- Haskell-Cafe - Haskell.org

Fwd: ANN: vector-buffer package 0.1
by Alexander McPhail 15 Feb '11

15 Feb '11

Hi list, Could someone explain why the error pointed out by Luke occurred? From: Luke Palmer <lrpalmer(a)gmail.com> I think this would be a very good question for the list. Don't worry, they're nice helpful folks. On Mon, Feb 14, 2011 at 10:10 PM, Alexander McPhail <haskell.vivian.mcphail(a)gmail.com> wrote: > Hi Roman, > > > Can you explain why the following code doesn't work? 'unsafePerformIO' is > used all the time in hmatrix. For example, adding two vectors, we create a > new Vector then use that as the target for an FFI C call. > > Is the difference because I'm using mutable Storable Vectors? I would have > thought that unsafeFreeze (at the end) would make sure the problem reported > by Luke wouldn't occur. > > Is there a problem with laziness in the let binding of Luke's example? > > I note that in Data.Vector.Storable there is a pure 'convert' function, > which is essentially what I am trying to emulate. > > -- | convert to a vector > toVector :: Storable a => Buffer a -> (V.Vector a) > toVector (B o n v) = unsafePerformIO $ do > w <- M.new n > i <- readIORef o > M.unsafeWith v $ \p -> > M.unsafeWith w $ \q -> do > let n' = n-i > copyArray q (p `advancePtr` i) n' > if i /= 0 > then copyArray (q `advancePtr` n') p i > else return () > V.unsafeFreeze w > {-# INLINE toVector #-} > > > Vivian > > From: Luke Palmer <lrpalmer(a)gmail.com> > > This interface is an outlaw. > > main = do > buf <- newBuffer 10 :: IO (Buffer Int) > pushNextElement buf 1 > let v1 = V.toList (toVector buf) > v2 = V.toList (toVector buf) > print v1 > pushNextElement buf 2 > print v2 > > Despite v1 and v2 being defined to equal the exact same thing, this > program prints two distinct lines. > > toVector depends on the current state of the buffer. If this is to be > a law-abiding interface, toVector must return a value in IO: > > toVector :: (Storable a) => Buffer a -> IO (Vector a) > > Luke >

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ANN: vector-buffer package 0.1
by Alexander McPhail 15 Feb '11

15 Feb '11

Hi, I have uploaded a small package, vector-buffer, to hackage. It provides a buffer that can be turned into a Data.Vector.Storable. The mapM* functions map from the oldest element, not the first. Similarly for the derived Vector. Feature requests etc. welcome. Vivian

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lhs2tex and line numbers
by Mitar 15 Feb '11

15 Feb '11

Hi! How could I convince lhs2tex to add in poly mode line numbers before each code line in code block? Mitar

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Re: [Haskell-cafe] Proving correctness
by kevin＠froglingo.com 15 Feb '11

15 Feb '11

On 14.02.2011, at 03:03 PM, Pavel Perikov [perikov(a)gmail.com] wrote: Sorely, Haskell can't prove logic with it. No predicates on values, guarantee that proof is not _|_. Haskell makes bug free software affordable, that's true. But it's not a proof assistant. Being equivalent to a class of total recursive functions, the EP (Enterprise-Participant) data model can assist users to enter arbitrary and meaningful expressions only. It is a new addition to programming languages. Check out here: http://www.froglingo.com/FroglingoPL.pdf Kevin

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Problems with (projects|community).haskell.org
by Erik de Castro Lopo 14 Feb '11

14 Feb '11

Hi all, Still a couple of problems with these servers. Firstly, community.haskell.org shows the default Apache "It works" page. It would be nice to have something better there. Secondly the mailman web interface on projects.haskell.org [0] is giving a Service Temporarily Unavailable message (and has been for a couple of days). Cheers, Erik [0] http://projects.haskell.org/cgi-bin/mailman/admindb/haskell-llvm -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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ghci command line execution
by Thomas Davie 14 Feb '11

14 Feb '11

A while ago I remember someone showing me some tool, I *think* ghci that allowed you to pass it a function of type String -> String as an input, and have it simply run that function on stdin (presumably using interact) to achieve useful things like this... $ cat myFile.txt | ghci -e 'unlines . zipWith (\x y -> show x ++ " " ++ y) [1..] . lines' Does anyone know what it was that I saw being used here? Thanks Tom Davie

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Why is there no "splitSeperator" function in Data.List
by Robert Clausecker 14 Feb '11

14 Feb '11

Is there any reason, that one can't find a function that splits a list at a seperator in the standard library? I imagined something like this: splitSeperator :: Eq a => a -> [a] -> [[a]] splitSeperator ',' "foo,bar,baz" --> ["foo","bar","baz"] Or something similar? This is needed so often, even if I can implement it in one line, is there any reason why it's not in the libs? Yours, Robert Clausecker

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Re: [Haskell-cafe] ANN: FunGEn-0.3 simple 2D game engine released
by Simon Michael 14 Feb '11

14 Feb '11

Hi Andre, good to hear from you. On Feb 13, 2011, at 5:27 PM, Andre Wilson Brotto Furtado wrote: > This is great, thanks Simon. I'm currently not involved in any > haskell projects anymore, but please do keep the ball rolling for > FunGEn. > > I'm currenty exploring how domain-specific development and software > product lines can be streamlined for game development. You can check > it out at http://sharpludus.codeplex.com. Volunteers to port such a > work to Haskell are welcome. Cool. Thanks, -Simon

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Vector library
by Pierre-Etienne Meunier 14 Feb '11

14 Feb '11

Hi, This is mostly a question for Roman : how do you use your vector library with multi-dimensional arrays ? I mean, the array library from the standard libraries does something more intelligent than the C-like solution with indirections. For instance, it allows you to program a single function that works for any dimensionality of the array. An example where it is useful ? Right : Some time ago I wrote a polynomial system solver for polynomials in Bernstein form. It needs to perform De Casteljau's algorithm to compute the coefficients of the restriction of its argument to a subinterval of [0,1]. The Ix thing (mainly the bounds function) allows for writing only one version of the function for any number of variables (ok, I needed to write another class to tell me there were n variables were when the index was a n-uple, and how to iterate over my data in some dimension between 0 and n-1). I know I could do it with vectors only by wrapping them in another type carrying the bounds information, but isn't there any more elegant way to do it ? Like an optional thing in the vector type, an alternative one, I don't know. Thanks, Pierre Some code example : instance Restrictable (Double,Double,Double,Double,Double,Double,Double,Double) (Int,Int,Int,Int,Int) where restriction (a,b,c,d,e,f,g,h) lower upper= let ((i0,i1,i2,i3,i4),(j0,j1,j2,j3,j4))=bounds lower dd=rangeSize (i4,j4) n0=rangeSize (i0,j0) n1=rangeSize (i1,j1) n2=rangeSize (i2,j2) n3=rangeSize (i3,j3) restr0=restrict 1 (n1*n2*n3) n0 dd a b coefs restr1=restrict n0 (n2*n3) n1 dd c d restr0 restr2=restrict (n0*n1) n3 n2 dd e f restr1 restr3=restrict (n0*n1*n2) 1 n3 dd g h restr2 in something Where restrict is written like : restrict bef aft nv dd a b coefs= let idx i j k d=((i*nv+j)*aft + k)*dd + d in something

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Haskell AES lazy vs. strict bytestrings
by tsuraan 14 Feb '11

14 Feb '11

By my reading of the documentation of the AES package in hackage (http://hackage.haskell.org/packages/archive/AES/0.2.7/doc/html/Codec-Crypto…) the sizes of the strict bytestring chunks of the lazy bytestring given to crypt must be multiples of 16 bytes or the resulting output will act as though the strict bytestrings were padded to 16 bytes with zeroes. I'm having trouble seeing this behaviour, so I'm wondering if I'm mis-reading the documentation or if I'm not understanding what's happening. Here's a ghci run: Prelude> import Codec.Crypto.AES Prelude Codec.Crypto.AES> let lconcat = Data.ByteString.Lazy.concat Prelude Codec.Crypto.AES> let lpack=Data.ByteString.Lazy.Char8.pack Prelude Codec.Crypto.AES> let strict = Data.ByteString.concat $ Data.ByteString.Lazy.toChunks $ lconcat [lpack "hi", lpack "there", lpack "everybody"] Prelude Codec.Crypto.AES> let lazy = lconcat [lpack "hi", lpack "there", lpack "everybody"] Prelude Codec.Crypto.AES> lazy Chunk "hi" (Chunk "there" (Chunk "everybody" Empty)) Prelude Codec.Crypto.AES> let pw = pack "0123456789abcdef" Prelude Codec.Crypto.AES> let sencode = crypt' CTR pw pw Encrypt strict Prelude Codec.Crypto.AES> let lencode = crypt CTR pw pw Encrypt lazy Prelude Codec.Crypto.AES> Data.ByteString.concat $ Data.ByteString.Lazy.toChunks lencode "\SUB\ESC\n\224{\174\152dv\194j\DC1\ESCf\209\FS" Prelude Codec.Crypto.AES> sencode "\SUB\ESC\n\224{\174\152dv\194j\DC1\ESCf\209\FS" So, I'm not seeing the padding of each strict bytestring of the lazy bytestring in the output encoding. I'd say that's a good thing, but it's not what I expect from the documentation, so I'd just like to know if I'm doing something wrong before I go and depend on lazy bytestrings working like this.

