November 2014 - Haskell-Cafe

DataKinds + KindSignatures question
by Wojtek Narczyński 27 Nov '14

27 Nov '14

Hello, The following code: > {-# LANGUAGE DataKinds #-} > {-# LANGUAGE KindSignatures #-} > > import Data.ByteString > > data CmdKind = GET | SET > > class Serialize (cmd :: CmdKind) where > serialize :: cmd -> ByteString Results in the following error: > wojtek@wojtek-desktop:~/src/he/snip$ ghc dk.hs > [1 of 1] Compiling Main ( dk.hs, dk.o ) > > dk.hs:9:17: > Expected a type, but ‘cmd’ has kind ‘CmdKind’ > In the type ‘cmd -> ByteString’ > In the class declaration for ‘Serialize’ Is this so by design, or should I write it in a different way, or is it just not implemented? -- Kind regards, Wojtek N.

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Re: [Haskell-cafe] Replacing IO with pure values
by Julian Arni 26 Nov '14

26 Nov '14

Besides, the aforementioned IOSpec, I would also suggest taking a look at "Purify Code Using Free Monads" [0], which gives a nice introduction to a general approach. If you're faced with library functions (i.e., ones you haven't written yourself) that do IO, it can become a little onerous to mock them all, though. In case the IO you're interested in testing is mostly stdin/stderr/stdout related, you could also look at the 'silently' package [1]. Finally, the 'knob' package [2] allows you to use in-memory file handles, so it helps if your tack ends up being passing Handles to pure functions (so that in the executable, those handles are stdin/stderr/stdout + files etc, but in the test they're just in-memory handles provided by 'knob'). 'System.Environment' has 'withArgs', which provides for the case of testing command-line args. [0] http://www.haskellforall.com/2012/07/purify-code-using-free-monads.html [1] https://hackage.haskell.org/package/silently-1.2.4.1 [2] http://hackage.haskell.org/package/knob-0.1.1/docs/Data-Knob.html

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Haskell Wiki engine.
by Nikita 26 Nov '14

26 Nov '14

Hello. It is apparently not possible to either register on the wiki or contact an administrator without an account, so I have to resort to writing here. What is the engine of the Haskell Wiki? Is it MediaWiki or what?

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suggestions about a library for numerical calculation
by felipe zapata 25 Nov '14

25 Nov '14

Hi all, I want to develop some tools on top of Vector and Repa, and I've wondered what tools could be useful that are not already on hmatrix. Any suggestions would be appreciated, Felipe Z.

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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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