- Haskell-Cafe - Haskell.org

Replacing IO with pure values
by martin 25 Nov '14

25 Nov '14

Hello all, in order to nail down my problem state in "Replacing the world in a State Machine with IO", I'd like to answer a much simpler question: If I have function which reads IO and I want to test this function by passing it a series of pure values, how would I do this? I suppose I can do this by splitting my function in two, such that the outer function does IO and the inner is a pure function. Is there any other way?

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"Flatten" JSON objects under toListOf
by Stuart Popejoy 25 Nov '14

25 Nov '14

Hi, I'd like to transform some JSON to "flatten" objects in a list to a single dimension. Say I have: [{ "Name": "Stuart", "Dimensions": { "H": 71, "W": 170 } }, { "Name": "Sam", "Dimensions": { "H": 72, "W": 180 } }] How do I get it to just [{ "Name": "Stuart", "W": 170 } }, { "Name": "Sam", "W": 180 } }] I've tried zipWith with various toListOf constructions to pick apart and rebuild a new JSON object. Is there a better way? Thanks, Stuart

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GHC: Discovery of source dependencies without --make
by Lars Hupel 25 Nov '14

25 Nov '14

Hi everyone, I've got another problem in our – admittedly complex – build process (for context, see [0]). Anyway, I'm struggling with the following: I have an 'out' folder which contains .hi and .o files from previous compilation stages. I now want to compile a bunch of .hs files which depend on the modules from previous stages. I can just say ghc -c -iout -outputdir out File1.hs File2.hs ... which works nicely if File*.hs are independent of each other, or are already topologically sorted wrt dependencies. If not, compilation of 'File1.hs' will fail when importing 'File2'. However, I can't make it to work if I don't know their precise ordering in advance. I tried ghc -c --make -iout -outputdir out ... but apparently '--make' makes GHC ignore .hi files in the 'out' folder. (Not even '-hidir' makes it respect them.) Reading a bit further, I discovered the '-M' flag, however, not only does it output Makefile-formatted output, it also ignores .hi files. Is there any way to get the dependency discovery of '--make' without the rest? Or alternatively, to get '--make' to not just respect source files [1]? I know I could probably make it work if I made a package out of the previous build products, but I really want to avoid that in order to not increase the build complexity even further. Cheers Lars [0] <http://article.gmane.org/gmane.comp.lang.haskell.cafe/113016> [1] After reading the section on "the search path" (§4.7.3, <https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/separate-com…>), that doesn't seem to be possible.

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Dutch functional programming day (NL-FP 2015)
by Christiaan Baaij 24 Nov '14

24 Nov '14

Dear all, The next Dutch Functional Programming day (NL-FP 2015) will take place on Friday, January 9, 2015 at the University of Twente, Enschede, The Netherlands. You are all cordially invited to participate and, of course, to give a presentation. The day will end with a joint dinner. *Request:* in order to make the necessary reservations, we would like to know as soon as possible how many people we may expect. More information, and the online registration form, can be found at: http://wwwhome.cs.utwente.nl/~jankuper/fp-dag/ Hope to meet you all in Twente at the next FP Day! Best regards, Christiaan Baaij

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parallel and concurrent Haskell programs
by Maarten Faddegon 24 Nov '14

24 Nov '14

Hello Café, Recently I became interested in parallel and concurrent Haskell and I wonder if there are any non-trivial open-source programs that are recommended to have a look at? Thanks, Maarten Faddegon

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Replacing the world in a State Machine with IO
by martin 24 Nov '14