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Re: [Haskell-cafe] : Gathering ideas for the Google Summer of Code
by Alexander Solla 14 Feb '11

14 Feb '11

> Last year we worked hard to come up with a good list of projects for the > 2010 Google Summer of Code. A list drafted at ZuriHac is here: > > > http://donsbot.wordpress.com/2010/04/01/the-8-most-important-haskell-org-gs… > > And we ended up with quite a few of those funded, > > > http://donsbot.wordpress.com/2010/04/26/the-7-haskell-projects-in-the-googl… > > Now is the time to think hard about student-sized projects that would be > valuable this year. > > A great place to collect these ideas, and discuss them, is the Haskell > Proposals Reddit, > > http://www.reddit.com/r/haskell_proposals/ > > -- Don > I don't have a reddit account, so I'll post here. One thing I would love to see is a "free theorem deriver" integrated into GHC. It would be lovely to have, say, a type class called "Free" or "Glue" which takes in two two types and whose function represents a glue from the first to the second. It would be nice if those injections were derived, say, by Djinn. Consider my comments (which I am reposting here, in abridged form) from http://www.palgorithm.co.uk/2010/05/why-isnt-haskell-popular-in-industry/ : I just ignore the “line noise”. Line noise is nearly always glue code. I don’t want that named. I just want it done. Yes, it can be hard to write from scratch. But any sane Haskell development environment uses an interactive environment like GHCi. If you use the type checker to guide you, writing the glue becomes trivial. Encode logic in data types, and let the glue write itself. If glue code type checks, it works, guaranteed. These are free theorems. Focus on the important things: the stuff you’re gluing together. ... class Glue a b where glue :: a -> b and hide all the line noise in there. And then your client code will look like blah :: StateT (MaybeT IO) (Integer) (Maybe Integer) blah = glue . parseInt . glue . getLine – or blah = glue $ parseInt $ glue $ getLine This is how “line noise” glue code looks to me. Except line noise is more maintainable. *fnord* Note that these glue functions would each require their own distinct type class instance. The nicer the client code looks, the more complexity you have to shovel into the Glue instances. But you are merely transferring complexity — not creating or destroying it. Complexity is created by data type definitions being glued. There is the potential for a combinatorial explosion, since you can have unused Glue instances, whereas you cannot have unused “line noise” glue code. Mathematicians don’t care about mathematical symbols. They care about the things the symbols join, because the semantics of joining “things” is dependent on — and usually determined by — the things being joined.

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ANN: FunGEn-0.3 simple 2D game engine released
by Simon Michael 14 Feb '11

14 Feb '11

All - inspired by #haskell-game, I'm pleased to announce that FunGEn has been revived as a community project. This makes Andre Furtado's 2002 work available to the new generation of haskell game developers. :) FunGEn (Functional Game Engine) is a platform-independent, BSD- licensed, easy-to-install 2D game engine currently based on OpenGL and GLUT. As of 2011 it is the only general-purpose game engine, and the easiest way to throw together simple 2D games, in Haskell. On the downside, I'm told GLUT can't handle simultaneous keypresses, but the included examples are quite playable. If Andre reads this and would like to get involved, or send corrections to my doc updates, that would be great. In any case, I hope to see your forks and patches making this better. This is also a chance to test darcsden.com's ability to serve as a lighter github. Release: http://hackage.haskell.org/package/FunGEn Code and docs: http://darcsden.com/simon/fungen Original home, more docs: http://www.cin.ufpe.br/~haskell/fungen Best, -Simon On Feb 10, 2011, at 12:40 PM, Simon Michael wrote: > Hi Andre, wman.. you guys haven't been responding, but FYI #haskell- > game IRC channel has been revived and I have just been updating > FunGEn's status on the wiki a bit: > > http://www.haskell.org/haskellwiki/Applications_and_libraries/Games#Game_En… > > http://www.haskell.org/haskellwiki/Game_Development > > If you're available/interested to join us on the channel, or update > the hackage package, you'd be welcome! Otherwise perhaps we'll take > a shot at it some time in future. > > Best, > -Simon

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Gathering ideas for the Google Summer of Code
by Don Stewart 14 Feb '11

14 Feb '11

Last year we worked hard to come up with a good list of projects for the 2010 Google Summer of Code. A list drafted at ZuriHac is here: http://donsbot.wordpress.com/2010/04/01/the-8-most-important-haskell-org-gs… And we ended up with quite a few of those funded, http://donsbot.wordpress.com/2010/04/26/the-7-haskell-projects-in-the-googl… Now is the time to think hard about student-sized projects that would be valuable this year. A great place to collect these ideas, and discuss them, is the Haskell Proposals Reddit, http://www.reddit.com/r/haskell_proposals/ -- Don

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Is Show special? Of course not but...
by Cristiano Paris 13 Feb '11

13 Feb '11

Hi cafè, given the following toy code: --------------- module Main where class Dumb p where dumb :: p -> String newtype Branded a b = Branded b unbrand :: Branded a b -> b unbrand (Branded x) = x wrong :: Dumb a => b -> Branded a b wrong = Branded right :: Show a => b -> Branded a b right = Branded --------------- Why: --------------- quarry:Haskell paris$ ghci -O1 GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help *Main> unbrand $ right True True *Main> unbrand $ right "Foo" "Foo" --------------- but: --------------- *Main> unbrand $ wrong True <interactive>:1:10: Ambiguous type variable `a' in the constraint: `Dumb a' arising from a use of `wrong' at <interactive>:1:10-19 Probable fix: add a type signature that fixes these type variable(s) --------------- ? Maybe it's a dumb question but... thank you for any explanation... -- Cristiano GPG Key: 4096R/C17E53C6 2010-02-22 Fingerprint = 4575 4FB5 DC8E 7641 D3D8 8EBE DF59 B4E9 C17E 53C6

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GHC and MinGW
by José Romildo Malaquias 13 Feb '11

13 Feb '11

Hello. How do I make ghc use my installation of MinGW on Windows? I have ghc-7.0.1 installed and the latest MinGW packages. I want ghc to automatically find libraries installed by the MinGW installer. Currently I am getting the error with my application: Linking Gui.exe ... C:\Program Files\Haskell\iconv-0.4.1.0\ghc-7.0.1/libHSiconv-0.4.1.0.a(hsiconv.o):hsiconv.c:(.text+0x8): undefined reference to `libiconv_open' where libiconv has been installed with the command mingw-get install mingw32-libiconv and the library is available at c:/devel/MinGW/lib/ Romildo

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ANNOUNCE: Save the date! Haskell hackathon in Philadelphia, July 29-31
by Brent Yorgey 12 Feb '11

12 Feb '11

I am very pleased to announce that Hac phi 3, third in an annual series of Haskell hackathons hosted at the University of Pennsylvania in Philadelphia, will take place from July 29-31. More details will be forthcoming, but put it on your calendar now. The past two years have been a blast -- you won't want to miss this one! Q: Do I have to already be a Haskell expert to attend? A: No! All are welcome. In fact, sitting in a room with a bunch of enthusiastic Haskell hackers is a great way to learn. Past attendees have included seasoned Haskell gurus and raw beginners alike. Q: What does one do at a Haskell hackathon? A: The goal is to create a fun, interactive, collaborative environment for writing Haskell code. You can work on a project of your own, or help someone else with theirs. Q: What will the cost be? A: We're still working out the details, but as in past years, registration will be free, and we hope to be able to provide some of the meals. You'll be on your own for transportation and lodging. The Hac Phi team: Chris Casinghino (ccasin) Daniel Wagner (dmwit) Brent Yorgey (byorgey)

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Beginner: (Another) Binary Search Tree Question
by htc2011 12 Feb '11

12 Feb '11

Hi all, I just started learning Haskell but am having some trouble getting the insertion into a binary search tree to work and was hoping that somebody could advise me on a possible solution. Thanks. Given the data type definition of the tree to be: Code: data Ord a => BST a = EmptyBST | Node ( BST a ) a ( BST a ) deriving (Show, Eq) And the insertion function to be: Code: insert v EmptyBST = Node EmptyBST v EmptyBST insert v (Node b a c) | v<= a = Node a ( insert v b ) c | otherwise = Node a b ( insert v c ) GHCI keeps telling me that I 'cannot construct an infinite type': ' Occurs check: cannot construct the infinite type: a = BST (BST a) Expected type: BST a -> BST (BST a) -> a Inferred type: BST a -> BST (BST a) -> BST (BST a) In the second argument of `Node', namely `(insert v b)' In the expression: Node a (insert v b) c' Upon adding the following signature: Code: insert :: Ord a => a -> BST a -> BST a I get: Occurs check: cannot construct the infinite type. Sorry, but I'm quite lost. Any thoughts? Thanks. Benjamin -- View this message in context: http://haskell.1045720.n5.nabble.com/Beginner-Another-Binary-Search-Tree-Qu… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANN: bytestring-trie 0.2.3
by wren ng thornton 12 Feb '11

12 Feb '11

-------------------------------------------- -- bytestring-trie 0.2.3 -------------------------------------------- A long-awaited release for efficient finite maps from (byte)strings to values. This version adds a number of new functions for taking advantage of the trie structure. At the Haskell Symposium 2010, Milan Straka presented a wonderful paper comparing the state of the @containers@ library pre-GHC7. In it he compared the new @hashmap@ against the buggy old @bytestring-trie@ 0.1.4 (the major bug was fixed in 0.2.2 released 2010-06-10), and I've been meaning to update the package ever since. I haven't had a chance to do an extensive performance analysis yet, but it's worth clearing up some misconceptions--- which is what this release is all about. If you are only interested in being able to associate strings to values, then you may prefer the @hashmap@ package which is faster for those only needing a map-like structure. This package is intended for those who need the extra capabilities that a trie-like structure can offer (e.g., structure sharing to reduce memory costs for highly redundant keys, taking the submap of all keys with a given prefix, contextual mapping, extracting the minimum and maximum keys, etc.) While hacking on another project I noticed that @bytestring-trie@ didn't actually export many functions taking advantage of the trie structure. So let's fix that! -------------------------------------------- -- Changes (available in 0.2.2, but not announced previously) -------------------------------------------- * Added some functions for treating tries like priority queues (minAssoc, maxAssoc, updateMinViewBy, updateMaxViewBy). Currently these are only exported by Data.Trie.Internal though future releases may re-export them from Data.Trie too. -------------------------------------------- -- Changes (since 0.2.2) -------------------------------------------- * Added Data.Trie.Internal.alterBy_: A variant of 'alterBy' which also allows modifying the sub-trie. This is useful for cases where you want to alter a number of values which are each prefixes of one another (e.g., "a", "ab", "abc",...). * Added a number of "contextual" mapping functions (contextualMap, contextualMap', contextualFilterMap, contextualMapBy) which give access to the sub-trie rooted at each value. Currently these are only exported by Data.Trie.Internal though future releases may re-export them from Data.Trie too. * Added strict variants of a number of functions in Data.Trie.Convenience (fromListWith', insertWith', insertWithKey', unionWith'). * Added some additional functions for building tries from association lists (fromListWith', fromListWithL, fromListWithL') as suggested by Ian Taylor. * Converted the definitions for fmap, foldMap, traverse, and filterMap to use the worker/wrapper transform. This appears to be an optimization, though I don't have a decent benchmarking suite for verifying this. * Lots of tweaks and improvements to the documentation. -------------------------------------------- -- Future work -------------------------------------------- * I'm still not leased with the situation of having so many different variants of fromList. There must be a nicer solution which generalizes over all of them and doesn't have the various pessimal corner cases... * The priority queue functions still need some work to make them general as well as comprehensive like the other functions in Data.Trie.Internal are. * I still need to do a thorough performance analysis. The code for Data.Trie was based very closely on the code for Data.IntMap. Milan Straka found a number of ways to optimize the Data.IntMap code, so I need to incorporate those into the Data.Trie code. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/bytestring-trie Darcs: http://community.haskell.org/~wren/bytestring-trie/ Haddock (Darcs version): http://community.haskell.org/~wren/bytestring-trie/dist/doc/html/bytestring… -- Live well, ~wren

