have docs (such as shortly after I've uploaded a new version). Ideas?

2 2

TASE 2009 - Call for Participation
by CRACIUN F. 05 Jun '09

05 Jun '09

TASE 2009 - CALL FOR PARTICIPATION ********************************************************** * 3rd IEEE International Symposium on * Theoretical Aspects of Software Engineering * (TASE 2009) * 29-31 July 2009, Tianjin, China * http://www.dur.ac.uk/ieee.tase2009 * * Early Registration Deadline : 18 June 2009 * For more information email: IEEE.TASE2009(a)durham.ac.uk ***********************=********************************** TASE 2009 Invited Speakers ========================== Kim G. Larsen, Aalborg University, Denmark Zhong Shao, Yale University, USA Jin-Song Dong, National University of Singapore TASE 2009 Programme ==================== Day 1: 29 July 2009 ------------------- Invited Talk: Verification and Performance Analysis of Embedded Systems Kim G. Larsen (Aalborg University) Session 1 : Real-Time and Embedded Systems Improving Responsiveness of Hard Real-Time Embedded Systems Hugh Anderson (Wellington Institute of Technology) and Siau-Cheng KHOO (National University of Singapore) Environmental Simulation of Real-Time Systems with Nested Interrupts Guoqiang Li (Shanghai Jiao Tong University), Shoji Yuen (Nagoya University) and Masakazu Adachi (Toyota Central R&D Labs. INC.). Semantics for Communicating Actors with Interdependent Real-Time Deadlines Istv¨¢n Knoll (Aalborg University), Anders P. Ravn (Aalborg University) and Arne Skou (Aalborg University). An Efficient Algorithm for Finding Empty Space for Reconfigurable Systems Zhenhua Duan (Xidian University) and Yan Xiao (Xidian University). Session 2 : Semantics State Visibility and Communication in Unifying Theories of Programming Andrew Butterfield (Trinity College Dublin), Pawel Gancarski (Trinity College Dublin) and Jim Woodcock (University of York). Semantics of Metamodels in UML Lijun Shan (National University of Defence Technology) and Hong Zhu (Oxford Brookes University). Refinement Algebra with Explicit Probabilism Tahiry Rabehaja (UNU/IIST) and Jeffrey Sanders (UNU/IIST). Session 3 : Model Checking Environment Abstraction with State Clustering and Parameter Truncating Hong Pan (Institute of Software, Chinese Academy of Sciences), Yi Lv (Institute of Software, Chinese Academy of Sciences) and Huimin Lin (Institute of Software, Chinese Academy of Sciences). Verification of Population Ring Protocols in PAT Yang Liu (National University of Singapore), Jun Pang (University of Luxembourg), Jun Sun (National University of Singapore) and Jianhua Zhao (Nanjing University). Bounded Model Checking of ACTL Formulae Wei Chen (Institute of Software, Chinese Academy of Sciences) and Wenhui Zhang (Institute of Software, Chinese Academy of Sciences). Day 2, 30 July 2009 ------------------- Invited Talk: Modular Development of Certified System Software Zhong Shao (Yale University) Session 4: Specification and Security Coarse Grained Retrenchment and the Mondex Denial of Service Attacks Richard Banach (Manchester University). Specifying and Enforcing Constraints of Artifact Life Cycles Xiangpeng Zhao (Peking University), Jianwen Su (University of California at Santa Barbara), Hongli Yang (Beijing University of Technology) and Zongyan Qiu (Peking University). Consistency Checking for LSC Specifications Hai-Feng Guo (University of Nebraska at Omaha), Wen Zheng (University of Nebraska at Omaha) and Mahadevan Subramaniam (University of Nebraska at Omaha). Integrating Specification and Programs for System Modeling and Verification Jun Sun (National University of Singapore), Yang Liu (National University of Singapore), Jin Song Dong (National University of Singapore) and Chunqing Chen (National University of Singapore). Session 5 : Software Testing I A Framework and Language