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Fwd: [Haskell-cafe] General function to count list elements?
by Alberto G. Corona 20 Apr '09

20 Apr '09

here goes a test of pointer equality (valid for functions too): samepointer :: a -> a -> IO Bool samepointer f1 f2 = do p1<- varHash f1 p2<- varHash f2 return $ p1 == p2 where varHash x= do sn <- makeStableName x return $ hashStableName sn 2009/4/18 <nowgate(a)yahoo.com>: > Hi John, > > Yes, I see now what the hullabaloo was about. > > I've only been hacking Haskell for a few weeks, so I'm still seeing the > "forest" rather than the "trees". But I'm coming along. > > Thanks. > > Michael > > --- On Sat, 4/18/09, John Dorsey <haskell(a)colquitt.org> wrote: > > From: John Dorsey <haskell(a)colquitt.org> > Subject: Re: [Haskell-cafe] General function to count list elements? > To: "michael rice" <nowgate(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Saturday, April 18, 2009, 2:36 PM > > Michael, > >> I had a count function that worked fine for an enum type, but thought >> why not go polymorphic with it. So I changed it and got the error I >> originally inquired about. > > For variety, I'll go a slightly different direction. > > If you generalize count to use any predicate, instead of always > equality... > > gcount :: (a -> a -> Bool) -> a -> [a] -> Int > gcount pred x0 xs = length (filter (pred x0) xs) > > count = gcount (==) > > This will work with any type that you can write a predicate for with the > type (a -> a -> Bool). I can even use this with functions, if I'm > careful. > > ghci> gcount (\f g -> True) (*2) [id,(const 1),(*3)] > 3 > ghci> gcount (\f g -> f 1 == g 1) (^2) [id,(const 1),(*3)] > 2 > > By the way, do you see why everyone's bothing you about comparing > functions? The type you gave count, which didn't have an Eq constraint, > was an assertion that you could compare two values of *any* type. If > there's a type that's not comparable, then count's type was wrong. > Functions are the canonical example of an incomparable type. > > When you're bored some time, read a bit about the Curry-Howard > correspondence. It's interesting, even if (like me) you don't grok all > of its implications. > > Regards, > John > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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RE: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
by Niemeijer, R.A. 20 Apr '09

20 Apr '09

If only it were that easy. Sadly, it's not. Let's look at the following example: data Test = Test { foo :: Int, bar :: Char, baz :: Bool } smallPrint t = concatMap (\f -> show $ f t) [foo, bar, baz] In this code the list [foo, bar, baz] should have the type [exists a. Show a => Test -> a]. If we explicitly specify the type, replacing the exists with a forall, then GHC complains about not being able to match Int, Char and Bool against type a. Forall is not the same as exists and GHC only implements the former. ________________________________________ From: Bulat Ziganshin [bulat.ziganshin(a)gmail.com] Sent: 19 April 2009 22:07 To: Niemeijer, R.A. Cc: haskell-cafe(a)haskell.org Subject: Re[2]: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release Hello R.A., Sunday, April 19, 2009, 11:46:53 PM, you wrote: > Does anybody know if there are any plans to incorporate some of > these extensions into GHC - specifically the existential typing ? it is already here, but you should use "forall" keyword instead odf "exists" -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
by Daniel Fischer 20 Apr '09

20 Apr '09

Am Sonntag 19 April 2009 13:09:17 schrieb Thomas Davie: > >> I don't understand what makes user installs more convenient. > >> Certainly, > >> my preference would be for global all the time I expect something > >> that > >> says it's going to "install" something to install it onto my > >> computer, > >> like any other installation program does. What is it that makes user > >> installs more convenient in this situation? > > > > You don't need 'sudo' access for user installs. This means that 'cabal > > install' works out of the box on every system, without needing > > admin/root privs (esp. important for students). > > But students will be used to needing to configure this in every > other installation system out there they need to tell it to install in > their user directory. Personally I find it more convenient to have > the "install" program do what it says it does! Install it! But it does install it, only not where you want it. Just for the record, I (no student, my own computer, sole user) prefer user-installs and cabal's default behaviour. Makes it so much easier to get rid of things I don't want anymore without any fear of buggering my system because something depends on it. > > This would save confusion about old tools that do things globally, and > not confuse students, because they're all already used to giving extra > flags to make install not install things system wide. Yes, it is bad that the runhaskell Setup interface has a different default. But, as Duncan said, too late to change it now. > > Bob

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Re: [Haskell] Helpful libraries to implement a board game?
by Conrad Meyer 20 Apr '09

20 Apr '09

On Sunday 19 April 2009 01:39:40 pm Frank Rosemeier wrote: > Dear Haskellers, > > I would like to implement a board game for a single player in Haskell. > The pieces may be moved one step in any direction if there is no > piece next to it, > and the goal is to rearrange the pieces to their home positions. > The Haskell program should find an optimal solution (with minimal > number of moves). > > Can anybody recommend some helpful libraries for this task? > Any hints are very welcome! > > > Frank Rosemeier (This should probably be on haskell-cafe, so I'm moving it there, CC-ing the author.) Hi, If you're interested in gametree search (i.e. game AI), which it sounds like you are, then the game-tree library on Hackage may interest you[0]. [0]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/game-tree Regards, -- Conrad Meyer <konrad(a)tylerc.org>

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Announce: A pragmatic Haskell .NET interop layer, 0.4.0
by Sigbjorn Finne 20 Apr '09

20 Apr '09

A new version of a Haskell .NET interop layer, hs-dotnet, has just been released and is now available for download, http://haskell.forkIO.com/dotnet It lets you access .NET functionality from Haskell and vice versa. Tool support is included in this release to aid such interop. The new version includes development done since the start of the year. Apart from rewriting the internals completely to put it all on a sounder footing, this release includes proper support for .NET generic types (classes and interfaces), mapping them naturally on to Haskell parameterized types. The support for generics enables for instance mixed Haskell-.NET LINQ programming; see the distribution for examples of this along with some other interesting applications of the hs-dotnet interop layer. enjoy --sigbjorn

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Using GHC as an arrows preprocessor?
by Peter Verswyvelen 20 Apr '09

20 Apr '09

I'm making some custom arrows and I'm getting bugs when using the GHC preprocessor, but not when using the old standalone preprocessor. In order to debug this, it would be nice to use GHC as an arrows preprocessor (so converting the arrow notation into plain Haskell and outputting the converted source code); is this possible? I also noticed that GHC's builtin preprocessor generates far more intermediate combinators than the old preprocessor. The benchmark in this paper <http://www.cs.yale.edu/homes/hl293/download/NEPLS-talk.pdf> seems to confirm this. Is this a known issue? Thanks, Peter

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Matrices
by Cetin Sert 19 Apr '09

19 Apr '09

http://ccnmtl.columbia.edu/projects/qmss/the_chisquare_test/about_the_chisq… given two matrices, Prelude Data.Matrix.Dense Data.Vector.Dense> m listMatrix (2,2) [46.0,37.0,71.0,83.0] Prelude Data.Matrix.Dense Data.Vector.Dense> es listMatrix (2,2) [40.9746835443038,42.0253164556962,76.0253164556962,77.9746835443038] how can I flatten them to do: [ (o-e)^2 / e | o <- m, e <- es ] or use a function that will apply a given function to every corresponding elements of 2 or n matrices and create a result matrix? If I should use a different matrix type or library altogether, please specify which one and how ^__^" Best Regards, Cetin Sert

