- Haskell-Cafe - Haskell.org

ANN: repa-linear-algebra = HMatrix + REPA
by Marcin Mrotek 16 Jul '15

16 Jul '15

Hello, Given that apparently HMatrix uses the same underlying representation as F arrays in REPA, I've wrote some utilities to convert to and from these types, and ported HMatrix functions to work with REPA arrays. http://hackage.haskell.org/package/repa-linear-algebra-0.0.0.0 All functions return F arrays. In general, unqualified functions take F array(s), and the postfixed functions take D array(s), allocating them before feeding to HMatrix: * -S functions allocate sequentially (using computeS) * -SIO functions allocate sequentially inside the IO monad (using computeIntoS) * -P functions allocate in parallel (using computeP) * -PIO functions allocate in parallel inside the IO monad (using computeIntoP) Writing all that boilerplate was quite mind-numbing, so sorry if there are any mistakes or inconsistencies (I wish this was a way to automate it, but it's not always straightforwards, for example I've decided that arguments that will be often supplied unchanged to multiple calls (like the kernel of convolution) shouldn't be changed to take D arrays in the preallocating functions). Please have in mind that the module is HIGHLY experimental, as it does just plain rude casts under the hood. Before you use it for anything serious first check if it works as you expect it to. Best regards, Marcin Mrotek

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Re: [Haskell-cafe] Haskell & Java/Scala interoperbility?
by Vasili I. Galchin 15 Jul '15

15 Jul '15

I am looking to suppprt Haskell (OCaml ??) for apache.spark.org. Currently there is support for programming language "R" which has functional features (not as rich as Haskell, OCaml, et. al.) and does lazy evaluation. I am going to drop back and look at the R->Scala/Java implementation BUT I will also read your link. Any more.suggestions are gratefully welcome. Kind thx, Vasili On Tuesday, July 14, 2015, Jason Dagit <dagitj(a)gmail.com> wrote: > Did you look at java bridge? > * https://hackage.haskell.org/package/java-bridge > > Perhaps counter-intuitively, you're probably better off defining a > protocol and using that for interop. Something like zeromq could simplify > the communication. > > The main hassle with a full interop is the difference between the notions > of types in the two languages. For instance, subtyping on the Java side > doesn't really map well to the Haskell side. > > I would say that it forces attempts at a general interop to go with a > least common denominator approach. In the extreme, this degenerates back to > having the code on each side pretend like it's talking to C. > > Whereas designing a message protocol for your task allows you to > incorporate domain specific knowledge and thus avoid a general solution and > also leave out things you will never use. > > On Tue, Jul 14, 2015 at 7:14 PM, Vasili I. Galchin <vigalchin(a)gmail.com > <javascript:_e(%7B%7D,'cvml','vigalchin(a)gmail.com');>> wrote: > >> Hello, >> >> I have been looking at Hackage for a package/library that >> supports Haskell interoperability with Java/Scala. Didn't see any such >> support. If true, is there any project in the works? >> >> Thanks, >> >> Vasili >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> <javascript:_e(%7B%7D,'cvml','Haskell-Cafe(a)haskell.org');> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > >

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Haskell & Java/Scala interoperbility?
by Vasili I. Galchin 15 Jul '15

15 Jul '15

Hello, I have been looking at Hackage for a package/library that supports Haskell interoperability with Java/Scala. Didn't see any such support. If true, is there any project in the works? Thanks, Vasili

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ANN: hledger 0.26
by Simon Michael 14 Jul '15

14 Jul '15

I'm pleased to announce hledger and hledger-web 0.26! This release restores non-regular-expression account aliases as the default, and brings miscellaneous bugfixes and cleanups and some performance work. Also the website and documentation have been improved, and now include many more examples. Full release notes: http://hledger.org/release-notes#hledger-0.26 . Release contributors: Simon Michael, Imuli, Carlos Lopez-Camey, Kyle Marek-Spartz, Rick Lupton, Simon Hengel. About: hledger (http://hledger.org) is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. It can also read CSV or timelog files, and output CSV. It provides useful reports, and can also help you record new transactions interactively. Add-on commands include hledger-web (a web interface), hledger-irr (for calculating internal rate of return) and hledger-interest (for generating interest transactions). hledger is inspired by and largely compatible with Ledger, and can be used with some Ledger files. Installation: $ cabal update [$ cabal sandbox init] $ cabal install hledger[-web]-0.26 or: install stack (https://github.com/commercialhaskell/stack/wiki/Downloads <https://github.com/commercialhaskell/stack/wiki/Downloads>) $ stack --resolver nightly-2015-07-13 install hledger [hledger-web] or: $ git clone https://github.com/simonmichael/hledger.git $ cd hledger $ git checkout hledger-0.26 $ stack install or see http://hledger.org/download for more options. Best! -Simon

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In " returnIO x = IO $ \ s -> (# s, x #)" what are the # symbols?
by Jeffrey Brown 14 Jul '15

14 Jul '15

That appears here: http://hackage.haskell.org/package/base-4.8.0.0/docs/src/GHC-Base.html#line…

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On Windows after I install the new cabal-install then when I do a cabal update it says there is a new ...
by KC 13 Jul '15

13 Jul '15

On Windows after I install the new cabal-install then when I do a cabal update it says there is a new cabal-install available. Yet cabal-install installed without any errors. Why do I get this message? -- -- Sent from an expensive device which will be obsolete in a few months! :D Casey

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Best way to return Bool based on a successful pattern match?
by Nikolay Amiantov 13 Jul '15

13 Jul '15

Hi Cafe, Every now and then I'm in a need to check if a value satisfies pattern with a Bool result. So far I know several ways to do this: 1. Use lens: `has _Left` 2. Make a function: `isLeft (Left _) = True; isLeft _ = False` 3. (When you need a lambda) Use LambdaCase: `\case Left _ -> True; _ -> False` Usually I go for (1) because I use lens extensively anyway, but for one-time usage all the extra machinery seems too much. I don't like (2) when I don't have a library defining that for me, and also (3) -- they seem inelegant. Is there any syntactic trick that I'm missing that makes it look nicer? -- Nikolay.