Support for Automatic Dynamic Testing of Workflow Management Systems Gwan-Hwan Hwang (National Taiwan Normal University), Che-Sheng Lin (National Taiwan Normal University), Li-Te Tsao (National Taiwan Normal University), Kuei-Huan Chen (National Taiwan Normal University) and Yan-You Li (National Taiwan Normal University). Fault-based Test Case Generation for Component Connectors Bernhard Aichernig (Graz University of Technology), Farhad Arbab (CWI), Lacramioara Astefanoaei (CWI), Frank de Boer (CWI), Meng Sun (CWI) and Jan Rutten (CWI). Test Data Generation for Derived Types in C Program Zheng Wang (East China Normal University), Xiao Yu (East China Normal University), Tao Sun (East China Normal University), Geguang Pu (East China Normal University) and Zuohua Ding (Zhejiang Sci-Tech University). Session 6 : Software Models Program Repair as Sound Optimization of Broken Programs Bernd Fischer (University of Southampton), Ando Saabas (Tallinn University of Technology) and Tarmo Uustalu (Tallinn University of Technology). Modeling Web Applications and Generating Tests: A Combination and Interactions-guided Approach Bo Song (Shanghai University) and Huaikou Miao (Shanghai University). Merging of Use Case Models: Semantic Foundations Stephen Barrett (Concordia University), Daniel Sinnig (Concordia University), Patrice Chalin (Concordia University) and Greg Butler (Concordia University). Day 3, 31 July 2009 ------------------- Invited Tutorial: Towards Expressive Specification and Efficient Model Checking Jin Song Dong (National University of Singapore) Session 7 : Verification Verifying Semistructured Data Normalization using SWRL Yuan Fang Li (University of Queensland), Jing Sun (University of Auckland), Gillian Dobbie (University of Auckland), Scott Uk-Jin Lee (University of Auckland) and Hai H. Wang (Aston University). Verifying Self-stabilizing Population Protocols with Coq Yuxin Deng (Shanghai Jiao Tong University) and Jean-fran?ois Monin (Universit¨¦ de Grenoble 1). The Logical Approach to Low-level Stack Reasoning Xinyu Jiang (University of Science and Technology of China), Yu Guo (University of Science and Technology of China) and Yiyun Chen (University of Science and Technology of China). Constructing Program Invariants via Solving QBF Shikun Chen (National University of Defence Technology), Zhoujun Li (Beihang University) and Mengjun Li (National University of Defence Technology). Session 8 : Concurrency Using Architectural Constraints for Deadlock-Freedom of Component Systems with Multiway Cooperation Moritz Martens (University of Mannheim) and Mila Majster-Cederbaum (University of Mannheim). Formal Reasoning about Concurrent Assembly Code with Reentrant Locks Ming Fu (University of Science and Technology of China), Yu Zhang (University of Science and Technology of China) and Yong Li (University of Science and Technology of China). Algorithms for Computing Weak Bisimulation Equivalence Weisong Li (Institute of Software, Chinese Academy of Sciences). Session 9 : Software Testing II Interpreting a Successful Testing Process: Risk and Actual Coverage Marielle Stoelinga (University of Twente) and Mark Timmer (University of Twente). Automated Test Case Generation based on Coverage Analysis Tim A. Majchrzak (University of Muenster) and Herbert Kuchen (University of Muenster). Exploring Topological Structure of Boolean Expressions for Test Data Selection Lian Yu (Peking University), Wei Zhao (IBM China Research Lab), Xiangdong Fan (Peking University) and Jun Zhu (IBM China Research Lab). On Testing 1-Safe Petri Nets Guy-Vincent Jourdan (University of Ottawa) and Gregor von Bochmann (University of Ottawa). The programme also include two poster sessions, comprising of 23 selected poster presentations. This list of posters can be found at http://www.dur.ac.uk/ieee.tase2009/