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Introducing monad-embed
by Tim Maxwell 12 Feb '11

12 Feb '11

Hi, monad-embed is an experimental programming language that parameterizes everything on a monad by default. Examples, source code, etc. are at <http://timmaxwell.org/pages/monad-embed/>. I posted about it on the Haskell subreddit a few months ago (see <http://bit.ly/cUZvrB>) but I was advised to re-post it in haskell-cafe as well. Tim Maxwell

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Re: [Haskell-cafe] Compiling network-bytestring with ghc7+hp2011
by Magnus Therning 12 Feb '11

12 Feb '11

On Fri, Feb 11, 2011 at 04:48:28PM -0800, Johan Tibell wrote: > Hi Magnus, > > network-2.3 supersedes network-bytestring as the modules provided by > the latter were merged into network. network-2.3 and later cannot be > used together with network-bytestring, in one package, as both > expose the same modules. Libraries that use network-bytestring > should eventually migrate to network-2.3. Excellent, I wasn't aware of this. I suggest that the Hackage entry for network-bytestring is modified to let users know this. I noticed that monads-fd has been marked as obsolete on Hackage, with a pointer to mtl, maybe it's possible to do something similar with network-bytestring? /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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Re: [Haskell-cafe] hledger: mtl vs transformers
by Simon Michael 11 Feb '11

11 Feb '11

On Feb 11, 2011, at 11:20 AM, Peter Simons wrote: > If hledger offers optional features by means of Cabal flags, then > users > of the library need the ability to depend on hledger with a specific > set > of features (flags) enabled or disabled, but unfortunately Cabal can't > do that. > > The new approach remedies that problem by placing optional features in > separate packages. That creates a different problem, however, because > users may now install hledger and hledger-web separately, which -- > as we > know -- may create all kinds of version conflicts that are non-trivial > to resolve. > > This is the problem you're now trying to solve by over-specifying the > dependencies of hledger. Arguably, though, Cabal should solve that > problem for the user, i.e. by offering to re-compile hledger to > resolve > the conflict etc., but unfortunately Cabal can't do that either. Hi Peter.. thanks for the follow-up. In case it wasn't clear, I do realise now that attempts to over-specify depedencies to avoid practical cabal install issues can backfire, and a package author should just accept that cabal install can't be made 100% reliable installation method for non-experts (for now - I know many people are thinking about this). And, I've fixed/relaxed the process dependency in hledger head, and only laziness/business has prevented that from being released to hackage. I'll do that asap. Cheers - Simon

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Haskell Help
by PatrickM 11 Feb '11

11 Feb '11

I'm writting a function that will remove tautologies from a fomula.The basic idea is that if in a clause, a literal and its negation are found, it means that the clause will be true, regardless of the value finally assigned to that propositional variable.My appoach is to create a function that will remove this but for a clause and map it to the fomula.Of course I have to remove duplicates at the beginning. module Algorithm where import System.Random import Data.Maybe import Data.List type Atom = String type Literal = (Bool,Atom) type Clause = [Literal] type Formula = [Clause] type Model = [(Atom, Bool)] type Node = (Formula, ([Atom], Model)) removeTautologies :: Formula -> Formula removeTautologies = map tC.map head.group.sort where rt ((vx, x) : (vy, y) : clauses) | x == y = rt rest | otherwise = (vx, x) : rt ((vy, y) : clauses) Now I have problems when I try to give it a formula (for example (A v B v -A) ^ (B v C v A)).Considering that example the first clause contains the literals A and -A. This means that the clause will always be true, in which case it can be simplify the whole set to simply (B v C v A) . But I get the following Loading package old-locale-1.0.0.2 ... linking ... done. Loading package time-1.1.4 ... linking ... done. Loading package random-1.0.0.2 ... linking ... done. [[(True,"A"),(True,"B")*** Exception: Assignment.hs:(165,11)-(166,83): Non-exhaustive patterns in function rt What should I do? -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-Help-tp3381647p3381647.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell Help
by Patrick M 11 Feb '11

11 Feb '11

I'm writting a function that will remove tautologies from a fomula.The basic idea is that if in a clause, a literal and its negation are found, it means that the clause will be true, regardless of the value finally assigned to that propositional variable.My appoach is to create a function that will remove this but for a clause and map it to the fomula.Of course I have to remove duplicates at the beginning. module Algorithm where import System.Random import Data.Maybe import Data.List type Atom = String type Literal = (Bool,Atom) type Clause = [Literal] type Formula = [Clause] type Model = [(Atom, Bool)] type Node = (Formula, ([Atom], Model)) removeTautologies :: Formula -> Formula removeTautologies = map tC.map head.group.sort where rt ((vx, x) : (vy, y) : clauses) | x == y = rt rest | otherwise = (vx, x) : rt ((vy, y) : clauses) Now I have problems when I try to give it a formula (for example (A v B v -A) ^ (B v C v A)).Considering that example the first clause contains the literals A and -A. This means that the clause will always be true, in which case it can be simplify the whole set to simply (B v C v A) . But I get the following Loading package old-locale-1.0.0.2 ... linking ... done. Loading package time-1.1.4 ... linking ... done. Loading package random-1.0.0.2 ... linking ... done. [[(True,"A"),(True,"B")*** Exception: Assignment.hs:(165,11)-(166,83): Non-exhaustive patterns in function rt What should I do?

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How to write type-level lambda-abstractions at the value level
by oleg＠okmij.org 11 Feb '11

