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GADT on the wiki: I'm lost
by Peter Verswyvelen 24 Apr '09

24 Apr '09

I was reading the explanation of GADTs on the wiki<http://en.wikibooks.org/wiki/Haskell/GADT> , and but can't make any sense of the examples. Sure I understand what a GADT is, but I'm looking for practical examples, and the ones on the wiki seem to show what you *cannot* do with them... For example, the article gives an alternative approach to the safeHead function (see code below) But now that does not work either, since Cons x y never evaluates to MarkedList x Safe, so safeHead (Cons 1 Nil) will give a type error... Am I missing something or is this wikibook just confusing? Does anybody have good links to examples of GADTs? Yampa surely seems a good example, but it's a bit too advanced. data NotSafe data Safe data MarkedList :: * -> * -> * where Nil :: MarkedList t NotSafe Cons :: t -> MarkedList t y -> MarkedList t z safeHead :: MarkedList x Safe -> x safeHead (Cons x _) = x silly 0 = Nil silly 1 = Cons () Nil silly n = Cons () $ silly (n-1)

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Base version on Hackage?
by Magnus Therning 24 Apr '09

24 Apr '09

I'm not sure why building of my recently uploaded version of dataenc fails to build on Hackage[1]. Where can I find out what version of base is available on Hackage? /M [1]: http://hackage.haskell.org/packages/archive/dataenc/0.12.1.0/logs/failure/g… -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe Haskell is an even 'redder' pill than Lisp or Scheme. -- PaulPotts

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ANN: Elerea, another FRP library
by Patai Gergely 24 Apr '09

24 Apr '09

Hi everyone, I'm pleased to announce Elerea, aka "Eventless reactivity", a minimalistic FRP implementation that - comes with a convenient applicative interface (similar to Reactive) - supports recursive definition of signals - supports signals fed from outside by IO actions - supports dynamism in the signal structure (I think ;) - seems to play nice with resources, especially memory - is based on some unsafePerformIO dark magic (that might easily break depending on many factors) - might have some parallelisation potential - has absolutely no formal foundations, it's just the result of some furious hacking over the last few days! There are working examples to show off the current capabilities of the library, found in the separate elerea-examples package. Have fun playing with it! Gergely -- http://www.fastmail.fm - A no graphics, no pop-ups email service

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Random number example
by michael rice 24 Apr '09

24 Apr '09

I pretty much followed the sequence of steps that led to this final code (see below), but will be looking it over for a while to make sure it sinks in. In the meantime, I get this when I try to use it (sumTwoDice) at the command line: [michael@localhost ~]$ ghci rand9 GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( rand9.hs, interpreted ) Ok, modules loaded: Main. *Main> sumTwoDice <interactive>:1:0: No instance for (Show (Seed -> (Int, Seed))) arising from a use of `print' at <interactive>:1:0-9 Possible fix: add an instance declaration for (Show (Seed -> (Int, Seed))) In a stmt of a 'do' expression: print it *Main> Can I employ a 'do' expression from the command line? Also, can I now use functions (>>) (>>=) and 'return' defined in the Prelude and still have this code work? Michael ================== {-# LANGUAGE NoImplicitPrelude #-} import Prelude hiding ((>>), (>>=), return) type Seed = Int type Random a = Seed -> (a, Seed) randomNext :: Seed -> Seed randomNext rand = if newRand > 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 rollDie :: Random Int rollDie seed = ((seed `mod` 6) + 1, randomNext seed) (>>) :: Random a -> Random b -> Random b (>>) m n = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) (>>=) :: Random a -> (a -> Random b) -> Random b (>>=) m g = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = (g result1) seed1 in (result2, seed2) return :: a -> Random a return x = \seed0 -> (x, seed0) sumTwoDice :: Random Int sumTwoDice = rollDie >>= (\die1 -> rollDie >>= (\die2 -> return (die1 + die2)))

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Can't find space leak in external sort with tournament trees (test case attached)
by Denis Bueno 23 Apr '09

23 Apr '09

Hello haskell-cafe, I'm running into what I think is a space leak. I've modified external-sort-0.2 [0] to use tournament trees for merging, and although the block sorting works in constant space, merging seems to produce a space leak. The external sort works by lazily consuming an input list, chopping it up into fixed size blocks, sorting each block in memory and writing each sorted block out to disk. At this point we have a big, on-disk array of k sorted blocks. To produce the final, sorted output list, we lazily read back each block into a tournament tree (a heap of sorts), which should only compare the first element of block to figure out the tree root. The merging algorithm in full is in `tourMerge' [1]. I've confirmed by profiling (with -hc) that the merge step gobbles up memory. I've attached a self-contained program (two haskell source files). It requires binary, mersenne-random*, and ArrayRef from hackage. $ tar xvzf leak.tar.gz leak.hs Data/TournamentTree.hs $ ghc -O --make leak.hs $ ./leak 5000000 # generate 5 million entries in ./entries.tmp, sort them, and output to ./leak.out Writing 5000000 entries to './entries.tmp' Done. Henceforth, if you omit the number argument to leak it will sort './entries.tmp'. Sorting 5000000 entries from './entries.tmp' External block sort using 1000 blocks: -- 5000 entries per block -- 240000 bytes per block 1000--blocks sorted. Merging blocks and writing sorted entries to './leak.out' Depending on how much RAM you have, you may need to tweak the argument to leak and/or numBlocks (at the top of leak.hs) -- a block needs to be able to fit in RAM. If the argument to leak is too low, the sort will succeed and the leak won't be apparent. Otherwise, the OS will kill the process because it allocated too much. Anyone have an idea how I can fix the leak? Denis *Only required to generate the initial input file to sort; it's much faster than System.Random. [0] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/external-sort-0.2 [1] tourMerge is in leak.hs, but I've also inlined it here for convenience. TT is the Data.TournamentTree module, included in the tarball I've attached. tourMerge :: (Ord a) => [[a]] -> [a] tourMerge [] = [] tourMerge blks = let t = {-SCC "TMfromList" #-} TT.fromList blks in tM (TT.minElem t) (TT.deleteMinF t) where tM :: (Ord a) => [a] -> TT.Forest [a] -> [a] tM l f | null f = l | otherwise = let t = TT.tournament f next = TT.minElem t (x, xs) = {-# SCC "TTspan" #-} span (<= head next) l in x ++ (tM next (if null xs then TT.deleteMinF t else (TT.singleton xs : TT.deleteMinF t)))

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Overriding a Prelude function?
by michael rice 23 Apr '09

