- Haskell-Cafe - Haskell.org

What's up on Win 8.1 64 bit, Haskell Platform?
by james 29 Nov '14

29 Nov '14

This is a transcript of the very first session I have after installing Haskell Platform 7.8.3, 64 bit. Why is cabal-install continuing to be confused about the version installed/available? How can I tell cabal to STFU about it? C:\Program Files (x86)\Microsoft Visual Studio 11.0>cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install C:\Program Files (x86)\Microsoft Visual Studio 11.0>cabal install cabal-install Resolving dependencies... Downloading Cabal-1.20.0.2... Configuring Cabal-1.20.0.2... Building Cabal-1.20.0.2... Installed Cabal-1.20.0.2 Downloading cabal-install-1.20.0.3... Configuring cabal-install-1.20.0.3... Building cabal-install-1.20.0.3... Installed cabal-install-1.20.0.3 C:\Program Files (x86)\Microsoft Visual Studio 11.0>cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install C:\Program Files (x86)\Microsoft Visual Studio 11.0>cabal install cabal-install Resolving dependencies... Configuring cabal-install-1.20.0.3... Building cabal-install-1.20.0.3... Installed cabal-install-1.20.0.3 C:\Program Files (x86)\Microsoft Visual Studio 11.0>cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install C:\Program Files (x86)\Microsoft Visual Studio 11.0>cabal install cabal-install Resolving dependencies... Configuring cabal-install-1.20.0.3... Building cabal-install-1.20.0.3... Installed cabal-install-1.20.0.3 C:\Program Files (x86)\Microsoft Visual Studio 11.0>cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install C:\Program Files (x86)\Microsoft Visual Studio 11.0>

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Why is type level (+) not commutative?
by Dominic Steinitz 28 Nov '14

28 Nov '14

I have a type signature (for an extended Kalman filter as it happens thought that is not relevant) > outer :: forall m n p q . > (KnownNat m, KnownNat n, KnownNat p, KnownNat q, > (1 <=? n) ~ 'True, (1 <=? m) ~ 'True, > (1 <=? p) ~ 'True, (n + q) ~ p) => > R n -> Sq n -> > L m n -> Sq m -> > (R p -> R n) -> (R n -> Sq n) -> Sq n -> > [R m] -> [R q] -> > [(R n, Sq n)] and my function compiles. If I change (n+ q) ~ p to (q + n) ~ p then I get a type error > Could not deduce (p ~ (n + q)) > from the context (KnownNat m, > KnownNat n, > KnownNat p, > KnownNat q, > (1 <=? n) ~ 'True, > (1 <=? m) ~ 'True, > (1 <=? p) ~ 'True, > (q + n) ~ p) Is expressing commutativity not possible? Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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Re: [Haskell-cafe] Generating valid html
by oleg＠okmij.org 28 Nov '14

28 Nov '14

I have sent the following message to Haskell-Cafe five days ago; it seems to have disappeared. I'm resending because HSXML answers your second question, of ensuring complex content constraints. I'm not sure why do you need Monad specifically? HSXML documents are particular monoids, which is what we want from documents. Generally speaking, all we need is a way to create primitive documents and to compose them into bigger ones. There are many more composition operations than monadic bind. Not everything is a monad and not everything has to be a monad. Old message: Somewhat related is the HSXML library for generating valid XML and HTML. To be precise, the library is designed to express in the type system content model constraints such as: block-level elements like DIV are allowed only in the block-level context; one cannot put DIV within H1, for example. Some items may be polymorphic: for example, TITLE appears in HEAD, it can be an attribute and it can be an element. It can be rendered differently in each case. The same HSXML document may be rendered as HTML or XML (or something else entirely, e.g., PDF). http://okmij.org/ftp/Scheme/xml.html#typed-SXML

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--hyperlinks-in-source ?
by Johannes Waldmann 27 Nov '14

27 Nov '14

I am using 'standalone-haddock --hyperlink-source' (and I cannot praise this enough) but ... is there some way to get hyperlinks *in* the source? currently, I have to jump back and forth between doc (which has all the links) and source (which I want to navigate - actually, present for others so they can browse) I understand there's a nontrivial design space but I could imagine a basic functionality like: for each identifier i (type, class, constructor, function) that appears in the source, generate the link that haddock would produce for " -- | 'i' " (3.8.7. Hyperlinked Identifiers https://www.haskell.org/haddock/doc/html/ch03s08.html#idm140354810770608 ) - J.W.

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Re: [Haskell-cafe] Increasing Haskell modularity
by oleg＠okmij.org 27 Nov '14

27 Nov '14

The issue with local instances is very complicated one. First, they are sorely missed. However, they present quite a problem. Let us assume they are implemented and consider the following function f1 :: Int -> String f1 x = let instance Show Int where show _ = "one" in show x I think we all agree (f1 1) should evaluate to "one". However, given f2 :: Show a => a -> String f2 x = let instance Show Int where show _ = "one" in show x what should (f2 (1::Int)) return? One answer is that it should still return "one" -- that is, local instances should sort of form a closure (like local variables in the real closure). That is a good answer, but may lead to unpleasant surprises because the fact of the closure is not at all reflected in the type of the function. Consider data T = T f3 :: Show a => a -> String f3 x = let instance Show T where show _ = "T" in show x and take the closure semantics. The type says that to make (f3 T) well-typed, there has to be an instance Show T in scope. The user of f3 has no way to know of the private local instance that is closed over (short of examining the source code). Since the user knows there is no global instance T, the user attempts to define in. The user will be quite surprised to discover that (f3 T) is well-typed but their global instance Show T has no effect. Let us assume the opposite of the closure semantics -- local instances are not closed. So, local instances should behave sort of dynamically scoped variables (which are not captured in closures). Therefore, (f2 (1::Int)) should return "1". Such a choice is also problematic. Suppose GHC, seeing our call to f2, notices that the polymorphic f2 is called at type Int and decides, to save time on indirect dictionary calls, to specialize f2 to the type Int. So, GHC will generate a specialized instance of f2: f2_Int :: Int -> String f2_Int x = let instance Show Int where show _ = "one" in show x and we have already decided that (f2_Int 1), should return "one" rather than "1" (since f2_Int looks just like f1). Previously, specialization of a function was easy to understand: GHC may, at will, create a copy of a polymorphic function and instantiate it at a specific type. Then all calls to the polymorphic function at that specific type are replaced with the calls to the specialized function. The user can even do such a specialization themselves. With unclosed local instances, such a specialization changes the program behavior. We have to complicate the matters. Further, what should (f4 (1::Int)) return? f4 :: Show a => a -> String f4 x = let instance Show Int where show _ = "two" in f2 x Stefan Kaes, the person who introduced ``parametric overloading'' (type classes, in modern term) chose the ``closure semantics''. His system and prototype implementation had only local instances (and even local type classes). http://okmij.org/ftp/Computation/typeclass.html#Kaes Perhaps the most comprehensive treatment of local instances is the old paper D. Duggan and J. Ophel. Open and closed scopes for constrained genericity (PDF). Theoretical Computer Science, 275, 215-258, 2002. http://dl.acm.org/citation.cfm?id=570578 They propose a method to have both closure and non-closure semantics, differentiating based on so-called open and closed types. Our HW 2004 paper with Chung-chieh Shan proposed a way to elide the problem by restricting the local instances in such a way that the problem never arises. The `reflection' facility was used to implement such local instances. The idea was that local instances can be thought of as ordinary global instances; however, the scope of the type in their head is restricted. That is, rather than make the instances local we make local the type for which they are defined (or one of the types, for multi-parameter type classes). Local types can be emulated via first-class polymorphism. (BTW, if one can leave with unsafeCoerce, local instances can be much more efficiently implemented.) Gesh wrote: > * Give up on non-associated open type families. This I have > researched less well, but it seems to me that these have no use > case. Since one can make whatever instances one wishes for these type > families, one can't do anything useful with values of their instances. On the contrary, we found them very useful in defining interpreters that provide their own interpretation of not only primitives but also of types. http://okmij.org/ftp/gengo/NASSLLI10/index.html See, for example, http://okmij.org/ftp/gengo/NASSLLI10/Lambda.hs for partial evaluation and http://okmij.org/ftp/gengo/NASSLLI10/CFG.hs In the latter case, extensibility is particularly important as we do want to add further syntactic categories and their semantic interpretations.