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To all users of FGL
by Ivan Lazar Miljenovic 13 Jul '15

13 Jul '15

As you may now, for the past few months I've been (finally!) cleaning up FGL, adding tests, etc. One of the issues that has arisen with this is that the behaviour of the delLEdge [1] function is not well specified when dealing with multiple edges. Specifically, the documentation states that the purpose is to "Remove *an* LEdge from the Graph." (emphasis added)... but the behaviour when dealing with multiple edges is to remove *all* such edges from the graph. The current version on GitHub is to instead just delete a single such labelled edge, with a new function "delAllLEdge" that replicates the current behaviour. Before releasing this change, however, I wanted to make sure that I wouldn't break people's code if they rely upon this functionality; I did try and search through GitHub to see who - if anyone - is using this function, but primarily found various copies of fgl embedded into other people's repositories. As such, please check your code and let me know if this change in behaviour might affect you (if this is the case, I might let delLEdge keep the current behaviour and have a new function delete just one edge). [1]: http://hackage.haskell.org/package/fgl-5.5.1.0/docs/Data-Graph-Inductive-Gr… -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Re: [Haskell-cafe] Announcing libravatar @ hackage
by fr33domlover 12 Jul '15

12 Jul '15

Forgot the link, just in case: http://hackage.haskell.org/package/libravatar On 2015-07-12 fr33domlover <fr33domlover(a)riseup.net> wrote: > Hello! > > Libravatar - http://libravatar.org - is a free-as-in-freedom and decentralized > avatar (profile image delivery service, a replacement of the centralized and > proprietary grava7ar. > > There has been a Haskell library for grava7ar for some time, but not for > Libravatar. I just finished writing one, and did a bit of testing which had > good results. > > More testing and feedback etc. very very welcome! I also enourage you all to > *integrate libravatar into your desktop/web applications and websites*! > Support openness, decentralization and freedom by using Libravatar. > > > > --- > fr33domlover <http://www.rel4tion.org/people/fr33domlover> > GPG key ID: 63E5E57D (size: 4096) > GPG key fingerprint: 6FEE C222 7323 EF85 A49D 5487 5252 C5C8 63E5 E57D --- fr33domlover <http://www.rel4tion.org/people/fr33domlover> GPG key ID: 63E5E57D (size: 4096) GPG key fingerprint: 6FEE C222 7323 EF85 A49D 5487 5252 C5C8 63E5 E57D

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Announcing libravatar @ hackage
by fr33domlover 12 Jul '15

12 Jul '15

Hello! Libravatar - http://libravatar.org - is a free-as-in-freedom and decentralized avatar (profile image delivery service, a replacement of the centralized and proprietary grava7ar. There has been a Haskell library for grava7ar for some time, but not for Libravatar. I just finished writing one, and did a bit of testing which had good results. More testing and feedback etc. very very welcome! I also enourage you all to *integrate libravatar into your desktop/web applications and websites*! Support openness, decentralization and freedom by using Libravatar. --- fr33domlover <http://www.rel4tion.org/people/fr33domlover> GPG key ID: 63E5E57D (size: 4096) GPG key fingerprint: 6FEE C222 7323 EF85 A49D 5487 5252 C5C8 63E5 E57D

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idea: <package>-latest URLs for Hackage
by Ömer Sinan Ağacan 12 Jul '15

12 Jul '15

It's very convenient to have a persistent URL linked to latest version. GHC user manual has this(https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/) and you can create a bookmark with that URL and it stays up-to-date, it appears in first place in Google queries etc. I'm wondering why not implement this for Hackage too? For example, hackage.haskell.org/package/base-latest, hackage.haskell.org/package/base-latest/docs/<module-name>.html etc. Any ideas?

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Fwd: SQL database schema diff tool
by Sergey Mironov 11 Jul '15

11 Jul '15

---------- Forwarded message ---------- From: Sergey Mironov <grrwlf(a)gmail.com> Date: 2015-07-11 22:17 GMT+03:00 Subject: Re: [Haskell-cafe] SQL database schema diff tool To: David Kraeutmann <kane(a)kane.cx> Thanks for the answer, I've looked at simple-migrations application. Unfortunately, it doesn't look like the solution for me. Term 'migration' in PostgreSQL seems to mean 'movement of data from one server to another using the same schema', but I need to move the data from one schema to another as safely as possible having the definitions of both schemas. Regards, Sergey 2015-07-11 21:05 GMT+03:00 David Kraeutmann <kane(a)kane.cx>: > You could try https://github.com/ameingast/postgresql-simple-migration, but I haven't used it myself. "schema migration" seems to be what you want. > On 7/11/2015 7:54 PM, Sergey Mironov wrote: >> Hi, list! I wish I have a tool which would compare two database >> sachemas, where each schema is represented as a list of 'CREATE smth' >> instructions. The result of such a comparison would be a list of >> 'ALTER smth' instructions. This instruction set would change the >> database from first state to second, if the transition is possible >> without data losses. The 'hard' mode - to prepare the transition for >> any inputs, with minimal data losses. >> >> The database of interest is PostgreSQL. Does such a tool exist? If >> not, which package do I better use to build the AST of initial >> schemas? >> >> The example schema is in attach. >> Thanks in advance, >> Sergey >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > >

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SQL database schema diff tool
by Sergey Mironov 11 Jul '15