23 Apr '09

I've been working through this example from: http://en.wikibooks.org/wiki/Haskell/Understanding_monads I understand what they're doing all the way up to the definition of (>>), which duplicates Prelude function (>>). To continue following the example, I need to know how to override the Prelude (>>) with the (>>) definition in my file rand.hs. Michael ============== [michael@localhost ~]$ cat rand.hs import System.Random type Seed = Int randomNext :: Seed -> Seed randomNext rand = if newRand > 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 toDieRoll :: Seed -> Int toDieRoll seed = (seed `mod` 6) + 1 rollDie :: Seed -> (Int, Seed) rollDie seed = ((seed `mod` 6) + 1, randomNext seed) sumTwoDice :: Seed -> (Int, Seed) sumTwoDice seed0 = let (die1, seed1) = rollDie seed0 (die2, seed2) = rollDie seed1 in (die1 + die2, seed2) (>>) m n = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) [michael@localhost ~]$

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Hac5 roundup
by Martijn van Steenbergen 23 Apr '09

23 Apr '09

Dear Haskell Hackers, Many thanks to all those who attended Hac5! We had a spectacular number of participants: over 50 hackers showed up, representing several countries. Many thanks also to the organising committee and sponsors! Those bagels on Sunday were delicious. :-) Please check out the updated wiki page [1] for: * a nice group photo of most attendants taken by Thomas Davie; * a list of blog posts/galleries/videos of the event; * a list of projects worked on and the progress that was made. Additionally, if you covered Hac5 in some way or contributed to any of the projects, please add your links or progress summary to the page. Thanks, Martijn. [1] http://www.haskell.org/haskellwiki/Hac5

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FRP, integration and differential equations.
by jean-christophe mincke 23 Apr '09

23 Apr '09

In a post in the *Elerea, another FRP library *thread*,* Peter Verswyvelen wrote: *>I think it would be nice if we could make a "reactive benchmark" or something: some tiny examples that capture the essence of reactive systems, and a way to compare each solution's >pros and cons.* * * *>For example the "plugging a space leak with an arrow" papers reduces the recursive signal problem to * * * *>e = integral 1 e* * * *>Maybe the Nlift problem is a good example for dynamic collections, but I guess we'll need more examples.* * * *>The reason why I'm talking about examples and not semantics is because the latter seems to be pretty hard to get right for FRP?* I would like to come back to this exemple. I am trying to write a small FRP in F# (which is a strict language, a clone of Ocaml) and I also came across space and/or time leak. But maybe not for the same reasons... Thinking about these problems and after some trials and errors, I came to the following conclusions: I believe that writing the expression e = integral 1 *something* where e is a Behavior (thus depends on a continuous time). has really two different meanings. 1. if *something *is independent of e, what the above expression means is the classical integration of a time dependent function between t0 and t1. Several numerical methods are available to compute this integral and, as far as I know, they need to compute *something *at t0, t1 and, possibly, at intermediate times. In this case, *something *can be a Behavior. 2. If *something *depends directly or indirectly of e then we are faced with a first order differential equation of the form: de/dt = *something*(e,t) where de/dt is the time derivative of e and *something*(e,t) indicates that *something* depends, without loss of generality, on both e and t. There exist specific methods to numerically solve differential equations between t0 and t1. Some of them only require the knowledge of e at t0 (the Euler method), some others needs to compute *something *from intermediate times (in [t0, t1[ ) *and *estimates of e at those intermediary times. 3. *something *depends (only) on one or more events that, in turns, are computed from e. This case seems to be the same as the first one where the integrand can be decomposed into a before-event integrand and an after-event integrand (if any event has been triggered). Both integrands being independent from e. But I have not completely investigated this case yet... Coming back to my FRP, which is based on residual behaviors, I use a specific solution for each case. Solution to case 1 causes no problem and is similar to what is done in classical FRP (Euler method, without recursively defined behaviors). Once again as far as I know... The second case has two solutions: 1. the 'integrate' function is replaced by a function 'solve' which has the following signature solve :: a -> (Behavior a -> Behavior a) -> Behavior a In fact, *something*(e,t) is represented by an integrand function from behavior to behavior, this function is called by the integration method. The integration method is then free to pass estimates of e, as constant behaviors, to the integrand function. The drawbacks of this solution are: - To avoid space/time leaks, it cannot be done without side effects (to be honest, I have not been able to find a solution without assignement). However these side effects are not visible from outside of the solve function. .. - If behaviors are defined within the integrand function, they are not accessible from outside of this integrand function. 2. Introduce constructions that looks like to signal functions. solve :: a -> SF a a -> Behavior a where a SF is able to react to events and may manage an internal state. This solution solves the two above problems but make the FRP a bit more complex. Today, I tend to prefer the first solution, but what is important, in my opinion, is to recognize the fact that e = integral 1 *something* really addresses two different problems (integration and solving of differential equations) and each problem should have their own solution. The consequences are : 1. There is no longer any need for my FRP to be able to define a Behavior recursively. That is a good news for this is quite tricky in F#. Consequently, there is no need to introduce delays. 2. Higher order methods for solving of diff. equations can be used (i.e. Runge-Kutta). That is also good news for this was one of my main goal in doing the exercice of writing a FRP. Regards, J-C

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Re: [Haskell-cafe] Re: Overriding a Prelude function?
by michael rice 23 Apr '09

23 Apr '09

OK, the 3rd and 4th responses below are the same, which is as it should be. I think we can put this thread to bed. Thanks to all who participated. Michael *Main> rollDie 362354 (3,1795116384) *Main> rollDie 1795116384 (1,523309185) *Main> rollDie 523309185 (4,1311937830) *Main> (rollDie ~>> (rollDie ~>> rollDie)) 362354 (4,1311937830) *Main> --- On Thu, 4/23/09, Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: From: Heinrich Apfelmus <apfelmus(a)quantentunnel.de> Subject: [Haskell-cafe] Re: Overriding a Prelude function? To: haskell-cafe(a)haskell.org Date: Thursday, April 23, 2009, 5:18 AM Achim Schneider wrote: > Well, you obviously need an initial seed: > > rollDie 0xdeadbeef ~>> (rollDie ~>> rollDie) Achim means (rollDie ~>> (rollDie ~>> rollDie)) 0xdeadbeef Regards, apfelmus -- http://apfelmus.nfshost.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Overriding a Prelude function?
by michael rice 23 Apr '09