1 0

understanding regex libs
by Simon Michael 05 Jun '09

05 Jun '09

Hi Chris.. thanks for your extensive work on haskell regex libs. I'm looking for a good way to make my regex-using app more portable to windows. I couldn't figure out the difference between the regex-pcre and regex- pcre-builtin on hackage. Could you clarify ? Best regards, -Simon

3 4

Text formatting question
by Evan Klitzke 05 Jun '09

05 Jun '09

I'm writing code with hslogger, and I'm finding myself frequently writing things like this: infoM $ printf "%s saw %s with %s" (show x) (show y) (show z) On #haskell I asked about why you can't have use %s with any instance of (Show a), and augustss, the Text.Printf maintainer, pointed out that it's not possible in Haskell 98 because of overlapping instances. So I'm trying to figure out the best way to work around this in my code. Some of the logging statements can get kind of long, so the ability to elide the calls to `show` would be nice. So what's the best way to do this? I was thinking of making my own variant of printf that only accepted the %s formatter and took (Show a)'s as arguments, but there might be a simpler way? Another thing is the code is already fairly GHC specific at this point, so if there are GHC extensions that might be useful here that I don't know about, I'm interested in hearing about them. Thanks. -- Evan Klitzke <evan(a)eklitzke.org> :wq

2 1

containers dependency on base
by Louis Wasserman 05 Jun '09

05 Jun '09

I have been unable to compile the stable version of containers-0.2.0.1, having to add the constraint base>=4.0.0.0. Can anybody else duplicate this problem? Should I patch it? (I'm working on another modification to containers and encountered this difficulty when attempting to build the whole package.) Louis Wasserman wasserman.louis(a)gmail.com

2 1

Re: [Haskell-cafe] Cabal/primes
by michael rice 05 Jun '09

05 Jun '09

Got it working. I downloaded two packages, primes and Numbers. Since Numbers has the three functions I want to use, primes, isPrime and isProbablyPrime, how do I uninstall the primes package so there won't be a conflict? Michael ================= [michael@localhost hackage.haskell.org]$ ghc-pkg hide primes Writing new package config file... done. [michael@localhost hackage.haskell.org]$ isPrime 7 bash: isPrime: command not found [michael@localhost hackage.haskell.org]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :m + Data.Numbers.Primes Prelude Data.Numbers.Primes> isPrime 7 Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package Numbers-0.2.1 ... linking ... done. True Prelude Data.Numbers.Primes> take 10 primes [2,3,5,7,11,13,17,19,23,29] Prelude Data.Numbers.Primes> --- On Wed, 6/3/09, michael rice <nowgate(a)yahoo.com> wrote: From: michael rice <nowgate(a)yahoo.com> Subject: Re: [Haskell-cafe] Cabal/primes To: "brian" <briand(a)aracnet.com> Cc: haskell-cafe(a)haskell.org Date: Wednesday, June 3, 2009, 3:46 PM This is working differently than it did last night, when at least import was working. Michael [michael@localhost ~]$ ghci GHCi, version 6.10.1: 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 Data.Numbers.Primes Could not find module `Data.Numbers.Primes': it was found in multiple packages: Numbers-0.2.1 primes-0.1.1 Prelude> :m + Data.Numbers.Primes Could not find module `Data.Numbers.Primes': it was found in multiple packages: Numbers-0.2.1 primes-0.1.1 Prelude> Data.Numbers.Primes> :browse <interactive>:1:21: parse error on input `:' Prelude> isPrime 3525266 <interactive>:1:0: Not in scope: `isPrime' Prelude> ========== [michael@localhost ~]$ cd .cabal [michael@localhost .cabal]$ ls -l total 20 -rw-rw-r-- 1 michael michael 1729 2009-06-02 18:59 config drwxrwxr-x 4 michael michael 4096 2009-06-02 22:14 lib drwxrwxr-x 2 michael michael 4096 2009-06-02 19:05 logs drwxrwxr-x 3 michael michael 4096 2009-06-02 18:59 packages drwxrwxr-x 3 michael michael 4096 2009-06-02 19:05 share [michael@localhost .cabal]$ cd packages [michael@localhost packages]$ ls -l total 4 drwxrwxr-x 4 michael michael 4096 2009-06-02 22:14 hackage.haskell.org [michael@localhost packages]$ cd hackage.haskell.org [michael@localhost hackage.haskell.org]$ ls -l total 12464 -rw------- 1 michael michael 11735040 2009-06-02 22:14 00-index.tar -rw------- 1 michael michael 992994 2009-06-02 22:14 00-index.tar.gz -rw-rw-r-- 1 michael michael 418 2009-06-02 22:14 build-reports.log drwxrwxr-x 3 michael michael 4096 2009-06-02 22:14 Numbers drwxrwxr-x 3 michael michael 4096 2009-06-02 19:05 primes [michael@localhost hackage.haskell.org]$ [michael@localhost hackage.haskell.org]$ pwd /home/michael/.cabal/packages/hackage.haskell.org [michael@localhost hackage.haskell.org]$ ============= --- On Wed, 6/3/09, brian <briand(a)aracnet.com> wrote: From: brian <briand(a)aracnet.com> Subject: Re: [Haskell-cafe] Cabal/primes To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Wednesday, June 3, 2009, 10:46 AM Prelude> :m + Data.Numbers.Primes Prelude Data.Numbers.Primes> :browse isPrime :: Integer -> Bool isProbablyPrime :: (System.Random.RandomGen g) => Integer -> g -> (Bool, g) primes :: [Integer] Prelude Data.Numbers.Primes> isPrime 3525266 Loading package syb ... linking ... done. Loading package base-3.0.3.0 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. Loading package Numbers-0.2.1 ... linking ... done. False Prelude Data.Numbers.Primes> On Jun 3, 2009, at 6:57 AM, michael rice wrote: > OK, I downloaded Numbers. Still missing the three functions I wanted, which are primes, isPrime, and isProbablyPrime. How do I get these? > > Michael > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