11 Jul '15

Hi, list! I wish I have a tool which would compare two database sachemas, where each schema is represented as a list of 'CREATE smth' instructions. The result of such a comparison would be a list of 'ALTER smth' instructions. This instruction set would change the database from first state to second, if the transition is possible without data losses. The 'hard' mode - to prepare the transition for any inputs, with minimal data losses. The database of interest is PostgreSQL. Does such a tool exist? If not, which package do I better use to build the AST of initial schemas? The example schema is in attach. Thanks in advance, Sergey

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First call to putStr takes far more time than subsequent calls
by Ben Gunton 11 Jul '15

11 Jul '15

This is a simplified version of an issue I'm running into when timing some functions. In this example, the first putStr inside the replicateM_ takes far longer than subsequent putStrs. If I add in a putStr at the top (the commented line), each iteration takes the same. (Though I presume I just shifted the long execution to the putStr on top). The same thing happens on the first iterations of a function when I need to evaluate some state, or deepseq some value. putStr was just a simpler version that still showed the same issue. Relevant code: http://lpaste.net/136250 What's going on here? Thanks! import Control.Monadimport Data.Time.Clock main :: IO ()main = do -- putStr "" replicateM_ 5 $ do t1 <- getCurrentTime putStr "" t2 <- getCurrentTime print . round $ 1000 * 1000 * diffUTCTime t2 t1 {- Sample output: 49 1 1 1 2 -}

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As reading, what are your favorite Haskell libraries?
by Jeffrey Brown 11 Jul '15

11 Jul '15

A wider question controls also for a library's applicability and (relative) absence of prerequisites, because the funnest reading is not only intrinsically fun, but also extrinsically rewarding and soon feasible (feasible to comprehend or to use). That is, I like simple, beautiful, powerful libraries that do not require me to first learn too many other things. I might value simplicity over power. I'm interested in answers to either question.

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GHC Memory Management wiki page
by Kyle Miller 10 Jul '15

10 Jul '15

I have been studying the garbage collection algorithms used in GHC, and it appears to me that section two of [1] is misleading, since thunks permit controlled mutation of the heap, and so objects in older generations may point to younger objects. In fact, the RTS maintains "remembered sets" to keep track of these relations. A Haskell could in principle be implemented in the way described (and execute in surprising complexity classes), but this does not seem to match reality. [1] https://wiki.haskell.org/GHC/Memory_Management Should the wiki be modified? Regards, Kyle Miller

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Confusion with GeneralizedNewtypeDeriving + MultiParamTypeClasses
by William Yager 10 Jul '15

10 Jul '15

Hello all, I have run into a bit of a strange case where I can't get `GeneralizedNewtypeDeriving` to work as I'd like. In the `vector` package, under `Data.Vector.Generic`, there is a generic vector typeclass of the form `Vector v a`, where `v` is the vector type and `a` is the type of items in the vector. I have a newtype wrapper of the form `newtype Neuron a = Neuron (Vector a)`. (Note that "Vector" here is a type "Vector", not the two-parameter constraint also called "Vector".) I would like to be able to use everything in `Data.Vector.Generic` on `Neuron`s. Using `GeneralizedNewtypeDeriving`, I immediately run up against the issue that the `Vector` typeclass doesn't have the correct form of `* -> Constraint`. According to section 7.5.5.1 of https://downloads.haskell.org/~ghc/7.8.4/docs/html/users_guide/deriving.html , we should be able to derive instances of multi-parameter type classes, provided the newtype is the last parameter of the typeclass. The problem here is that it's the first parameter. Using `ConstraintKinds` to make a constraint synonym (of the form `FlippedVectorClass a v`) so that I can do something like `newtype Neuron a = Neuron (Vector a) deriving (FlippedVectorClass a)` also doesn't seem to work. Does anyone have any advice for how to make this work? It's quite tempting just to make `type Neuron a = Vector a`, but I'd rather do a proper wrapper type. Cheers, Will

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darcs 2.10.1 release
by Guillaume Hoffmann 09 Jul '15

09 Jul '15

Hi all, The darcs team is pleased to announce the release of darcs 2.10.1 ! # Downloading # The easiest way to install darcs 2.10.1 from source is by first installing the Haskell Platform (http://www.haskell.org/platform) If you have installed the Haskell Platform or cabal-install, you can install this release by doing: $ cabal update $ cabal install darcs-2.10.1 Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.10.1.tar.gz and build it by hand as explained in the README file. The 2.10 branch is also available as a darcs repository from http://darcs.net/releases/branch-2.10 # What's new in 2.10.1 (since 2.10.0) # - generalized doFastZip for darcsden support - support terminfo 0.4, network 2.6, zlib 0.6, quickcheck 2.8 and attoparsec 0.13 - errorDoc now prints a stack trace (if profiling was enabled) (Ben Franksen) - beautified error messages for command line and default files (Ben Franksen) - fixed the following bugs: - issue2449: test harness/shelly: need to handle mis-encoded/binary data (Ganesh Sittampalam) - issue2423: diff only respecting --diff-command when a diff.exe is present (Alain91) - issue2447: get contents of deleted file (Ben Franksen) - issue2307: add information about 'darcs help manpage' and 'darcs help markdown' (Dan Frumin) - issue2461: darcs log --repo=remoterepo creates and populates _darcs (Ben Franksen) - issue2459: cloning remote repo fails to use packs if cache is on a different partition (Ben Franksen) # Feedback # If you have an issue with darcs 2.10.0, you can report it on http://bugs.darcs.net/ . You can also report bugs by email to bugs(a)darcs.net, or come to #darcs on irc.freenode.net. # Known issues # - issue2269: rebase should warn about stolen patches at suspend, not unsuspend - issue2272: darcs rebase unsuspend should automate or semi-automate handling unrecorded changes - issue2276: darcs rebase unsuspend needs UI improvements for "You are not... Amend anyway?" - issue2359: convert --export mishandles Unicode filenames - issue2372: Please remove "HINT: I could not reach..." message - issue2436: rollback --patches takes ages before first prompt - issue2445: internal error if suspended patch is pulled into repository again