11 Feb '11

HList programming -- as programming with too revealing type systems in general -- is quite tedious. Since the type of an HList value reveals its structure, including the number of the elements and their sequence, we often have to write an HList algorithm twice. We have to program the operation on HList values, using ordinary Haskell. We also have to write the corresponding operation on types, using this time the emergent programming language built, by accident, into the type system. In HList programming, the value- and type- operations are almost the same. For example, to take the head of an HList, we have to extract the first element of the value-level and of the type-level cons-cells. It offends the lazy programmer having to implement the same algorithm twice. What adds injury to the insult is that the type-level language is quite different from the value-level one. The type-level language is untyped, leading to difficult-to-find errors and puzzling error messages. Second, it is not quite higher-order: there seem to be no type-level lambdas. We offer a sedative, showing how to write even higher-order HList operations only once, in one piece of code, from which value- and type- level transformations are derived by the compiler. The key was to bring the type-level language closer to Haskell, in particular, by using lambda-abstractions with named, humanly readable variables. Our unified type-value code relies on the applicative notation, taking it to unintended extremes (hence we call it ultra-applicative). Along the way we find that a form of the first-class, bounded polymorphism has existed in Haskell all along. The goals of unifying value- and type-level HList programming and improving error diagnosis have been set and achieved (albeit perhaps a bit painfully) in Jeff Polakow's recent article: Improving HList programming/debugging (longish) http://www.haskell.org/pipermail/haskell-cafe/2011-January/088168.html which has been the inspiration for the present message. The main difference is ours use of lambda-calculus with named variables to program HList operations. Jeff Polakow has used combinator calculus (the point-free notation), which may become abstruse. Whereas Jeff Polakow relied on GADTs to add a compiler-run--time checking, we avoid GADTs, using the standard tagging approach. At least on Jeff Polakow's examples, GHC reports the same error diagnostics. We borrow Jeff Polakow's main example, writing filter in terms of fold. Here is the (value-level) code for ordinary Haskell lists: > vfilter :: (a -> Bool) -> [a] -> [a] > vfilter p = foldr f [] > where f x v = if p x then x : v else v Here is Jeff Polakow's filter code for HLists: > hFltr p = hFoldr ((Uncurry :$ (HIf Fst Snd)) :. (p :&&& (Cons :&&& > (Flip :$ Const)))) Nil Here is our filter for HLists: > hFilter3 p = hFoldr f nil > where f = lam (X,(V,())) > (HIF (p <$> var X) > (HCONS <$> var X <*> var V) > (var V)) The code ostensibly specifies the operation on HList values. However, it also specifies the corresponding operation on HList types, which we can see from the inferred type of hFliter3: > *TypeLambdaVal> :t hFilter3 > hFilter3 > :: (HFOLDR > (LAM > (X, (V, ())) > (HIF > (A (HConst x) (Lookp X)) > (A (A (HConst HCONS) (Lookp X)) (Lookp V)) > (Lookp V))) > (HList Nil) > xs > res) => > x -> HList xs -> res Here is the sample application, to the HList > l1 = (hTrue *: "a" *: () *: nil) *: > (hFalse *: 'b' *: nil) *: > (hTrue *: nil) *: > nil with the following type and printed value > *TypeLambdaVal> :t l1 > l1 > :: HList > (HList (HBool HTrue :* ([Char] :* (() :* Nil))) > :* (HList (HBool HFalse :* (Char :* Nil)) > :* (HList (HBool HTrue :* Nil) :* Nil))) > *TypeLambdaVal> l1 > [<[<HTrue, "a", ()>], [<HFalse, 'b'>], [<HTrue>]>] The result of filtering has the different printed value, and the matching, also filtered, type: > thF3 = hFilter3 HHead l1 > > *TypeLambdaVal> :t thF3 > thF3 > :: HList > (HList (HBool HTrue :* ([Char] :* (() :* Nil))) > :* (HList (HBool HTrue :* Nil) :* Nil)) > *TypeLambdaVal> thF3 > [<[<HTrue, "a", ()>], [<HTrue>]>] The complete code of the article is available at http://okmij.org/ftp/Haskell/TypeLambdaVal.hs We comment on the salient points below. We skip the discussion of error diagnostics; please see the comments in the code. Our workhorse is the class Apply, which has been defined and used already in the HList library. Its other notable applications are second-order typeclass programming http://okmij.org/ftp/Haskell/types.html#poly2 type-level lambda-calculus http://okmij.org/ftp/Haskell/types.html#computable-types > class Apply f x res | f x -> res where > apply :: f -> x -> res Jeff Polakow's message also relies on Apply, calling it HEval. We prefer the name Apply, because of the following, `natural' instance > instance a ~ a' => Apply (a -> b) a' b where > apply = ($) We starts with a simpler example, of > safe_head :: [a] -> Maybe a > safe_head lst = if null lst then Nothing else Just (head lst) and its implementation for HLists, which is subtle. The standard Haskell function `head' is partial, which is a source of much grief -- but also permitting the simple safe_head code above, which includes (head lst) as a sub-expression. The argument lst may well be the empty list. The type checker does permit applications of `head' to a potentially empty list. That application is guarded by the 'null lst' test and so it will not be executed, and so safe_head is safe indeed. At type-level, things are different: hHead is the total function. The application to the empty HList won't type, regardless of whether it will be executed. We solve the problem with the applicative IF expression, and write hsafe_head simply as > hsafe_head lst = > hif HNull (const HNothing) > (\lst -> HJust (apply HHead lst)) > lst When it comes to iterative computations, we have no such luck. We try > hFilter1 p = hFoldr f nil > where > f (x,(v,())) = hif p (\x -> x *: v) (const v) x (assuming the appropriately defined foldr on HList), which does type check. The inferred type however hFilter1 :: (Apply test t (HBool b), Apply (HIF' (x -> HList (x :* xs)) (b1 -> HList xs)) (HBool b, t) res, HFOLDR ((t, (HList xs, ())) -> res) (HList Nil) xs1 res1) => test -> HList xs1 -> res1 points out the problem, which shows up we try to apply hFilter1: > thF1 = hFilter1 HHead l1 > Couldn't match expected type `HFalse' against inferred type `HTrue' The type of hFilter1 can be written as hFilter1 :: forall t x xs b1 b test xs1 res1. (Apply test t (Bool b), ...) => test -> HList xs1 -> res1 or, in other words, hFilter1 :: forall test xs1 res1. (exists t x xs b1 b. Apply test t (Bool b), ...) => test -> HList xs1 -> res1 But we need hFilter1 :: forall test xs1 res1. (forall t x xs b1 b. Apply test t (Bool b), ...) => test -> HList xs1 -> res1 We need first-class polymorphism. Moreover, we need first-class bounded polymorphism, a first-class polymorphic value with type-class constraints. We eventually get it, by programming the lambda-calculus interpreter with human-readable variable names. The result, the hFilter3 code, was shown at the beginning of the message. Please see the complete code for further details. We only mention the implementation of the beta-reduction: > instance Apply body (ENV v arg) res => Apply (LAM v body) arg res where > apply (LAM body) arg = apply body ((ENV arg) :: ENV v arg) We have confirmed that everything is indeed up to interpretation. We may introduce any notation so long as we could interpret it. Solving a problem in an unusual language by first writing a lambda-calculus interpreter in that language is a good strategy. Interpreting lambda-calculus is sure subtle, yet can be done in many languages, even the ones that were never intended as programming languages, such as C++ templates, Haskell multi-parameter type classes with functional dependencies, or Scheme syntax-rules. Scheme syntax-rules is another example of making use of an odd-ball language by first writing a lambda-calculus interpreter for it. The approach was taken quite far, to the Scheme -to- syntax-rules compiler http://okmij.org/ftp/Scheme/macros.html#Macro-CPS-programming Given (a well-tested) procedure written in a subset of Scheme, the compiler outputs a syntax-rule macro that effects the same computation at the compile-time. Syntax-rules macros are _nothing_ like value-level Scheme code (for example, syntax-rule macros are not applicative). The only publicly known practical application is stress-testing macro-expanders: http://okmij.org/ftp/Scheme/macros.html#syntax-rule-stress-test executing the (compiled from Scheme) syntax-rule that tests if 5 is prime. The macro implements the sieve of Eratosthenes. There has been one macro-expander that crashed, taking down the OS, FreeBSD, with it. That is the only case known to me of a macro and a primality test crashing the operating system.

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Haskell Summers of Code retrospective (updated for 2010)
by Gwern Branwen 11 Feb '11

11 Feb '11

2 years ago in February 2009, I wrote up a history of Summers of Code through 2008 (http://www.haskell.org/pipermail/haskell-cafe/2009-February/055489.html) But the Wheel turns, and years come and pass, leaving memories that fade into 404s; a wind rose in Mountain View, whispering of the coming Summer... I have considerably expanded and updated the coverage: http://www.gwern.net/Haskell%20Summer%20of%20Code.html It now covers the 2009 & 2010 SoCs, adds scads of links, flips the appraisal of some of the older SoCs as time passed and more info came to light, and adds a section discussing 12 proposals from the subreddit & Trac. (It's long enough that I don't feel comfortable copying it inline as I did 2 years ago.) -- gwern http://www.gwern.net

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MissingH won't upgrade
by Andrew Pennebaker 10 Feb '11

10 Feb '11

Specs: - cabal-install 0.8.2 - Cabal 1.8.0.6 - GHC 6.12.3 - Mac OS X 10.6.6 - MacBook Pro 5,1 $ cabal upgrade missingh Resolving dependencies... Configuring regex-base-0.93.2... Preprocessing library regex-base-0.93.2... Building regex-base-0.93.2... [1 of 4] Compiling Text.Regex.Base.RegexLike ( Text/Regex/Base/RegexLike.hs, dist/build/Text/Regex/Base/RegexLike.o ) [2 of 4] Compiling Text.Regex.Base.Context ( Text/Regex/Base/Context.hs, dist/build/Text/Regex/Base/Context.o ) [3 of 4] Compiling Text.Regex.Base ( Text/Regex/Base.hs, dist/build/Text/Regex/Base.o ) [4 of 4] Compiling Text.Regex.Base.Impl ( Text/Regex/Base/Impl.hs, dist/build/Text/Regex/Base/Impl.o ) Registering regex-base-0.93.2... Installing library in /Users/andrew/.cabal/lib/regex-base-0.93.2/ghc-6.12.3 Registering regex-base-0.93.2... Configuring regex-posix-0.94.4... Preprocessing library regex-posix-0.94.4... Building regex-posix-0.94.4... [1 of 6] Compiling Text.Regex.Posix.Wrap ( dist/build/Text/Regex/Posix/Wrap.hs, dist/build/Text/Regex/Posix/Wrap.o ) [2 of 6] Compiling Text.Regex.Posix.String ( Text/Regex/Posix/String.hs, dist/build/Text/Regex/Posix/String.o ) [3 of 6] Compiling Text.Regex.Posix.Sequence ( Text/Regex/Posix/Sequence.hs, dist/build/Text/Regex/Posix/Sequence.o ) [4 of 6] Compiling Text.Regex.Posix.ByteString ( Text/Regex/Posix/ByteString.hs, dist/build/Text/Regex/Posix/ByteString.o ) [5 of 6] Compiling Text.Regex.Posix.ByteString.Lazy ( Text/Regex/Posix/ByteString/Lazy.hs, dist/build/Text/Regex/Posix/ByteString/Lazy.o ) [6 of 6] Compiling Text.Regex.Posix ( Text/Regex/Posix.hs, dist/build/Text/Regex/Posix.o ) Registering regex-posix-0.94.4... Installing library in /Users/andrew/.cabal/lib/regex-posix-0.94.4/ghc-6.12.3 Registering regex-posix-0.94.4... Configuring regex-compat-0.93.1... Preprocessing library regex-compat-0.93.1... Building regex-compat-0.93.1... [1 of 1] Compiling Text.Regex ( Text/Regex.hs, dist/build/Text/Regex.o ) Registering regex-compat-0.93.1... Installing library in /Users/andrew/.cabal/lib/regex-compat-0.93.1/ghc-6.12.3 Registering regex-compat-0.93.1... Configuring MissingH-1.1.0.3... Preprocessing library MissingH-1.1.0.3... Preprocessing executables for MissingH-1.1.0.3... Building MissingH-1.1.0.3... [ 1 of 47] Compiling System.Console.GetOpt.Utils ( src/System/Console/GetOpt/Utils.hs, dist/build/System/Console/GetOpt/Utils.o ) [ 2 of 47] Compiling System.Debian ( src/System/Debian.hs, dist/build/System/Debian.o ) [ 3 of 47] Compiling System.IO.WindowsCompat ( src/System/IO/WindowsCompat.hs, dist/build/System/IO/WindowsCompat.o ) [ 4 of 47] Compiling System.Posix.Consts ( src/System/Posix/Consts.hs, dist/build/System/Posix/Consts.o ) [ 5 of 47] Compiling System.IO.StatCompat ( src/System/IO/StatCompat.hs, dist/build/System/IO/StatCompat.o ) [ 6 of 47] Compiling System.IO.PlafCompat ( src/System/IO/PlafCompat.hs, dist/build/System/IO/PlafCompat.o ) [ 7 of 47] Compiling Data.Compression.Inflate ( src/Data/Compression/Inflate.hs, dist/build/Data/Compression/Inflate.o ) [ 8 of 47] Compiling Data.Hash.MD5.Zord64_HARD ( src/Data/Hash/MD5/Zord64_HARD.lhs, dist/build/Data/Hash/MD5/Zord64_HARD.o ) src/Data/Hash/MD5/Zord64_HARD.lhs:4:2: Bad interface file: /Users/andrew/.cabal/lib/haskell98-1.1.0.0/ghc-6.12.3/Word.hi Something is amiss; requested module haskell98:Word differs from name found in the interface file haskell98-1.1.0.0:Word cabal: Error: some packages failed to install: MissingH-1.1.0.3 failed during the building phase. The exception was: ExitFailure 1 Cheers, Andrew Pennebaker