4 3

ANN: wp-archivebot 0.1 - archive Wikipedia's external links in WebCite
by Gwern Branwen 05 Jun '09

05 Jun '09

I'd like to announce wp-archivebot. # What wp-archivebot is a relatively simple little script which follows all the links in a RSS feed, combs the destination for http:// links, and submits them to WebCite. WebCite https://secure.wikimedia.org/wikipedia/en/wiki/WebCite is an organization much like the more famous Internet Archive. Unlike the Wayback Machine, however, WebCite will archive pages on-demand.* # Why This is good, since link-rot and 404 errors are a fact of life on Wikipedia. Links go stale, fall dead, get banned, edited, censored, etc. If those links are being used as a reference for some important fact or detail, then there is a very big problem. Even the hit-or-miss Internet Archive has proven to be very useful for editors**, so a more reliable way of archiving links would be even better. # Limitations The WebCite FAQhttp://webcitation.org/faq mentions that a good project would be to > develop a wikipedia bot which scans new wikipedia articles for cited URLs, submits an archiving request to WebCite®, and then adds a link to the archived URL behind the cited URL Adding a link would be both quite difficult and require community approval; further, although I have thought about this for years, there's no obvious good way to add a link. Any method is either visually awkward, possibly otiose (if [[Google]] links to google.com as the homepage in its infobox, there's no purpose to have an archived version of google.com!) and certainly will bloat up the markup - even if there's any way to insert links without bolloxing templates and other such constructs. So I'm satisfied to just archive the link. WebCite is searchable, after all. If enough people run bots like this and achieve enough coverage, then perhaps editors can be educated to always check in WebCite as well. # Download & Install As ever, wp-archivebot is Free and is available from Hackage at: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/wp-archivebot You can install with ease by a simple 'cabal install wp-archivebot', or download the tarball and compile it yourself with the usual 'runhaskell Setup configure && runhaskell Setup build && runhaskell Setup install' dance. # Usage wp-archivebot takes one mandatory argument, an email address; WebCite needs to have somewhere to send notices of archival success/failure. wp-archivebot takes a second, optional, argument. This is a RSS feed to use. It defaults to Special:NewPages on the English Wikipedia, but one could just as well follow, say, RecentChanges. Here's an example: > wp-archivebot gwern0(a)gmail.com 'http://en.wikipedia.org/w/index.php?title=Special:RecentChanges&feed=rss' (This sets my email address as the recipient, and follows RecentChanges. This may not be a good idea as RecentChanges is *much* busier than NewPages.) ## Example Here's an example session's output: [12:35 PM] 829Mb$ wp-archivebot gwern0(a)gmail.com "http://www.webcitation.org/archive?url=http://en.wikisource.org/wiki/Berkel…" "http://www.webcitation.org/archive?url=http://www.baseball-reference.com/pl…" "http://www.webcitation.org/archive?url=http://www.baseball-reference.com/pl…" "http://www.webcitation.org/archive?url=http://www.baseball-reference.com/mi…" "http://www.webcitation.org/archive?url=http://www.baseball-reference.com/mi…" "http://www.webcitation.org/archive?url=http://www.timesargus.com/apps/pbcs.…" "http://www.webcitation.org/archive?url=http://www.timesargus.com/apps/pbcs.…" "http://www.webcitation.org/archive?url=http://www.timesargus.com/apps/pbcs.…" "http://www.webcitation.org/archive?url=http://www.timesargus.com/apps/pbcs.…" "http://www.webcitation.org/archive?url=http://www.erniestires.net/&email=gw…" "http://www.webcitation.org/archive?url=http://www.erniestires.net/&email=gw…" "http://www.webcitation.org/archive?url=http://thepeerage.com/p4893.htm#i489…" "http://www.webcitation.org/archive?url=http://thepeerage.com/p4893.htm#i489…" "http://www.webcitation.org/archive?url=http://www.leighrayment.com/commons/…" "http://www.webcitation.org/archive?url=http://www.leighrayment.com/commons/…" "http://www.webcitation.org/archive?url=http://thepeerage.com/p4893.htm#i489…" "http://www.webcitation.org/archive?url=http://thepeerage.com/p4893.htm#i489…" "http://www.webcitation.org/archive?url=http://www.esec.edu&email=gwern0@gma…" "http://www.webcitation.org/archive?url=http://www.esec.edu&email=gwern0@gma…" ... # Related The development version (HEAD) of the Gitit wiki has plugin support; one of those plugins, WebArchiver.hs, will on every page-save comb through for off-wiki links and submit them to WebCite in the same way as this bot. It's nice to know that if those links ever disapear, you can retrieve them from WebCite and 'see' the revision with the same set of external links as when the revision was created. * Technically, the Internet Archive will archive on demand as well - but you need to pay them. ** In many more ways than one might expect. For example, not infrequently someone will visit an article and claim it is plagiarizing some other webpage. With the IA, it's easy to go back to the first version of that webpage and crosscheck against the article's history - quite often it is the other website that plagiarized us! -- gwern