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(no subject)
by None 09 Jul '15

09 Jul '15

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[GHC 7.8.4] IO Monad leak space?
by Baojun Wang 09 Jul '15

09 Jul '15

Hi, First of all, I found it interesting that loopM_ f k n s = when (k <= n) (f k >> loopM_ f (s+k) n s) loopM_ seems faster than mapM_ ( mapM_ f [k, k+s..n])) I think mapM_ is used very commonly, why it's performance is even lower than a hand-written loop function? 2nd, even I replace mapM_ with loopM_ from above, when chain IO action, it still can leak space. ( Because IO Monad (>>) need keep ``RealWorld s'' updated so that I/O actions can be done in-order? ) Consider below function: f3 :: UArray Int Int -> IOUArray Int Int64 -> Int -> IO () f3 u r i = let !v = u ! i in go (f31 v) i i where f31 v j = readArray r j >>= \v1 -> writeArray r j (v1 + (fromIntegral i) * (fromIntegral v)) f31 :: Int -> Int -> IO () go g k s = when (k <= maxn) ( g k >> go g (s+k) s ) When call f3: loopM_ (f3 uu res) 1 1 1000000 Which will have blow profiling output: individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc ... loopM_ Main 104 4000002 7.4 10.1 100.0 99.3 f3 Main 113 1000000 1.0 2.0 70.2 69.1 f3.go Main 116 14970034 32.7 67.1 68.8 67.1 f3.f31 Main 117 13970034 34.5 0.0 36.1 0.0 f3.f31.\ Main 118 13970034 1.7 0.0 1.7 0.0 f3.f31 Main 114 0 0.3 0.0 0.4 0.0 f3.f31.\ Main 115 0 0.1 0.0 0.1 0.0 ... Why f3.go consumes so much space (67.1%)? The only reason I can think of is IO Monad chain (>>) isn't space free as I thought. Did I get something fundamentally wrong? Thanks baojun

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Problems with type families
by Leza Morais Lutonda 09 Jul '15

09 Jul '15

Hi All! Consider the following definitions: type family TFunIn t e type family TFunOut t e data IdT data ComposeT t1 t2 -- etc... type instance TFunIn (ComposeT t1 t2) e = TFunIn t2 e type instance TFunOut (ComposeT t1 t2) e = TFunOut t1 e -- etc... class SignalT t e where transform :: t -> TFunIn t e -> TFunOut t e instance (SignalT t1 e, SignalT t2 e) => SignalT (ComposeT t1 t2) e where transform _ x = (transform (undefined :: t1) . transform (undefined :: t2)) x Due to the `TFunIn` and `TFunOut` type families instances for [(ComposeT t1 t2) e], the `transform` function for [(ComposeT t1 t2) e] instance of SignalT should be inferred as: transform :: t -> TFunIn (ComposeT t1 t2) e -> TFunOut (ComposeT t1 t2) e <==> :: t -> TFunIn t2 e -> TFunOut t1 e But I cannot figure out why it does not compile. Any ideas? Thanks. -- Leza Morais Lutonda, Lemol-C http://lemol.github.io 50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964 http://cujae.edu.cu

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You know, I am tired of it.
by Stefan Reich 09 Jul '15

09 Jul '15

Whatever "community" I go, they all pretend like their old shit is SO important.

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The evil of "mailing list removals" - and Microsoft-sponsored s**t ^^
by Stefan Reich 09 Jul '15

09 Jul '15

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List Moderation
by Gershom B 08 Jul '15

08 Jul '15

Stefan Reich has been removed as a member from this list due to a stream of unapologetically off-topic posts. Best, Gershom

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[Haskell-cafe] tinybrain.blog.de/2015/07/08/the-evil-of-mailing-list-removals-20645305/
by Stefan Reich 08 Jul '15

08 Jul '15

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They scrubbed my link??
by Stefan Reich 08 Jul '15

08 Jul '15

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Announcing: monadic FRP using stm
by Silvio Frischknecht 08 Jul '15

08 Jul '15

Hi Haskellers, I know there are already a lot of FRP libraries out there, but still. This is my take on it. It's a very simple library. There are portable Signals that are basically TVars with some update Notifiers. Then you can combine them using a continuation like monad. Because you have a Monad and not only Applicative it's very intuitive and powerful. Check it out. github: https://github.com/yokto/Updater hackage: https://hackage.haskell.org/package/Updater Be warned though. There are probably still some bugs around, but the basics seem to work and there is an example program. Silvio

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The Internet is insane
by Stefan Reich 08 Jul '15

08 Jul '15

They thumb up the shit and destroy the good stuff. I am really, seriously, interested how anyone can even try to explain how JavaX is supposedly worse (??) than Java. Cheers

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Request for maintainer(s) : language-javascript / hjsmin
by Alan & Kim Zimmerman 08 Jul '15

08 Jul '15

Hi all I am the author of language-javascript and its companion hjsmin. Unfortunately I do not use the libraries myself, and I am falling behind in maintaining them, all the more so with the new ECMASCRIPT spec out. So I am calling for volunteers to take over maintenance of these two packages. Regards Alan

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ambiguous module name Network.URI
by Kees Bleijenberg 08 Jul '15

08 Jul '15

If i import these 2 packages in a project: import Network.HTTP import qualified Network.URI as NU I get: testNetw.hs:7:18: Ambiguous module name `Network.URI': it was found in multiple packages: network-uri-2.6.0.3 network-2.4.2.3 I'am using the latest 32 bits haskell platform on Windows 7 64 bits. I need the NU.parseURI function. What can I do? Kees

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References on haskellwiki
by Alexis Praga 07 Jul '15