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Building Plaftorm 2010 2 on 64bit Linux
by Dušan Kolář 10 Feb '11

10 Feb '11

Hello all, I'm trying to build platform on Linux 2.6.32.22 #1 SMP Tue Sep 21 09:44:04 CEST 2010 x86_64 x86_64 x86_64 GNU/Linux. I've downloaded binary of ghc6.12.3 for the purpose, but ghc doesn't work. ghci does work, gcc does work, but for ghc I get: ghc -O2 --make test.hs -o test [1 of 1] Compiling Main ( test.hs, test.o ) ghc: could not execute: /usr/bin/gcc Even if I can easily compile gcc testc.c -o testc and run testc. Could anyone point me to a solution? Helps found by google pointed me to try use option -pgmc gcc, which doesn't work. All other threads with similar issue stayed unresoleved. :-( Thanks Dusan

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Re: [Haskell-cafe] Cabal && license combinations
by Vivian McPhail 10 Feb '11

10 Feb '11

> > It seems then that a package should be the least restrictive > > combination of all the licenses in all the contained modules. > > Omit the words "least restrictive" and I think you are correct. > > To combine licences, just aggregate them. There is no lattice of > subsumption; no "more" or "less" restrictive ordering. > I was thinking that the lattice was already flattened into a list of licences. Currently the top-level package has a single licence field which is an arbitrary disjunctive choice. Much better is a conjunctive aggregation which is just as or less restrictive than the arbitrary disjunctive choice. Cheers, Vivian P.S. OK := acknowledge [ACK]

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Re: [Haskell-cafe] Cabal && license combinations
by Vivian McPhail 10 Feb '11

10 Feb '11

> On Mon, 2011-02-07 at 14:42 +0000, Malcolm Wallace wrote: > > > It seems then that a package should be the least restrictive > > > combination of all the licenses in all the contained modules. > > > > Omit the words "least restrictive" and I think you are correct. > OK. > > > > To combine licences, just aggregate them. There is no lattice of > > subsumption; no "more" or "less" restrictive ordering. It's simple: > > you must obey all of them. Some aggregations introduce a > > contradiction of terms, so you cannot legally aggregate those modules > > without breaking some term. But if the terms of the aggregated > > licences are compatible rather than contradictory, then all is good. > > Right, so the effect of per-file/mixed licenses could be achieved by > letting packages specify a list of licenses: > > license: Foo, Bar > Could this be computed automatically from the source files by Cabal? > > Meaning you may copy/distribute provided you comply with all these > licenses. > > Note that this does not cover dual licensing, e.g. Foo or Bar at > distributor's choice. > > Duncan > Looking specifically at hmatrix, there are three kinds of modules i) bindings to GSL GPL ii) bindings to LAPACK BSD iii) pure Haskell hmatrix author's choice 1) Am I correct in thinking that even the bindings modules (the Haskell parts, not the C files) can be under any licence, FOO, chosen by the author, but the binary _linked_ to, say, GSL has to comply with FOO and GPL? 2) If someone uses hmatrix but no GSL functions (hence there are no GSL functions in the linked binary) can they get away with not complying with the GSL requirement? Cheers, Vivian

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Haskell Weekly News: Issue 168 - February 09, 2011
by Daniel Santa Cruz 10 Feb '11

10 Feb '11

Welcome to issue 168 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of January 30 to February 5, 2011. Announcements Maciej Piechotka [2]announced version 0.1.1 of nanoparsec. "Nanoparsec is currently simply a port of attoparsec on ListLike (the abstraction of lists used by iteratee)." Graham Hutton [3]announced the availability of studenships in Functional Programming at the University of Nottingham, UK. Ashley Yakeley [4]made a small update to time (1.2.0.4) that "fixes a bug in parseTime." George Giorgidze [5]announced the release to Hackage of Database Supported Haskell (DSH). Simon Hengel [6]announced the release of HackageOneFive, "a tiny Snap app that provides reverse dependency lookup for all packages on Hackage." Quotes of the Week * alpounet: map succ/pred is a scandinavian name generator or what? ... well, scandinavian and aztec * dons: Think of a monad as a spacesuit full of nuclear waste in the ocean next to a container of apples. Now, you can't put oranges in the space suite or the nuclear waste falls in the ocean, *but* the apples are carried around anyway, and you just take what you need. * Apocalisp: a unit of clarity is a clarinet * roconnor: My coq has failed me * roconnor: I mean, I could, and do, say that a lens is just a monoid natural transformation from the coalgebra functiors from haskell functors to haskell types. * Saizan: "not being ML" is a quite sound principle syntax-wise * lpsmith: well, getting rid of airliners certainly *is* one way of eliminating their potential use as weapons against larger buildings * ddarius: The reason edwardk writes so many Haskell packages is to ward the Perl from penetrating his heart. * monochrom: most people tend to deny the usefulness of useful alternative perspectives --- their entrenched vested interests demand it. as they grow older they also have more influence and power over "the state of the art" to preserve their self-fulfilling prophecy that "the alternative is useless" --- by controlling what practice looks like, they can control what looks useless. * edwardk: i learned to program becaise i'd lied and told a kid that i'd written a disassembler, then had to make good on the claim. Top Reddit Stories * Quine Central ... because everyone needs to be able to make nth order quines Domain: blog.sigfpe.com, Score: 40, Comments: 7 On Reddit: http://www.reddit.com/r/haskell/comments/fc4cs/quine_central_because_everyo… Original: http://blog.sigfpe.com/2011/01/quine-central.html * jhc 0.7.7 is out Domain: haskell.org, Score: 38, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/fbqo7/jhc_077_is_out/ Original: http://www.haskell.org/pipermail/jhc/2011-January/000874.html * Hulk: A Haskell IRC server Domain: chrisdone.com, Score: 32, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/fbrie/hulk_a_haskell_irc_server/ Original: http://chrisdone.com/posts/2011-01-30-hulk-haskell-irc-server.html * Status update on {code, trac, projects, planet, community}.haskell.org Domain: haskell.org, Score: 25, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/fdnjp/status_update_on_code_trac_p… Original: http://www.haskell.org/pipermail/haskell-cafe/2011-February/088829.html * All about MVars : Inside T5 Domain: blog.ezyang.com, Score: 23, Comments: 4 On Reddit: http://www.reddit.com/r/haskell/comments/fduxb/all_about_mvars_inside_t5/ Original: http://blog.ezyang.com/2011/02/all-about-mvars/ * Text: a visual programming language based on Haskell, demo Domain: vimeo.com, Score: 21, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/fbgxd/text_a_visual_programming_la… Original: http://vimeo.com/19273744 * [SO] Why is GHC so big? Domain: stackoverflow.com, Score: 21, Comments: 10 On Reddit: http://www.reddit.com/r/haskell/comments/fczht/so_why_is_ghc_so_big/ Original: http://stackoverflow.com/questions/4858585/why-is-ghc-so-large-big * Haskell Weekly News: Issue 167 Domain: contemplatecode.blogspot.com, Score: 21, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/fe63a/haskell_weekly_news_issue_16… Original: http://contemplatecode.blogspot.com/2011/02/haskell-weekly-news-issue-167.h… * The F# Team are Hiring (functional programming jobs) Domain: haskell.org, Score: 20, Comments: 6 On Reddit: http://www.reddit.com/r/haskell/comments/fcvec/the_f_team_are_hiring_functi… Original: http://www.haskell.org/pipermail/haskell/2011-January/022523.html * Recently Uploaded: DSH-0.4.2 Natural Sql Queries without the need to write Sql strings. Domain: hackage.haskell.org, Score: 20, Comments: 2 On Reddit: http://www.reddit.com/r/haskell/comments/fesjj/recently_uploaded_dsh042_nat… Original: http://hackage.haskell.org/package/DSH * Specifying behaviors in a purely functional language Domain: io7m.com, Score: 16, Comments: 0 On Reddit: http://www.reddit.com/r/haskell/comments/fdvux/specifying_behaviors_in_a_pu… Original: http://io7m.com/articles/itefs/ Top StackOverflow Answers * Why is GHC so large/big? votes: 29 Read on SO: http://stackoverflow.com/questions/4858585/why-is-ghc-so-large-big/4867236#… It's a bit silly really. Every library that comes with GHC is provided in no less than 4 flavours: static dynamic profiled GHCi The GHCi version is just the static version linked together in a single .o file. The other three versions all have their own set of interface files (.hi files) too. The profiled versions seem to be about twice the size of the ... * Why `(map digitToInt) . show` is so fast? votes: 24 Read on SO: http://stackoverflow.com/questions/4841078/why-map-digittoint-show-is-so-fa… Seeing as I can't add comments yet, I'll do a little bit more work and just analyze all of them. I'm putting the analysis at the top; however, the relevant data is below. (Note: all of this is done in 6.12.3 as well - no GHC 7 magic yet.) Analysis: Version 1: show is pretty good for ints, especially those as short as we have. Making strings actually ... * What Justification for the type of f x = f x in Haskell is there? votes: 15 Read on SO: http://stackoverflow.com/questions/4847008/what-justification-for-the-type-… Okay, starting from the function definition f x = f x, let's step through and see what we can deduce about the type of f. Start with a completely unspecified type variable, a. Can we deduce more than that? Yes, we observe that f is a function taking one argument, so we can change a into a function between two unknown type variables, which we'll call b -> ... * . versus $ in haskell votes: 14 Read on SO: http://stackoverflow.com/questions/4876828/versus-in-haskell/4876900#4876900 Prelude> :t ($) ($) :: (a -> b) -> a -> b Prelude> :t (.) (.) :: (b -> c) -> (a -> b) -> a -> c $ applies a function to a value. . composes two functions. So I can write f $ g x which is "apply f to (g of x)" or f . g $ x which is "apply the composition of f and g to x". One common style is to pile up dots on the left with a ... * Starting a game development in an exotic language I am just learning (Lisp, Haskell, ...) votes: 12 Read on SO: http://stackoverflow.com/questions/4867656/starting-a-game-development-in-a… If you could write a game in C, you can write one in Haskell. The biggest difficulties many people have with it are: Lack of support for OOP, which is what most people are familiar with these days. Old-fashioned procedural code, on the other hand, is actually very pleasant to write in Haskell; the IO type distinguishes effect-free functions from procedures ... * Haskell: Check if Int is in a list of Int's votes: 11 Read on SO: http://stackoverflow.com/questions/4846974/haskell-check-if-int-is-in-a-lis… First find the type of the function you need. To "Check if" means to return either True or False, a Bool. So the function takes an Int, a list of Int (aka [Int]) and returns Bool: Int -> [Int] -> Bool Now ask hoogle. elem :: Eq a => a -> [a] -> Bool Hoogle is a very useful tool. You can integrate it with ghci. Top StackOverflow Questions * Why is GHC so large/big? votes: 32, answers: 5 Read on SO: http://stackoverflow.com/questions/4858585/why-is-ghc-so-large-big * Starting a game development in an exotic language I am just learning (Lisp, Haskell, …) votes: 16, answers: 10 Read on SO: http://stackoverflow.com/questions/4867656/starting-a-game-development-in-a… * Why `(map digitToInt) . show` is so fast? votes: 8, answers: 2 Read on SO: http://stackoverflow.com/questions/4841078/why-map-digittoint-show-is-so-fa… * Transforming expression given in prefix notation, identifying common subexpressions and dependencies votes: 7, answers: 6 Read on SO: http://stackoverflow.com/questions/4843237/transforming-expression-given-in… * Practical use of curried functions? votes: 7, answers: 6 Read on SO: http://stackoverflow.com/questions/4888480/practical-use-of-curried-functio… About the Haskell Weekly News You can find an html version of this newsletter at: http://contemplatecode.blogspot.com/2011/02/haskell-weekly-news-issue-168.h… To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85781 3. https://groups.google.com/d/topic/fa.haskell/rURgYMjEZhU/discussion 4. https://groups.google.com/d/topic/fa.haskell/l-I05W30DWU/discussion 5. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85961 6. https://groups.google.com/d/topic/fa.haskell/vp2P5w8GbkM/discussion