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Zippers for gui elements?
by Gü?nther Schmidt 19 Apr '09

19 Apr '09

Hi all, I need to control some gui elements, ie. stuff like when dis button is clicked make dat one inactive. Some time ago, wish I'd remember where, I read about using Zippers for that sort of thing. Does anybody know where I might have read that? Günther

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Re: [Haskell-cafe] General function to count list elements?
by nowgate＠yahoo.com 19 Apr '09

19 Apr '09

Hi John, Yes, I see now what the hullabaloo was about. I've only been hacking Haskell for a few weeks, so I'm still seeing the "forest" rather than the "trees". But I'm coming along. Thanks. Michael --- On Sat, 4/18/09, John Dorsey <haskell(a)colquitt.org> wrote: From: John Dorsey <haskell(a)colquitt.org> Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, April 18, 2009, 2:36 PM Michael, > I had a count function that worked fine for an enum type, but thought > why not go polymorphic with it. So I changed it and got the error I > originally inquired about. For variety, I'll go a slightly different direction. If you generalize count to use any predicate, instead of always equality... gcount :: (a -> a -> Bool) -> a -> [a] -> Int gcount pred x0 xs = length (filter (pred x0) xs) count = gcount (==) This will work with any type that you can write a predicate for with the type (a -> a -> Bool). I can even use this with functions, if I'm careful. ghci> gcount (\f g -> True) (*2) [id,(const 1),(*3)] 3 ghci> gcount (\f g -> f 1 == g 1) (^2) [id,(const 1),(*3)] 2 By the way, do you see why everyone's bothing you about comparing functions? The type you gave count, which didn't have an Eq constraint, was an assertion that you could compare two values of *any* type. If there's a type that's not comparable, then count's type was wrong. Functions are the canonical example of an incomparable type. When you're bored some time, read a bit about the Curry-Howard correspondence. It's interesting, even if (like me) you don't grok all of its implications. Regards, John

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Re: [Haskell-cafe] General function to count list elements?
by michael rice 19 Apr '09

19 Apr '09

I wasn't comparing functions, you were. I was just trying to answer your questions. I had a count function that worked fine for an enum type, but thought why not go polymorphic with it. So I changed it and got the error I originally inquired about. Thanks. Michael --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: From: Eugene Kirpichov <ekirpichov(a)gmail.com> Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, April 18, 2009, 1:18 PM 2009/4/18 michael rice <nowgate(a)yahoo.com>: >> To compare two functions in C, I would compare their machine addresses. >> > > Why would you need that at all? > > How would *you* do it? Do what? As for comparing functions, I would not do it at all, because comparing functions makes no sense: one way of comparison (extensional equality) is not implementable, whereas the other (object identity) breaks referential transparency and is impossible to reason about and generally doesn't have a single reason to work correctly at all. Object identity has no reason to exist in a language without mutable objects, like Haskell. That's probably the key. As for implementing the count function, I would implement it in the way that people already suggested to you: by putting a constraint onto its type, namely a constraint that makes it polymorphic *over types whose values can be compared for equality*, t.i. over members of the Eq class. This is perfectly correct and does not cause any mathematical unsoundness. > > Michael > > ================ > > --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: > > From: Eugene Kirpichov <ekirpichov(a)gmail.com> > Subject: Re: [Haskell-cafe] General function to count list elements? > To: "michael rice" <nowgate(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Saturday, April 18, 2009, 12:39 PM > > 2009/4/18 michael rice <nowgate(a)yahoo.com>: >> I know functions can be compared in Scheme >> >> Welcome to DrScheme, version 4.1 [3m]. >> Language: Swindle; memory limit: 128 megabytes. >>> (equal? equal? equal?) >> #t >>> >> > > That's not the functions being compared, but the memory addresses of > the code implementing them. If your goal is comparing functions to > answer a question "Are these two values indistinguishable?", equal? > doesn't help you, because it may answer 'false' even if these two > values are indistinguishable from a mathematical point of view. > >> >> but apparently not in Haskell >> >> [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> (==) (==) (==) >> >> <interactive>:1:0: >> No instance for (Eq (a -> a -> Bool)) >> arising from a use of `==' at <interactive>:1:0-13 >> Possible fix: add an instance declaration for (Eq (a -> a -> Bool)) >> In the expression: (==) (==) (==) >> In the definition of `it': it = (==) (==) (==) >> Prelude> >> >> though I'm new at Haskell and may not be posing the question properly. >> >> I would think a language with 1st-class support for functions would >> certainly include comparing them. >> > > Again, this is first-class support for memory addresses of code > representing functions. > >> To compare two functions in C, I would compare their machine addresses. >> > > Why would you need that at all? > >> Michael >> >> >> --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: >> >> From: Eugene Kirpichov <ekirpichov(a)gmail.com> >> Subject: Re: [Haskell-cafe] General function to count list elements? >> To: "michael rice" <nowgate(a)yahoo.com> >> Cc: haskell-cafe(a)haskell.org >> Date: Saturday, April 18, 2009, 11:39 AM >> >> Could you then provide an example of two functions that *are* equal, >> or, even better, a definition of equality for arbitrary functions? >> Since Haskell may be compiled into C, this must be a definition that >> is implementable in C. >> >> 2009/4/18 michael rice <nowgate(a)yahoo.com>: >>> Though I haven't tried it out, it's trying to use my function to count >>> functions. >>> >>> The first argument is the identity function. >>> >>> The second argument is a list of a different form of the identity >>> function. >>> >>> Though the two identity functions, given the same input, would produce >>> the >>> same output, I doubt they would be equal. >>> >>> So my guess at an answer would be zero. >>> >>> Michael >>> >>> --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: >>> >>> From: Eugene Kirpichov <ekirpichov(a)gmail.com> >>> Subject: Re: [Haskell-cafe] General function to count list elements? >>> To: "michael rice" <nowgate(a)yahoo.com> >>> Cc: haskell-cafe(a)haskell.org >>> Date: Saturday, April 18, 2009, 11:03 AM >>> >>> What should >>> >>> count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined)) >>> >>> return? >>> >>> 2009/4/18 michael rice <nowgate(a)yahoo.com>: >>>> Is there a general function to count list elements. I'm trying this >>>> >>>> count :: a -> [a] -> Int >>>> count x ys = length (filter (== x) ys) >>>> >>>> with this error upon loading >>>> >>>> ============= >>>> >>>> [michael@localhost ~]$ ghci count >>>> 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. >>>> [1 of 1] Compiling Main ( count.hs, interpreted ) >>>> >>>> count.hs:2:29: >>>> Could not deduce (Eq a) from the context () >>>> arising from a use of `==' at count.hs:2:29-32 >>>> Possible fix: >>>> add (Eq a) to the context of the type signature for `count' >>>> In the first argument of `filter', namely `(== x)' >>>> In the first argument of `length', namely `(filter (== x) ys)' >>>> In the expression: length (filter (== x) ys) >>>> Failed, modules loaded: none. >>>> Prelude> >>>> >>>> ============= >>>> >>>> Not sure what it's trying to tell me other than I need an (Eq a) >>>> somewhere. >>>> >>>> Michael >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>>> >>> >>> >>> >>> -- >>> Eugene Kirpichov >>> Web IR developer, market.yandex.ru >>> >>> >> >> >> >> -- >> Eugene Kirpichov >> Web IR developer, market.yandex.ru >> >> > > > > -- > Eugene Kirpichov > Web IR developer, market.yandex.ru > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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Re: [Haskell-cafe] General function to count list elements?
by michael rice 19 Apr '09