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Nested Lists
by Paul Keir 04 Jun '09

04 Jun '09

Hi all, If I have a list, and I'd like to convert it to a list of lists, each of length n, I can use a function like bunch: bunch _ [] = [] bunch n as = let (c,cs) = splitAt n as in c:bunch n cs > bunch 8 [1..16] [[1,2,3,4,5,6,7,8],[9,10,11,12,13,14,15,16]] If I now want to do the same for the nested lists, I can compose an application involving both map and bunch: > map (bunch 4) . bunch 8 $ [1..16] [[[1,2,3,4],[5,6,7,8]],[[9,10,11,12],[13,14,15,16]]] and I can "bunch" the new length 4 lists again: > map (map (bunch 2)) . map (bunch 4) . bunch 8 $ [1..16] [[[[1,2],[3,4]],[[5,6],[7,8]]],[[[9,10],[11,12]],[[13,14],[15,16]]]] Clearly there is a pattern here involving the bunch function and latterly, three Int parameters; 2, 4 and 8. My question is, can I create a function that will take such parameters as a list, and give the same result, for example: > f [2,4,8] [1..16] [[[[1,2],[3,4]],[[5,6],[7,8]]],[[[9,10],[11,12]],[[13,14],[15,16]]]] or perhaps: > f [bunch 2, bunch 4, bunch 8] [1..16] [[[[1,2],[3,4]],[[5,6],[7,8]]],[[[9,10],[11,12]],[[13,14],[15,16]]]] I think it may not be possible because the type signature of f would depend on the length of its list parameter; but I'm not sure. -Paul

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Re: Strange type error with associated type synonyms
by John Lato 04 Jun '09