23 Apr '09

Here's what I get: [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> import Prelude hiding ((>>)) <interactive>:1:0: parse error on input `import' Prelude> ===== I was passing seed0 to rollDie and getting back (r1,seed1) passing seed1 to rollDie and getting back (r2,seed2) passing seed2 to rollDie and getting back (r3,seed3) Just based on the problem text, I would guess that passing rollDie and seed0 to (>>) I would get back (r3,seed3), losing the intermediate random numbers r1 and r2 along the way, at least that's what I understood it to say. So, I know that next I'm probably going to have to do something to remedy that, but I haven't gotten to that next step yet. What is unsugar? Thanks in advance for your patience. Michael --- On Wed, 4/22/09, Dan Weston <westondan(a)imageworks.com> wrote: From: Dan Weston <westondan(a)imageworks.com> Subject: Re: [Haskell-cafe] Overriding a Prelude function? To: "Ross Mellgren" <rmm-haskell(a)z.odi.ac> Cc: "michael rice" <nowgate(a)yahoo.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Date: Wednesday, April 22, 2009, 12:37 PM Be aware that the do unsugars to (Prelude.>>), not your (>>), even if you hide (Prelude.>>): import Prelude hiding ((>>)) m >> f = error "Call me!" main = putStrLn . show $ do [3,4] [5] The desugaring of the do { [3,4]; [5] } is (Prelude.>>) [3,4] [5] = [5,5], whereas you might have hoped for [3,4] >> [5] = error "Call me!" Dan Ross Mellgren wrote: > I think > > import Prelude hiding ((>>)) > > does that. > > -Ross > > On Apr 22, 2009, at 11:44 AM, michael rice wrote: > >> I've been working through this example from: http://en.wikibooks.org/wiki/Haskell/Understanding_monads >> >> I understand what they're doing all the way up to the definition of (>>), which duplicates Prelude function (>>). To continue following the example, I need to know how to override the Prelude (>>) with the (>>) definition in my file rand.hs. >> >> Michael >> >> ============== >> >> [michael@localhost ~]$ cat rand.hs >> import System.Random >> >> type Seed = Int >> >> randomNext :: Seed -> Seed >> randomNext rand = if newRand > 0 then newRand else newRand + 2147483647 >> where newRand = 16807 * lo - 2836 * hi >> (hi,lo) = rand `divMod` 127773 >> >> toDieRoll :: Seed -> Int >> toDieRoll seed = (seed `mod` 6) + 1 >> >> rollDie :: Seed -> (Int, Seed) >> rollDie seed = ((seed `mod` 6) + 1, randomNext seed) >> >> sumTwoDice :: Seed -> (Int, Seed) >> sumTwoDice seed0 = >> let (die1, seed1) = rollDie seed0 >> (die2, seed2) = rollDie seed1 >> in (die1 + die2, seed2) >> >> (>>) m n = \seed0 -> >> let (result1, seed1) = m seed0 >> (result2, seed2) = n seed1 >> in (result2, seed2) >> >> [michael@localhost ~]$ >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: [Haskell-cafe] Re: Overriding a Prelude function?
by michael rice 23 Apr '09

23 Apr '09

I forgot the type signiture on the last last: (~>>) :: (Seed -> (a,Seed)) -> (Seed -> (b,Seed)) -> (Seed -> (b,Seed)) (~>>) m n = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) ======== With it I get this: *Main> rollDie 362354 ~>> (rollDie ~>> rollDie) <interactive>:1:0: Couldn't match expected type `Seed -> (a, Seed)' against inferred type `(Int, Seed)' In the first argument of `(~>>)', namely `rollDie 362354' In the expression: rollDie 362354 ~>> (rollDie ~>> rollDie) In the definition of `it': it = rollDie 362354 ~>> (rollDie ~>> rollDie) *Main> Michael --- On Wed, 4/22/09, Achim Schneider <barsoap(a)web.de> wrote: From: Achim Schneider <barsoap(a)web.de> Subject: [Haskell-cafe] Re: Overriding a Prelude function? To: haskell-cafe(a)haskell.org Date: Wednesday, April 22, 2009, 9:52 PM michael rice <nowgate(a)yahoo.com> wrote: > OK, I changed the operator from (>>) to (~>>). When I try to use it I > get this: > > [michael@localhost ~]$ ghci rand > GHCi, version 6.10.1: http://www.haskell.org/ghc/__ :? for help > Loading package ghc-prim ... linking ... done. > Loading package integer ... linking ... done. > Loading package base ... linking ... done. > [1 of 1] Compiling Main________________________ ( rand.hs, interpreted ) > Ok, modules loaded: Main. > *Main> rollDie ~>> (rollDie ~>> rollDie) > > <interactive>:1:0: > ______ No instance for (Show (Seed -> (Int, Seed))) > __________ arising from a use of `print' at <interactive>:1:0-32 > ______ Possible fix: > __________ add an instance declaration for (Show (Seed -> (Int, Seed))) > ______ In a stmt of a 'do' expression: print it Well, you obviously need an initial seed: rollDie 0xdeadbeef ~>> (rollDie ~>> rollDie) -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Re: Overriding a Prelude function?
by michael rice 23 Apr '09

23 Apr '09

I'm just copying code from this web page: http://en.wikibooks.org/wiki/Haskell/Understanding_monads ================ Using their initial seed I get this: *Main> rollDie 362354 ~>> (rollDie ~>> rollDie) <interactive>:1:0: Couldn't match expected type `t -> (t1, t2)' against inferred type `(Int, Seed)' In the first argument of `(~>>)', namely `rollDie 362354' In the expression: rollDie 362354 ~>> (rollDie ~>> rollDie) In the definition of `it': it = rollDie 362354 ~>> (rollDie ~>> rollDie) *Main> =================== Passing their initial seed through successive calls to rollDie: *Main> rollDie 362354 Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. (3,1795116384) *Main> rollDie 1795116384 (1,523309185) *Main> rollDie 523309185 (4,1311937830) <== ISN'T THIS SUPPOSED TO BE THE RESULT? *Main> Michael --- On Wed, 4/22/09, Achim Schneider <barsoap(a)web.de> wrote: From: Achim Schneider <barsoap(a)web.de> Subject: [Haskell-cafe] Re: Overriding a Prelude function? To: haskell-cafe(a)haskell.org Date: Wednesday, April 22, 2009, 9:52 PM michael rice <nowgate(a)yahoo.com> wrote: > OK, I changed the operator from (>>) to (~>>). When I try to use it I > get this: > > [michael@localhost ~]$ ghci rand > GHCi, version 6.10.1: http://www.haskell.org/ghc/__ :? for help > Loading package ghc-prim ... linking ... done. > Loading package integer ... linking ... done. > Loading package base ... linking ... done. > [1 of 1] Compiling Main________________________ ( rand.hs, interpreted ) > Ok, modules loaded: Main. > *Main> rollDie ~>> (rollDie ~>> rollDie) > > <interactive>:1:0: > ______ No instance for (Show (Seed -> (Int, Seed))) > __________ arising from a use of `print' at <interactive>:1:0-32 > ______ Possible fix: > __________ add an instance declaration for (Show (Seed -> (Int, Seed))) > ______ In a stmt of a 'do' expression: print it Well, you obviously need an initial seed: rollDie 0xdeadbeef ~>> (rollDie ~>> rollDie) -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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WHNF versus HNF (was: Optimizing unamb by determining the "state" of a thunk?)
by Peter Verswyvelen 23 Apr '09