19 Apr '09

> To compare two functions in C, I would compare their machine addresses. > Why would you need that at all? How would *you* do it? Michael ================ --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: From: Eugene Kirpichov <ekirpichov(a)gmail.com> Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, April 18, 2009, 12:39 PM 2009/4/18 michael rice <nowgate(a)yahoo.com>: > I know functions can be compared in Scheme > > Welcome to DrScheme, version 4.1 [3m]. > Language: Swindle; memory limit: 128 megabytes. >> (equal? equal? equal?) > #t >> > That's not the functions being compared, but the memory addresses of the code implementing them. If your goal is comparing functions to answer a question "Are these two values indistinguishable?", equal? doesn't help you, because it may answer 'false' even if these two values are indistinguishable from a mathematical point of view. > > but apparently not in Haskell > > [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> (==) (==) (==) > > <interactive>:1:0: > No instance for (Eq (a -> a -> Bool)) > arising from a use of `==' at <interactive>:1:0-13 > Possible fix: add an instance declaration for (Eq (a -> a -> Bool)) > In the expression: (==) (==) (==) > In the definition of `it': it = (==) (==) (==) > Prelude> > > though I'm new at Haskell and may not be posing the question properly. > > I would think a language with 1st-class support for functions would > certainly include comparing them. > Again, this is first-class support for memory addresses of code representing functions. > To compare two functions in C, I would compare their machine addresses. > Why would you need that at all? > Michael > > > --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: > > From: Eugene Kirpichov <ekirpichov(a)gmail.com> > Subject: Re: [Haskell-cafe] General function to count list elements? > To: "michael rice" <nowgate(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Saturday, April 18, 2009, 11:39 AM > > Could you then provide an example of two functions that *are* equal, > or, even better, a definition of equality for arbitrary functions? > Since Haskell may be compiled into C, this must be a definition that > is implementable in C. > > 2009/4/18 michael rice <nowgate(a)yahoo.com>: >> Though I haven't tried it out, it's trying to use my function to count >> functions. >> >> The first argument is the identity function. >> >> The second argument is a list of a different form of the identity >> function. >> >> Though the two identity functions, given the same input, would produce the >> same output, I doubt they would be equal. >> >> So my guess at an answer would be zero. >> >> Michael >> >> --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: >> >> From: Eugene Kirpichov <ekirpichov(a)gmail.com> >> Subject: Re: [Haskell-cafe] General function to count list elements? >> To: "michael rice" <nowgate(a)yahoo.com> >> Cc: haskell-cafe(a)haskell.org >> Date: Saturday, April 18, 2009, 11:03 AM >> >> What should >> >> count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined)) >> >> return? >> >> 2009/4/18 michael rice <nowgate(a)yahoo.com>: >>> Is there a general function to count list elements. I'm trying this >>> >>> count :: a -> [a] -> Int >>> count x ys = length (filter (== x) ys) >>> >>> with this error upon loading >>> >>> ============= >>> >>> [michael@localhost ~]$ ghci count >>> 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. >>> [1 of 1] Compiling Main ( count.hs, interpreted ) >>> >>> count.hs:2:29: >>> Could not deduce (Eq a) from the context () >>> arising from a use of `==' at count.hs:2:29-32 >>> Possible fix: >>> add (Eq a) to the context of the type signature for `count' >>> In the first argument of `filter', namely `(== x)' >>> In the first argument of `length', namely `(filter (== x) ys)' >>> In the expression: length (filter (== x) ys) >>> Failed, modules loaded: none. >>> Prelude> >>> >>> ============= >>> >>> Not sure what it's trying to tell me other than I need an (Eq a) >>> somewhere. >>> >>> Michael >>> >>> >>> >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >>> >> >> >> >> -- >> Eugene Kirpichov >> Web IR developer, market.yandex.ru >> >> > > > > -- > Eugene Kirpichov > Web IR developer, market.yandex.ru > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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General function to count list elements?
by michael rice 19 Apr '09

19 Apr '09

Is there a general function to count list elements. I'm trying this count :: a -> [a] -> Int count x ys = length (filter (== x) ys) with this error upon loading ============= [michael@localhost ~]$ ghci count 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. [1 of 1] Compiling Main ( count.hs, interpreted ) count.hs:2:29: Could not deduce (Eq a) from the context () arising from a use of `==' at count.hs:2:29-32 Possible fix: add (Eq a) to the context of the type signature for `count' In the first argument of `filter', namely `(== x)' In the first argument of `length', namely `(filter (== x) ys)' In the expression: length (filter (== x) ys) Failed, modules loaded: none. Prelude> ============= Not sure what it's trying to tell me other than I need an (Eq a) somewhere. Michael

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Computing a sorted list of products lazily
by Jason Dagit 18 Apr '09

18 Apr '09

Hello, A colleague of mine recently asked if I knew of a lazy way to solve the following problem: Given two sets of sorted floating point numbers, can we lazily generate a sorted list of the products from their Cartesian product? The algorithm should return the same result as: sortProduct a b = sort [ x * y | x <- a, y <- b ] But, my friend is not satisfied with the strictness of sort. In particular, he doesn't want to have to hold the whole list in memory to sort it. He would like to compute the results one product at a time and in the correct order. Is there an existing algorithm for this problem? We searched but we don't seem to know the right search terms. Additionally, my friend is not writing this in Haskell, but a Haskell solution is fine because we should be able to transform it into a Java solution by hand. We think we know an algorithm that will produce the right result but it's a bit messy and we doubt we're the first to tackle this problem. Any help would be greatly appreciated. Thanks in advance! Jason

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Synchronising cabal package version with self-reported version
by Denis Bueno 18 Apr '09

18 Apr '09

Hi all, In a command-line app of mine I want to have a --version flag, like many GNU apps do to report their version. The only way I know to provide the version number is to hardcode it in the source code somewhere. That means I have the version number in two places: the .cabal file and the source code. I just know that one day they'll get unsynchronised, and --version will be wrong. Is there any way to put the version number in _one_ place, and have both the .cabal and source refer to it? Or even any easier way to synchronise both version numbers? Thanks, Denis

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Debugging a RecSel Error
by Gü?nther Schmidt 18 Apr '09

18 Apr '09

Hi all, I'm trying to find the spot in my source code that triggered a RecSel Exception ("No match in record selector

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Parsec question
by Michael P Mossey 18 Apr '09

18 Apr '09

I want to write a parser that can read a file with this format: the file has sections which are demarcated by keywords. Keywords always begin with two forward slashes and consist of letters, digits, and underscore. The text can be anything, including special characters. For instance: //keyword some text and more text //another_keyword and) some { more text //ya_keyword $$ -- text I'm not sure how to write a parser that considers anything but a double slash to be a valid part of the text. Thanks, Mike

