- Haskell-Cafe - Haskell.org

ANNOUNCE: pdf-slave-1.0 tool for PDF generation from haskintex templates
by NCrashed . 21 Dec '16

21 Dec '16

I am glad to announce first release of pdf-slave, a tool for generation of PDF documents from LaTeX templates with embedded Haskell. Hackage: http://hackage.haskell.org/package/pdf-slave Github: https://github.com/NCrashed/pdf-slave Description ======= In many real-world systems we need to produce reports, cheque and other documents to users. One possible solution is rendering LaTeX files, but it is not flexible enough as you need to solve burden with generation of LaTeX file from varying inputs, manage static files like images and handling dependencies between pieces of document. The new tool takes the burden away. Now all static files and generation code is located within a single template project, that describes how to build your PDF document in a declarative way. Common work flow with pdf-slave: * Develop template from several LaTeX files with embedded Haskell (thanks to haskintex tool) * Pack the template project into bundle all-in file. * Distribute the bundle (for instance, to your service server). * Call pdf-slave with bundle and inputs in JSON file to generate new PDF document. Status ==== pdf-slave is designed as part of commercial Haskell system. The tool is very young and need time approval, although it already has all needed features one may need from such tool. Special thanks to hint authors and Daniel Díaz, developer of awesome HaTex and haskintex, without these packages pdf-slave wouldn't be possible! Reports, feature request, PR are always welcome! Anton Gushcha aka NCrashed

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stack on Windows and C Preprocessor
by Olaf Klinke 20 Dec '16

20 Dec '16

Dear Cafe, not sure who the culprit is, thus trying on this list: My project builds fine with stack on Linux. I installed the latest (1.3.0 x86_64) stack on a Windows box and ran 'stack.exe setup' inside my project's directory. The build fails at the depencency HaXml-1.25.3 with a lexer error on line 34 of this file: http://hackage.haskell.org/package/HaXml-1.25.3/docs/src/Text-XML-HaXml-Wra… lexical error in string/character literal at character '\r' It seems the line (VERSION) is processed by the C preprocessor which somehow messes it up. Unfortunately, stack deletes its temporary files after running into the error, which deprives me of handles to further debug this. Does anyone have an idea how to continue from here? Thanks, Olaf

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Re: [Haskell-cafe] Foo.Bar.hs filenames poll
by Dimitri DeFigueiredo 20 Dec '16

20 Dec '16

I believe it is a mistake to tie the module structure of a software system to a file structure on a hierarchical file system. I see those too structures as different entities. Why can't I coalesce two tiny modules into a single file? I also don't like having to capitalize my file names (Foo.hs instead of foo.hs) in linux, that seems rather ugly. It see this "ugliness" as a symptom of the "arbitrarily forced link" between the module structure and the file structure. Obviously, some conventions should apply, but a one-to-one correspondence seems too strict. Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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memoizing with a default
by Michael George 20 Dec '16

20 Dec '16

I've come across a slight variant of memoization and I'm curious if either there is a better way to think about (I have yet to reach Haskell Enlightement), or if it's been thought about before. I'm writing a program that does DFA minimization, as described here: https://www.tutorialspoint.com/automata_theory/dfa_minimization.htm. For me, the simplest way to understand the algorithm is we form an equivalence relation, where two states are different if either one is final and the other isn't, or if there are transitions from them to states that are different. different a b = isFinal a && not (isFinal b) || isFinal b && not (isFinal a) || exists chars (\c -> different (step a c) (step b c)) This definition diverges; it never decides that two states are the same. The standard algorithm builds a table, starting with the assumption that all states are the same, and iteratively applying this definition until it converges. I'm thinking of that algorithm as a memoized version of the implementation of different above. But it's not quite: different = memoFix2 different' where different' different'' a b = isFinal a && not (isFinal b) || isFinal b && not (isFinal a) || exists chars (\c -> different'' (step a c) (step b c)) for the same reason; it diverges. But with memoization I can detect the diversion by putting a marker in the memo table. If the computation makes a recursive call on an input that is started but not computed, then (in this case) I want to return false. In general, I think what I want is memoFixWithDefault :: (a -> b) -> ((a->b) -> (a -> b)) -> (a -> b) Where (memoFixWithDefault default f) is like (memoFix f) except that if (memoFix f a) would loop, then (memoFixWithDefault default f a) = default a. I suspect there's a nice denotational way of describing this, but my denotational semantics is rusty (maybe it's the least fixed point of f that is greater than g or something, but that's not it). I suspect that there needs to be a nice relationship between f and g for memoFixWithDefault f g to be sensible. I also suspect that there's a cute 3-line implementation like all the other implementations of memoize out there, but I don't see it. I also suspect that depth-first search has a cute implementation using this combinator. So I was wondering if other people had thoughts or references before I went off and thought about it. Thanks! -M

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Foo.Bar.hs filenames poll
by Christopher Done 19 Dec '16

19 Dec '16

Short version: here is the poll < https://docs.google.com/forms/d/e/1FAIpQLSdniOfoaX7xflgdWRnjVQ6_VLtk1oxA00S… > I noticed recently that Foo.Bar.hs is supported by GHC. I had always assumed it wasn't because people always use directories. I've never liked having separate directories for each level of hierarchy. It's easier to just list a list of files and script (e.g. even copying a file X.hs to Y.hs is a bummer). When opening them on GitHub you have to click through to get a complete picture of a project. Other languages do and don't do this. Lispers, for example, don't. How do other Haskellers feel about it? Would it mess with anybody's tooling or mojo if I switched to that style in my packages? For one I know that Stack (my own implementation), actually assumes hierarchical filenames. So I'd have to patch that to implement this. E.g. > Unable to find a known candidate for the Cabal entry "HIndent.Types", but did find: HIndent.Types.hs. If you are using a custom preprocessor for this module with its own file extension, consider adding the file(s) to your .cabal under extra-source-files. I suppose the real question is, as a language standard and a community preference, should this be considered a bug? Should people be free to use X.Y.hs or X/Y.hs styles? Ciao!