07 Jul '15

Hi, I've begun to do some formatting on old Monad Readers editions on Haskell Wiki. Unfortunately, there is no support for proper references support. Can the admin add the "Cite" extension ? It would help a lot. Thanks -- Alexis Praga, PhD Student (CERFACS) GPG key : AD4A AF6D BB5C 042F 9422 1223 06E1 C1BF E287 65D0

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The world actually IS insane.
by Stefan Reich 07 Jul '15

07 Jul '15

So if I think "hey, these people are insane and I'm not" - I'm actually right. Because I woke up from insanity. Come join me, it's better here. You lose the pain. Cheers Stefan PS: Somebody now tell me that "this list is only for Haskell!!11!!!", once again proving your insanity. :)

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The sad thing is, you don't see the new stuff.
by Stefan Reich 07 Jul '15

07 Jul '15

Haskell was - maybe - the most elegant mathematical logic-oriented language out there. It's why I used it. Now there is my approach, which is reciprocal simplification of the programming process. THAT is elegant. Cheers

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Symmetric Difference for Data.Set and Data.IntSet?
by Zemyla 07 Jul '15

07 Jul '15

Could this be implemented as a primitive? I know it can be made from union and difference, or doing toList on both of them, taking the symmetric difference, and using fromDistinctAscList on the result, but it'd probably be much faster if it could be done natively, like can be done with Data.Map.mergeWithKey. Alternatively, could we get a merge interface for Set and IntSet roughly matching Map and IntMap?

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Strange "ambiguity" problems thanks to GADTs
by wren romano 07 Jul '15

07 Jul '15

Hi all, In my latest project I've been using a bunch of GADTs, which unfortunately disables let-polymorphism (i.e., where-polymorphism) in the most annoying way. For the most part I've been able to hack around that limitation via judicious use of ScopedTypeVariables, but I've run into an issue that I can't for the life of me figure out why GHC doesn't like it. I have something like the following function, where @a@ and @b@ happen to be GADTs: foo :: (A a, B b) => a i -> M (b i) foo a = case view a of ... SomePattern a1 a2 -> do b1 <- foo a1 b2 <- foo a2 return . unview $ SomePattern b1 b2 It seems to me that the recursive calls should work just fine, using the same @b@ as we'll ultimately be returning. However, for some reason GHC complains about the recursive calls using some other @b0@ can can't deduce @B b0@ from @B b@. Why doesn't GHC unify these two types? How can I force them to unify without adding type annotations at every recursive call? -- Live well, ~wren

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stm onCommit
by Silvio Frischknecht 06 Jul '15

06 Jul '15

I need a onCommit functionality for stm. I know there is the stm-io-hooks [1] package. But it seems so big for such a small thing and then I have to translate everything to their monad. lift lift lift ... So I thought of a little hack to solve the issue and I'm wondering if this is safe to use. My understanding of stm internals is very limited. It's basically just a TChan with IO actions in it and there is another thread waiting to execute anything inserted into it. import Control.Concurrent.STM.TChan onCommitChan :: TChan (IO ()) {-# NOINLINE onCommit #-} onCommitChan = unsafePerformIO $ do chan <- newTChanIO forkIO $ forever $ atomically $ readTChan chan return chan onCommit :: IO () -> STM () onCommit = writeTChan onCommitChan It would be cool if an onCommit hook could be directly added to the stm package. Silvio [1] https://hackage.haskell.org/package/stm-io-hooks-1.0.1

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OCL 2015: Final Call for Papers - Only 10 Days Left
by Achim D. Brucker 06 Jul '15