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Announce: funsat-0.6
by Denis Bueno 18 Apr '09

18 Apr '09

Hello haskell-cafe, funsat is a modern, DPLL-style SAT solver written in Haskell. Funsat solves formulas in conjunctive normal form and produces a total variable assignment for satisfiable problems. Funsat is intended to be reasonably efficient for practical problems and convenient to use as a constraint-solving backend in other software. Version 0.6 is available from Hackage: * http://hackage.haskell.org/cgi-bin/hackage-scripts/package/funsat New in 0.6: * A representation for logical circuits (and, or, not, onlyif, iff, if-then-else) supporting efficient conversion to CNF (for solving) has been added. * Now uses the BSD3 license. Please report any bugs to the github page linked from hackage. Denis

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Haskell Weekly News: Issue 114 - April 17, 2009
by Brent Yorgey 18 Apr '09

18 Apr '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090417 Issue 114 - April 17, 2009 --------------------------------------------------------------------------- Welcome to issue 114 of HWN, a newsletter covering developments in the [1]Haskell community. The [2]5th Haskell Hackathon is underway in Utrecht! Happy Haskell hacking! An early HWN this week since I will be traveling this weekend (but not, unfortunately, to the Hackathon). Announcements Reminder: Haskell Communities and Activities Report. Janis Voigtlaender [3]reminded everyone that the deadline for the [4]May 2009 edition of the Haskell Communities and Activities Report is only two weeks away. If you haven't already, please write an entry for your new project, or update your old entry. primes. Sebastian Fischer [5]announced the release of the [6]primes package, which implements lazy wheel sieves for efficient, purely functional generation of prime numbers in Haskell. level-monad-0.3. Sebastian Fischer [7]announced version 0.3 of the package [8]level-monad, which implements breadth-first search directly as an instance of MonadPlus (without using an intermediate tree representation). Version 0.3 adds a MonadPlus instance for iterative deepening inspired by Michael Spivey's paper on [9]Algebras for combinatorial search. hgettext 0.1.10. Vasyl Pasternak [10]announced a new release of the [11]hgettext package, which now has bindings to all gettext functions. Haskell logo in TeX. Philip Hölzenspies [12]announced a version of the new [13]Haskell logo design prepared using TikZ, for inclusion in LaTeX documents. The Monad.Reader (14) - Call for copy. Wouter Swierstra [14]issued a call for copy for Issue 14 of [15]The Monad.Reader. The deadline for submissions is May 15, 2009. Let Wouter know if you intend to submit something -- the sooner, the better. time 1.1.2.4. Ashley Yakeley [16]announced the release of [17]time 1.1.2.4, which should now compile on Windows. Discussion Code Golf. Sebastian Fischer [18]started a lively round of code golf with his code for list diagonalization. Converting IO [XmlTree] to [XmlTree]. rodrigo.bonifacio [19]asked how to convert an IO [XmlTree] into an [XmlTree], leading to a discussion of Haskell pedagogy. Blog noise [21]Haskell news from the [22]blogosphere. * Roman Cheplyaka: [23]Fun in Utrecht. * Roman Cheplyaka: [24]Utrecht: first impressions. * >>> Daniel van den Eijkel: [25]Hommage: Haskell Offline Music Manipulation And Generation EDSL. * >>> Brandon Simmons: [26]Some initial tests of Tries. * Christopher Lane Hinson: [27]Trends in Profiling Haskell. * >>> Larry O'Brien: [28]Windows & .NET Watch: Haskell: It's like Klingon, but with math!. * Sean Escriva: [29]Why Haskell is a joy to learn. * GHC / OpenSPARC Project: [30]Instruction counts on x86 vs SPARC. * Sebastian Fischer: [31]Barefaced pilferage of monadic bind. * Benjamin L. Russell: [32]Climbing the Towers of Hanoi with Haskell-style Curry from a Monadic Container (While Sparing the Sugar!). Quotes of the Week * Gracenotes: And then the type system goes all crazy and demands that x and 1 are both Word32s! * mauke: data What a = No; instance Monad What where { return _ = No; No >>= _ = No } * pumpkin: makes the next internet hit video, 2 natural transformations, 1 functor * mmorrow: a functor is like an analogy between two analogies * FliPPeh: @faq Can Conficker be rewritten in Haskell? <lambdabot> <no location info>: parse error on input `:' * HairyDude: The Haskell Type System is a Harsh Mistress.. there ain't no such thing as a free theorem. * LeCamarade: Now, let's say the set is {Haskell, SML, Ruby, Tomatoes, Human, Cabbage, Noise, IRC}. * pjdelport: YO DAWG I HERD YOU LIKE CARS SO WE PUT A PAIR IN YO CAR SO YOU CAN CAR WHILE YOU CAR * Babelfish: And there you travel: a beam tracer! Naturally, there are many things that ought to be amend. About the Haskell Weekly News New editions are posted to [33]the Haskell mailing list as well as to [34]the Haskell Sequence and [35]Planet Haskell. [36]RSS is also available, and headlines appear on [37]haskell.org. To help create new editions of this newsletter, please see the information on [38]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [39]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://haskell.org/haskellwiki/Hac5 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/57075 4. http://www.haskell.org/pipermail/haskell/2009-April/021180.html 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/57042 6. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/primes 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56981 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/level-monad 9. http://spivey.oriel.ox.ac.uk/mike/search-jfp.pdf 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56940 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hgettext 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17094 13. http://www.cs.utwente.nl/~holzensp/haskelllogo.sty 14. http://article.gmane.org/gmane.comp.lang.haskell.general/17090 15. http://haskell.org/haskellwiki/The_Monad.Reader 16. http://article.gmane.org/gmane.comp.lang.haskell.libraries/10921 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/time 18. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/56957 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/56898 21. http://planet.haskell.org/ 22. http://haskell.org/haskellwiki/Blog_articles 23. http://ro-che.blogspot.com/2009/04/fun-in-utrecht.html 24. http://ro-che.blogspot.com/2009/04/utrecht-first-impressions.html 25. http://substitut-fuer-feinmotorik.net/projects/haskellommage/introduction 26. http://coder.bsimmons.name/blog/2009/04/some-initial-tests-of-tries/ 27. http://blog.downstairspeople.org/2009/04/16/trends-in-profiling-haskell/ 28. http://www.sdtimes.com/content/article.aspx?ArticleID=33399 29. http://www.webframp.com/2009/04/15/why-haskell-is-a-joy-to-learn/ 30. http://ghcsparc.blogspot.com/2009/04/instruction-counts-on-x86-vs-sparc.html 31. http://www-ps.informatik.uni-kiel.de/~sebf/haskell/stealing-bind.html 32. http://dekudekuplex.wordpress.com/2009/04/13/climbing-the-towers-of-hanoi-w… 33. http://www.haskell.org/mailman/listinfo/haskell 34. http://sequence.complete.org/ 35. http://planet.haskell.org/ 36. http://sequence.complete.org/node/feed 37. http://haskell.org/ 38. http://haskell.org/haskellwiki/HWN 39. http://code.haskell.org/~byorgey/code/hwn/