04 Jun '09

Hello, Apologies for the late contribution; I've been away recently. I have a very strong opinion on this, though. > From: Bulat Ziganshin <bulat.ziganshin(a)gmail.com> > > Thursday, May 28, 2009, 2:06:36 AM, HT wrote: > > Prelude>> let a = 'a'; b = "b" in a==b > >> <interactive>:1:27: >> Couldn't match expected type `Char' against inferred type `[Char]' >> .... > >> Is the type of 'a' wrong or that of 'b'? > > it is not important, well, at least we can live with it. Compiler > should say: > > First argument of == should be of type String > while a is of type Char I find this to be a somewhat important issue. If there weren't so many polymorphic functions it wouldn't be as big of a problem. The only reason the first argument of == should be a String is because the second argument is (or alternatively 2nd should be Char, etc.). For polymorphic functions, in which the type of one argument is fixed by another, I think the error message should ideally point out how the compiler is getting expected/inferred types. E.g. Couldn't match expected type `Char' against inferred type `[Char]' Type of [first argument] to `==' must match type of [second argument] where [first|second argument] is filled in with the appropriate expressions. I've become somewhat adept at figuring this out, but I think it would be helpful for those who haven't experienced it before. A related issue is with multiple function definitions. Here's a short example: foo n = ... where bar (Just a) = return $ somefunc a bar (Nothing) = somefunc n a = otherfunc assume that 'a' and 'n' have the same type. Now the type checker won't allow bar, because the two definitions have differing types. GHC's error message would ideally state that the definitions for 'bar' have differing types, but instead we get a message about mismatched types within one of the definitions. I know this example is rather contrived, but I don't have any real code to hand because I've fixed it already. You get a better message if you give a type signature for 'bar', but that requires lexical scoping (at least in my case), which means the code would no longer be Haskell98 (which it otherwise is IIRC). Cheers, John Lato

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Scary type inference for monadic function definitions
by Ahn, Ki Yung 04 Jun '09

04 Jun '09

Scary type inference for monadic function definitions (or, why you'd want to annotate types for monadic function definitions) This is a real example that I've experienced. I defined the following function. > checkOneVerseByLineWith readLine v = > do mg <- readLine > case mg of > Just g -> return Just (v==g) > Nothing -> return Nothing My intention was to use it something like this: checkOneVerseByLineWith (readline "% ") where readline is the library function from System.Console.Readline. As you can see, there is an obvious mistake which I forgot to group (Just (v==g)) in parenthesis. However, GHC or any other Haskell 98 compliant implementation will infer a type for you and this will type check! Try it yourself if in doubt. Of course, checkOneVerseByLineWith (readline "% ") won't type check because checkOneVerseByLineWith has strange type. The reason why the above definition type checks is because ((->) r) is an instance of Monad.

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cabalizing "legacy' source
by Vasili I. Galchin 04 Jun '09

04 Jun '09

Hello, I have cabalizing some "legacy" Haskell source. By "legacy", I totally don't mean in a pejorative sense ... code has been designed and written well. I am making good progress ... I have succeeded in building an archive file(library) on a POSIX platform, i.e. Linux. The original author intended to build several executables from this .a file. In addition there are many pairings of .hs/.lhs along with "main" test files. Currently I have only one .cabal file. This is inadequate for the author's intent: 1) I need to build "n" test executables. 2) I need to build 1/2 CLI executables. I need advice on the cabal front. E.g. should I first run the library .cabal and then run several subsidary .cabal files, e.g. "n"test .cabals and also 1/2 CLI executable .cabals??? I am looking for advice so that I don't have a clumsy cabal build process/tree! Thanks, Vasili

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type checking that I can't figure out ....
by Vasili I. Galchin 04 Jun '09

