- Beginners - Haskell.org

Embed haskell
by Rosario Borda 22 Apr '12

22 Apr '12

Hi All, There is a simple method for embed haskell in other languages, like latex? An example in quasiquotation notation: \documentclass{article} \title{Hello World} \begin{document} \maketitle Hello world! and reversed: [haskell: putStrLn reverse "Hello, World!"] \end{document} Many thanks, :) Rosario

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Re: [Haskell-beginners] Training tasks
by umptious 21 Apr '12

21 Apr '12

On 20 April 2012 19:47, Michael Litchard <michael(a)schmong.org> wrote: > A copy would be most helpful. Thanks :) > The most useful part of the Elite/Traveller trading engine problem is that it's extremely easy to come up with changes to the requirements once you've finished the thing. We all have some idea how international trade works, so you can think about the domain and say - "Hey - I'm going to add tariffs! And I'm going to add alliances - most worlds will belong to one, and if you're registered with Alliance A then you don't pay tariffs at their ports, but you do pay extra at B's, and you can't dock at C's at all. But those relationships may change at an any moment." Politics and trade are complex enough so that you can always come up with some tweak to the model that you never thought of before. Which means you get to see how easy it is to refactor for completely unanticipated requirements - it's not a hard test that gives you numbers, but it does give you a feel for how the language being tested copes with the sort of unanticipated change you can get in real world requirements. Jay

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Re: [Haskell-beginners] Training tasks
by umptious 20 Apr '12

20 Apr '12

On 19 April 2012 20:20, Michael Litchard <michael(a)schmong.org> wrote: > Hi, I noticed one of your suggestions > > > - A version of the classic Traveller/Elite trading system - a really nice > > little toy business rules system that can be code either minimally or to > > show off the funkier features of a language to get more flexibility and > more > > interesting pricing rules. Minimal version would be a few tens of lines > and > > the config file declaring item > > I couldn't find anything on Elite, could you give me a reference? > I found a reference concerning Traveller, but if you had anything in > mind, it may be helpful. > > Thanks :) > If you download Oolite, available from most Linux repositories, it should have the same trading rules as the original Elite (which it stole from the Traveller pen and paper game.) There's a space trading game for the Palm Pilot that has improved more interesting rules - maybe you can find a Palm emulator if you don't have one? - http://en.wikipedia.org/wiki/Space_Trader_%28Palm_OS%29 Basically, you like goods that can be traded and you have worlds to sell them on. Worlds have government types and economic types - and in more complex variants ecologies and events - that affect the price and supply of goods. Eg Fish will be cheap on a Farming world with an Ocean environment, but expensive on an Industrial Desert world especially if it is Rich; Guns will sell for more if a War is going on; Survival Equipment and any Food Item will sell for more in the aftermath of a disaster. You can also model legality based on a Government type. The challenge is to get a system that allows trade rules to be concise and expressive while being written in a form that can be edited by a non-programmer - the config can be an .hs file, but it should be one that even a non-coder can understand and edit. It's a nice problem because it's scalable (you increase the complexity of the rules) and it has a lot of characteristics of real business systems. I knocked out the core of a solution last night - together with the recursive Shape Editor it's one of the best language test programs I know - and should have something nice around Monday if you want a copy. - Jay

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cmath Install troubles
by Tom Murphy 20 Apr '12

20 Apr '12

Any advice? If you can't even install a package with no dependencies except base, I think there's trouble: $ cabal install cmath Resolving dependencies... Configuring cmath-0.3... Preprocessing library cmath-0.3... Building cmath-0.3... [1 of 1] Compiling Foreign.C.Math.Double ( dist/build/Foreign/C/Math/Double.hs, dist/build/Foreign/C/Math/Double.o ) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2sB_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: ‘gamma’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: (Each undeclared identifier is reported only once /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2850:0: error: for each function it appears in.) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2t0_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:2896:0: error: ‘gamma’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2uH_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3088:0: error: ‘isinf’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2v6_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3134:0: error: ‘isinf’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2vv_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3180:0: error: ‘isnan’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2vU_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3226:0: error: ‘isnan’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2wj_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3272:0: error: ‘finite’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2wI_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:3318:0: error: ‘finite’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2Kh_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:4856:0: error: ‘significand’ undeclared (first use in this function) /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc: In function ‘s2KG_ret’: /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/ghc70799_0/ghc70799_0.hc:4902:0: error: ‘significand’ undeclared (first use in this function) cabal: Error: some packages failed to install: cmath-0.3 failed during the building phase. The exception was: ExitFailure 1 Thanks for any help! amindfv / Tom

