- Haskell-Cafe - Haskell.org

Building pattern and trying monads
by L Corbijn 28 May '12

28 May '12

Hello cafe, I'm working on a project where the main goal of the program is building some complex output (e.g. a Haskell function, module, etc.). In this process there is almost always some partially finished product on which to work. Currently I'm modelling this with a wrapper around StateT containing the partial product but I'm having some doubts about this approach. On one of the projects there is some rule-based transformation which needs to be done. The problem is with the matching and applying the rules. The matches need to retrieve information from the monads in the stack, and the application of a rule changes values in the stack. However when a match fails the stack should be left *untouched* and another rule should be tried. The solution I've in mind depends on the stack being pure. When the monad stack is pure a rule can be applied, returning a maybe value (or having a MaybeT wrapper) and when returning Nothing (failed rule) reverting the stack to it's point before applying the rule. As I'm not quite sure about the design (nor good at software design) I've some questions about this approach. 1. Is there a better approach then using a state monad for building the 'products'? 2. My solution with saving/reverting monad-stacks seems quite a hassle/hack, so is it a good approach or is there something better? Thanks in advance, Lars

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Re: [Haskell-cafe] Building pattern and trying monads
by wren ng thornton 27 May '12

27 May '12

On 5/27/12 8:21 AM, L Corbijn wrote: > 2. My solution with saving/reverting monad-stacks seems quite a > hassle/hack, so is it a good approach or is there something better? One good solution for backtracking is to use logict[1]. I've used it with various state-like monads and it works well (e.g., the IntBindingT monad in unification-fd[2]). [1] http://hackage.haskell.org/package/logict http://okmij.org/ftp/papers/LogicT.pdf [2] http://hackage.haskell.org/package/unification-fd http://hackage.haskell.org/packages/archive/unification-fd/0.7.0/doc/html/C… -- Live well, ~wren

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Record syntax, reopening a can of worms.
by timothyhobbs＠seznam.cz 27 May '12

27 May '12

Hello list, As my first post. I'd like to open a can of worms that I sure has been opened before. That is record syntax. As we all know, there are some type safety problems with our current record syntax. The most obvious is that this compiles without even giving you a warning: >data MyData = > A {a::Int, > b::Int} | > B {c::Int} >foo :: MyData -> Int >foo a@A{} = c a >main :: IO () >main = print $ foo (A 1 2) This compiles, then crashes at run time. Rediculus, why didn't the glorious GHC catch this? This is not some ambigious scenario... One way I found to improve type checking is this: >data MyData = A A' | B B' >data A' = A'{a::Int, > b::Int} >data B' = B'{c::Int} >foo :: MyData -> Int >foo (A a) = c a >main :: IO () >main = print $ foo (A (A' 1 2)) This doesn't compile anymore because a :: A' and c :: B' -> X It seems like the later method is simply better. Better type checking = fewer runtime errors(I actually ran into a crash in a real project because of this lack of proper type checking...). Can we have a nice sugar for doing the later method? Thank you for your time, Timothy

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Problem with packet documentation generated by cabal on windows 7
by Nicu Ionita 27 May '12

27 May '12

Hi cafe, I have a problem with haddock documentation created when installing new packages with cabal on windows. The generated html files have all links in the form j:\Users\...\doc\...\xxx.html, but firefox says, it cannot open that link. Actually all links should be prefixed by "file:///". I wonder if this is only on windows so and if there is a solution to this. Is there a cabal or haddock flag for this? Nicu

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ANN: format-0.1.0.0
by Scott Lawrence 26 May '12