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An opportunity in Singapore
by Camps, Tony 27 Nov '14

27 Nov '14

Good morning Haskellers! Hope all is well; I understand that the natural reaction to recruiters posting on these mailing lists would be spite and malice so I took some advice from a Haskell Developer I know. He said to cut the buzzword bingo, be transparent about remuneration and be honest about location. So here we go: - This is an opportunity to relocate to Singapore on a permanent basis to work for a leading Investment Bank (I think we all know who I am talking about...) - The role is working on the development & support of Cortex/ Mu(Haskell) based applications for the Process Automation and Efficiency team including a proprietary reporting tool. - The salary is set at SGD 105,000 basic with a good bonus and benefits – more details on request. All relocation is handled by the bank. This role is perfect for someone with a passion or FP and keen to have an adventure in Asia! I hope I have been brief but to the point – further details are available on request via return email: tcamps(a)teksystems.com<mailto:tcamps@teksystems.com> Look forward to hearing from you! Regards, Tony [http://www.allegisgroup.co.uk/images/tek/tek_sig_logo.gif] Tony Camps T +44 (0)207 997 1026 | M +44 (0)781 790 3478 [http://www.allegisgroup.co.uk/images/tek/tek_sig_linkedin.gif]<https://www.linkedin.com/company/teksystems> [http://www.allegisgroup.co.uk/images/tek/tek_sig_facebook.gif] <https://www.facebook.com/TEKsystemsJobs> [http://www.allegisgroup.co.uk/images/tek/tek_sig_twitter.gif] <https://twitter.com/TEKsystemsJobs> Our People Make IT Possible. Learn why.<http://www.teksystems.com> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Trading under the TEKsystems brand. Registered in: England: Allegis Group Ltd, OTV House, East Wing, Wokingham Road, Bracknell, Berkshire, RG42 1NG, United Kingdom. No. 2876353. Belgium: Allegis Group SA 9 Rue Guimard, 1040 Brussels, Belgium. No. 0543691037. France: Aston Carter International SAS. 2e étage, 40-42 Rue La Boétie, 75008 Paris, France. Co Reg/Siren 499 449 494 RCS Paris. Germany: TEKsystems ist ein Geschäftsbereich der Allegis Group GmbH mit Sitz in Frankfurt am Main. Amtsgericht Frankfurt Am Main, HRB 52009, Geschäftsführer Christopher Lee Hartman. Netherlands: Allegis Group B.V., Godebaldkwartier 365, Kantoor Janssoenborch, 3e Verdieping, 3511 DT Utrecht, Nederland. Company No. 14109133. Allegis Contracting B.V., Godebaldkwartier 365, Kantoor Janssoenborch, 3e Verdieping, 3511 DT Utrecht, Nederland. Company No. 34254913. Sweden: Aston Carter International Ltd UK Filial. Grev Turegatan 3, 114 46 Stockholm, Sweden. No. 516404-0163. This email is confidential and intended solely for the use of the individual to whom it is addressed. Any views or opinions presented are solely those of the author and do not necessarily represent those of Allegis Group, Allegis Group companies or Allegis Group brands. If you are not the intended recipient, be advised that you have received this email in error and that any use, dissemination, forwarding, printing or copying of this email is strictly prohibited. If you have received this email in error please notify postmaster(a)teksystems.com<mailto:postmaster@teksystems.com> ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- This electronic mail (including any attachments) may contain information that is privileged, confidential, and/or otherwise protected from disclosure to anyone other than its intended recipient(s). Any dissemination or use of this electronic mail or its contents (including any attachments) by persons other than the intended recipient(s) is strictly prohibited. If you have received this message in error, please notify us immediately by reply e-mail so that we may correct our internal records. Please then delete the original message (including any attachments) in its entirety. Thank you

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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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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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Monads: external questions
by Jeffrey Brown 22 Nov '14

22 Nov '14

As opposed to the internal logic of monads, how they work, I hope to start a discussion about their external logic: how and why to use monads. design ------ How do monads change the way one * thinks about a problem? * structures data? * refactors? * tests? Should I always be giving the monads a lot of cognitive bandwidth, because they reorder the way everything should be, or is it an investment with a high initial cognitive cost but requiring little maintenance thereafter? what is their common framework? ------------------------------- Monads let data reach farther than it otherwise would. Subjectively, they feel like a controlled way of violating encapsulation. Are there other, deeper or more specific, commonalities that explain why monads are a good way to implement exceptions, output, state, and perhaps other services? varieties --------- In practice, are all monads either for exceptions, state or output? If not, what other goals are monads well suited to? How should multiple contexts coexist? What's a simple Haskell program that uses more than one kind of monad in the same scope? Should, say, output and state be nestled in parallel into a three-attribute monad (the third variable being the type variable), or would it instead be better to create separate output and state monads? If each context should have a separate monad, is it a good idea to nest them, or somehow otherwise let them overlap, within a particular function or scope? temptations to err ------------------ I have a vague sense that a common pitfall is to use monads to do things in a procedural way that eliminates the benefits of a functional language, but I'm not sure I would recognize it if I were making that error. I feel like I could learn from a comparison of code that works and uses monads badly to code that does better.

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Re: [Haskell-cafe] Generating valid html
by Tobias Dammers 21 Nov '14

21 Nov '14

On Wed, Nov 19, 2014 at 03:45:37PM +0100, Wojtek Narczyński wrote: > On 19.11.2014 13:17, Tobias Dammers wrote: > >That's pretty much what my tamper library > >(http://hackage.haskell.org/package/tamper) is supposed to be. It's > >pretty rough around the edges still, and nowhere near as complete as > >Blaze itself, but it works well enough for basic HTML templating, and > >unlike Blaze, it is implemented as a monad transformer, which means you > >can integrate it with whatever monad stack you like. I couldn't find an > >existing solution to this myself, which is why I rolled my own. > Tobias, > > > I'm puzzled how your library can ensure that, for example <head> has only > one <title>, how / where are such rules enforced? Do you have a sample or > testcase? Ah, sorry for getting your hopes up. No, it doesn't ensure valid parent/child combinations or anything, it just does what basic Blaze-HTML does, but generalized to a monad transformer. > > > On the practical side, wouldn't you prefer to write, for example? > > {-#LANGUAGE Rank2Types #-} > > type ElementType = (Monad m, Ord t, IsString t) => TamperT t m () -> TamperT > t m () > > a, abbr, address, area, article, aside, audio :: ElementType > a = tag "a" > abbr = tag "abbr" > address = tag "address" > area = tagS "area" > article = tag "article" > aside = tag "aside" > audio = tag "audio" Good point; patches welcome in case I forget doing it myself ;)

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Generating valid html
by Wojtek Narczyński 20 Nov '14

20 Nov '14

Hello list, I have a question. How to create a DSL for HTML generation, that would (statically) allow only valid HTML to be generated? Parent - child relations, valid attributes only. Or does such a DSL already exist, perhaps? -- Kind regards, Wojtek Narczynski

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NYC Haskell Developer Position
by Christopher Lewis 20 Nov '14

20 Nov '14

Karamaan Group, a principal investment firm based in Manhattan, is looking for an outstanding software developer to develop tools for financial analysis and knowledge management. We are a growth oriented firm that values people who take a craftsman's pride in their work. Our ideal candidate is an experienced software developer with strong analytical, organizational, and communication skills. A candidate who demonstrates an intense focus on quality, but has the ability to recognize and make the tradeoffs that are a necessary part of day-to-day software development. Candidates should have at least a degree in a quantitative field and a keen interest in building robust and elegant computer programs. This is a high-impact, high-visibility position where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influences our workflow. The ideal candidate will have experience with Haskell, relational database technologies, and Java. All new development is performed in Haskell, but our legacy Java code base occasionally requires maintenance. Unlike most finance companies, our atmosphere is informal and intellectual. We don't require previous experience in finance or business, but knowledge in those areas is a plus. Karamaan Group is situated in Manhattan, we are an investment adviser to hedge funds and family offices. We value innovative thinking, encourage an entrepreneurial atmosphere, and enjoy spirited conversations. We also offer lunch, and every once in a while we go bowling. Please send your CV and cover letter to recruitment at karamaan dot com.