23 Apr '09

I'm having difficulty to understand the difference between WHNF and HNF. Is this explanation<http://encyclopedia2.thefreedictionary.com/Weak+Head+Normal+Form> the correct one? Or is WHNF and HNF equivalent in Haskell land? The GHC documentation of seq says: Evaluates its first argument to head normal form, and then returns its second argument as the result. Let's try in GHCi *Main> let f = trace "\\x" $ \x -> ((trace "\\y" $ \y -> trace "y" y + trace "x" x) $ trace "2" 2) *Main> f `seq` () \x () *Main> That did not evaluate anything inside the body of the first lambda, so according to the article, seq reduces to weak head normal form, not hnf...

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Trouble with type signatures and type families
by Stephan Friedrichs 23 Apr '09

23 Apr '09

Hi, consider the following code: ---------------------------- class Foo f where type Bar f :: * type Baz f :: * from :: Bar f -> Baz f to :: Baz f -> Bar f data Tree a b c = Empty | Tree b c (Tree a b c) (Tree a b c) --singleton :: (Foo f) => Bar f -> Tree a (Bar f) (Baz f) singleton x = Tree x (from x) Empty Empty ---------------------------- what type does 'singleton' have? ghci-6.10.2 says: > :t singleton singleton :: forall f a. (Foo f) => Bar f -> Tree a (Bar f) (Baz f) which is no surprise. But when I uncomment the type signature in the above code sample, loading it to ghci gives the following error: > :r [1 of 1] Compiling Main ( T.hs, interpreted ) T.hs:14:22: Couldn't match expected type `Baz f1' against inferred type `Baz f' In the second argument of `Tree', namely `(from x)' In the expression: Tree x (from x) Empty Empty In the definition of `singleton': singleton x = Tree x (from x) Empty Empty T.hs:14:27: Couldn't match expected type `Bar f' against inferred type `Bar f1' In the first argument of `from', namely `x' In the second argument of `Tree', namely `(from x)' In the expression: Tree x (from x) Empty Empty Failed, modules loaded: none. I don't get it... what's wrong, I just copied the inferred type? //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

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Optimizing unamb by determining the "state" of a thunk?
by Peter Verswyvelen 23 Apr '09

23 Apr '09

I was wandering if it would be possible to optimize unamb<http://hackage.haskell.org/packages/archive/unamb/0.1.9/doc/html/Data-Unamb…> by checking if a value is already evaluated to head normal form. So f `unamb` g would then be extremely fast if either f or g is already evaluated to head normal form. Maybe using some vacuum tricks? This function would need to be in IO since it is of course not referentially transparent. Although threads are lightweight in Haskell, forking/waiting/killing surely must have more overhead than just checking the thunk of an expression? Of course one could also make unamb a primitive :-)

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ANNOUNCE: dataenc 0.12.1.0
by Magnus Therning 23 Apr '09

23 Apr '09

I've just uploaded version 0.12.1.0 of dataenc with the following (visible) changes compared to the previous version (0.12): - implementation of a bunch of new encodings: - xxencode - hexadecimal - quoted-printable - python escaping - url encoding - squashing of a bug in the yEncoding implementation that only manifested on 32-bit systems - an attempt to conform to the guidelines for the Haskell Platform regarding versioning and dependencies This release brings the full list of encodings to - Base16 - Base32 - Base32Hex - Base64 - Base64Url - Base85 - Hexadecimal - Python string escaping - quoted-printable - URL encoding - uuencoding - xxencoding - yEncoding The hackage page[1] now reads: Versions 0.9, 0.10.1, 0.10.2, 0.11, 0.11.1, 0.12, 0.12.1.0 Dependencies array (>=0.2.0 && <0.3), base (>=4.0.0 && <4.1), containers (>=0.2.0 && <0.3) License BSD3 Copyright Magnus Therning, 2007-2009 Author Magnus Therning Maintainer magnus(a)therning.org Category Codec Home page http://www.haskell.org/haskellwiki/Library/Data_encoding Upload date Wed Apr 22 19:40:22 UTC 2009 Uploaded by MagnusTherning /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe Haskell is an even 'redder' pill than Lisp or Scheme. -- PaulPotts

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Re: [Haskell-cafe] Getting the x out
by michael rice 23 Apr '09

23 Apr '09

Got it! I figured there must be some way to unpack it. My goodness, there are so many functions I'm not even aware of. Has anyone ever counted them all? Thanks. Michael --- On Tue, 4/21/09, Tony Morris <tonymorris(a)gmail.com> wrote: From: Tony Morris <tonymorris(a)gmail.com> Subject: Re: [Haskell-cafe] Getting the x out To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Tuesday, April 21, 2009, 8:54 PM You mean, the x out of *Maybe* x even. In the very literal sense, the assumption that there is an x in "Maybe x" is false -- there may not be one since it is maybe, but not necessarily, x. IT's a bit like the use of null that you might have seen in other languages where you might have a value or you might have null. What you can do however, is say "give me the x if there is one, otherwise, use this value". This is the fromMaybe function. Prelude Data.Maybe> let safeDivision x y = if y == 0 then Nothing else Just (x/y) Prelude Data.Maybe> 3 + (42 `fromMaybe` safeDivision 10 5) 5.0 Prelude Data.Maybe> 3 + (42 `fromMaybe` safeDivision 10 0) 45.0 michael rice wrote: > How do I get the x out of Just x? > > Michael > > ============= > > safeDivision :: Float -> Float -> Maybe Float > safeDivision x y = if y == 0 then Nothing else Just (x/y) > > *Main Data.List> safeDivision 10 5 > Just 2.0 > *Main Data.List> 3 + (safeDivision 10 5) > > <interactive>:1:0: > No instance for (Num (Maybe Float)) > arising from a use of `+' at <interactive>:1:0-22 > Possible fix: add an instance declaration for (Num (Maybe Float)) > In the expression: 3 + (safeDivision 10 5) > In the definition of `it': it = 3 + (safeDivision 10 5) > *Main Data.List> > > > ------------------------------------------------------------------------ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Tony Morris http://tmorris.net/