26 May '12

Hey all, I've just uploaded to hackage format-0.1, a small library for handling format strings similar to those used by nginx's log_format directive. The idea is to allow easy parsing of text rendered with such a format string, to make parsing simple, automatically generated text files as easy as possible. So for example, if you want to parse lines consisting of two bits of text, separated by " -- ", then map (scanFormatString "$a -- $b") $ lines text will do so, generating associative arrays with the variable names as keys and their content as values. A `renderFormatString` function, doing the obvious, is also provided. Obligatory blog post with some more details: http://bytbox.net/blog/2012/05/format-haskell.html I hope somebody finds this useful. (This seems the sort of thing which should already have existed, but I couldn't find anything. Did I miss it?) I'm using parsec underneath, both for parsing the format string and the generated parser, and I've payed no attention to optimization. Some simple testing indicates that a reasonably fast computer takes about 1 second to parse 5000 lines with nginx's (and apache's) default format string - pretty slow. If there's demand, I'll be happy to pay more attention to that. -- Scott Lawrence

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How to translate Repa 2 program to efficient Repa 3 code?
by Michael Serra 26 May '12

26 May '12

Hi Haskellers, I've posted this question (and my code) to stack overflow as well ( http://stackoverflow.com/questions/10747079/what-are-the-key-differences-be…) so if anyone here has the answer, I'll post it to that site for the world's reference. Using the Repa 2 API, I have written some simple image convolution tests which run more than fast enough. The trick to getting good performance was to call 'force' after every array transformation. I can't quite figure out the analogous thing to do with Repa 3 - at stackoverflow you can see my Repa 3 code, which runs correctly but very slowly. It is not clear to me how exactly the monadic "computeP" functions in Repa 3 are intended to be used - I have several calls to 'force' in my Repa 2 code, but only 1 call to computeP in the Repa 3 version. I've read the excellent "Numeric Haskell" Repa tutorial by Don S, but it doesn't cover Repa 3. My thanks in advance!

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gona to join in haskell-cafe
by Dante.py 26 May '12

26 May '12

Hello,friends! I come from China, wishing to enjoy haskell with you. -- 张晔 Dante.py 中山大学09级数学与应用数学专业个人主页：http://dantepy.yslsg.org/

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Correspondence between libraries and modules
by Alvaro Gutierrez 26 May '12

26 May '12

Hi there, I've only dabbled in Haskell, so please excuse my ignorance: why isn't there a 1-to-1 mapping between libraries and modules? As I understand it, a library can provide any number of unrelated modules, and conversely, a single module could be provided by more than one library. I can see how this affords library authors more flexibility, but at a cost: there is no longer a single, unified view of the library universe. (The alternative would be for every module to be its own, hermetic library.) So I'm very interested in the rationale behind that aspect of the library system. Thanks! Alvaro

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Template Haskell antiquotation in user-defined quasiquoters
by Sam Lindley 25 May '12

25 May '12

Template Haskell supports antiquotation for built-in quasiquotes, e.g.: [| \x -> x + $([|3 * 4|]) |] However, as far as I can tell, there is no way of supporting antiquotation in user-defined quasiquoters, because the only way to specify a new quasiquoter is through a quoteExp function of type String -> Q Exp. Of course, it is perfectly possible to write a parser for some fragment of Haskell inside your quoteExp function, but that seems crazy given that Template Haskell or rather GHC already implements a parser for the whole language. I know about Language.Haskell.Exts.Parser in haskell-src-exts, which provides parseExp :: String -> ParseResult Exp, but that Exp is a different type to the one provided by Template Haskell.<http://hackage.haskell.org/packages/archive/haskell-src-exts/1.9.0/doc/html…> I'm also aware of Dominic Orchard's syntax-trees package, which supports converting between the two representations using a cunning hack that pretty-prints the haskell-src-exts representation to a string and uses Template Haskell to parse it back. Is there a saner way of simulating antiquotation in user-defined quasiquoters? Sam -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

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webcam library on github
by . 25 May '12

25 May '12

Hello Cafe, since I haven't found anything like that, I wrote a small library [1] to read from a webcam in Haskell, using V4L on Linux. It uses the v4l2 package and repa for images. Is anyone interested in contributing to that, or giving some hints on how it could be made faster (I am not sure if I use repa correctly for best performance at all times), or just in emitting any other constructive comments? If yes, please give me a shout. Cheers, Christian PS I would put the whole thing on hackage, but my cabal currently doesn't want to do an sdist. [1] https://github.com/cgo/hsimage

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