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Space leak in WAI 3.0 application
by Thomas Koster 20 Nov '14

20 Nov '14

Hello list, I am quite new to Haskell and I love the language and community, but I am frustrated by a space leak in a WAI 3.0 Application that for now just echoes the request entity back in the response. Specifically, I am having trouble understanding *why* I have the space leak. I intend to pass some or all of the request entity on to another web service whose response will influence the HTTP status code and headers of my service's response. At the moment, I am preparing the request entity as a lazy bytestring using lazy I/O just like Data.ByteString.Lazy.hGetContents does, at least until I can get around to learning pipes or conduit. When I use this technique to echo the request entity back in the response, it looks like two copies of the entire request entity are being accumulated in memory, presumably the original from the request and a copy for the response. The heap profile says it is all in "PINNED", which I am assuming are the bytestring buffers. However, the efficacy of this technique turns out to be irrelevant as I have been able to distill the problem down to a much simpler example: a WAI Application that responds with 100 MB of zeros read from /dev/zero using Data.ByteString.Lazy.hGetContents. I have prepared two variations that differ only in the composition of the operations. Both applications create identical responses, but version A accumulates the entire 100 MB entity in memory (heap profile shows a huge "PINNED" cost just over 100 MB in size) whereas version B streams the entity in constant space. Source code and a cabal file follow. I am using GHC 7.8.3. It is not necessary to make heap profiles - the symptoms are evident with "+RTS -s". ==== BEGIN Zeros.hs ==== {-# LANGUAGE OverloadedStrings #-} import Blaze.ByteString.Builder import Control.Concurrent import qualified Data.ByteString.Lazy as LBS import Data.Int import Network.HTTP.Types import Network.Wai import Network.Wai.Handler.Warp import System.IO -- | This version binds the large LBS of zeros /outside/ of the -- 'responseStream' body lambda. -- -- This version has the space leak. -- -- @ -- curl -v -o \/dev\/null localhost:3000\/zeros\/a -- @ zerosAppA :: Application zerosAppA _req respond = withZeros 100000000 $ \ largeLBS -> respond $ responseStream status200 [] $ \ write _flush -> write $ fromLazyByteString largeLBS -- | This version binds the large LBS of zeros /inside/ of the -- 'responseStream' body lambda. -- -- This version streams the response entity in constant space. -- -- @ -- curl -v -o \/dev\/null localhost:3000\/zeros\/b -- @ zerosAppB :: Application zerosAppB _req respond = respond $ responseStream status200 [] $ \ write _flush -> withZeros 100000000 $ \ largeLBS -> write $ fromLazyByteString largeLBS -- | Do something with /n/ bytes read lazily from @\/dev\/zero@. -- -- This part is common to both 'zerosAppA' and 'zerosAppB'. withZeros :: Int64 -> (LBS.ByteString -> IO a) -> IO a withZeros n f = withBinaryFile "/dev/zero" ReadMode $ \ h -> do zeros <- LBS.hGetContents h let largeLBS = LBS.take n zeros f largeLBS main :: IO () main = do _ <- forkIO $ run 3000 app putStrLn "Using port 3000. Press ENTER to exit..." _ <- getLine putStrLn "Exit." app :: Application app req respond = case pathInfo req of ["zeros", "a"] -> zerosAppA req respond ["zeros", "b"] -> zerosAppB req respond _ -> respond $ responseLBS status404 [] "Not found." ==== END Zeros.hs ==== ==== BEGIN zeros.cabal ==== name: zeros version: 0.1.0.0 build-type: Simple cabal-version: >=1.10 executable zeros main-is: Zeros.hs build-depends: base >=4.7 && <4.8, blaze-builder ==0.3.3.4, bytestring ==0.10.4.0, http-types ==0.8.5, wai ==3.0.2, warp ==3.0.2.3 default-language: Haskell2010 ghc-options: -Wall -rtsopts ==== END zeros.cabal ==== Why does version A not process the LBS in constant space? What in version A is preventing the GC from collecting the LBS chunks after they have been fed to Warp? What is it about version B that permits the LBS chunks to be collected? Although I believe the issue is not actually specific to WAI or Warp, I am unable to reproduce the space leak without them. But because I am new to Haskell, I suspect I have missed something obvious about lambda bindings, laziness (or strictness) of IO, and GC. Thanks. -- Thomas Koster

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Forcing recomputation
by Todd Wilson 20 Nov '14

20 Nov '14

I'm curious: is there a reliable way to force Haskell to recompute a function application instead of sharing the result of the forced thunk? For example, suppose I have a function f and two arguments e1 and e2, and I want to compare the time it takes to compute f e1 vs. f e2, but ghci reports that each takes 0.00 sec (i.e., less than 0.005 sec) to compute. The traditional approach would be to compute each value a large number of times so that the total time becomes significant, except that all of the ways I can think of to do that in Haskell don't actually recompute the function application, but rather compute it once and share, or otherwise optimize away the redundant computation. Todd Wilson, PhD Department of Computer Science California State University, Fresno