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Seq question
by Lee Short 19 Apr '12

19 Apr '12

I've got the following code, and I'm not sure why the seq seems to be doing nothing. The code should never terminate if the seq were forcing the full evaluation of y. Instead, it runs just fine (though stack overflows if I ask for too many elements of primes). I didn't see anything in the tutorial section of haskell.org that seems to explain this -- pointers to a useful source would be welcome. sieve :: [Int] -> [Int] sieve xs = let ys = filter (\z -> 0 /= z `mod` head xs) xs in seq ys $ head xs:sieve ys primes = sieve [2..] thanks Lee

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Cross-platform .hs files on Linux and Windows
by Vinay Sajip 19 Apr '12

19 Apr '12

I've been given a set of .hs files which contain the shebang line #!/usr/bin/env runhaskell and I would like them to work on Windows, but none of the Windows binaries seem to be able to process them without a <script.hs>:1:1: parse error on input `#!' These files came from a Linux machine, where they run without trouble. Is there any way I can get the Windows executables to run these files? I asked on IRC and it was suggested that I change the files to literate Haskell, but I'd rather have some way of having cross-platform operation which does not involve making changes to the scripts themselves. Is there something that can be done e.g. by using particular command line options or configuration settings? Thanks & regards, Vinay Sajip

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Re: [Haskell-beginners] Training tasks
by Nikita Beloglazov 19 Apr '12

19 Apr '12

> > Yes, I get that. It's a bad idea. Showing atypical "promotional" cases for > each language encourages faddishness and silver bullet cults; it's > irresponsible and unprofessional. If you're shopping around for a language > then nothing is worse than selected sweet spot cases. Unless they are a > fair-ish benchmark for a language that excels in a clear area, like R, APL > and Awk and you're looking for a glorified DSL. > > I'd say that its better to have the same tasks for each language if you're > evaluating a general purpose tool. Obvious ones would be, from simple to > complex: > > - Some of the unix command line utilities like wc (some of these might be > single liners) > > - OXO, Caesar cipher (10 lines or so) > > - The Markov chain program from the Practice Of Programming > > - Life > > - A simple spheres only ray tracer like the one in Graham's Lisp book > > - A simple calculator program like the one in Hutton > > - The recursive 2D shape program I've mentioned in other posts today > > - A version of the classic Traveller/Elite trading system - a really nice > little toy business rules system that can be code either minimally or to > show off the funkier features of a language to get more flexibility and > more interesting pricing rules. Minimal version would be a few tens of > lines and the config file declaring items. > > If you want to see a language comparison done responsibly, look at > Kernighan and Pike's "Practice Of Programming." Thank you for criticism and tasks examples. But I still think it isn't such a bad idea. Because if somebody get excited about some language (even if it is based on "promotional" cases) and he learns this language more by himself then it's great. I don't think somebody will judge about whole language by few tasks. He may get interested or dislike the language, but I doubt there will be silver bullet cult based on few tasks. By shopping language I mean choosing some new fun language to learn, not to solve particular practical task. Nikita

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Program reliability and multiple data constructors; polymorphism
by umptious 19 Apr '12