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astronomy projects in haskell
by Michael Litchard 23 Apr '09

23 Apr '09

I remember reading some website, that dons (probably) posted once. I'd like to find them again for a report I'm doing. So, if you know of any astronomy websites that talk about projects using haskell, please let me know. thanks Michael Litchard

3 2

haxr build failure from cabal
by Jeff Heard 23 Apr '09

23 Apr '09

haxr will no longer compile from cabal because of the dependency marked HTTP > 1.0. The current version of the library requires HTTP < 4000.0.0 as it stands. Can it be updated real quick in hackage? -- Jeff

3 3

Using Data.Complex
by michael rice 22 Apr '09

22 Apr '09

Just exploring. How to load? Michael [michael@localhost ~]$ ghci Data.Complex GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. <no location info>: module `Data.Complex' is a package module Failed, modules loaded: none. Prelude>

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Re: [Haskell-cafe] Getting the x out
by michael rice 22 Apr '09

22 Apr '09

Hi Neil, I just tried out Hoogle for the first time and it works great. It seems there are a lot of resources available for Haskell; it just takes time to find out about them and how they work. Thanks. Michael --- On Wed, 4/22/09, Neil Mitchell <ndmitchell(a)gmail.com> wrote: From: Neil Mitchell <ndmitchell(a)gmail.com> Subject: Re: [Haskell-cafe] Getting the x out To: "michael rice" <nowgate(a)yahoo.com> Cc: "Tony Morris" <tonymorris(a)gmail.com>, haskell-cafe(a)haskell.org Date: Wednesday, April 22, 2009, 1:26 AM Hi It's not too hard. You wanted a function that converted Maybe a -> a, you just Hoogle for it: http://haskell.org/hoogle/?hoogle=Maybe+a+->+a Thanks Neil On Wed, Apr 22, 2009 at 2:07 AM, michael rice <nowgate(a)yahoo.com> wrote: > Got it! I figured there must be some way to unpack it. > > My goodness, there are so many functions I'm not even aware of. Has anyone > ever counted them all? > > Thanks. > > Michael > > --- On Tue, 4/21/09, Tony Morris <tonymorris(a)gmail.com> wrote: > > From: Tony Morris <tonymorris(a)gmail.com> > Subject: Re: [Haskell-cafe] Getting the x out > To: "michael rice" <nowgate(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Tuesday, April 21, 2009, 8:54 PM > > You mean, the x out of *Maybe* x even. In the very literal sense, the > assumption that there is an x in "Maybe x" is false -- there may not be > one since it is maybe, but not necessarily, x. IT's a bit like the use > of null that you might have seen in other languages where you might have > a value or you might have null. What you can do however, is say "give me > the x if there is one, otherwise, use this value". > > This is the fromMaybe function. > > Prelude Data.Maybe> let safeDivision x y = if y == 0 then Nothing else > Just (x/y) > Prelude Data.Maybe> 3 + (42 `fromMaybe` safeDivision 10 5) > 5.0 > Prelude Data.Maybe> 3 + (42 `fromMaybe` safeDivision 10 0) > 45.0 > > > > michael rice wrote: >> How do I get the x out of Just x? >> >> Michael >> >> ============= >> >> safeDivision :: Float -> Float -> Maybe Float >> safeDivision x y = if y == 0 then Nothing else Just (x/y) >> >> *Main Data.List> safeDivision 10 5 >> Just 2.0 >> *Main Data.List> 3 + (safeDivision 10 5) >> >> <interactive>:1:0: >> No instance for (Num (Maybe Float)) >> arising from a use of `+' at <interactive>:1:0-22 >> Possible fix: add an instance declaration for (Num (Maybe Float)) >> In the expression: 3 + (safeDivision 10 5) >> In the definition of `it': it = 3 + (safeDivision 10 5) >> *Main Data.List> >> >> >> ------------------------------------------------------------------------ >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > -- > Tony Morris > http://tmorris.net/ > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] Getting the x out
by michael rice 22 Apr '09

22 Apr '09

It makes sense, once you understand the terminology. Thanks. Michael --- On Wed, 4/22/09, Cristiano Paris <frodo(a)theshire.org> wrote: From: Cristiano Paris <frodo(a)theshire.org> Subject: Re: [Haskell-cafe] Getting the x out To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Wednesday, April 22, 2009, 7:55 AM On Wed, Apr 22, 2009 at 2:49 AM, michael rice <nowgate(a)yahoo.com> wrote: > How do I get the x out of Just x? Hi Michael, in your code you're using Maybe to inform the caller of safeDivision about an exceptional situation. This way, you made a full coverage of all the input cases and nothing is left out, i.e. you created a total function (which is GOOD). If you introduced the Nothing case, you just don't want to ignore it. Also, the type system is forcing you to take the Nothing case into account so you can handle it properly. Hence, you might try something like the "maybe" function, which accounts for the Nothing case. If you'd use fromJust, the Nothing case would remain uncovered, leading you to an unhandled exception, which conflicts with your safeDivision definition. Cristiano

1 0

Getting the x out
by michael rice 22 Apr '09

22 Apr '09

How do I get the x out of Just x? Michael ============= safeDivision :: Float -> Float -> Maybe Float safeDivision x y = if y == 0 then Nothing else Just (x/y) *Main Data.List> safeDivision 10 5 Just 2.0 *Main Data.List> 3 + (safeDivision 10 5) <interactive>:1:0: No instance for (Num (Maybe Float)) arising from a use of `+' at <interactive>:1:0-22 Possible fix: add an instance declaration for (Num (Maybe Float)) In the expression: 3 + (safeDivision 10 5) In the definition of `it': it = 3 + (safeDivision 10 5) *Main Data.List>

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Issues with IO and FFIs
by Jon Harrop 22 Apr '09

22 Apr '09

Does anyone have any comments on the following criticism of some difficulties with FFI, including IO, in Haskell: http://groups.google.com/group/comp.lang.functional/msg/6d650c086b2c8a49?hl… In particular, is it not always possible to write IO libraries safely in Haskell? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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ANN: list-tries-0.0 - first release
by Matti Niemenmaa 22 Apr '09