4 5

Haskell Weekly News: Issue 312
by Daniel Santa Cruz 20 Nov '14

20 Nov '14

Welcome to issue 312 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from October 26 to November 15, 2014 Quotes of the Week * barsoap: There's no place for half measures in overkill. Top Reddit Stories * Category Theory for Programmers: The Preface Domain: bartoszmilewski.com, Score: 141, Comments: 68 Original: [1] http://goo.gl/nTaVM5 On Reddit: [2] http://goo.gl/9cnWqN * Idris 0.9.15 released: partial evaluator, uniqueness types, library cleanups, and fancier docs. Domain: idris-lang.org, Score: 111, Comments: 55 Original: [3] http://goo.gl/nL9Ptr On Reddit: [4] http://goo.gl/mkHVEl * HaskForce - The Haskell Plugin for IntelliJ IDEA Domain: carymrobbins.github.io, Score: 104, Comments: 34 Original: [5] http://goo.gl/Oa0jBA On Reddit: [6] http://goo.gl/W2jrFW * Haskell for all: How to desugar Haskell code Domain: haskellforall.com, Score: 97, Comments: 50 Original: [7] http://goo.gl/1ZfiyP On Reddit: [8] http://goo.gl/hK3xTo * Functional programming and condescension Domain: superginbaby.wordpress.com, Score: 87, Comments: 147 Original: [9] http://goo.gl/erVkNx On Reddit: [10] http://goo.gl/ncQcxX * Quick Reminder to be Courteous Domain: self.haskell, Score: 85, Comments: 39 Original: [11] http://goo.gl/pmEODB On Reddit: [12] http://goo.gl/pmEODB * Category Theory Applied to Functional Programming Domain: eafit.edu.co, Score: 84, Comments: 14 Original: [13] http://goo.gl/ax59vm On Reddit: [14] http://goo.gl/xZYksD * New Book - Game Programming in Haskell Domain: leanpub.com, Score: 84, Comments: 31 Original: [15] http://goo.gl/hAqaaU On Reddit: [16] http://goo.gl/XePpcJ * "hasql" is up to 2x and 7x faster than "postgresql-simple" and "HDBC" Domain: nikita-volkov.github.io, Score: 72, Comments: 22 Original: [17] http://goo.gl/YwU4OQ On Reddit: [18] http://goo.gl/fnfFAC * Typing Haskell in Haskell (markdownified with syntax highlighting and updated links) Domain: gist.github.com, Score: 68, Comments: 3 Original: [19] http://goo.gl/U6Jnvr On Reddit: [20] http://goo.gl/QeLcak * Using Haskell at Work Domain: self.haskell, Score: 66, Comments: 66 Original: [21] http://goo.gl/265kRm On Reddit: [22] http://goo.gl/265kRm * Category: The Essence of Composition (First section of the Category Theory for Programmers) Domain: bartoszmilewski.com, Score: 60, Comments: 37 Original: [23] http://goo.gl/pdwr4X On Reddit: [24] http://goo.gl/AtYhI3 * PureScript 0.6 released, plus new website Domain: github.com, Score: 55, Comments: 23 Original: [25] http://goo.gl/TSuiEM On Reddit: [26] http://goo.gl/ZRBCHz * Tomatoes are a subtype of vegetables Domain: blog.ezyang.com, Score: 53, Comments: 58 Original: [27] http://goo.gl/1evzHU On Reddit: [28] http://goo.gl/qiiwSS * The Guts of a Spineless Machine Domain: jozefg.bitbucket.org, Score: 49, Comments: 11 Original: [29] http://goo.gl/KxW1iq On Reddit: [30] http://goo.gl/CNQbwk * A Large Scale Study of Programming Languages and Code Quality in Github Domain: macbeth.cs.ucdavis.edu, Score: 47, Comments: 48 Original: [31] http://goo.gl/TEoSBV On Reddit: [32] http://goo.gl/18UfCM * ghci-ng - GHCi plus extra goodies Domain: github.com, Score: 45, Comments: 14 Original: [33] http://goo.gl/g6VMiK On Reddit: [34] http://goo.gl/5Mg7n8 Top StackOverflow Questions * Haskell's type checker is allowing a very wrong type replacement, and the program still compiles votes: 68, answers: 2 Read on SO: [35] http://goo.gl/ikcCmv * Subsumption in polymorphic types votes: 29, answers: 1 Read on SO: [36] http://goo.gl/clKuju * Rewriting as a practical optimization technique in GHC: Is it really needed? votes: 23, answers: 3 Read on SO: [37] http://goo.gl/MNNuhW * Is there an unsigned integer type that will warn about negative literals? votes: 21, answers: 1 Read on SO: [38] http://goo.gl/SmRO3t * How can I make GHCI release memory votes: 19, answers: 1 Read on SO: [39] http://goo.gl/U3qroZ * How much of Pascal's triangle does this evaluate? votes: 15, answers: 1 Read on SO: [40] http://goo.gl/PsaQ3X * Why is super-compilation not implemented more prevalent? votes: 14, answers: 3 Read on SO: [41] http://goo.gl/MFNsrE Until next time, [42]+Daniel Santa Cruz References 1. http://bartoszmilewski.com/2014/10/28/category-theory-for-programmers-the-p… 2. http://www.reddit.com/r/haskell/comments/2kkrd3/category_theory_for_program… 3. http://www.idris-lang.org/idris-0-9-15-released/ 4. http://www.reddit.com/r/haskell/comments/2kfosg/idris_0915_released_partial… 5. http://carymrobbins.github.io/intellij-haskforce/ 6. http://www.reddit.com/r/haskell/comments/2kvzuz/haskforce_the_haskell_plugi… 7. http://www.haskellforall.com/2014/10/how-to-desugar-haskell-code.html 8. http://www.reddit.com/r/haskell/comments/2kf61f/haskell_for_all_how_to_desu… 9. http://superginbaby.wordpress.com/2014/10/28/suddenly-the-opposite-appeared/ 10. http://www.reddit.com/r/haskell/comments/2klj3b/functional_programming_and_… 11. http://www.reddit.com/r/haskell/comments/2lfofw/quick_reminder_to_be_courte… 12. http://www.reddit.com/r/haskell/comments/2lfofw/quick_reminder_to_be_courte… 13. http://www1.eafit.edu.co/asicard/pubs/cain-screen.pdf 14. http://www.reddit.com/r/haskell/comments/2kowzu/category_theory_applied_to_… 15. https://leanpub.com/gameinhaskell 16. http://www.reddit.com/r/haskell/comments/2m253y/new_book_game_programming_i… 17. http://nikita-volkov.github.io/hasql-benchmarks/ 18. http://www.reddit.com/r/haskell/comments/2lwx9y/hasql_is_up_to_2x_and_7x_fa… 19. https://gist.github.com/chrisdone/0075a16b32bfd4f62b7b 20. http://www.reddit.com/r/haskell/comments/2lo881/typing_haskell_in_haskell_m… 21. http://www.reddit.com/r/haskell/comments/2lcx8c/using_haskell_at_work/ 22. http://www.reddit.com/r/haskell/comments/2lcx8c/using_haskell_at_work/ 23. http://bartoszmilewski.com/2014/11/04/category-the-essence-of-composition/ 24. http://www.reddit.com/r/haskell/comments/2la0cx/category_the_essence_of_com… 25. https://github.com/purescript/purescript/releases/tag/v0.6.0 26. http://www.reddit.com/r/haskell/comments/2lt7oc/purescript_06_released_plus… 27. http://blog.ezyang.com/2014/11/tomatoes-are-a-subtype-of-vegetables/ 28. http://www.reddit.com/r/haskell/comments/2meyxf/tomatoes_are_a_subtype_of_v… 29. http://jozefg.bitbucket.org/posts/2014-10-28-stg.html 30. http://www.reddit.com/r/haskell/comments/2kswnp/the_guts_of_a_spineless_mac… 31. http://macbeth.cs.ucdavis.edu/lang_study.pdf 32. http://www.reddit.com/r/haskell/comments/2lb3oz/a_large_scale_study_of_prog… 33. https://github.com/chrisdone/ghci-ng 34. http://www.reddit.com/r/haskell/comments/2l9bvb/ghcing_ghci_plus_extra_good… 35. http://stackoverflow.com/questions/26770247/haskells-type-checker-is-allowi… 36. http://stackoverflow.com/questions/26806653/subsumption-in-polymorphic-types 37. http://stackoverflow.com/questions/26827663/rewriting-as-a-practical-optimi… 38. http://stackoverflow.com/questions/26574302/is-there-an-unsigned-integer-ty… 39. http://stackoverflow.com/questions/26712188/how-can-i-make-ghci-release-mem… 40. http://stackoverflow.com/questions/26729146/how-much-of-pascals-triangle-do… 41. http://stackoverflow.com/questions/26605431/why-is-super-compilation-not-im… 42. https://plus.google.com/105107667630152149014/about

