February 2011 - Haskell-Cafe

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

11 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 10 Feb '11

10 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

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forkIO on GHCI does not seem to spawn the thread in the background on some OSs
by C K Kashyap 10 Feb '11

10 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

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haskell prop. logic
by Patrick M 10 Feb '11

10 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.

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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

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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

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