24 Nov '14

Hello all, at my shop some folks are implementing things using a java tool called "activiti". AFAIK, they create a bunch of processes and a "process engine" takes care of activating or deactivating processes. Among the things which can cause a state transition are external events. I wonder how one could do this in haskell. (1) Async IO I thought the collection of all process states can be seen as the system state of a state machine. Some state transitions can be done without doing IO, e.g. in situations where the termination of one processes immediatlely activates another process. However other transitions block and occur as soon as an external event is received. Yet other transitions occur WHENEVER an external event gets received. I suppose I could handle this using threads, but I don't trust threads. They look simple first, but then the trouble they cause typically exceeds my worst expectations. My primary problem is that I haven't yet found a proper way to fully capture the problem. It appears to me that a state machine without IO is essentially a discrete event simulation. A state machine where ALL state transitions are triggered by IO also seems easy, everything would be blocking and I just dispatch external events. But I cannot picture a state machine where some (but not all) transitions are triggered by IO. (2) Replacing the real world In the long run, I'd like to replace the "world" from which these events originate by a simulated world. This should allow the process engine to proceeed in fast motion. I suppose I will then also make the clock part of the simulated world. This raises the question: how does one replace IO? The simulated world will not do any real IO (but still communicate to the process engine). Considering that "IO a" basically means "World -> (World, a)" this should be easy, but I cannot see how to get a hold on "World". If someone could give my thoughts a push in the right direction, I'd much appreciate it.

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Re: [Haskell-cafe] Programming videogames in haskell
by Sean Seefried 24 Nov '14

24 Nov '14

I just updated the wiki [1] with a build script I wrote in Docker [2]. Putting together the build script in Docker was a very pleasant experience. I broke the script up into many small pieces so that if one particular part failed I could simply go back to the last checkpoint and start from there. Once I had finished the script, I also had very high assurance that it would work with no problems whatsoever when I ran it again from scratch. Normally putting together build scripts is difficult because of the inherently stateful nature of a file system. If you are working on a build script that takes a long time to run the temptation is not to start again from scratch every time something goes wrong. The turn-around times are simply too long. You can't be waiting 50 minutes every time you make a small change. Therein lies madness. But because Docker uses a copy-on-write filesystem under the hood, the filesystem is really a kind of persistent data structure. When one particular part of the script failed I was able to go back to *exactly* the state it was in before it failed. The boon to my productivity and my sanity was immense. Sean [1] https://www.haskell.org/haskellwiki/Android [2] https://github.com/sseefried/docker-build-ghc-android

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Munich Haskell Meeting, 2014-11-26 @ 19:30
by Christian Neukirchen 23 Nov '14

23 Nov '14

Dear all, next week, on Wednesday, 26th of November, our monthly Munich Haskell Meeting will take place again at Cafe Puck at 19:30 CET. For details see here: http://www.haskell-munich.de/dates Since Heinrich is not around much in Munich anymore, I have taken over organizing our Haskell meeting. The website will be updated soon. For the meantime, if you plan to join, please add yourself to this dudle so we can reserve enough seats: https://dudle.inf.tu-dresden.de/haskell-munich-nov-2014/ Everybody is welcome! Looking forward to see you, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Improving *> and >> for Data.Sequence
by David Feuer 23 Nov '14

23 Nov '14

I'd like to define (*>) and (>>) for Data.Seq.Seq in a "clever" way, like replicate, but I'm a bit stuck. It kind of looks like this is the purpose behind the applicativeTree function, which bills itself as a generalization of replicateA, but something seems to have gotten stuck and the only time I see applicativeTree actually used is to define replicateA. With all the fancy nesting, I'm a bit lost as to how to go about this, and having only one example doesn't really help. Can someone help give me a clue? Thanks, David

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GC times with +RTS -N2 -s
by Erik Rantapaa 22 Nov '14

22 Nov '14

When I run the sudoko2 program from the parconc-examples package, I get output like: ... SPARKS: 2 (1 converted, 0 overflowed, 0 dud, 0 GC'd, 1 fizzled) INIT time 0.00s ( 0.02s elapsed) MUT time 1.75s ( 1.44s elapsed) GC time 0.73s ( 0.14s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 2.48s ( 1.60s elapsed) ... My full command line is: ./sudoku2 sudoku17.1000.txt +RTS -N2 -s I understand how to interpret the Total time - 2.48 seconds is the total time spent by both cores and 1.6 seconds is the running time of the program as a whole. But how should interpret the GC times? What do 0.73s and 0.14s actually represent? This is with GHC 7.8.3, OSX 10.8.5, 64-bit Haskell Platform.

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