1 0

Re: [Haskell-cafe] Generating valid html
by Tobias Dammers 19 Nov '14

19 Nov '14

On Wed, Nov 19, 2014 at 03:45:37PM +0100, Wojtek Narczyński wrote: > On 19.11.2014 13:17, Tobias Dammers wrote: > >That's pretty much what my tamper library > >(http://hackage.haskell.org/package/tamper) is supposed to be. It's > >pretty rough around the edges still, and nowhere near as complete as > >Blaze itself, but it works well enough for basic HTML templating, and > >unlike Blaze, it is implemented as a monad transformer, which means you > >can integrate it with whatever monad stack you like. I couldn't find an > >existing solution to this myself, which is why I rolled my own. > Tobias, > > > I'm puzzled how your library can ensure that, for example <head> has only > one <title>, how / where are such rules enforced? Do you have a sample or > testcase? Ah, sorry for getting your hopes up. No, it doesn't ensure valid parent/child combinations or anything, it just does what basic Blaze-HTML does, but generalized to a monad transformer. I considered writing something that could do this, but figured that 1) it would be pretty tricky to get right, and 2) it would be quite restricting, for little gain. The kind of incorrectness this would prevent isn't that common, and when it does happen, it rarely causes severe breakage, meanwhile locking things down would make it harder to adapt your code to the moving target that is HTML. So I dropped the idea. One thing I might want at some point though is checking for valid attributes, somehow. Not sure how to go about that yet though, not without going completely crazy with types, anyway. > > > On the practical side, wouldn't you prefer to write, for example? > > {-#LANGUAGE Rank2Types #-} > > type ElementType = (Monad m, Ord t, IsString t) => TamperT t m () -> TamperT > t m () > > a, abbr, address, area, article, aside, audio :: ElementType > a = tag "a" > abbr = tag "abbr" > address = tag "address" > area = tagS "area" > article = tag "article" > aside = tag "aside" > audio = tag "audio" Good point; patches welcome in case I forget doing it myself ;)

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Re: [Haskell-cafe] Parsing error in Haskell2010
by Stuart A. Kurtz 19 Nov '14

19 Nov '14

Dear Andy, > If that is the actual indentation in the file (and not just one or both of our mail clients breaking it), the problem is that the `let` line needs to start in the same column as the lines with `gen` and `putStr`. The gen, let, and putStr were properly aligned. After all, it compiled as Haskell98 ;-). > (Also, the inner do-block needs to be indented strictly further than the word `log`, in case it isn't in the original file.) This seems to be the issue. Haskell98 didn't require this, Haskell2010 does, and this seems less desirable to me. Isn't it reasonable to assume that the it's the do that dominates syntactically here, and not the let? Peace, Stu

3 4

Dependently typed fold in GHC/Haskell
by Christiaan Baaij 19 Nov '14

19 Nov '14

Dear Cafe, I'm trying to convert the following Idris code, which compiles: > gfold : {p : Nat -> Type} > -> ((l : Nat) -> a -> p l -> p (S l)) > -> p 0 -> Vect k a -> p k > gfold _ z Nil = z > gfold f z ((::) {n} x xs) = f n x (gfold f z xs) > > gconcat : {m : Nat} -> Vect n a -> Vect m a -> Vect (n + m) a > gconcat {m} xs ys = gfold {p} f ys xs > where > p : Nat -> Type > p l = Vect (l + m) a > > f : (l : Nat) -> a -> Vect (l + m) a -> Vect (S (l + m)) a > f _ x r = x :: r To Haskell + various GHC extensions, which gives me type errors: > {-# LANGUAGE DataKinds, GADTs, KindSignatures, TypeOperators, RankNTypes, > ScopedTypeVariables, TypeFamilies #-} > > import Data.Proxy > > data Nat = Z | S Nat > > type family Plus (x :: Nat) (y :: Nat) :: Nat where > Plus Z y = y > Plus (S n) y = S (Plus n y) > > data Vec :: * -> Nat -> * where > Nil :: Vec a Z > (:>) :: a -> Vec a n -> Vec a (S n) > > gfold :: Proxy (p :: Nat -> *) > -> (forall l . Proxy l -> a -> p l -> p (S l)) > -> p Z -> Vec a k -> p k > gfold Proxy f z Nil = z > gfold Proxy f z (x :> xs) = f Proxy x (gfold Proxy f z xs) > > type family P (a :: *) (m :: Nat) (l :: Nat) :: * where > P a m l = Vec a (Plus l m) > > gconcat :: forall n m a . Vec a n -> Vec a m -> Vec a (Plus n m) > gconcat xs ys = gfold p f ys xs > where > p :: Proxy (P a m) > p = Proxy > > f :: forall (l :: Nat) . Proxy l -> a -> Vec a (Plus l m) > -> Vec a (S (Plus l m)) > f _ x r = x :> r I get the following errors: > VecTest.hs:25:25: > Couldn't match type ‘l’ with ‘Plus l m’ > ‘l’ is a rigid type variable bound by > a type expected by the context: > Proxy l -> a -> Vec a l -> Vec a ('S l) > at VecTest.hs:25:17 > Expected type: Proxy l -> a -> Vec a l -> Vec a ('S l) > Actual type: Proxy l > -> a -> Vec a (Plus l m) -> Vec a ('S (Plus l m)) > Relevant bindings include > p :: Proxy (P a m) (bound at VecTest.hs:28:5) > f :: forall (l :: Nat). > Proxy l -> a -> Vec a (Plus l m) -> Vec a ('S (Plus l m)) > (bound at VecTest.hs:32:5) > ys :: Vec a m (bound at VecTest.hs:25:12) > gconcat :: Vec a n -> Vec a m -> Vec a (Plus n m) > (bound at VecTest.hs:25:1) > In the second argument of ‘gfold’, namely ‘f’ > In the expression: gfold p f ys xs > > VecTest.hs:25:27: > Couldn't match type ‘m’ with ‘'Z’ > ‘m’ is a rigid type variable bound by > the type signature for > gconcat :: Vec a n -> Vec a m -> Vec a (Plus n m) > at VecTest.hs:24:21 > Expected type: Vec a 'Z > Actual type: Vec a m > Relevant bindings include > p :: Proxy (P a m) (bound at VecTest.hs:28:5) > f :: forall (l :: Nat). > Proxy l -> a -> Vec a (Plus l m) -> Vec a ('S (Plus l m)) > (bound at VecTest.hs:32:5) > ys :: Vec a m (bound at VecTest.hs:25:12) > gconcat :: Vec a n -> Vec a m -> Vec a (Plus n m) > (bound at VecTest.hs:25:1) > In the third argument of ‘gfold’, namely ‘ys’ > In the expression: gfold p f ys xs Is this kind of type-level programming just not yet possible in GHC? Or am I simply doing it wrong? Cheers, Christiaan

3 3

How to reverse ghc encoding of command line arguments
by Ben Franksen 19 Nov '14

19 Nov '14

I have a question about how to reverse the text encoding as done by ghc and the base library for stuff that comes from the command line or the environment. Assume the user's environment specifies a non-Unicode locale, e.g. some latin encoding. In this case, the String we get from e.g. System.Environment.getArgs does *not* contain the Unicode code points of the characters the user has entered. Instead the input bytes are mapped one-to- one to Char. This has probably been done for compatibility reasons, and I do not want to discuss this choice here. Rather, I want to find out how I can convert such a string back to some proper Unicode representation, for instance in order to store the value in a file with a defined encoding such as utf-8. This should be done in a generic way, i.e. without making ad-hoc assumptions about what the user's encoding might be. There is the iconv package. However, it takes ByteString as input and output and it also requires that I give it the encoding as input. How do I find out which is this encoding? On the command line I could simply do ben@sarun[1]: ~ > locale charmap ISO-8859-1 Is there a Haskell function that does the equivalent or do I have to use getEnv "LC_CTYPE", then parse the result? Let's assume I get this to work, so now I have a String that represents the user's encoding, such as "ISO-8859-1". Now, in order to use iconv, I have to convert the string I got via getArgs into a ByteString. But to do that properly, I would have to decode it according to the user's current locale, which is exactly what I want to achieve in the first place. How do I break this cycle? Perhaps it is simpler to write our own getArgs/getEnv functions and directly convert the data we get from the system to a proper (Unicode) String? Any suggestions would be highly appreciated. Cheers Ben -- "Make it so they have to reboot after every typo." -- Scott Adams