22 Apr '09

Announcing the first public release of list-tries! Version 0.0, woo! list-tries is a library providing implementations of finite sets and maps for list keys using tries, both simple and of the Patricia kind. The data types are parametrized over the map type they use internally to store the child nodes: this allows extending them to support different kinds of key types or increasing efficiency. To get at it, head over either to Hackage or my homepage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/list-tries http://iki.fi/matti.niemenmaa/list-tries/ It's Hugs-compliant to boot! I haven't tested UHC, but I suspect it'd work as well. I've tested compilation with both GHC 6.10 releases; 6.8 should also be fine. Being at version 0.0, I'm happy to get feedback, although I'm fairly busy with school for the foreseeable future so I can't promise I'll act on it very quickly. In particular, the API might be a bit unobvious due to the map type parametrization (and a bit cluttered due to the strict versions of just about every operation), but I hope it's not much trouble. I've actually had this release ready for a while now. I originally planned to coincide its release with that of GHC 6.10.2 since it needs a new version of 'containers' to build (see "Warning", below), but of course 'containers' wasn't updated since the changes that list-tries requires are API changes. I figured I might as well wait until 6.12.1, but with some recent buzz regarding tries I decided to just shove it out now, even as a crippled version due to the lack of a new enough 'containers'. Warning ======= In order to run properly, list-tries needs a version of 'containers' that hasn't yet been released. I incorporated a little hack which makes it compile even with 0.2, but some calls will fail by calling 'error': 30 of my 1014 test cases do so. If you have the latest Darcs version of 'containers' (which you probably do if you're using the GHC HEAD, for instance), make sure it's registered as a 0.3 version and then ask Cabal to build list-tries with the '-fcontainers03' flag. That way you'll get a fully-featured version of list-tries! Unfinished ========== I had hoped to get some benchmarks out before I released list-tries, to see if there's still much work to be done with respect to performance (and if there's any point in releasing it), but I didn't find the time. Data.ListTrie.Base.Map is largely undocumented, but it's not meant to be used much anyway so that shouldn't matter a great deal.

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Books recommendation for LINQ?
by Günther Schmidt 22 Apr '09

22 Apr '09

Hi, I'll be trying Sigbjorns haskell-dotnet package together with LINQ. What's a good book for LINQ? Günther

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ANN: HTTP-4000.0.6
by Sigbjorn Finne 22 Apr '09

22 Apr '09

Hi, a refresh release of the HTTP package has been uploaded to Hackage, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP Robust handling of ill-formed cookies + squash a bug in normalization of certain proxy-bound requests would be the (minor) headlines of this release. enjoy --sigbjorn

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Re: [Haskell-cafe] Getting the x out
by michael rice 22 Apr '09

22 Apr '09

There's a lot of well thought out stuff in Haskell, but getting familiar with it all seems like a huge task. Thanks for the help. Michael --- On Tue, 4/21/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Getting the x out To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Tuesday, April 21, 2009, 8:53 PM If you want to just get the value out, meaning you'll get a program error if it happens to be Nothing, then you can use Data.Maybe.fromJust. But usually, you'd want to preserve the Nothing. Applicative or Monad is pretty good for this: import Control.Applicative (3+) <$> safeDivision 10 5 the result will be Just 5.0 in this case, but if the division was incorrect it would be nothing. If you want to do something else, you can either pattern match on it: case safeDivision 10 5 of Just x -> -- do something with x Nothing -> -- do something else or use some functions from Data.Maybe. Say you want to evaluate to 1 instead of Nothing: import Data.Maybe fromMaybe 1 (safeDivision 10 5) -Ross On Apr 21, 2009, at 8:49 PM, michael rice wrote: How do I get the x out of Just x? Michael ============= safeDivision :: Float -> Float -> Maybe Float safeDivision x y = if y == 0 then Nothing else Just (x/y) *Main Data.List> safeDivision 10 5 Just 2.0 *Main Data.List> 3 + (safeDivision 10 5) <interactive>:1:0: No instance for (Num (Maybe Float)) arising from a use of `+' at <interactive>:1:0-22 Possible fix: add an instance declaration for (Num (Maybe Float)) In the expression: 3 + (safeDivision 10 5) In the definition of `it': it = 3 + (safeDivision 10 5) *Main Data.List> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Comments from two weeks of using Leksah
by Jeff Heard 22 Apr '09

22 Apr '09

I've been using the latest and greatest version of leksah for the last couple of weeks and I wanted to give a short report on the things I've found. First of all, it's crashed only once, and the error was an actual segfault, so I'm not sure what went wrong there. All in all, I like the eyecandy and have left it on for everything I do, but I've noticed that arrows have an extra space after them, no matter which arrow. Also, the candy for =<< and >>= is the same. Only one "missing feature" that I've noticed: regex find and replace. I'd like very much to be able to find and replace using regexes, as this is pretty much why I keep coming back to vi and emacs. I really really like the modules pane which gives me quick views into the doc of everything that's installed and the signatures. I wonder, though, could it be limited to the packages made available by the dependencies in configuring the current working package? That way, it would be easy to tell whether a dependency was missing. I wonder if there's a way to setup the preferences so that autocomplete can be accelerator based (like Ctrl+Space) instead of constantly on, because I've noticed that occasionally I'll hit enter at the end of a line and have it autocorrect my last identifier incorrectly. It'd be kind of neat if the error lines from the continuous background build showed up highlighted in the editor in some way, like they do in oh say Eclipse, but that's just a nice feature. Only other feature i'd like to see added is the ability to have multiple packages open at once, allow them to depend on each other, and handle configuring, building, and installing each of them be a one-step process. That's about it. For large projects, very neat and very useful! I like it. -- Jeff

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A haskell error message
by siso dagbovie 22 Apr '09

22 Apr '09

Hi, I have been trying to specify the depth-first numbering algorithm as follow: >_dfNum :: Int -> [Tree a] -> [Tree Int] >_dfNum i ((Node n ts):us) = (Node i vs):(_dfNum (i+1+(length ts)) us) where > vs=_dfNum (i+1) ts But when I test it, I receive the message: Non-exhaustive patterns in function _bfNum Please, could you give me an idea of how I could fix this? Thanks Kind regards, Ayawoa Sitsopé Dagbovie African Institute for Mathematical Sciences #6 Melrose Road Muizenberg 7945 South Africa. Phone: +27 782 960453