06 Jul '15

(Apologies for duplicates) If you are working on the foundations, methods, or tools for OCL or textual modelling, you should now prepare your submission for the OCL workshop! CALL FOR PAPERS 15th International Workshop on OCL and Textual Modeling Tools and Textual Model Transformations Co-located with ACM/IEEE 18th International Conference on Model Driven Engineering Languages and Systems (MODELS 2015) September 28th, 2015, Ottawa, Canada http://ocl2015.lri.fr Modeling started out with UML and its precursors as a graphical notation. Such visual representations enable direct intuitive capturing of reality, but some of their features are difficult to formalize and lack the level of precision required to create complete and unambiguous specifications. Limitations of the graphical notations encouraged the development of text-based modeling languages that either integrate with or replace graphical notations for modeling. Typical examples of such languages are OCL, textual MOF, Epsilon, and Alloy. Textual modeling languages have their roots in formal language paradigms like logic, programming and databases. The goal of this workshop is to create a forum where researchers and practitioners interested in building models using OCL or other kinds of textual languages can directly interact, report advances, share results, identify tools for language development, and discuss appropriate standards. In particular, the workshop will encourage discussions for achieving synergy from different modeling language concepts and modeling language use. The close interaction will enable researchers and practitioners to identify common interests and options for potential cooperation. Topics of interest include (but are not limited to) =================================================== - Mappings between textual modeling languages and other languages/formalisms - Algorithms, evaluation strategies and optimizations in the context of textual modeling languages for -- validation, verification, and testing, -- model transformation and code generation, -- meta-modeling and DSLs, and -- query and constraint specifications - Alternative graphical/textual notations for textual modeling languages - Evolution, transformation and simplification of textual modeling expressions - Libraries, templates and patterns for textual modeling languages - Tools that support textual modeling languages (e.g., verification of OCL formulae, runtime monitoring of invariants) - Complexity results for textual modeling languages - Quality models and benchmarks for comparing and evaluating textual modeling tools and algorithms - Successful applications of textual modeling languages - Case studies on industrial applications of textual modeling languages - Experience reports -- usage of textual modeling languages and tools in complex domains, -- usability of textual modeling languages and tools for end-users - Empirical studies about the benefits and drawbacks of textual modeling languages - Innovative textual modeling tools - Comparison, evaluation and integration of modeling languages - Correlation between modeling languages and modeling tasks This year, we particularly encourage submissions describing tools that support - in a very broad sense - textual modeling languages (if you have implemented OCL.js to run OCL in a web browser, this is the right workshop to present your work) as well as textual model transformations. Venue ===== The workshop will be organized as a part of MODELS 2015 Conference in Ottawa, Canada. It continues the series of OCL workshops held at UML/MODELS conferences: York (2000), Toronto (2001), San Francisco (2003), Lisbon (2004), Montego Bay (2005), Genova (2006), Nashville (2007), Toulouse (2008), Denver (2009), Oslo (2010), Zurich (2011, at the TOOLs conference), 2012 in Innsbruck, 2013 in Miami, and 2014 in Valencia, Spain. Similar to its predecessors, the workshop addresses both people from academia and industry. The aim is to provide a forum for addressing integration of OCL and other textual modeling languages, as well as tools for textual modeling, and for disseminating good practice and discussing the new requirements for textual modeling. Workshop Format =============== The workshop will include short (about 15 min) presentations, parallel sessions of working groups, and sum-up discussions. Submissions =========== Three types of papers will be considered: * short papers (between 6 and 8 pages) describing ideas, * tool papers (between 6 and 8 pages), and * full papers (between 12 and 16 pages) in LNCS format. Submissions should be uploaded to EasyChair (https://easychair.org/conferences/?conf=ocl20150) The program committee will review the submissions (minimum 2 reviews per paper, usually 3 reviews) and select papers according to their relevance and interest for discussions that will take place at the workshop. Accepted papers will be published online in a pre-conference edition of CEUR (http://www.ceur-ws.org) Important Dates =============== Submission of papers: July 17, 2015 Notification: August 21, 2015 Workshop date: September 28, 2015 Organizers ========== Achim D. Brucker, SAP SE, Germany Marina Egea, Indra Sistemas S.A., Spain Martin Gogolla, University of Bremen, Germany Frederic Tuong, Univ. Paris-Sud - IRT SystemX - LRI, France Programme Committee =================== Mira Balaban, Ben-Gurion University of the Negev, Israel Tricia Balfe, Nomos Software, Ireland Achim D. Brucker, SAP SE, Germany Fabian Buettner, Inria - Ecole des Mines de Nantes, France Jordi Cabot, Inria - Ecole des Mines de Nantes, France Dan Chiorean, Babes-Bolyai University, Romania Robert Clariso, Universitat Oberta de Catalunya, Spain Tony Clark, Middlesex University, UK Manuel Clavel, IMDEA Software Institute, Spain Carolina Dania, IMDEA Software Institute, Spain Birgit Demuth, Technische Universitat Dresden, Germany Marina Egea, Indra Sistemas S.A., Spain Geri Georg, Colorado State University, USA Martin Gogolla, University of Bremen, Germany Shahar Maoz, Tel Aviv University, Israel Istvan Rath, Budapest University of Technology and Economics, Hungary Bernhard Rumpe, RWTH Aachen, Germany Frederic Tuong, Univ. Paris-Sud - IRT SystemX - LRI, France Claas Wilke, Technische Universitat Dresden, Germany Edward Willink, Willink Transformations Ltd., UK Burkhart Wolff, Univ. Paris-Sud - LRI, France Steffen Zschaler, King's College, UK -- Dr. Achim D. Brucker, SAP SE, Vincenz-Priessnitz-Str. 1, D-76131 Karlsruhe Phone: +49 6227 7-52595, http://www.brucker.ch/

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Fwd: Efficient records with arbitrarily many fields [was: Extensible states]
by Alexander V Vershilov 06 Jul '15