2 1

Re: [Haskell-cafe] RE: [Announce] primes
by michael rice 18 Apr '09

18 Apr '09

You're right. Since I'm not familiar with Cabal, I didn't use it. Is there a good tutorial? Docs? Also, I'm running a 32-bit Linux OS. Does one get a significant speed increase by switching to a 64-bit OS? Thanks for the feedback. Michael --- On Fri, 4/17/09, Sebastian Fischer <sebf(a)informatik.uni-kiel.de> wrote: From: Sebastian Fischer <sebf(a)informatik.uni-kiel.de> Subject: Re: [Haskell-cafe] RE: [Announce] primes To: "Haskell Cafe mailing list" <haskell-cafe(a)haskell.org> Date: Friday, April 17, 2009, 4:03 AM Oh, I just remembered, I'm using ghci. I'll bet that's why I'm so slow. I also did, but after installing the package using cabal. IIRC, cabal compiles with -O2 by default. But if you downloaded the tarball and then loaded the module in ghci without installing it, this is probably the reason for the slow response. Cheers,Sebastian -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

types and braces
by Conor McBride 17 Apr '09

17 Apr '09

Hi folks In search of displacement activity, I'm trying to tweak Language.Haskell.Exts to support a few more perfidious Exts I have in mind -- they only need a preprocessor, but I do need to work on parsed programs, ideally. I was hoping to add a production to the grammar of types to admit expressions, delimited by braces: { exp } The idea is that instead of writing, (er, hi Claus), data True data False one just re-uses yer actual Bool (which becomes kind {Bool}) and writes {True} or {False}. The trouble is, the production I've added causes a reduce/reduce conflict in the grammar, but I don't get any more precise complaint than that. I guess what I'd like to know is whether I just need to debug my grammar extension, or whether the notation I'm proposing actually introduces a serious ambiguity that I'm too dim to notice. I'm mostly sending this in the hope that I have one of those "d'oh" moments you sometimes get when you articulate a stupid question in public. Put me out of my misery, please... Cheers Conor

5 6

glut installation using cabal failed
by Raja Koduru 17 Apr '09

17 Apr '09

hi, I am a beginner to haskell. I am trying to install glut using "cabal install glut" Pasting here a tail of the output --------------------------------------------------------------------- checking for unistd.h... yes checking windows.h usability... yes checking windows.h presence... yes checking for windows.h... yes checking GL/glut.h usability... no checking GL/glut.h presence... no checking for GL/glut.h... no checking for GLUT library... no checking for GL/glut.h... (cached) no checking GLUT/glut.h usability... no checking GLUT/glut.h presence... no checking for GLUT/glut.h... no configure: error: no GLUT header found, so this package cannot be built See `config.log' for more details. cabal: Error: some packages failed to install: GLUT-2.1.1.2 failed during the configure step. The exception was: exit: ExitFailure 1 --------------------------------------------------------------------------------- But I do have "glut.h" in my "D:\ghc\ghc-6.10.2\include\mingw\GL". Here is my `ghc-pkg list` -------------------------------------------------------------------------------- D:\ghc\ghc-6.10.2>ghc-pkg list d:/ghc/ghc-6.10.2\package.conf: Cabal-1.6.0.3, GLFW-0.3, HUnit-1.2.0.3, OpenGL-2.2.1.1, QuickCheck-1.2.0.0, Win32-2.2.0.0, array-0.2.0.0, base-3.0.3.1, base-4.1.0.0, bytestring-0.9.1.4, containers-0.2.0.1, directory-1.0.0.3, (dph-base-0.3), (dph-par-0.3), (dph-prim-interface-0.3), (dph-prim-par-0.3), (dph-prim-seq-0.3), (dph-seq-0.3), filepath-1.1.0.2, (ghc-6.10.2), ghc-paths-0.1.0.5, ghc-prim-0.1.0.0, haddock-2.4.2, haskell-src-1.0.1.3, haskell98-1.0.1.0, hpc-0.5.0.3, html-1.0.1.2, integer-0.1.0.1, mtl-1.1.0.2, network-2.2.1, old-locale-1.0.0.1, old-time-1.0.0.2, packedstring-0.1.0.1, parallel-1.1.0.1, parsec-2.1.0.1, pretty-1.0.1.0, process-1.0.1.1, random-1.0.0.1, regex-base-0.72.0.2, regex-compat-0.71.0.1, regex-posix-0.72.0.3, rts-1.0, stm-2.1.1.2, syb-0.1.0.1, template-haskell-2.3.0.1, xhtml-3000.2.0.1 -------------------------------------------------------------------------------- I am on windows XP using ghc-6.10.2 Please advice, regards, raja

3 3

wxHaskell not in scope
by Tsunkiet Man 17 Apr '09

17 Apr '09

Hello, I'm trying to create a GUI by using wxHaskell. However I get the weird error message of "Not in scope "dt"", well so I sorted them so that my so called "dt" was in scope, however it failed. Can someone please tell me how I can solve this error? ... A lot of code that is not relevant in my opinion, if I'm wrong please correct me and I will post my full code --Debug text -- dt <- staticText f [text := "Hello world!"] imagePanel <- panel f [position := Point 2 2, clientSize := Size 100 100, tooltip := "This is a drawPanel", bgcolor := rgb 255 255 255] set f [ clientSize := Size 700 500, menuBar := [mFile, mHelp], visible := True, on (menu exit) := close f, on (menu open) := onOpen f vFile ] return () where onOpen :: Frame a -> Var b -> IO () onOpen frame var = do file <- fileOpenDialog frame False True "Open File" [("PGM bestanden (*.pgm)",["*.pgm"]),("Alle bestanden (*.*)",["*.*"])] "" "" case file of Nothing -> return () Just file -> set dt [text := "HELLO"] return () Thank you for your help, I really owe haskell-cafe. Greetings Tsunkiet Man

4 10

Re: [Haskell-cafe] RE: [Announce] primes
by michael rice 17 Apr '09

17 Apr '09

Oh, I just remembered, I'm using ghci. I'll bet that's why I'm so slow. Thanks. Michael --- On Thu, 4/16/09, Andrew Coppin <andrewcoppin(a)btinternet.com> wrote: From: Andrew Coppin <andrewcoppin(a)btinternet.com> Subject: Re: [Haskell-cafe] RE: [Announce] primes To: haskell-cafe(a)haskell.org Date: Thursday, April 16, 2009, 4:38 PM Jake McArthur wrote: > michael rice wrote: >> Just curious, what kind of super-cooled processor is this guy running on? I got the same answer but it took almost three minutes (2:43:15). > > Only took a few seconds on my machine (Core 2 Duo 2.53GHz). Maybe it's the version of GHC that matters? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

2 1

"O LANGUAGE DESIGNER, REMEMBER THE POOR USER""
by Matt Morrow 17 Apr '09

17 Apr '09

This is interesting (and from 1990): http://groups.google.co.uk/group/comp.lang.functional/msg/655bb7bbd0fd8586 (Not sure if this is well-known. It seems like it either is, or it should be. Either way, I just stumbled across it.)