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Call for Participation: BOB Conference in Berlin (Jan 23, 2015)
by Torsten Grust 19 Nov '14

19 Nov '14

================================================================ BOB 2015 Conference "What happens if we simply use what's best?" January 23.2015 Berlin http://bobkonf.de/2015/ Program: http://bobkonf.de/2015/programm.html Registration: http://bobkonf.de/2015/registration.html ================================================================ BOB is the conference for developers, architects and decision-makers to explore technologies beyond the mainstream in software development, and to find the best tools available to software developers today. Our goal is for all participants of BOB to return home with new insights that enable them to improve their own software development experiences. The program features 14 talks and 8 tutorials on current topics: http://bobkonf.de/2015/programm.html The subject range of talks includes functional programming, microservices, package management, and data management. The tutorials feature introductions to Erlang, Haskell, Swift, and ClojureScript, and their applications. Anil Madhavapeddy will hold the keynote talk - about unikernels and functional programming. Registration is open online: http://bobkonf.de/2015/registration.html NOTE: The early-bird rates expire on Dec. 19, 2014! BOB cooperates with the :clojured conference on the following day. There is a registration discount available for participants of both events. http://www.clojured.de/

1 0

Code review: list-like datatype with constant time appending based on function composition.
by Nicola Gigante 19 Nov '14

19 Nov '14

Hello everybody I would kindly ask you to take a look at this code and tell me your opinions: https://gist.github.com/nicola-gigante/43533ce907f88a6aba16 <https://gist.github.com/nicola-gigante/43533ce907f88a6aba16> The file try to implement a list-like data type, based on function composition to achieve a constant-time appending operation (in contrast to linear (++)). I’m sure this is something old. If you have material that covers an approach like this, please tell me. I’ve tried to implement all the type classes that I currently use, and it seems to work. Also, all the operations of those typeclasses seems to preserve the original laziness properties, despite I was expecting the contrary. A list in this code is basically a function that’s constructed by composing a lot of partially applied cons operators. To obtain a plain list from it, it’s sufficient to apply the empty list at the end. So I was expecting that to evaluate even the first element of the list, the code would have to evaluate all the function composition chain to obtain the entire function to being able to apply the final argument. It seems not true, since this code can still manipulate infinite lists. For example: > fmap (*2) . fromList $ repeat 42 This code prints an infinite stream of “84” numbers in GHCI. It doesn’t wait to have the entire list to then print it out. In my understanding that means I’m building the list sufficiently lazily, or am I missing something? In the case this actually means my datatype is lazy enough, why isn’t it more strict instead, following the above reasoning? Is it the case that haskell laziness extends to partially constructed functions? Also, the implementation is very simple. Basically I use the list functions and go back and forth with toList/fromList. It all seems too easy… am I wasting a lot of time or space somewhere? Thank you a lot for your help, Greetings, Nicola

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NOTE: the primary webserver is going down for immediate maintenance.
by Austin Seipp 19 Nov '14

19 Nov '14

Hello *, The primary haskell.org domain, www.haskell.org, is hosted on a system which seems to have lost one of its RAID disks completely. We were planning on moving this machine in the next few weeks to new infrastructure, but we are now expediting this plan and will be doing it ASAP. As we move this server, both the webserver and the mailing system will be going down. Please don't be alarmed if your emails aren't delivered or things go quiet. Many services will continue to work, but we do realize this will be upsetting for many. You can follow the progress on #haskell-infrastructure on Freenode, and see updates on https://status.haskell.org If you need to download something like a GHC binary or Haskell Platform package, you can use https://downloads.haskell.org in the mean time, which is a new service we were hoping to announce more officially soon, but is already working today. Unfortunately we cannot give an expected time of completion for the move, but we'll try to keep people well informed through IRC or something like Reddit. Thanks -- Regards, Austin Seipp, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

2 1

[ANNOUNCE] New release of SBV (v3.2)
by Levent Erkok 19 Nov '14

19 Nov '14

I'm pleased to announce v3.2 release of SBV, providing facilities for SMT based theorem proving in Haskell. This is mainly a bug-fix/maintenance release, together with one new feature: SBV now implements 'sAssert', which works similar to 'assert' calls in Haskell or other languages to check that certain program invariants always hold. Except, SBV allows the condition to be symbolic, and all violations will be caught and reported as the underlying program is symbolically executed. (For instance, this facility can be useful in other DSL's built on top of SBV to make sure the code they generate never divides by 0, or indexes out of bounds.) Full release notes: https://github.com/LeventErkok/sbv /blob/master/CHANGES.md SBV web page: http://leventerkok.github.io/sbv/ As usual, bug reports and feedback are most welcome! -Levent.

1 0

Parsing error in Haskell2010
by Stuart A. Kurtz 18 Nov '14

18 Nov '14

Dear Cafe, I recently ran into a Haskell2010 parsing error with code that parses just fine under Haskell98, and it seems to me that this might be an unintended consequence. Briefly, I have runModel :: StdGen -> Model () -> Result where Model is a monadic type, and then my main was main :: IO () main = do gen <- getStdGen let log = runModel gen $ do initialize 72 report replicateM_ 50 $ do replicateM_251 migrate report putStr . format $ log Haskell 2010 doesn't like the additional level of indentation after the "let log" line. If I pull the content of the "do" into a separate definition, so this becomes let log = runModel gen simulate the parsing error goes away. Questions: 1) Is this intended? 2) Can this code be formatted in a way to make Haskell 2010 happy? Many thanks. Peace, Stu

3 2

ISO 8601 Date time format without old-locale?
by Bram Neijt 18 Nov '14

18 Nov '14

I want to do ISO8601 formatting of a date with a numeric zone offset and I can't seem to find a way of doing it without requiring the old-locale package[1]. What is the current method of obtaining and formatting time representation if old-locale is old? Greetings, Bram [1] A sketch of the code using old-locale: -- 2014-11-18T06:37:04+00:00 iso8601DateTimeFormat :: String iso8601DateTimeFormat = "%FT%T%z" main = do now <- getCurrentTime putStrLn $ formatTime defaultTimeLocale iso8601DateTimeFormat now Which requires "defaultTimeLocale" from the old-locale package.

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The next 7000... abstractions to learn!
by Gautier DI FOLCO 18 Nov '14

18 Nov '14

Hi all, I'm using Haskell since nearly three years now and I think I have well understood that types are the most important part of our programs. I have looked all around to learn as many datatypes/typeclasses as possible. But I suffer for a lack of direction to pursue my learning. I have well understood Prelude's one and some random ones (include someones for Category Theory), but I can't handle Kmett's code. Are there some intermediates abstractions I can learn? (For example in some librairies). Thanks by advance for your help. Regards.

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What could possibly be the reason of new GC-related problems after upgrading from ghc-7.6 to ghc-7.8.3 ?
by ducis 18 Nov '14

18 Nov '14

Recently I revived one of my projects set aside a year ago. As I'm an archlinux user, the ghc version has now inevitably become 7.8.3. Unfortunatly, the program starts to somehow have very bad GC performance (less than 50% productivity) . While I don't have the statistics of the older versions I'm pretty sure the current version has become visually less responsive. And if the user performs a series of actions in quick succession, the heap is not garbage collected in time and overflows. If the user performs the same actions with longer pauses then everything is fine. The GC seems to have become more loose in 7.8. There is quite a large underdocumented pile of code so I don't think posting it here would help. But can anyone give me a hint about what could be the reason?