06 Jul '15

(forgot to reply to cafe list Hi. You can take a look at vector-fixed-hetero [1], that can act as anonymous structure with arbitrary number of fields, convertion to and from datatypes with the same structure and many more features. It's missing field names though and syntactic sugar that 'records' package have. If you like to reinvent a wheel, then you can use following appoach: introduce some typelevel info data a :? (b :: Symbol) data FieldName (t :: Symbol) = FieldName introduce generic structure for records with any number of fiels newtype ARec (m :: [*]) = ARec { unRec :: Array Any } type instance TProxy (ARec m) = m type instance VProxy (ARec m) = ARec rerec :: ARec a -> ARec b rerec = ARec . unRec indexRec :: (KnownNat (FieldIndex name m), FieldType name m ~ a) => proxy name -> ARec m -> a indexRec name v = unsafeCoerce $ indexArray (unRec v) (fromIntegral (natVal (indexProxy name v))) updateRec :: (KnownNat (Length m), KnownNat (FieldIndex name m), FieldType name m ~ a) => proxy name -> a -> ARec m -> ARec m updateRec name b v = runST $ do out <- newArray len undefined copyArray out 0 (unRec v) 0 len writeArray out idx (unsafeCoerce b) ARec <$> unsafeFreezeArray out where idx = fromIntegral (natVal (indexProxy name v)) len = fromIntegral (natVal (lengthProxy v)) you'll need some typelevel magic for that: type family FieldType (n :: Symbol) m where FieldType n ((a :? n) ': z) = a FieldType n (b ': z) = FieldType n z type family FieldIndex (n::Symbol) m :: Nat where FieldIndex n ((a :? n) ': z) = 0 FieldIndex n ( b ': z) = 1 + FieldIndex n z indexProxy :: (KnownNat c, FieldIndex n m ~ c) => proxy1 n -> proxy2 m -> Proxy c indexProxy _ _ = Proxy type family Length m where Length '[] = 0 Length (x ': xs) = 1 + Length xs lengthProxy :: (KnownNat c, Length n ~ c) => proxy n -> Proxy c lengthProxy _ = Proxy then you can implement lenses: instance (KnownNat (FieldIndex n m), KnownNat (Length m), FieldType n m ~ a) => HasField (n :: Symbol) (ARec m) a where getField = indexRec updateField p = flip (updateRec p) fieldLens' :: (HasField name z a, FieldType name (TProxy z) ~ a) => FieldName name -> Lens z z a a fieldLens' name = \f m -> fmap (updateField name m) (f $ getField name m) type family UpdateType (n :: Symbol) z a b where UpdateType n ((a :? n) ': z ) a b = (b :? n) ': z UpdateType n ( z ': zs) a b = z ': UpdateType n zs a b fieldLens :: ( ARec m ~ z, ARec m' ~ z', m' ~ UpdateType name m a b , FieldType name m ~ a, FieldType name m' ~ b , KnownNat (FieldIndex name m), KnownNat (FieldIndex name m') , KnownNat (Length m), KnownNat (Length m') ) => FieldName name -> Lens z z' a b fieldLens name = \f m -> fmap (updateField name (rerec m)) (f $ getField name m) this approach is more or less the same as records package with only one datastructure and almost the same syntactic sugar can be applied, the only missing thing is that pattern matching will be more difficult that with records. At this point it's not possible to write strict fields, but it can be easily extended. If someone is interested in this sort of wheel, I can prepare a package and some docs about and with coercion with other solutions like fixed-vector-hetero and records. [1] https://hackage.haskell.org/package/fixed-vector-hetero On Sat, Jul 4, 2015, 16:08 Ben Franksen <ben.franksen(a)online.de> wrote: > > Marcin Mrotek wrote: > > Okay, perhaps I'm too newbie to understand the big picture, but it > > seems to me you can get either: > > > > a) O(1) access to any, arbitrarily selected (at runtime) field > > b) O(1) append > > > > I guess option a) is better performance-wise, as appending is usually > > done less often than selecting (an O(1) slice is already possible with > > independently typed regular Haskell records) but > > dependently-typed-list-based implementation, or at the very least > > Vinyl (I haven't ever used HList) has the advantage of being dead > > simple in both implementation and usage. I mean, with Vinyl, you can > > write manual recursion over Rec's like: > > > > foo :: Rec ... -> Rec ... > > foo RNil = ... > > foo (r :& rs) = ... > > > > whenever GHC's typechecker gives up and goes on a strike; and I dare > > to say, with commonly used record sizes (have you ever used a record > > with more than, let's say, 10 fields?) the speed tradeoff is not > > noticeable. > > While more than 10 fields in a record is uncommon for typical library APIs > and simple programs, real world projects can grow much larger records. One > example is configuration data for complex programs (like Darcs or even GHC) > with many options. It would be so nice if we could use record types for the > configuration! Another application could in control system toolkits like > EPICS [1], which currently has (actually: generates) C records with > potentially hundreds of fields. > > If lookup is / remains linear we can never efficiently support these kinds > of applications and that would be very sad. > > I think the most reasonable default is O(1) for lookup and O(n) for > extension, like in Nikita Volkov's record package. It is quite unfortunate > that this package limits the number of fields! If GHC would offer generic > support for tuples of arbitrary size (with the same efficiency as today) > this limitation could be avoided and all would be well. > > Cheers > Ben > > [1] http://www.aps.anl.gov/epics/ > -- > "Make it so they have to reboot after every typo." ― Scott Adams > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe -- Alexander

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New Functional Programming Job Opportunities
by Functional Jobs 06 Jul '15

06 Jul '15

Here are some functional programming job opportunities that were posted recently: Senior Developer at Elsen, Inc. http://functionaljobs.com/jobs/8840-senior-developer-at-elsen-inc Cheers, Sean Murphy FunctionalJobs.com

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SPJ's Venn diagram on type-correctness
by Mathijs Kwik 06 Jul '15

06 Jul '15

Hi all, Sometime in the past, I came across a presentation from SPJ in which he showed a Venn diagram showing "programs that compile" and "programs that are correct". Unfortunately, I cannot remember the exact wording and I'm unable to find the slides/talk on google/youtube. Does anyone remember the exact details of the diagram? Or the title of the talk? Is there still a link to it? Thanks, Mathijs

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ANN: stack-0.1.2.0
by Emanuel Borsboom 05 Jul '15

05 Jul '15

New release of stack, a build tool. Changes: https://github.com/commercialhaskell/stack/releases/tag/v0.1.2.0 General information and download links (now with package repositories for CentOS, Fedora, and Debian): https://github.com/commercialhaskell/stack#readme If you already have stack-0.1.1.0, you can upgrade using: stack upgrade --git && stack upgrade [normally just 'stack upgrade' will suffice, but due to my flub in the last release we ended up with a stack-9.9.9 on hackage which can't be removed (see https://github.com/haskell/hackage-server/issues/382) and you have to get the stack version that knows to ignore it first]

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ANN: zoom-refs - zoom and pairing for mutable references
by Marcin Mrotek 05 Jul '15

05 Jul '15

Hello, I've just uploaded my package zoom-refs: http://hackage.haskell.org/package/zoom-refs-0.0.0.0 It's a "port" of State monad zoom (from lens) to mutable references: zoomTVar :: Lens' a b -> TVar a -> TVar b These TVars aren't actually the raw TVars from STM, but wrappers that provide the same functionality. Similar functions are provided for STRefs and IORefs. Additionally, TVars and STRefs can be paired to create composite references: pairTVars :: TVar a -> TVar b -> TVar (a,b) No such functionality is provided for IORefs, as there would be no way to guarantee atomicity of operations on the underlying references. Together. mutable references can be used sort of like Functors and Applicatives, though one needs to use lenses rather than plain functons to map them. Finally, there are multiple references that use traversals instead of lenses to zoom: readMultiTVar :: Monoid a -> MultiTVar a -> STM a readMultiTVarList :: MultiTVar a -> STM [a] readMultiTVarHead :: MultiTVar a -> STM (Maybe a) Best regards, Marcin Mrotek

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Extensible states
by Alberto G. Corona 05 Jul '15

05 Jul '15

Hi, Anyone used some of the extensible record packages to create a kind of extensible state monad? I mean something that besides having "get", "gets" and "put" would have some kind of "add" and "gets": add :: a -> State () gets :: State (Maybe a) or add :: LabelName -> a -> State () gets :: LabelName -> State (Maybe a) So that I can extend the state without using additional monad transformers. Monad transformers are very hard for beginners and scramble error messages I did the first option for MFlow, hplayground and Transient packages (setSData and getSData). But my solution uses a map indexed by type and this requires a lookup for each access. I would like to know if there is an alternative with no lookups. I´m not obsessed with speed but In some applications the speed may be important.... Anyone? -- Alberto.