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Where is "All about monads"?
by Magicloud Magiclouds 10 Feb '11

10 Feb '11

Hi, Just noticed that the link was deprecated. Googling shows that this thing does not exist on haskell.org any more? -- 竹密岂妨流水过山高哪阻野云飞

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Synthetic values?
by Cristiano Paris 09 Feb '11

09 Feb '11

Hi all, I've a type problem that I cannot solve and, before I keep banging my head against an unbreakable wall, I'd like to discuss it with the list. Consider the following code: ------------ module Main where class PRead p where {} class PWrite p where {} newtype Sealed p a = Sealed a instance Monad (Sealed p) where return = Sealed (Sealed x) >>= f = f x mustRead :: PRead p => a -> Sealed p a mustRead = Sealed mustWrite :: PWrite p => a -> Sealed p a mustWrite = Sealed readLog :: PRead p => String -> Sealed p String readLog = mustRead . id writeLog :: PWrite p => String -> Sealed p String writeLog = mustWrite . id appendLog l e = do l <- readLog l writeLog $ l ++ e ------------ The central type of this code is Sealed, which boxes a value inside a newtype with a phantom type which represents a "set of permissions". This set of permissions is implemented through a series of type classes (PRead and PWrite in this case) which are attached to the permission value p of the Sealed newtype. This way I can define which set of permissions I expect to be enforced when trying to peel off the Sealed value. The use of the Monad class and type classes as permissions behaves nicely when combining functions with different permission constraints, as it's the case of appendLog, whose type signature is: appendLog :: (PRead p, PWrite p) => String -> [Char] -> Sealed p String Very nice, the permissions accumulates as constraints over the p type. Now for the "peel-off" part: ------------ unseal :: p -> Sealed p a -> a unseal _ (Sealed x) = x ------------ Basically this function requires a witness value of the type p to peel-off the Sealed value. Notice that: ------------ unseal undefined $ appendLog "Foo" "Bar" ------------ won't work as the undefined value is unconstrained. That's good, because otherwise it'd very easy to circumvent the enforcing mechanism. So, I defined some "roles": ------------ data User = User data Admin = Admin instance PRead User where {} instance PRead Admin where {} instance PWrite Admin where {} ------------ If I try to unseal the Sealed value passing User, it won't succeed, as the type checker is expecting the value of a type which is also an instance of the PWrite class: ------------ *Main> unseal User $ appendLog "Foo" "Bar" <interactive>:1:14: No instance for (PWrite User) arising from a use of `appendLog' at <interactive>:1:14-34 ------------ while works perfectly if I pass Admin as a value: ------------ *Main> unseal Admin $ appendLog "Foo" "Bar" "FooBar" ------------ The idea is to hide the Admin and User constructor from the programmer and having two factory functions, checkAdmin and checkUser, which checks whether the current user has the named role, something like: ------------ checkAdmin :: IO Admin checkUser :: IO User ------------ where role checking happens in the IO Monad (or something similar), a-là trusted kernel. So far so good and I'm very happy with that. Now the problem. I would like to enforce permissions not at the role level, but at the permissions level. Let's say that I want to leave "unseal" unchanged, I'd like to "construct" a p-value for unseal "combining" functions checking for single permissions, that is, in pseudo-code: unseal (checkPRead .*. checkPWrite) $ appendLog "Foo" "Bar" where .*. is some kind of "type aggregation" operator. Or maybe something like: (checkPRead .*. checkPWrite) $ appendLog "Foo" "Bar" So far I got only frustration. In principle it seems possible to achieve this result because everything is known at compile time and the type-checked should have all the information available to enforce the security constraints. Anyhow, I couldn't write any usable code. Any help would be appreciated, even pointers to papers discussing this approach. Thank you, -- Cristiano GPG Key: 4096R/C17E53C6 2010-02-22 Fingerprint = 4575 4FB5 DC8E 7641 D3D8 8EBE DF59 B4E9 C17E 53C6

5 10

forkIO on GHCI does not seem to spawn the thread in the background on some OSs
by C K Kashyap 09 Feb '11

09 Feb '11

Hi, I found that on windows and my ubuntu box, when I did this ghci> t <- forkIO someAction "someAction" started executing in the foreground - as in, the ghci prompt did not come back (until I pressed Ctrl-C) On my mac however, when I ran the same thing, the action started executing in the background and ghci> prompt returned instantaneously. How can I get the thread backgrounded explicitly? Regards, Kashyap

3 5

haskell prop. logic
by Patrick M 09 Feb '11

09 Feb '11

Hello all I'm working on a project and I'm down to my last 3 functions.I need some help(in any form) with them as I'm a beginner in haskell.I tried to create a description on how the functions should behave below ,if you still have some questions don't hesitate to ask. Also here is the code for the function I've written so far(some with help). http://pastebin.com/fQp40ucg propagateUnits :: Formula->Formula If a clause in a propositional formula contains only one literal, then that literal must be true (so that the particular clause can be satisfied). When this happens,we can remove the unit clauses (the ones that contain only one literal), all the clauses where the literal appears and also, from the remaining clauses, we can delete the negation of the literal (because if P is true, -P will be false).For example, in the formula (P v Q v R) ^ (-P v Q v -R) ^ (P) we have one unit clause (the third clause(P) ). Because this one has to be true for the whole formula to be true we assign True to P and try to find a satisfying assignment for the remaining formula. Finally because -P cannot be true (given the assigned value of P) then the second clause is reduced by eliminating the symbol -P . This simplification results in the revised formula (Q v -R). The resulting simplification can create other unit clauses. For example in the formula (-P v Q) ^ (P) is simplified to (Q) when the unit clause (P) is propagated. This makes (Q) a unit clause which can now also be simplified to give a satisfying assignment to the formula. The function should apply unit propagation until it can no longer make any further simplifications. Note that if both P and -P are unit clauses then the formula is unsatisfiable. In this case the function should return a formula with an empty clause in it to indicate that the formula could not be satisfied. update :: Node -> [Node] The update function should take in a Node and return a list of the Nodes that result from assigning True to an unassigned atom in one case and False in the other (ie. a case split). So the list returned should have two nodes as elements. One node should contain the formula with an atom assigned True and the model updated with this assignment, and the other should contain the formula with the atom assigned False and the model updated to show this. The lists of unassigned atoms of each node should also be updated accordingly. This function should use your implemented assign function to make the assignments. It should also use the chooseAtom function provided to select the literal to assign. search :: (Node -> [Node]) -> [Node] -> Int -> (Bool, Int) The search function should perform a backtracking search. The function takes the update function as input and uses it to generate nodes in the search space. The search function also takes in a list which has one element, the initial node consisting of the formula along with an initial model. It should generate nodes using the update function and check nodes using the given check function. If a node is unsatisfiable then it should abandon that branch of the search. If a node is satisfiable then a satisfying assignment has been found and so it should return True. If a node is neither satisfiable or unsatisfiable then it should generate new nodes from this node. If all possible branches of the search space have been tried, i.e. the list of nodes to try has become the empty list, then it should return False since all possible assignments have been tried. The search function also has an Int argument which should be an integer that tracks how many calls to the update function have been made. The search function should return a pair consisting of the truth value of the formula and the number of calls to update made.