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UPDATE: haskellmode for Vim now at projects.haskell.org (+screencast; -)
by Claus Reinke 22 Apr '09

22 Apr '09

I have become aware that many Haskellers are not aware of the Haskell mode plugins for Vim, in spite of the >100 downloads per month I saw when I last checked. Since the plugins have just completed their move to their new home at http://projects.haskell.org/haskellmode-vim/ this seems to be a good opportunity to mention them again (now with 100% more screencasts!-). They do much (though certainly not all) of the stuff people often wish for here, such as showing or adding types, looking up source where available locally or looking up documentation where source isn't available. Mostly, they collect my attempts to automate tasks when I have to do them often enough. Here is a section of the quick reference (:help haskellmode-quickref): |:make| load into GHCi, show errors (|quickfix| |:copen|) |_ct| create |tags| file |_si| show info for id under cursor |_t| show type for id under cursor |_T| insert type declaration for id under cursor |balloon| show type for id under mouse pointer |_?| browse Haddock entry for id under cursor |:IDoc| {identifier} browse Haddock entry for unqualified {identifier} |:MDoc| {module} browse Haddock entry for {module} |:FlagReference| {s} browse Users Guide Flag Reference for section {s} |_.| qualify unqualified id under cursor |_i| add 'import <module>(<identifier>)' for id under cursor |_im| add 'import <module>' for id under cursor |_iq| add 'import qualified <module>(<identifier>)' for id under cursor |_iqm| add 'import qualified <module>' for id under cursor |_ie| make imports explit for import statement under cursor |_opt| add OPTIONS_GHC pragma |_lang| add LANGUAGE pragma |i_CTRL-X_CTRL-O| insert-mode completion based on imported ids (|haskellmode-XO|) |i_CTRL-X_CTRL-U| insert-mode completion based on documented ids (|haskellmode-XU|) |i_CTRL-N| insert-mode completion based on imported sources |:GHCi|{command/expr} run GHCi command/expr in current module For those who have never used these plugins, or haven't used Vim at all, it has often been difficult to imagine what editing in Vim can be like. The old quick tour of features available has now been updated from screenshots to screencasts (my first venture into this area - please let me know whether that is useful or a waste of time!-), so you can watch them on Youtube before deciding to invest time into learning Vim. For those who are using Vim, the only reason not to use my haskellmode plugins would be if you had your own (not uncommon among Vim users;-), in which case I hope you make yours available as well (feel free to adopt features from my plugins, and let me know if you have some useful features to contribute), here: http://www.haskell.org/haskellwiki/Libraries_and_tools/Program_development#… For those who have happily been using these plugins: in the process of moving the site, I noticed that I hadn't updated the published version for a quite some time - apparently, noone had missed the fixes, but in case you want to check, the relevant section of my update log is appended below. Happy Vimming!-) Claus ------------- updates since last published version on old site 04/04/2009 haskell_doc.vim: when narrowing choices by qualifier for _?, take lookup index from un-narrowed list (else we could end up in the docs for the wrong module) 02/04/2009 ghc.vim: actually, we can try to make a reasonable guess at the parent type for constructors in _ie, from their type signature 01/04/2009 ghc.vim: try a bit harder to escape " and ' in :GHCi eliminate duplicates in _ie and mark data constructor imports as ???(Cons) - we can't reliably figure out the parent type for the constructor:-( handle Prelude as special case in _ie (can't just comment out import, need to import [qualified] Prelude [as X]() ) haskell_doc.vim: fix keys (no namespace tags) and urls (modules) for :MDoc 31/03/2009 all files: new home page at projects.haskell.org 28/03/2009 haskell_doc.vim: in ProcessHaddockIndexes2, fix a case where making new entries could lose old ones (eg, zipWith's base package locations got lost when adding its vector package locations) 07/12/2008 haskell_doc.vim: since we're now reading from multiple haddock indices in DocIndex, we need to extend, not overwrite entries.. 03/12/2008 ghc.vim: do not reset b:ghc_static_options on every reload

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ANNOUNCE: Runge-Kutta library -- solve ODEs
by Uwe Hollerbach 21 Apr '09

21 Apr '09

Hello, all, I'm pleased to announce a small Runge-Kutta library for numerically solving ordinary differential equations, which I'm hereby unleashing upon an unsuspecting world. The README is as follows: This is a small module collecting about a dozen Runge-Kutta methods of different orders, along with a couple of programs to exercise them. Build and run testrk, volterra, volterra2, and arenstorf: o testrk exercises all of the methods in a non-adaptive way, solving a test problem with a known analytic solution, to check convergence. (This was what first indicated that there was a problem with the Fehlberg 7(8) listing in HNW.) o volterra uses a non-adaptive method to solve the Lotka-Volterra equations from t=0 to t=40: either from a built-in starting point, or from a starting point specified on the command line. o volterra2 does the same, except it uses an adaptive solver o arenstorf solves the restricted 3-body problem (earth+moon+satellite) using an adaptive solver with some specific initial conditions which yield periodic orbits The volterra2 and arenstorf examples use an "oracle" function to decide what is a good step size. Right now that oracle function is in each test file; arguably it should be in the RungeKutta module. Eventually it will be, but I haven't spent much time yet on making that oracle especially good. I have so far only tested it with ghc 6.8.3 on MacOS 10.3.9 (powerPC), but I know of no reason why it wouldn't work with other versions and OSs. It's BSD licensed, in fact I stole the LICENSE file from ghc (and filed off the serial numbers). I'm afraid I haven't messed with cabal much yet, so it's not cabalized; neither is it uploaded to hackage, as I have no account there. If someone wants to do either of those, please feel entirely free to do so. In addition to attaching a tarball to this message, I'm also putting this onto my (sadly neglected) web site: it will live at http://www.korgwal.com/software/rk-1.0.0.tar.gz. Enjoy! Uwe

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regex-tdfa: mkRegex and matchRegex in ghci
by j.romildo＠gmail.com 21 Apr '09