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Posted code snippet for Ex. 12 of Ch. 5 of "Fun w/ Phantom Types".
by David Banas 05 Jul '15

05 Jul '15

Hi all, I’ve spent the last few days working on Exercise 12, at the end of Chapter 5 of Ralf Hinze’s paper, “Fun with Phantom Types”. (Thanks, Conal, for all the help!) I thought I’d share my code, in case anyone else happens to be in the same Haskell space-time as me, right now. :) https://github.com/capn-freako/Haskell_Misc/blob/master/norm_by_eval.hs I hope everyone is enjoying their weekend. Cheers, -db

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List-based ini parser?
by Mike Meyer 04 Jul '15

04 Jul '15

Ok,I've looked at the packages google and hackage found (ini, hsini & ConfigFile), and can't use any of them for dealing with the ini files I'm being handed. The problem is they all parse the config file into Maps, and that doesn't seem to be an option. I need lists, because I have multiple sections with the same name that turn into a list of objects, as well as sections that can have multiple options with the same name that turn into multiple objects. Any chance I overlooked a parser? Or maybe some parsing options in ConfigFile? Any other advice on a library to do this?

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MVars and locks
by Tom Ellis 04 Jul '15

04 Jul '15

In the following section of the book "Parallel and Concurrent Programming in Haskell" http://chimera.labs.oreilly.com/books/1230000000929/ch07.html#sec_conc-phon… Simon Marlow explains that MVars can be used to implement something like locks on shared functional state: "To acquire the lock, we take the MVar, whereas, to update the variable and release the lock, we put the MVar." Am I right in thinking this only holds if we are careful to ensure both those operations happen in the given order? For example, if I take the MVar (lock) and I am about to put something back in (unlock) but someone else puts something in before I do, then the lock system becomes broken, doesn't it? So if we want to be sure we have a robust lock system we have to wrap up MVars in another abstraction? Tom

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ANNOUNCE: SubHask-0.1
by Mike Izbicki 01 Jul '15

01 Jul '15

SubHask is an alternative prelude for programming in subcategories of Hask. The documentation includes examples on: * the category of polynomials * how to give set a monad instance in two different categories * basic linear algebra using the category of linear functions More information at: http://github.com/mikeizbicki/subhask I'd like to emphasize that SubHask still has a number of bugs that need to be fixed. This release should be seen as just a technology preview.

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A datatype with a single constructor and forall to multiple constructors without forall
by Hemanth Kapila 01 Jul '15

01 Jul '15

Hi, Occassionally, in (*good*) haskell libraries, I came across data types that are defined with a single constructor and a forall. For instance, Iteratee[0] is defined as below *newtype Iteratee s m a =* * Iteratee {runIter :: forall r. (a -> Stream s -> m r) -> ((Stream s -> Iteratee s m a) -> Maybe SomeException -> m r) -> m r}* I *think* it is equivalent to the following. But am not sure. *data Iteratee s m a = IOne (m (a , Stream s))* * | ITwo (Stream s -> m (Iteratee s m a)) (Maybe SomeException)* Can some one please explain what are the advantages of using the forall version? Does it significantly improve performance because it is newtype? Or does it help in reasoning about code (in some proof tools or such)? And can some one kindly point me to some references that help me convert from one form to another reliably? Many Thanks, Hemanth

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Re: [Haskell-cafe] haskell-src-exts maintenance
by Roman Cheplyaka 01 Jul '15

01 Jul '15

Thanks for the update, Peter. Forwarding it to cafe. On 01/07/15 10:36, Peter A Jonsson wrote: > Hi Roman, > > I’ve inserted what I replied to Matthew a couple of days ago below. > Unfortunately my reply bounced from both libraries and haskell-cafe > since I’m not a subscriber. Matthew should have gotten the reply > though. > > Kind Regards, > > Peter > > > Dear Matthew, > >> I think it is important to get this work merged and update the >> parser to support modern language features introduced in GHC 7.8 >> and 7.10. > > We all agree about the importance of HSE supporting modern language > features that people actually use. The simple reason for my > inactivity lately is that I’ve been swamped at work so HSE has been > on the backburner. I just started my vacation though so I’m slowly > working through my backlog of things that I haven’t had time to do > previously. Without having looked at any patches in the bug tracker I > can say that the current state of HSE is stable so we should be able > to merge the simple things quickly and the more involved things > should be doable. Once that is done it’s probably time to make a new > yearly major release.

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Advice gfx library
by Nicholls, Mark 30 Jun '15

30 Jun '15

I am interested in writing some real code now! What i could do with is a gfx library that Can render mpeg/mov files Scale Transform etc Is simple! Runs on windows in some form Ive got hudaks book about multimedia, and understand the general reactive idea behind it....i want to build on top of that Ive nosed around gloss and grapefruit, but the mpeg piece seems missing from these Any ideas? Excuse the spelling, sent from a phone with itty bitty keys, it like trying to sow a button on a shirt with a sausage. CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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