19 Apr '12

One of the programming exercises I keep evolving as I learn Haskell is a toy 2D shape editor with four primitives: data Shape = Circle {origin::Pt2, radius::Float} | Square {origin::Pt2, side ::Float} | Rect {origin::Pt2, other ::Pt2} | Composite {shapes::[Shape]} deriving (Show, Read) The intent is Composites can contain Shapes of any kind, including other Composites so that you can apply transformations to a Composite and these will be applied to the contained Shapes recursively. So an arm might contain a hand which constains a dozen or so Rects. Transform the arm and the hand and rects should transform; transform the hand and its rects should transform but the not arm. Big deal. And the above makes that really easy when you know you're talking to a Composite. But now I've hit an intellectual stumbling point and the books and source I have don't seem to address it: I can apply the destructuring command "shapes" defined in the cstr "Composite" to ANY Shape. And if I do that to say a circle, BLAM! Or if I apply "radius" to Rect, BLAM! At runtime. No type checking support (because yes, they're the same type.) To me this seems alarming. It means that I can't reason about the safety of my program based on type checking as I'd like. And none of the answers I can think seem at all elegant: - I could use exception handling, but that means carefully checking which data declarations are potential bombs and using exceptions only when they are involved - hideously error prone - or using exceptions everywhere. Which is just hideous. - I could hack run time type checking using the ctsr info in "show". But again I'd have to know when to use it or use it everywhere. And it seems like a ridiculous kludge to bring to a language that has been designed for elegance. ..So what is the Haskell idiom for dealing with this??? In fact I suppose I'm asking two questions: 1. How do I re-design this program so it is safe (use class and instance maybe, abandoning use of a single data type? but I then have to have separate Lists for each type, even if they derived from a common class?) 2. How can one use compile time checking or (less good) coding practices to eliminate the possibilty of such runtime exceptions? And come to think of it 3. is there a Haskell book which addresses design and structural problems like this one - which I would have thought was both obvious and fundamental - because of the books I've looked at so far seem to do a tolerable job. The best of them present an adequate "on rails" tour, but none of them seem to give you the tools to address issues like this one. Whereas with C++and Stroustrupp, Common Lisp and Graham, the Smalltalk book, and I Erlang and Armstrong I'd know exactly what to do. Admittedly the C++ solutions wouldn't be pretty, but anything the compiled would be safe to run (unless I went to great efforts otherwise..)

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Training tasks
by Nikita Beloglazov 18 Apr '12

18 Apr '12

Hi. I'm building website where people can try and "taste" new languages by solving small or mediums size tasks. Tasks are language specific and should show best features of the language. Website is not meant to teach new language but to give idea what is this language good for. Now I want to add Haskell. I need about 7-10 tasks for now. First three of four tasks are introductory, they should show/check basics of haskell. E.g. given n, return sum of squares of first n even numbers. Other tasks are more complicated and show advantages of functional programming in general or some specific haskell features. I don't have any experience with haskell and I need you help. Could you help me with ideas for tasks? Thank you, Nikita Beloglazov

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Re: [Haskell-beginners] Looking for vim text obejct plugin
by John Tyree 18 Apr '12

18 Apr '12

There are some decent object plugins on github. Look at vim-textobj-user for the parent plugin and then vim-textobj-indent for some useful ones based on indent level. I don't think there's anything that can handle haskell style arguments... although I think they are always either single identifiers or something wrapped in parens, so yiw and yib should handle it usually. -John ------------------------------ Message: 6 Date: Wed, 18 Apr 2012 13:23:00 +0900 From: Mait <mattengi(a)gmail.com> Subject: [Haskell-beginners] Looking for vim text obejct plugin for haskell. To: beginners(a)haskell.org Message-ID: <CABQgwkXyHdad4_Btef10N9nFSWXBo9EKB-Yom3jfmp0cRTfJ_A(a)mail.gmail.com> Content-Type: text/plain; charset=UTF-8 Any good one there everybody knows except me? :) -- Mait ------------------------------ Message: 7 Date: Wed, 18 Apr 2012 13:34:20 +0900 From: Mait <mattengi(a)gmail.com> Subject: Re: [Haskell-beginners] Looking for vim text obejct plugin for haskell. To: beginners(a)haskell.org Message-ID: <CABQgwkWcfa9mL3fSb7RkTuRG8OXhjG3q2Qke=yvP9ZoZoo+mHw(a)mail.gmail.com> Content-Type: text/plain; charset=UTF-8 Especially looking for 'argument' object plugin. function x:xs ^ <- cursor ysaa) or ysia) -> function (x:xs) 2012/4/18 Mait <mattengi(a)gmail.com>: > Any good one there everybody knows except me? :) > > -- > Mait -- Mait

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