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How to Kill Haskell
by Eric Crockett 18 Nov '14

18 Nov '14

For the purposes of profiling a partially evaluated program, I'm interested in knowing the best way to terminate a GHC program. This is useful for profiling programs that take a long time to run, possibly as long as forever. It appears that *if* the program can be terminated with a single ^c, then profiling information is written to output. Sometimes, a single ^c doesn't kill the program, at not before I get impatient and hit ^c again. Usually two ^c will kill the program, but then no profiling data is written to output. As a simple example, Main.hs fib n = fib (n - 1) + fib (n - 2) main = print $ fib 100 Compiling with -prof and running with +RTS -p, I can kill this program with a *single* ^c in the first approximately 10 seconds of execution, but after that only two ^c will do the job. Looking at my resources, this change appears to coincide with the program using all of my physical memory and moving to swap space, however that could also be coincidental. Why does ^c work sometimes, but not other times for the same program? What is the easiest way to ensure that profiling data will get printed when the program does not terminate on its own?

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OS X 10.10
by Hans Aberg 18 Nov '14

18 Nov '14

The URL [1] says the platform “Works even on 10.10 beta!”, however, OS X 10.10.1 has just been released. Is there anyone out there who does not run the beta? :-) 1. https://www.haskell.org/platform/mac.html

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[ANN] elevator: getting to a top of a monad transformer stack
by Fumiaki Kinoshita 17 Nov '14

17 Nov '14

Hello everybody, I created an elevator to navigate a monad transformer stack, liberating us from creating a Monad* class and instances. The key function, elevate, can be whatever you need: elevate :: IO a -> IO a elevate :: IO a -> StateT s IO a elevate :: State s a -> StateT s IO a elevate :: IO a -> MaybeT (StateT s IO) a If you want to elevate actions above your monad, just write instance Tower YourMonad to declare that YourMonad is one-storied. Hackage: http://hackage.haskell.org/package/elevator-0.1/docs/Control-Elevator.html Cheers Fumiaki Kinoshita

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[Haskell] ANNOUNCE: Haskell Communities and Activities Report (27th ed., November 2014)
by Mihai Maruseac 17 Nov '14

17 Nov '14

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (27th edition, November 2014) is now available, in PDF and HTML formats: http://haskell.org/communities/11-2014/report.pdf http://haskell.org/communities/11-2014/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of April 2015: target deadline for contributions to the May 2015 edition of the HC&A Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in October (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Mihai Maruseac and Alejandro Serrano Mena <hcar at haskell.org> -- Mihai Maruseac (MM) "If you don't know, the thing to do is not to get scared, but to learn." -- Atlas Shrugged.

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Using wreq Session handling in a monad transformer stack
by Cody Goodman 17 Nov '14

17 Nov '14

Could I somehow add a Wreq.Session.Session to my monad transformer stack? Should I use something other than Wreq? How would I create a default Session, Wreq doesn't seem to export the constructor. Basically I want to achieve Sessions/cookie handling. lpaste of my code: http://lpaste.net/114405 my full code (for those who want to view in email): {-# LANGUAGE OverloadedStrings #-} module Network.Scraper.State where import Control.Lens ((^.)) import Control.Monad import Control.Monad.IO.Class (liftIO) import qualified Control.Monad.Trans.State.Strict as ST import qualified Data.ByteString.Lazy as LBS import Data.Maybe (fromJust, fromMaybe) import Data.Monoid import qualified Data.Text as T import qualified Data.Text.IO as TIO import Network.Wreq (FormParam (..)) import qualified Network.Wreq as Wreq import Network.Wreq.Session (Session (..), withSession) import qualified Network.Wreq.Session as Sesh import Network.Wreq.Types import Safe import Text.HTML.DOM (parseLBS) import Text.XML.Cursor import qualified Text.XML.Cursor.Generic as CG data ScraperState = PS { currentOptions :: Wreq.Options , currentHtml :: LBS.ByteString , currentCursor :: Maybe Cursor , currentSession :: Session } deriving (Show) type Scraper = ST.StateT ScraperState IO toCursor = fromDocument . parseLBS initialSt = PS { currentOptions = Wreq.defaults , currentHtml = ("" :: LBS.ByteString) , currentCursor = Nothing -- , currentSession = ... how do I get a Session? Wreq doesn't seem to export this type } setCurrentOptions :: Wreq.Options -> Scraper () setCurrentOptions o = do scraper <- ST.get ST.put $ scraper { currentOptions = o } -- getCurrentPage :: Shpider Page getCurrentCursor :: Scraper (Maybe Cursor) getCurrentCursor = do scraper <- ST.get return $ currentCursor scraper getCurrentSession :: Scraper (Session) getCurrentSession = do scraper <- ST.get return $ currentSession scraper setCurrentSession :: Session -> Scraper () setCurrentSession s = do scraper <- ST.get ST.put $ scraper { currentSession = s} setCurrentCursor :: Cursor -> Scraper ( ) setCurrentCursor c = do scraper <- ST.get ST.put $ scraper { currentCursor = Just c } setCurrentHtml :: LBS.ByteString -> Scraper () setCurrentHtml html = do scraper <- ST.get ST.put $ scraper { currentHtml = html } runScraper :: Scraper a -> IO a runScraper k = evalScraperWith k initialSt evalScraperWith :: Scraper a -> ScraperState -> IO a evalScraperWith k s = withSession $ \sesh -> do -- set the current session to the mutable session variable return $ setCurrentSession sesh ST.evalStateT k s formShortInfo' f = formInfo' where go Nothing = "N/A" go (Just x) = x formInfo = (headMay . attribute "name" $ f, headMay . attribute "action" $ f) formInfo' = (\(x,y) -> (go x, go y)) formInfo ppTuple :: (T.Text, T.Text) -> T.Text ppTuple = \(x,y) -> "[" <> x <> "]" <> ": " <> y -- move to ../Spider.hs printFormNames :: Scraper () printFormNames = do c <- getCurrentCursor let c' = fromMaybe (error "No cursor set") c forms = c' $// element "form" formInfo = map (ppTuple . formShortInfo') forms liftIO $ mapM_ (TIO.putStrLn) formInfo getFormByName :: T.Text -> Scraper [Cursor] getFormByName name = do c <- getCurrentCursor let c' = fromMaybe (error "No cursor set") c return $ c' $// element "form" >=> attributeIs "name" name get :: String -> Scraper (LBS.ByteString) get url = do r <- liftIO $ Wreq.get url let html = r ^. Wreq.responseBody setCurrentHtml html setCurrentCursor (toCursor html) return html post :: Postable a => String -> a -> Scraper (LBS.ByteString) post url params = do r <- liftIO $ Wreq.post url params let html = r ^. Wreq.responseBody setCurrentHtml html setCurrentCursor (toCursor html) return html test :: Scraper () test = do get "https://www.google.com" >> printFormNames

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Re: [Haskell-cafe] data analysis question
by Dominic Steinitz 17 Nov '14

17 Nov '14

I’d much prefer that. I really dislike the google group experience. I’ll drop Austin a note. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com On 15 Nov 2014, at 15:42, Ben Gamari <ben(a)smart-cactus.org> wrote: > On November 15, 2014 1:54:38 AM EST, Dominic Steinitz <dominic(a)steinitz.org> wrote: >> Mark Fredrickson <mark.m.fredrickson <at> gmail.com> writes: >> >>> >>> Is there a mailing list for statistics/analytics/simulation/numerical >>> analysis/etc. using Haskell? If not, I purpose we start one. (Not to >>> take away from general discussion, but to provide a forum to hash out >>> these issues among the primary user base). >> >> Sadly not but I think there are sufficient numbers of people >> interested in this subject that it is probably worth setting one up. I >> really don't like the google group experience but maybe that is the >> best place to start? >> > I agree that this would be a worthwhile forum to have. Why not just stay with Haskell.org infrastructure? I'm sure Austin would set up a mailing list for the cause. > > Cheers, > > - Ben > > >