1 0

Possible bug in Control.Concurrent
by Krzysztof Skrzętnicki 09 Feb '11

09 Feb '11

Hello Cafe, Here is a simple program that yields strange results: module Main where import Control.Concurrent import Control.Concurrent.Chan import Control.Monad main = do c <- newChan writeChan c 1 forkIO $ forever $ do i <- readChan c print ("forkio",i) isEmptyChan c >>= print First of all, if we try to run it via runhaskell, it will hang: runhaskell deadlock.hs ("forkio",1) -- no more output -- Compiled version OTOH behaves differently dependent on compilation flags. Without -threaded: ./deadlock ("forkio",1) False With -threaded: ./deadlock False Now, this is strange thing: we put single element into the channel. We take it out. And then we see the channel isn't really empty. Perhaps there is a race condition here? So we put an delay, so that we will be sure the check for empty channel occurs 1 second later than the channel is emptied. import Control.Concurrent import Control.Concurrent.Chan import Control.Monad main = do c <- newChan writeChan c 1 forkIO $ forever $ do i <- readChan c print ("forkio",i) threadDelay 1000000 isEmptyChan c >>= print This program will misbehave. Invariably of -threaded flag it will go like this: ./deadlock ("forkio",1) deadlock: thread blocked indefinitely in an MVar operation I have no idea what is the problem here. Perhaps I'm not using the library in the right way. Does anyone has any idea what's going on here? Best regards, Krzysztof Skrzętnicki

3 3

ANN: monad-control-0.2 & a challenge
by Bas van Dijk 09 Feb '11

09 Feb '11

Dear all, I just released control-monad-0.2 a library for lifting control operations, like exception catching, through monad transformers: http://hackage.haskell.org/package/monad-control-0.2 darcs get http://bifunctor.homelinux.net/~bas/monad-control/ To quote the NEWS file: * Use RunInBase in the type of idLiftControl. * Added this NEWS file. * Only parameterize Run with t and use RankNTypes to quantify n and o -liftControl :: (Monad m, Monad n, Monad o) => (Run t n o -> m a) -> t m a +liftControl :: Monad m => (Run t -> m a) -> t m a -type Run t n o = forall b. t n b -> n (t o b) +type Run t = forall n o b. (Monad n, Monad o, Monad (t o)) => t n b -> n (t o b) Bumped version from 0.1 to 0.2 to indicate this breaking change in API. * Added example of a derivation of liftControlIO. This derivation of liftControlIO is really enlightening. It shows the recursive structure of liftControlIO applied to a stack of three monad transformers with IO as the base monad: t1 (t2 (t3 IO)) a: (Note that the derivation in the API documentation also shows the types of the intermediate computations) liftControlIO = liftLiftControlBase $ -- instance MonadControlIO t1 liftLiftControlBase $ -- instance MonadControlIO t2 liftLiftControlBase $ -- instance MonadControlIO t3 idLiftControl -- instance MonadControlIO IO = \f → liftControl $ \run1 → -- Capture state of t1 liftControl $ \run2 → -- Capture state of t2 liftControl $ \run3 → -- Capture state of t3 -- At this point we've captured the state of all transformers -- and have landed in the base (IO) monad. -- So we can start executing f: let run ∷ RunInBase (t1 (t2 (t3 IO))) IO run = -- Restore state liftM (join ∘ lift) ∘ liftM (join ∘ lift) -- Identity conversion ∘ liftM (join ∘ lift) ∘ liftM return -- Run ∘ run3 ∘ run2 ∘ run1 in f run This derivation clearly shows what is happening: first the state of all transformers is captured using the liftControl operations. Each capture supplies us a run function that allows us to run a computation of the respected monad transformer. When we arrive in the base (IO) monad we can start executing f. f expects a run function :: ∀ b. t1 (t2 (t3 IO)) b → IO (t1 (t2 (t3 IO)) b) which allows it to run a t1 computation in IO. The IO computation then returns a new t1 computation that has the final state of the given t1 computation. This can later be used to restore that final state. Although the derivation is correct, I'm not really happy with it from a performance perspective. In the created run function there are two places where it can do better: * The identity conversion: (liftM (join ∘ lift) ∘ liftM return) could be replaced with something more efficient like (liftM (join ∘ lift ∘ return)) or (liftM id) or just id. * The restore conversion: (liftM (join ∘ lift) ∘ liftM (join ∘ lift)) could be replaced with (liftM (join ∘ lift ∘ join ∘ lift)). This transformation is correct according to the Functor law: liftM f ∘ liftM g = liftM (f ∘ g). I haven't figured out yet how to change monad-control in such a way that it generates this more efficient derivation. So I'm posting this as a challenge to you! Good luck, Bas

1 0

Split function
by C K Kashyap 09 Feb '11

09 Feb '11

Hi, Is this a good implementation of split function? ... I was wondering about the "lines" function, it basically a special case of split right? split c xs = foldr f [[]] xs where f x list@(l:ls)= if x == c then []:list else (x:l):ls Regards, Kashyap

2 1

Re: [Haskell-cafe] semantics
by kevin＠froglingo.com 09 Feb '11

09 Feb '11

On Tue, Feb 8, 2011 at 07:55 pm, Conal Elliott <conal(a)conal.net> wrote: Here's my personal denotational answer to question 2: I think of a type as denoting a collection of (mathematical) values. If an expression e has type T, then the meaning (value) of e is a member of the collection denoted by T. This simple principle, which is fundamental to how I think of functional programming, has consequences in library design, which I've discussed at http://conal.net/blog/posts/notions-of-purity-in-haskell/ . When we consider a class of partial recursive functions as the type T, then what are the expressions (such as e in your statement above) for T? It seems that the type definition missed operations. For example, if a and b are two variables declared as integers, then we would not know how to calculate a + b if the type integers didn't include the plus operator. Defining a type as a set of values and a set of operations on the values is perfectly fine mathematically. But in the context of programming languages, a type seems to need its syntax anyhow in addition to be a set of values and a set of operations on the values. For example the type definition in C: typedef mydata { int a; char c; } The definition itself is the syntax while we may view the infinite many records {<0, 'a'>, <2, 'a'>, ..., <0, 'b>, ....} as its semantics (or the semantics was called a type earlier).

2 1

Re: [Haskell-cafe] semantics
by kevin＠froglingo.com 09 Feb '11

09 Feb '11

On Tue, Feb 8, 2011 at 2:01 PM, Alexander Solla <alex.solla(a)gmail.com> wrote: > It is hard to come up with a single operational semantic for types, because they do so many different things, and there is a semantic for each interpretation. G. Hillebrand, P. C. Kanellakis (in paper "Functional Database Query Languages as Typed Lambda Calculi of Fixed Order" and others) encoded tuples, lists, tables, the relational algebra, complex objects, and others in typed lambda calculus. The different things (I guess we can call them types here) were able to be compared with the uniformed (opeartional) semantics.

1 0

semantics
by Patrick Browne 09 Feb '11

09 Feb '11

Consider the following definitions: 1. Denotational semantics can be considered as relation between syntax and mathematical objects (the meaning of the syntax). 2. Operational semantics can be considered as set of rules for performing computation. Question 1 Applying these definitions in a Haskell context can I say that the lambda calculus provides both the operational and denotational semantics for Haskell programs at value level for definition and evaluations. Question 2 Does it make sense to use these definitions at type level? If so, what exactly do they represent? Thanks, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

3 2

Unrecognized function symbols in .prof
by Lee Pike 09 Feb '11

09 Feb '11

Hi, I am profiling a Haskell program using GHC, and after executing the program with +RTS -p, I get a .prof file that lists the cost centers of the program. Some of the cost centers listed are for function symbols that do not exist in the given module. For example, I have in my .prof file: COST CENTRE MODULE %time %alloc ==_a2MT Foo 19.8 0.0 ==_a2R8 Foo 17.8 0.0 ==_a2Bg Bar 13.7 0.0 .... .... It appears these are generated symbols; how do I find out what those function symbols correspond to? (In this case, my assumption is that these symbols refer to instances of the Eq class, but I'm not sure.) [I couldn't find anything about this in the GHC online documentation or via Googling, but sorry if I missed it.] Thanks, Lee

4 4

ACM SIGPLAN Workshop on ML
by Chung-chieh Shan 08 Feb '11

08 Feb '11

ACM SIGPLAN Workshop on ML Sunday, 18 September 2011, Tokyo, Japan (co-located with ICFP) http://conway.rutgers.edu/ml2011/ CALL FOR CONTENT The ML family of programming languages includes dialects known as Standard ML, Objective Caml, and F#. These languages have inspired a large amount of computer-science research, both practical and theoretical. This workshop aims to provide a forum for discussion and research on ML and related technology (higher-order, typed, or strict languages). The format of ML 2011 will continue the return in 2010 to a more informal model: a workshop with presentations selected from submitted abstracts. Presenters will be invited to submit working notes, source code, and extended papers for distribution to the attendees, but the workshop will not publish proceedings, so any contributions may be submitted for publication elsewhere. We hope that this format will encourage the presentation of exciting (if unpolished) research and deliver a lively workshop atmosphere. SCOPE We seek research presentations on topics related to ML, including but not limited to * applications: case studies, experience reports, pearls, etc. * extensions: higher forms of polymorphism, generic programming, objects, concurrency, distribution and mobility, semi-structured data handling, etc. * type systems: inference, effects, overloading, modules, contracts, specifications and assertions, dynamic typing, error reporting, etc. * implementation: compilers, interpreters, type checkers, partial evaluators, runtime systems, garbage collectors, etc. * environments: libraries, tools, editors, debuggers, cross-language interoperability, functional data structures, etc. * semantics: operational, denotational, program equivalence, parametricity, mechanization, etc. Research presentations should describe new ideas, experimental results, significant advances in ML-related projects, or informed positions regarding proposals for next-generation ML-style languages. We especially encourage presentations that describe work in progress, that outline a future research agenda, or that encourage lively discussion. In addition to research presentations, we seek both Status Reports and Demos that emphasize the practical application of ML research and technology. Status Reports: Status reports are intended as a way of informing others in the ML community about the status of ML-related research or implementation projects, as well as communicating insights gained from such projects. Status reports need not present original research, but should deliver new information. In the abstract submission, describe the project and the specific technical content to be presented. Demos: Live demonstrations or tutorials should show new developments, interesting prototypes, or work in progress, in the form of tools, libraries, or applications built on or related to ML. In the abstract submission, describe the demo and its technical content, and be sure to include the demo's title, authors, collaborators, references, and acknowledgments. (Please note that you will need to provide all the hardware and software required for your demo; the workshop organizers are only able to provide a projector.) Each presentation should take 20-25 minutes, except demos, which should take 10-15 minutes. The exact time will be decided based on the number of accepted submissions. We plan to make videos of the presentations available on ACM Digital Library. SUBMISSION INSTRUCTIONS Email submissions to ccshan AT cs.rutgers.edu. Submissions should be at most two pages, in PDF format, and printable on US Letter or A4 sized paper. Persons for whom this poses a hardship should contact the program chair. Submissions longer than a half a page should include a one-paragraph synopsis suitable for inclusion in the workshop program. IMPORTANT DATES * 2011-06-17: Submission * 2011-07-22: Notification * 2011-09-18: Workshop PROGRAM COMMITTEE Amal Ahmed (Indiana University) Andrew Tolmach (Portland State University) Anil Madhavapeddy (University of Cambridge) Chung-chieh Shan (chair) (Rutgers University) Joshua Dunfield (Max Planck Institute for Software Systems) Julia Lawall (University of Copenhagen) Keisuke Nakano (University of Electro-Communications) Martin Elsman (SimCorp) Walid Taha (Halmstad University) STEERING COMMITTEE Eijiro Sumii (chair) (Tohoku University) Andreas Rossberg (Google) Jacques Garrigue (Nagoya University) Matthew Fluet (Rochester Institute of Technology) Robert Harper (Carnegie Mellon University) Yaron Minsky (Jane Street)

1 0

Second call for submissions: Monad.Reader special poetry and fiction edition
by Brent Yorgey 08 Feb '11

08 Feb '11

Ten days until the submission deadline. "Nein": what I won't take for an answer. Eighth to eighteenth February, Seven thousand minutes twice, Sick's he who can't write aught. Five submissions now; Four more, at least? Three'd be fair To no- One. http://themonadreader.wordpress.com/2011/01/19/call-for-copy-special-poetry…

1 0

File position
by Permjacov Evgeniy 08 Feb '11

08 Feb '11

Is there any way to get current position from System.IO.Handle as Integer? If no, what other <b> windows/unix portable</b> IO function set of file io with this functionality you can suggest? keeping track of current position by hand is not an option, sorry. I'm thinking about c's low-level functions such as open, read, write, but there is no such functions in windows and I do not know if libc from haskell platform provides emulation. If it does, I'll use lowlevel c functions brainfree.