04 Jun '09

Hello Haskellers, I isolated to a not so small piece: > {-# OPTIONS -fglasgow-exts #-} > {-# LANGUAGE UndecidableInstances #-} > import Control.Monad.Identity > import Control.Monad.Reader > import Control.Monad.State > import qualified Data.List as L > import qualified Data.Map as M > import Data.Array import IOExts The type of a regular expression. > data Re t > = ReOr [Re t] > | ReCat [Re t] > | ReStar (Re t) > | RePlus (Re t) > | ReOpt (Re t) > | ReTerm [t] > deriving (Show) The internal type of a regular expression. > type SimplRe t = Int > data SimplRe' t > = SReOr (SimplRe t) (SimplRe t) > | SReCat (SimplRe t) (SimplRe t) > | SReStar (SimplRe t) > | SReLambda > | SReNullSet > | SReTerm t > deriving (Eq, Ord, Show) The regular expression builder monad. > data (Ord t) => ReRead t > = ReRead { > rerNullSet :: SimplRe t, > rerLambda :: SimplRe t > } > data (Ord t) => ReState t > = ReState { > resFwdMap :: M.Map (SimplRe t) (ReInfo t), > resBwdMap :: M.Map (SimplRe' t) (SimplRe t), > resNext :: Int, > resQueue :: ([SimplRe t], [SimplRe t]), > resStatesDone :: [SimplRe t] > } > type ReM m t a = StateT (ReState t) (ReaderT (ReRead t) m) a TEMP WNH Dfa construction > data ReDfaState t > = ReDfaState { > dfaFinal :: Bool, > dfaTrans :: [(t, SimplRe t)] > } > deriving (Show) TEMP WNH The ReInfo type > data ReInfo t > = ReInfo { > reiSRE :: SimplRe' t, > reiNullable :: Bool, > reiDfa :: Maybe (ReDfaState t) > } > deriving (Show) TEMP WNH > class (Monad m, Ord t) => ReVars m t where { } > instance (Monad m, Ord t) => ReVars m t where { } TEMP WNH > remLookupFwd :: (ReVars m t) => SimplRe t -> ReM m t (ReInfo t) > remLookupFwd re > = do fwd <- gets resFwdMap > -- let { Just reinfo = M.lookup fwd re } -- PROBLEM > reinfo <- M.lookup fwd re -- PROBLEM > return reinfo When I "compile" with ghci I get: Dfa_exp.lhs:91:32: Couldn't match expected type `M.Map (M.Map (SimplRe t) (ReInfo t)) t1' against inferred type `SimplRe t2' In the second argument of `M.lookup', namely `re' In a 'do' expression: reinfo <- M.lookup fwd re In the expression: do fwd <- gets resFwdMap reinfo <- M.lookup fwd re return reinfo I trimmed the original code down a lot! But still can't why I am getting type check errors!!! Help! Kind regards, Vasili

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ANN: memscript-0.0.0.2 (Command line utility for memorizing scriptures or any other text)
by Ahn, Ki Yung 04 Jun '09

04 Jun '09

memscript: Command line utility for memorizing scriptures or any other text http://hackage.haskell.org/cgi-bin/hackage-scripts/package/memscript "memscript <filename>" Run memscript with a UTF-8 (or ASCII since ASCII is a subset of UTF8) plain text file. Try to exactly guess the text line by line. If your guess is wrong it will show you a diff like output comparing your guess and the original verse. This will repeat until you get all the verses right. For the test data I included four beloved Psalms (1,23,121,127) from the Old Testament, each in Revised Korean Version (RKV) and New International Version (NIV), which I happened to have had to memorize. You can use it for any other text you'd want to memorize, such as your favorite poems, quotes, or whatsoever. No craft or ticks, really simple and straightforward utility but serves well for the purpose. I used readline because that was the only sane way I know of handling multibyte inputs. ==== an example usage ==== kyagrd@kyagrd:~/cscs/memscript$ memscript 001en.txt % Blessed is the man who does not walk in the counsel of the wicked or stand in the way of sinners or sit in the seat of mockers. % But his delight is in the law of the LORD, and on his law he meditates night and day. === < But his delight is in the law of the LORD, and on his law he meditates day and night. --- > But his delight is in the law of the LORD, and on his law he meditates night and day. === % But his delight is in the law of the LORD, and on his law he meditates day and night. % He is like a tree planted by streams of water, which yields its fruit in season and whose leaf does not wither. Whatever he does prospers. % Not so the wicked! They are like chaff that the wind blows away. % The wicked will not stand in the judgment, nor sinners in the assembly of the righteous. === < Therefore the wicked will not stand in the judgment, nor sinners in the assembly of the righteous. --- > The wicked will not stand in the judgment, nor sinners in the assembly of the righteous. === % Therefore the wicked will not stand in the judgment, nor sinners in the assembly of the righteous. % For the LORD watches over the way of the righteous, but the way of the wicked will perish. kyagrd@kyagrd:~/cscs/memscript$

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