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Call for Presentations: Compose Conference [New York, Jan 30-Feb 1]
by Gershom B 16 Nov '14

16 Nov '14

Compose is a new conference for typed functional programmers, focused specifically on Haskell, OCaml, F#, and related technologies. It will be held in New York from Jan 30-Feb 1, and registration is opening shortly. http://www.composeconference.org/ Below is our call for presentations. We recognize the deadline is tight, so feel free to submit proposals and ideas on the less-polished side. Call for Presentations and Speakers. http://www.composeconference.org/call/index.html --- The audience for Compose is Haskell, OCaml, or F# developers who are looking to increase their skills or learn new technologies and libraries. Presentations should be aimed at teaching or introducing new ideas or tools. We are also interested in presentations aiming at taking complex concepts, such as program derivation, and putting them into productive use. However proposals on anything that you suspect our audience may find interesting are welcome. The following are some of the types of talks we would welcome: Library/Tool Talks — Exploring the uses of a powerful toolkit or library, be it for parsing, testing, data access and analysis, or anything else. Production Systems — Experience reports on deploying functional techniques in real systems; insights revealed, mistakes made, lessons learned. Theory made Practical — Just because it’s locked away in papers doesn’t mean it’s hard! Accessible lectures on classic results and why they matter to us today. Such talks can include simply introducing the principles of a field of research so as to help the audience read up on it in the future; from abstract machines to program derivation to branch-and-bound algorithms, the sky’s the limit. We also welcome proposals for more formal tutorials for the Sunday unconference. Such tutorials should be aimed at a smaller audience of beginner-to-novice understanding, and ideally include hands-on exercises. The due date for submissions is November 30, 2014. We will send out notice of acceptance by 10 December. We prefer that submissions be via the EasyChair website (https://easychair.org/conferences/?conf=compose2015) Please suggest a title, and describe the topic you intend to speak on. Additional information may be included on both your expertise and the interesting elements of your topic, going on what might be included in a public abstract. Furthermore, if your abstract doesn't feel "final"—don't worry! We'll work with you to polish it up. If you want to discuss your proposal(s) before submitting, or to further nail down what you intend to speak on, please feel free to contact us at info(a)composeconference.org. We're happy to work with you, even if you are a new or inexperienced speaker, to help your talk be great. —Gershom

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How can I improve the pipes's performance with a huge file?
by zhangjun.julian 16 Nov '14

16 Nov '14

Dear cafe I have 2 file, I want zip the 2 file as couple, and then count each couple's repeat times? The file had more than 40M rows, I use pipe to write code as blow. When I test with 8768000 rows input, it take 30 secs When I test with 18768000 rows input, it take 74 secs But when I test with whole file (40M rows), it take more than 20 minutes and not finished yet. It take more than 9G memorys, and the disk is also busy all time. The result will less than 10k rows, so I had no idea why the memory is so huge. I had use the “http://hackage.haskell.org/package/visual-prof” to profile and improve the performance with the small file But I don’t know how to deal with the “hang” situation. Anyone can give me some help, Thanks. =================================== import System.IO import System.Environment import Pipes import qualified Pipes.Prelude as P import qualified Data.Map as DM import Data.List emptyMap = DM.empty::(DM.Map (String,String) Int) keyCount num = do readHandle1 <- openFile "dataByColumn/click" ReadMode readHandle2 <- openFile "dataByColumn/hour" ReadMode writeHadle <- openFile "output" AppendMode rCount num readHandle1 readHandle2 writeHadle hClose writeHadle hClose readHandle1 hClose readHandle2 mapToString::DM.Map (String,String) Int-> String mapToString m = unlines $ map eachItem itemList where itemList = DM.toList m eachItem ((x,y),i) = show x ++ "," ++ show y ++ "," ++ show i --rCount::Int -> [String] -> Handle->Handle -> IO() rCount num readHandle1 readHandle2 writeHadle = do rt <- P.fold (\x y -> DM.unionWith (+) x y) emptyMap id $ P.zipWith (\x y -> DM.singleton (x,y) 1) (P.fromHandle readHandle1) (P.fromHandle readHandle2) >-> P.take num hPutStr writeHadle $ mapToString rt main = do s<- getArgs let num = (read . head) s keyCount num

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data analysis question
by Tobias Pflug 15 Nov '14

15 Nov '14

Hi, just the other day I talked to a friend of mine who works for an online radio service who told me he was currently looking into how best work with assorted usage data: currently 250 million entries as a 12GB in a csv comprising of information such as which channel was tuned in for how long with which user agent and what not. He accidentally ran into K and Q programming language (*1) which apparently work nicely for this as unfamiliar as it might seem. This certainly is not my area of expertise at all. I was just wondering how some of you would suggest to approach this with Haskell. How would you most efficiently parse such data evaluating custom queries ? Thanks for your time, Tobi [1] (http://en.wikipedia.org/wiki/K_(programming_language) [2] http://en.wikipedia.org/wiki/Q_(programming_language_from_Kx_Systems)

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Re: [Haskell-cafe] Programming videogames in haskell
by sean.seefried＠gmail.com 15 Nov '14

15 Nov '14

Hi Quentin, I am currently building a game in Haskell that I plan to deploy to iOS and Android. It’s built using a combination of SDL2 and Cairo (for vector graphics) and Hipmunk (for physics via the Chipmunk C library). As in Ivan Perez's case I’m not using the Hackage versions of the SDL2 bindings. Instead I’m using the versions found on Github at [1] and [2]. I have recently managed to deploy my game to Android but this has not been a straightforward process. Building a GHC cross compiler for ARM was a time consuming process but greatly aided by the using the script found in the stable-ghc-snapshot branch of joehy’s repo on Github [3]. Building all the libraries I needed was even less straightforward. (I’ve written more about this in a post script) Deploying to iOS should be a much more straightforward process since there is official GHC support for iOS and you can download a binary distribution of the cross compiler at [4]. (Scroll down to find Luke Iannini’s distribution. Luke Iannini, Steven Blackheath put in a lot of hard work over many months to provide iOS support for GHC.) I’ve also had a play around with Helm which uses the Elerea FRP library, SDL2 and Cairo. I think it’s a pretty good little library for prototyping, although you will need to learn FRP (Functional Reactive Programming) in the process. Cheers, Sean p.s. Here is a little more detail on targeting ARM. Many of the libraries, notably the SDL2 binding, did not work out of the box with a cross compiler since they did not use Cabal’s Distribution.Simple and required that Setup.hs was compiled and run. Unfortunately this just does not work with a cross-compiler since you can’t run the code on your host machine. The work around is to build it using GHC targeting the host machine and to run that. This works okay, but a straight “cabal install” will not. A more serious problem I ran into was that the SDL2 binding requires the use of hsc2c. hsc2c has a --cross-compile flag but it does not support all of the hsc2c directives, specifically the “let” directive which is used by SDL2. [1] https://github.com/Lemmih/hsSDL2 [2] https://github.com/jdeseno/hs-sdl2-mixer [3] https://github.com/joeyh/ghc-android [4] https://www.haskell.org/ghc/download_ghc_7_8_3

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Programming videogames in haskell
by Quentin Le Guennec 14 Nov '14

14 Nov '14

Hello, We're building a indie game studio with some friends. I am the lead developper and i am planning to write the game logic and graphics in haskell. I would like to know if some of you have some experience in game developpement in haskell, functionnal reactive programming in particular. Furthermore, knowing which tools/DSL's/libraries i could use and where to get good documentation could really help me. Elise Huard recently wrote an ebook about game programming in haskell, did some of you read it ? Is it worth buying it ? Thanks for your feedback. -- Quentin Le Guennec

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