2 1

Re: [Haskell-cafe] Byte Histogram
by John Lato 08 Feb '11

08 Feb '11

From: Johan Tibell <johan.tibell(a)gmail.com> > > > > > (The OOP people, of course, just don't bother trying. They use typecasts > > everywhere...) > > > > Do associated types solve this? Or are there still problems? > > Duncan showed me a definition using associated types, which I have > unfortunately forgotten. > Yes, associated types (or fundeps) solve this problem. This problem of container types is one of the motivating examples behind both extensions. I think the real problem we have with container classes has a lot more to do with what we would use them for. That is, Haskell already has Monoid, Foldable and Traversable. These three (especially Foldable) cover nearly everything OOP programmers would expect out of generic container operations. What's missing are classes for specific data types. That is, a Map/Dict interface, a Queue interface, and a Heap interface (probably others too, but these are the first that come to mind). But the standard Data.Map and List (for a queue) seem good enough for most people, so there seems to be a lot of inertia to overcome for these to be popular. John

3 5

Froglingo – a functional programming language
by kevin＠froglingo.com 08 Feb '11

08 Feb '11

Hi, I have recently finished a paper about Froglingo, a programming language designed to make it easier to write data-intensive business applications. It is a monolith consolidating programming, database/file management, data exchange, web server, and access control. What makes Froglingo unique is a sub language that is the first known as semantically equivalent to a class of TOTAL recursive functions. Froglingo shares with Haskell a declarative approach to programming, and strong mathematical foundations, so I thought that people on this list might find it of some interest. You can find the paper at: http://www.froglingo.com/FroglingoPL.pdf. If you have any thoughts about it, I’d love to hear from you. Thank you for your attention. Kevin

1 0

ICFP 2011: Second Call for Papers
by Wouter Swierstra 08 Feb '11

08 Feb '11

===================================================================== Second Call for Papers ICFP 2011: International Conference on Functional Programming Tokyo, Japan, Monday 19 -- Wednesday 21 September 2011 http://www.icfpconference.org/icfp2011 ===================================================================== Important Dates ~~~~~~~~~~~~~~~ Titles, abstracts & keywords due: Thursday 17 March 2011 at 14:00 UTC Submissions due: Thursday 24 March 2011 at 14:00 UTC Author response: Tuesday & Wednesday 17-18 May Notification: Monday 30 May 2011 Final copy due: Friday 01 July 2011 Conference: Monday-Wednesday 19-21 September 2011 Scope ~~~~~ ICFP 2011 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Particular topics of interest include * Language Design: type systems; concurrency and distribution; modules; components and composition; metaprogramming; relations to imperative, object-oriented, or logic programming; interoperability * Implementation: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; memory management; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources * Software-Development Techniques: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling * Foundations: formal semantics; lambda calculus; rewriting; type theory; mathematical logic; monads; continuations; delimited continuations; global, delimited, or local effects * Transformation and Analysis: abstract interpretation; partial evaluation; program transformation; program calculation; program proofs; normalization by evaluation * Applications and Domain-Specific Languages: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed-systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; multimedia programming; scripting; system administration; security; education * Functional Pearls: elegant, instructive, and fun essays on functional programming * Experience Reports: short papers that provide evidence that functional programming really works or describe obstacles that have kept it from working in a particular application Abbreviated instructions for authors ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * By 17 March 2011, 14:00 UTC, submit a title, an abstract of at most 300 words, and keywords. * By 24 March 2011, 14:00 UTC, submit a full paper of at most 12 pages (6 pages for a Functional Pearl and for an Experience Report), including bibliography and figures. The deadlines will be strictly enforced and papers exceeding the page limits will be summarily rejected. * Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. * Each submission must adhere to SIGPLAN's republication policy, as explained on the web at http://www.acm.org/sigplan/republicationpolicy.htm In addition, authors of resubmitted (but previously rejected) papers have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the program chair will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no rewiewer will read the annotated copies of the previous reviews. Overall, a submission will be evaluated according to its relevance, correctness, significance, originality, and clarity. It should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. Functional Pearls and Experience Reports are separate categories of papers that need not report original research results and must be marked as such at the time of submission. Detailed guidelines on both categories are on the conference web site. Proceedings will be published by ACM Press. Authors of accepted submissions are expected to transfer the copyright to the ACM. Presentations will be videotaped and released online if the presenter consents by signing an additional permission form at the time of the presentation. Formatting: Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. If this requirement is a hardship, make contact with the program chair at least one week before the deadline. Papers must adhere to the standard ACM conference format: two columns, nine-point font on a ten-point baseline, with columns 20pc (3.33in) wide and 54pc (9in) tall, with a column gutter of 2pc (0.33in). A suitable document template for LaTeX is available from SIGPLAN at http://www.acm.org/sigs/sigplan/authorInformation.htm Submission: Submissions will be accepted electronically at a URL to be named later. Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. Author response: Authors will have a 48-hour period, starting at 14:00 UTC on Tuesday 17 May 2010, to read reviews and respond to them. Special Journal Issue: There will be a special issue of the Journal of Functional Programming with papers from ICFP 2011. The program committee will invite the authors of select accepted papers to submit a journal version to this issue. Conference Chairs: Manuel M T Chakravarty, University of New South Wales, Australia Zhenjiang Hu, National Institute of Informatics (NII), Japan Program Chair: Olivier Danvy, Aarhus University, Denmark Program Committee: Kenichi Asai, Ochanomizu University, Japan Josh Berdine, Microsoft Research, UK Adam Chlipala, Harvard University, USA William Cook, University of Texas at Austin, USA Maribel Fernandez, King's College London, UK Ronald Garcia, Carnegie Mellon University, USA Neal Glew, Intel Labs, USA Jacques Garrigue, Nagoya University, Japan Suresh Jagannathan, Purdue University, USA Sam Lindley, University of Edinburgh, UK Frank Pfenning, Carnegie Mellon University, USA Paola Quaglia, University of Trento, Italy Alexis Saurin, University of Paris VII, France Mike Spivey, Oxford University, UK Kristian Stoevring, University of Copenhagen, Denmark Doaitse Swierstra, Utrecht University, The Netherlands David Van Horn, Northeastern University, USA Rene Vestergaard, JAIST, Japan Edwin Westbrook, Rice University, USA

1 0

AES on 32-bit system
by Michael Snoyman 08 Feb '11

08 Feb '11

Hi everyone, Does anyone else have trouble installing the AES package on a 32-bit system? My system at home installs it just fine, but my VPS chokes with the following error messages (plus a bunch of warnings): cbits/ctr_inc.c:11:0: error: 'uint_64t' undeclared (first use in this function) cbits/ctr_inc.c:11:0: error: (Each undeclared identifier is reported only once cbits/ctr_inc.c:11:0: error: for each function it appears in.) cbits/ctr_inc.c:11:0: error: 'ctr' undeclared (first use in this function) cbits/ctr_inc.c:11:0: error: expected expression before ')' token It's actually for this very reason that I'm still maintaining the OpenSSL backend for http-enumerator: I think the tls package is stable enough now to be used in production environments (kudos to Vincent by the way). However, I can't use it in production if I can't build one of its dependencies. This bug is also preventing me from adding some nice features to http-enumerator, such as checking validity of SSL certificates. Anyone have any thoughts? Michael

4 7

coding style vs. foreign interfaces
by Donn Cave 08 Feb '11

08 Feb '11

I just noticed a handful of spelling errors, in a package that isn't all that obscure. Enums from a C interface - data AlarmingNews = -- ALARM_STUFF_WENT_WRONG AlarmStufWentWrong | ... (oops, should have been Stuff, not Stuf.) Who can blame the guy, after all, because this renaming work is not only kind of a mind-numbing and trivial job, it's quite gratuitous. Of the people who are apt to be interested, a sizeable percentage already will be familiar with ALARM_STUFF_WENT_WRONG, and as the "nice" Haskell spelling offers no practical advantage at all, it's purely a waste of their time to translate from one to the other. Is screenflicker_frequency() going to be screenFlickerFrequency, or screenflickerFrequency? gah! I know it's enshrined in many years of convention by now, but if anyone might ever consider the matter again, my vote would be, when adopting foreign interfaces essentially unchanged, that the spellings also be preserved to whatever extent practicable. thanks! Donn Cave

4 4

Building doc for yesod-json
by Magnus Therning 08 Feb '11

08 Feb '11

Michael, I've now gotten part of the way through the *long* list of dependencies for yesod 0.7.0, and I'm stuck on yesod-json. It's the documentation that fails actually, but I'm not sure why it would: dist/build/tmp20668/Yesod/Json.hs:4:6: parse error on input `-- ^ Convert from a JSON value' It's the very first bit in that file: module Yesod.Json ( -- ^ Convert from a JSON value defaultLayoutJson Do you have any clue as to why this would fail? I should probably mention that I'm compiling this on a system with GHC 6.12.3. /M -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus

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