2 2

REMINDER: Haskell Communities and Activities Report
by Janis Voigtlaender 17 Apr '09

17 Apr '09

Dear Haskellers, The deadline for the May 2009 edition of the Haskell Communities and Activities Report is only two weeks away. If you haven't already, please write an entry for your new project, or update your old entry. Please mail your entries to hcar(a)haskell.org in LaTeX format. More information can be found in the original Call for Contributions at: http://www.haskell.org/pipermail/haskell/2009-April/021180.html I look forward to receiving your contributions. Thanks a lot, Janis (current editor) -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

1 0

Best text editor
by Melanie_Green 17 Apr '09

17 Apr '09

Hi I would like to follow the crowd and find out what text editor everyone uses for haskell on windows. Thx in advanced -- View this message in context: http://www.nabble.com/Best-text-editor-tp23018470p23018470.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

24 29

Re: wxHaskell not in scope (Source available)
by Tsunkiet Man 17 Apr '09

17 Apr '09

Note: the source can be found here: http://www.students.cs.uu.nl/~jtkman/Application.hs (It's my own work, please tell me what I did wrong) 2009/4/16 Tsunkiet Man <temp.tsun(a)gmail.com> > Hello, > > I'm trying to create a GUI by using wxHaskell. However I get the weird > error message of "Not in scope "dt"", well so I sorted them so that my so > called "dt" was in scope, however it failed. Can someone please tell me how > I can solve this error? > > ... A lot of code that is not relevant in my opinion, if I'm > wrong please correct me and I will post my full code > > --Debug text -- > dt <- staticText f [text := "Hello world!"] > > imagePanel <- panel f [position := Point 2 2, clientSize := > Size 100 100, tooltip := "This is a drawPanel", bgcolor := rgb 255 255 255] > set f [ clientSize := Size 700 500, > menuBar := [mFile, mHelp], > visible := True, > on (menu exit) := close f, > on (menu open) := onOpen f vFile ] > > > return () > > where > onOpen :: Frame a -> Var b -> IO () > onOpen frame var = do file <- fileOpenDialog frame False > True "Open File" [("PGM bestanden (*.pgm)",["*.pgm"]),("Alle bestanden > (*.*)",["*.*"])] "" "" > case file of > Nothing -> return () > Just file -> set dt [text := > "HELLO"] > return () > > Thank you for your help, I really owe haskell-cafe. > > Greetings Tsunkiet Man >

1 0

Re: [Haskell-cafe] RE: [Announce] primes
by michael rice 17 Apr '09

17 Apr '09

Just curious, what kind of super-cooled processor is this guy running on? I got the same answer but it took almost three minutes (2:43:15). Michael --- On Thu, 4/16/09, Geoffrey Irving <irving(a)naml.us> wrote: From: Geoffrey Irving <irving(a)naml.us> Subject: Re: [Haskell-cafe] RE: [Announce] primes To: "Daniel Fischer" <daniel.is.fischer(a)web.de> Cc: haskell-cafe(a)haskell.org Date: Thursday, April 16, 2009, 12:57 PM >> > The implementation is reasonably efficient. The query >> > >> > > primes !! 1000000 >> > >> > 15485867 >> > >> > answers after a few seconds. >> >

3 3

How the following works
by Tsunkiet Man 17 Apr '09

17 Apr '09

Hello, I can hardly imagine how the following code works: cinits :: [a] -> [[a]] cinits [] = [[]] cinits (x:xs) = [] : map (x:) (cinits xs) can someone give me a good explaination? (I understand it a bit, but it's really hard for me to figure out how a map in a map function works.) Thank you for your time, Tsunkiet

4 8

google-like "do you mean?" feature
by Michael Mossey 17 Apr '09

17 Apr '09

I'm thinking of writing a parser to load files that my customers have created. I'm a software requirements engineer; the data consists of the customers' thoughts in response to the latest release of the requirements doc. In fact, the files will probably be copies of the requirements doc itself, into which customers have entered their notes and made changes. The original requirements doc will have a format that can be parsed; probably something simple like lines marked with codes like //customer={customer name goes here} //requirement= {requirement text goes here} When I parse the documents that come back from the customers, they are likely to contain some errors. Field names may be mangled or misspelled. Customer names may be entered in unrecognizable variants (e.g. someone named "Michael" is indicated as "Mike") and so forth. I was thinking that it might be useful to have a Google-like "do you mean this?" feature. If the field name is //customer=, then the parser might recognize a huge list of variants like //ustomer=, //customor=, etc... that is, recognize them well enough to continue parsing and give a decent error message in context. Any ideas how to go about this? I don't think I would create a parser language that includes every variant, but instead the field names would be tokens that could be passed to another routine. The variants could be generated ahead of time. I would limit the number of variants to something manageable, like 10,000 for each field name. Thanks, Mike

5 5

Re: [Haskell-cafe] Issues with running Ghci from emacs
by Patai Gergely 16 Apr '09

16 Apr '09

> I end up doing > > :set -i../Documents/haskell/SOE/src > > To set the search directory so that ghci can find the source. > > I've not been how to tailor that 'cd' which is run at start > up (but I've not looked to hard) It is looking for the closest ancestor directory with a file ending in .cabal, and that's where cd goes. Probably the simplest way to trick it without chaniging any setting is to create an empty dummy.cabal in the directory of your program. Gergely -- http://www.fastmail.fm - Send your email first class

3 2

[Announce] primes
by Sebastian Fischer 16 Apr '09

16 Apr '09

I am pleased to announce the package 'primes' that implements lazy wheel sieves for efficient, purely functional generation of prime numbers in Haskell. Following the current discussion about primes in Haskell, I packaged up an implementation inspired by the papers "Lazy wheel sieves and spirals of primes" by Colin Runciman and "The Genuine Sieve of Eratosthenes" by Melissa O'Neil. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/primes The package does not implement all that was wished for in the mentioned discussion. It only exports two operations: The global constant primes :: [Integer] is an infinite list of primes and wheelSieve :: Int -> [Integer] is the function used to build this list. If you are worried about the memory requirements of using a global constant, you can use `wheelSieve 6` instead of `primes`. The implementation is reasonably efficient. The query > primes !! 1000000 15485867 answers after a few seconds. Feel free to contribute more functionality to this package. The sources are on Github: http://github.com/sebfisch/primes If you fork my project, I'll be happy to merge your changes. Cheers, Sebastian

1 0

understanding generics
by Anatoly Yakovenko 16 Apr '09

16 Apr '09

i am trying to understand how data.data works. How would i traverse the subterms of one type and build another type out of them. something like this: > gmapQ (\a -> Right a) (Just "hello") <interactive>:1:0: Inferred type is less polymorphic than expected Quantified type variable `d' escapes In the first argument of `gmapQ', namely `(\ a -> Right a)' In the expression: gmapQ (\ a -> Right a) (Just "hello") In the definition of `it': it = gmapQ (\ a -> Right a) (Just "hello") Thanks, Anatoly

2 1

Re: Looking for the fastest Haskell primes
by Niemeijer, R.A. 16 Apr '09

16 Apr '09

Heinrich Apfelmus wrote: > +1 except that exporting the potentially infinite list of primes is > problematic in that it may become a memory leak. > > I'd suggest to export two versions > > primes :: [Integer] > primes' :: () -> [Integer] > > for casual (i.e. throwaway program to solve a Project Euler problem) and > for memory aware use respectively. I'm afraid I don't quite follow. Would you mind explaining what the first parameter is for and how it would solve the memory leak?