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ANN: New Haskell.org Committee Members
by Gershom B 19 Dec '16

19 Dec '16

Following the self-nomination period and discussion, the Haskell.org committee has selected new members: Full term: * Mitsutoshi Aoe * Jasper Van der Jeugt * Niki Vazou Short term: * Alan Zimmerman As per the rules of the committee, this discussion was held among the current members of the committee, and the outgoing members of the committee who were not seeking reappointment. Thank you to all the many candidates who submitted a self-nomination. We recieved many strong nominations, from a broad range of geographies and backgrounds. The choice amongst them was not easy, and we would encourage all those who nominated themselves to consider self-nominating again in the future. The outgoing members are: Adam Foltzer, Edward Kmett, Andres Loeh, and Nicolas Wu. Thanks all for your service! These are tough shoes to fill, but I have confidence that our incoming members will more than rise to the occasion. Cheers, Gershom

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Re: [Haskell-cafe] Absolutely confused with error/exception handling
by Saurabh Nanda 18 Dec '16

18 Dec '16

My recommendation is to catch the exception in IO first, before it is lifted into the ActionCtxT. If your business logic is decoupled from the web framework (as it should be) then it should be easy enough to do this. I'm trying to write a "global" error handler which logs unhandled errors to Airbrake along with information from the request object (domain, URL, params, cookies, etc). I understand that I can make this part of my app monad, but if I don't understand why I'm unable to catch errors in MonadIO I'm bound to get stuck again, or implement it incorrectly (for example, what are the resource leaks that were being referred to?) Also what if a particular handler is not in my app monad? How do I prevent someone from accidentally adding a handler without the airbrake integration? -- Saurabh.

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regression in memory usage between 6.10 and 7.x?
by Ben Franksen 17 Dec '16

17 Dec '16

In the functional pearl "A Play on Regular Expressions" by Sebastian Fischer, Frank Huch, and Thomas Wilke, the authors explain how to implement a Glushkov automaton in a very elegant way. They claim that when they compile their code with ghc-6.10.4 and with -O2 optimization they get: """ 1 MB total memory in use ... Total time 0.06s (0.21s elapsed) """ for a program that matches stdin against the equivalent (in grep -E syntax) of ^(a?){500}a{500}$ When I try their code on my machine with ghc-7.10.3 (and also -O2) I get: """ ben@sarun[1]: .../play/regexfp > ghc Reg.hs -O2 -o re [1 of 1] Compiling Main ( Reg.hs, Reg.o ) Linking re ... ben@sarun[1]: .../play/regexfp > ./re 500 +RTS -s True ... 155 MB total memory in use ... Total time 1.031s ( 1.037s elapsed) """ The details of the RTS report furthermore tell me that GC is responsible for more than 80% of the runtime. Now, I certainly have a different machine and perhaps OS than the authors of the paper, but that can't explain a factor of 16 in runtime and of 150 in memory use. I have double and triple checked my code for possible mistakes. I /do/ get the expected results (same as in the paper). I tried different numbers and found n total memory in use / MB 100 8 200 28 500 169 1000 698 which shows that memory consumtion is roughly quadratic in n. I tried heap profiling but that tells me only that all the allocation is done by the function 'shift', which I already knew (or rather guessed). I also tried several compiler versions: 7.0.4, 7.10.3, 7.4.2, 7.6.3, 8.0.1. They all result in similar performance. The ghc version the authors used is 6.10.4 and I don't have a package for that compatible to my distribution. Any hint as to where these differences might come from are appreciated. I have attached the program for reference. Cheers Ben -- "Make it so they have to reboot after every typo." ― Scott Adams

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Absolutely confused with error/exception handling
by Saurabh Nanda 17 Dec '16

17 Dec '16

Hi, What started as a simple exercise of writing an Airbrake integration for Spock, has turned into a day of reading about errors, Control.Exception, MonadThrow, MonadCatch, sync, asynchronous, etc. Even after all that reading I haven't been able to find answers to the following -- * What exactly is done by the "error" function? How does one "trap" those errors and react to it? Can "catch" trap those errors? What is "e" in the case of errors raised by "error"? * How is "error" different from "throw" and "throwIO" (in the Control.Exception package, I believe) * What does MonadThrow and MonadCatch bring to the table? My understanding is that they are bringing a unified API to representing short-circuitable computations based on the inherently short-circuitable value of the host monad. Eg throwM in Maybe just results in a Nothing. throwM in List results in [] Therefore, in most of the cases throwM is not really using the error machinery built into the Haskell language (what said machinery is, is the very first question that I still don't completely understand) All of this might be obvious, and I might have just reached a mind-block after hours of reading. Help would be really appreciated :) -- Saurabh.

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Fwd: Your Assembly at 33C3
by Profpatsch 16 Dec '16

16 Dec '16

We will organize a “Functional Hot-Pot” at 33C3, where friends of functional languages, Haskellistas, Lambdroids and Scalafists can share their experience and have a good time. https://events.ccc.de/congress/2016/wiki/Assembly:Functional_Hot-Pot If you want to maybe organize a workshop or have other ideas, don’t hesitate to contact me (this mail or DECT LBDA). -- Proudly written in Mutt with Vim on NixOS. Q: Why is this email five sentences or less? A: http://five.sentenc.es May take up to five days to read your message. If it’s urgent, call me.

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