10 10

Data.ByteString woes
by David Carter 16 Apr '09

16 Apr '09

Hi, I am battling with Data.ByteString.Lazy.Char8, and currently losing. Any help gratefully received. Here's my code: import qualified Data.ByteString.Lazy.Char8 as B import Text.Regex.Posix ((=~)) test = B.pack "abc" =~ B.pack "b" :: Bool This works fine without the ".Lazy", and is also fine under GHC 6.8.1, but with GHC 6.10.1, it fails with: No instances for (Text.Regex.Base.RegexLike.RegexLike Text.Regex.Posix.Wrap.Regex B.ByteString, Text.Regex.Base.RegexLike.RegexMaker Text.Regex.Posix.Wrap.Regex Text.Regex.Posix.Wrap.CompOption Text.Regex.Posix.Wrap.ExecOption B.ByteString) I then thought I might work around the problem by converting lazy ByteStrings to strict ones in order to do the regex match. I discovered toChunks, which is advertised in the documentation as type ByteString -> [ByteString], but it actually appears to be of type toChunks :: ByteString -> [Data.ByteString.Internal.ByteString] and I can't see any way of converting a Data.ByteString.Internal.ByteString to a (strict) ByteString. Does anyone know whether this is (a) bug(s) in GHC, Data.ByteString, Text.Regex.Posix or (maybe most likely) my own limited understanding? And any ideas for fixes or workarounds (other than the obvious one of downgrading to GHC 6.8)? Thanks in advance David

4 4

how to upgrade?
by Johannes Waldmann 16 Apr '09

16 Apr '09

Is there a nice way of upgrading ghc: I mean does cabal-upgrade know to install exactly the packages that I had with the previous ghc version? - J.W.

2 1

Issues with running Ghci from emacs
by Daryoush Mehrtash 16 Apr '09

16 Apr '09

I am having problem running GHCI with Haskell files that include imports. I am running emacs22 on Ubuntu, with haskell-mode-2.4 extensions. I load my file (Fal.lhs in this case) in emacs. Then try to run the Ghci by doing C-c C-l. The result is shown below. Ghci fails to find the Draw.lhs which is also on the same directory as the Fal.lhs. Note: If I go to the directory where Fal.lhs is and run Ghci directly it all works fine. Any idea how I can get the interepreter to work form emacs? 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> :cd ~/.cabal/ Prelude> :load "../Documents/haskell/SOE/src/Fal.lhs" * ../Documents/haskell/SOE/src/Fal.lhs:76:9: Could not find module `Draw': Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude> Thanks, Daryoush

2 1

Cabal and WinHugs
by Iavor Diatchki 16 Apr '09

16 Apr '09

Hello, What is the preferred way to install a cabal package so that it works with winhugs? When I tried "cabal install --user --hugs" I got an error that it could not find "hugsffi". I managed to get things working by manually downloading the package, and extracting the appropriate source directories to the "packages" folder of winhugs (i.e., basically skipping cabal) but this is kind of clunky. So to summarize, here are my two questions: 1. Am I doing something wrong, or does "cabal" not support WinHugs?, and 2. Does WinHugs have a user specific "packages" folder (so that each user can have their own set of packages)? Thanks for any info that anyone might have, Iavor

2 1

Re: [Haskell-cafe] Enum to String, and back?
by michael rice 16 Apr '09

16 Apr '09

Thanks guys. It works like a charm. Michael --- On Wed, 4/15/09, Rahul Kapoor <rk(a)trie.org> wrote: From: Rahul Kapoor <rk(a)trie.org> Subject: Re: [Haskell-cafe] Enum to String, and back? To: Cc: haskell-cafe(a)haskell.org Date: Wednesday, April 15, 2009, 3:20 PM On Wed, Apr 15, 2009 at 3:13 PM, michael rice <nowgate(a)yahoo.com> wrote: > Using Show it is possible to establish a relationship between an enum type > and a String type to display it. > Can one as easily establish a reverse relationship, i.e., convert a String > type like "Red" back to its corresponding Color type? > Make it an instance of the Read type class. http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t%3ARead That will allow you to write - read "Red" :: Color => Red HTH Rahul _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Brackets and Asynchronous Exceptions
by Andrew Gallagher 16 Apr '09

16 Apr '09

Hi, In a program I am writing, I have many locations where I acquire a resource, perform an operation with it, then release it. I have been using the 'bracket' function so that the "release" operation would be performed even if the operation threw an exception. This seems to work nicely. In the event of an asynchronous exception, however, is there a possible scenario where a release is not performed after an acquire? Looking at the example given in bracket documentation: bracket (openFile "filename" ReadMode) (hClose) (\fileHandle -> do { ... }) Is it possbile that an asynchronous exception could be raised in this thread after openFile executes but *before* the appropriate handlers are installed and the operation is run, preventing hClose from executing? If 'bracket' does not handle this case, should I be using the block/unblock functions to disable asynchronous exceptions: block (bracket (openFile "filename" ReadMode) (hClose) (\fileHandle -> do unblock ({ ... }))) Thanks, Andrew

2 2

Mondrian and Haskell
by Vasili I. Galchin 16 Apr '09

16 Apr '09

Hello, Several days ago I posted a question about retargeting GHC to generate CIL so that Haskell could be a .NET language. One objection was Haskell "lazy" nature didn't fit well with .NET's CLR. I found a couple publications .... http://www.ddj.com/windows/184404967;jsessionid=0AVCEATVAEGKMQSNDLOSKH0CJUN… ... more popular presentation http://docs.msdnaa.net/ark_new/Webfiles/WhitePapers/ECOOP.pdf ... this paper seems to be saying that Mondrian's "lazy" nature fits in quite well Any opinions from Mondrian people "listening" in? What lessons were learned from the Mondrian experience that could speak to Haskell in particular GHC? Regards, Vasili

1 0

Enum to String, and back?
by michael rice 16 Apr '09

16 Apr '09

Hi, Using Show it is possible to establish a relationship between an enum type data Color = Red | Blue | Green | Yellow | Orange | Brown | White | Black deriving (Show, Eq, Enum, Bounded) and a String type to display it. *Main> Red Red *Main> [Red,Green,Blue] [Red,Green,Blue] *Main> which is Red :: Color -> "Red" :: [Char] Can one as easily establish a reverse relationship, i.e., convert a String type like "Red" back to its corresponding Color type? So that "Red" :: [Char] -> Red :: Color Michael

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Maybe off-topic -- Writing contracts or software specifications
by Maurício 16 Apr '09

16 Apr '09

Hi, I'm an engineer, and as a programmer I'm just an amateur. This easied things to me, since I could take decisions about practices based on what made sense to me. But now I need to take responsability for some formal programming tasks, and I don't know which examples to follow. I need, for instance, to write a contract with a programmer we are hiring for a task. But the only example I have of such contracts seemed to me (as I said, an amateur. I may be completely wrong) impractical. It was a 150 pages document with every possible user action and every imaginable allowed consequences. But it would be easier to me write the software than such contract itself. Are there ways already accepted by practice on how to write software contracts? It's a small program to acquire and show data from a device, a one person, two months work. I like the guy we are going to hire, and much to my surprise he sugested using Haskell for the task. But he also do not have experience writing such documents. Thanks, Maurício

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Fast mutable arrays of ByteString?
by Maxime Henrion 15 Apr '09

15 Apr '09

Hello all, I have been rewriting a small utility program of mine from C to Haskell for fun. This tool reads lines from stdin or from files, shuffles them one or more times using the Fisher-Yates algorithm, and outputs the result to stdout. Since this algorithm is based on in-place updates, I've been storing my strings in a mutable array in the ST monad. Since it's holding strings I could not use an unboxed array. The resulting program works fine and seems to run at a decent speed, even though it is much slower than the original C version, slightly more so than I expected. While trying to optimize it using profiling, and playing with the number of shuffling passes, I noticed that this operation was responsible for a significant amount of the runtime, much more so than with the C version. I also noticed that the %GC time was around 56%. In order to do more tests, I wrote another version of this program which keeps the strings in a pure and immutable array, and stores the indices of this array in an unboxed mutable ST array. The shuffling is then done on this indices array instead of the strings array. This version runs much faster and only spends ~21% of its time in the garbage collector, at the cost of consuming more memory for the indices array. I'm attaching both versions of the code to this e-mail, and I'd be curious to hear about any possible improvements to it, and whether the performance of STArray of ByteString I'm observing corresponds to people's expectations. Thanks in advance, Maxime Henrion

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ANN: level-monad-0.3
by Sebastian Fischer 15 Apr '09

15 Apr '09

I am pleased to announce version 0.3 of the package level-monad. This package implements breadth-first search directly as an instance of MonadPlus (without using an intermediate tree representation). In version 0.3 I have added a MonadPlus instance for iterative deepening inspired by Michael Spivey's paper on "Algebras for combinatorial search"[1]. The package is on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/level-monad Sources are on Github: http://github.com/sebfisch/level-monad Cheers, Sebastian [1]: http://spivey.oriel.ox.ac.uk/mike/search-jfp.pdf

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cabal install vs. profiling
by David F. Place 15 Apr '09

15 Apr '09

Hi, Suppose I have installed a number of libraries and have written a program using them. Now, I want to profile my program. What is the best way to get the profiling versions of the libraries installed? Thanks, David

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GHC including System.Time but not Data.Time?
by John Goerzen 15 Apr '09

15 Apr '09

Hi folks, Apologies in advance because this sounds rantish... So I went to my friendly API reference at http://www.haskell.org/ghc/docs/latest/html/libraries/index.html and noticed that I couldn't find Data.Time there anymore. Though it was still at http://www.haskell.org/ghc/docs/6.10.1/html/libraries/index.html After some checking on IRC, apparently this is on purpose. I find it *incredibly* annoying, and leaves us with the following unfortunate set of circumstances: 1) GHC ships with NO way to do date/time calculations in the preferred way (Data.Time) 2) GHC's docs reference only the obsolete (System.Time) way of doing things, with no reference to the preferred way. 3) I can't tell people to just install GHC and expect them to be able to perform date & time calculations the preferred way. Just about EVERY other language (C, Perl, Java, Python, etc.) come with this as part of the base install. 4) I can't update all my apps to use Data.Time without worrying about Yet Another Dependency. 5) Result: black eye on us. As I saw on IRC: <quicksilver> the system isn't supposed to work out the box. <quicksilver> the haskell platform is supposed to work out of the box. <quicksilver> (shame it doesn't exist) Which is a fine goal, but until then, pretty please don't go dropping Data.Time out of GHC. I understand the goal of removing stuff from GHC, but the practical implications can be rather annoying. -- John

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Ambiguous reified dictionaries
by Martijn van Steenbergen 15 Apr '09

15 Apr '09

Good morning, The [1]GHC user's guide, section 8.4.5 says: "The new feature is that pattern-matching on MkSet (as in the definition of insert) makes available an (Eq a) context. In implementation terms, the MkSet constructor has a hidden field that stores the (Eq a) dictionary that is passed to MkSet; so when pattern-matching that dictionary becomes available for the right-hand side of the match." But what happens if there are several dictionaries available? Consider these three modules: ReifyMonoid.hs: > {-# LANGUAGE GADTs #-} > > module ReifyMonoid where > > import Data.Monoid > > data MonoidInst a where > MkMonoidInst :: Monoid a => MonoidInst a ReifySum.hs: > module ReifySum where > > import ReifyMonoid > import Data.Monoid > > instance Monoid Int where > mempty = 0 > mappend = (+) > > intSum :: MonoidInst Int > intSum = MkMonoidInst ReifyProd.hs: > module ReifyProd where > > import ReifyMonoid > import Data.Monoid > > instance Monoid Int where > mempty = 1 > mappend = (*) > > intProd :: MonoidInst Int > intProd = MkMonoidInst Now a function > emp :: MonoidInst a -> a > emp MkMonoidInst = mempty works as you'd expect: *ReifySum ReifyProd> emp intSum 0 *ReifySum ReifyProd> emp intProd 1 But what about this function? > empAmb :: MonoidInst a -> MonoidInst a -> a > empAmb MkMonoidInst MkMonoidInst = mempty Now there are two dictionaries available. GHC consistently picks the one from the second argument: *ReifySum ReifyProd> empAmb intProd intSum 1 *ReifySum ReifyProd> empAmb intSum intProd 0 My questions are: 1) Shouldn't GHC reject this as being ambiguous? 2) Should class constraints only be available on existentially qualified type variables to prevent this from happening at all? 3) Is it possible to implement the following function? > mkMonoidInst :: a -> (a -> a -> a) -> MonoidInst a > mkMonoidInst mempty mappend = ... Thank you, Martijn. [1] http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extension…

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Re: Looking for the fastest Haskell primes algorithm
by Johannes Waldmann 15 Apr '09

15 Apr '09

for the API design, always check others before rolling your own. E.g. the (three) functions with "Prime" in their name from http://java.sun.com/javase/6/docs/api/java/math/BigInteger.html I hope they are there for a reason. While we're at it - do we have modPow? modInverse? And of course check their implementation as well (should be straightforward, but you never know). J.W.

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understanding typeable
by Anatoly Yakovenko 15 Apr '09

15 Apr '09

any idea why this is True data Foo = FooC Int | BarC Int deriving (Data, Typeable, Show) > fromJust $ funResultTy (typeOf FooC) (typeOf (1::Int)) Loading package syb ... linking ... done. ParseG.Foo > typeRepTyCon $ fromJust $ funResultTy (typeOf FooC) (typeOf (1::Int)) ParseG.Foo > let a = typeRepTyCon $ fromJust $ funResultTy (typeOf FooC) (typeOf (1::Int)) > :t a a :: TyCon > typeRepTyCon $ typeOf $ BarC 2 ParseG.Foo > let b = typeRepTyCon $ typeOf $ BarC 2 > a == b True I thought that TyCon can distinguish constructors. it seems no different then a typerep

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