21 Apr '09

Hello. How mkRegex and matchRegex can be used with the regex-tdfa backend in ghci? The following fails: $ ghci GHCi, version 6.10.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> import Text.Regex Prelude Text.Regex> import Text.Regex.TDFA Prelude Text.Regex Text.Regex.TDFA> let re = mkRegex "jos. romildo" Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package bytestring-0.9.1.4 ... linking ... done. Loading package mtl-1.1.0.2 ... linking ... done. Loading package regex-base-0.93.1 ... linking ... done. Loading package regex-posix-0.94.1 ... linking ... done. Loading package regex-compat-0.92 ... linking ... done. Prelude Text.Regex Text.Regex.TDFA> let text = "josé romildo" Prelude Text.Regex Text.Regex.TDFA> matchRegex re text Nothing Clearly regex-posix is being used here. The standalone version of the program works as expected: $ cat t.hs module Main (main) where import Text.Regex import Text.Regex.TDFA re = mkRegex "jos. romildo" text = "josé romildo" result = matchRegex re text main = print result $ ghc --make t [1 of 1] Compiling Main ( t.hs, t.o ) Linking t ... $ ./t Just [] Any clues? Romildo

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Building regex-tdfa-1.1.1 documentation
by j.romildo＠gmail.com 21 Apr '09

21 Apr '09

Hello. I want to build regex-tdfa-1.1.1 haddock documentation locally, but the procedure fails: $ ./setup haddock Preprocessing library regex-tdfa-1.1.1... Running Haddock for regex-tdfa-1.1.1... Warning: The documentation for the following packages are not installed. No links will be generated to these packages: rts-1.0 Text/Regex/TDFA/Common.hs:106:25: parse error on input `-- ^ identity of Position tag to set during a transition -- | Internal use to indicate type of tag and preference for larger or smaller Positions' $ haddock --version Haddock version 2.4.2, (c) Simon Marlow 2006 Ported to use the GHC API by David Waern 2006-2008 Any clues? Romildo

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ANN: curl-1.3.5
by Sigbjorn Finne 21 Apr '09

21 Apr '09

Hi, a refresh release of the Haskell (lib)curl package has been uploaded to Hackage, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/curl It enables 6.10.2 use, taking into account the updated story on how to register Haskell-based finalizers. Thanks to Carl Howells for reporting this problem. Bug reports / feature requests etc. most welcome, btw. enjoy --sigbjorn

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Petr Ročkai's darcs project accepted for Haskell GSoC
by Eric Kow 21 Apr '09

21 Apr '09

Hi everybody, I'm pleased to announce that Petr Ročkai's project on optimising darcs has been accepted for Google Summer of Code. More details can be found here: http://socghop.appspot.com/org/home/google/gsoc2009/haskell Petr's project fits into an ongoing effort by the darcs community to improve our performance so that one day we will able to recommend darcs for medium sized and larger projects. Until early 2008, we have been focused on darcs 2: which provides the darcs 2 theory (to solve most of the exponential merge problems), and also the "hashed" format repositories (including darcs 2). The project for this summer will allow us to complete the work on hashed repositories. In their current state, hashed format repositories make darcs a lot more robust and also allow us to fetch repositories faster (with darcs get --lazy and with a global patch cache). But in their current form, hashed repositories can can also slow down day-to-day operations like darcs whatsnew and darcs record. Petr has identified the cause of these delays has provided a work plan which we think will get hashed storage right. If his project is successful, future versions of darcs should make operations like darcs record and whatsnew much more comfortable in practice. Congratulations and good luck to Petr! And many thanks to the darcs and Haskell communities for their support. -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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RE: [Announce] primes
by Niemeijer, R.A. 21 Apr '09

21 Apr '09

Sebastian Fischer wrote: > I am pleased to announce the package 'primes' that implements lazy > wheel sieves for efficient, purely functional generation of prime > numbers in Haskell. > > The implementation is reasonably efficient. The query > > > primes !! 1000000 > 15485867 > > answers after a few seconds. > > Feel free to contribute more functionality to this package. The > sources are on Github: > > http://github.com/sebfisch/primes > > If you fork my project, I'll be happy to merge your changes. I have just finished benchmarking all the implementations provided in http://www.cs.hmc.edu/~oneill/code/haskell-primes.zip (the zip file linked to from the Haskell wiki article on primes). NaurPrimes.hs is by far the fastest version, and at least 2 or 3 times faster than your current implementation. I'm pretty sure it also uses less memory. I want to find efficient algorithms for the other proposed functions before forking, but I figured I'd let you know in the meantime.

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[ANN] Hack: a sexy Haskell Webserver Interface ^^
by Jinjing Wang 21 Apr '09

21 Apr '09

Simplest app should look like this module Main where import Hack import Hack.Handler.Kibro hello :: Application hello = \env -> return $ Response { status = 200 , headers = [ ("Content-Type", "text/plain") ] , body = "Hello World" } main = run hello Hack is a brainless port of the brilliant Ruby Rack framework. Rack is very modular and that's very important. Rack utilities and middlewares should be able to be ported with minimal effort, I hope. link / source: http://github.com/nfjinjing/hack/tree/master -- jinjing

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Re: [Haskell-cafe] CPS and the product function
by michael rice 21 Apr '09

21 Apr '09

That last definition multiplied zeros coming out of the recursion, but I believe this one is good: myprod l = prod l id id where prod [] k b = k 1 prod (x:xs) k b = if x == 0 then b 0 else prod xs (\ z -> k (x * z)) b My goal was to use CPS to do it. Did I succeed? Also, is there another way to do the same thing without passing b through all those recursions? I'm not good enough with Haskell's syntax to see how to do it. Michael --- On Mon, 4/20/09, michael rice <nowgate(a)yahoo.com> wrote: From: michael rice <nowgate(a)yahoo.com> Subject: Re: [Haskell-cafe] CPS and the product function To: haskell-cafe(a)haskell.org, "Tillmann Rendel" <rendel(a)cs.au.dk> Date: Monday, April 20, 2009, 11:28 AM This also seems to work: myprod l = prod l id where prod [] k = k 1 prod (x:xs) k = if x == 0 then 0 else prod xs (\ z -> k (x * z)) *Main Data.List> :load prod [1 of 1] Compiling Main ( prod.hs, interpreted ) Ok, modules loaded: Main. *Main Data.List> myprod [1,2,3,4,5,0,6,7,8,9] 0 *Main Data.List> myprod [1,2,3,4,5] 120 *Main Data.List> myprod [0..] 0 *Main Data.List> Michael --- On Mon, 4/20/09, Tillmann Rendel <rendel(a)cs.au.dk> wrote: From: Tillmann Rendel <rendel(a)cs.au.dk> Subject: Re: [Haskell-cafe] CPS and the product function To: haskell-cafe(a)haskell.org Date: Monday, April 20, 2009, 11:07 AM michael rice wrote: > I've been looking at CPS in Haskell and wondering how many multiplications the product function performs if it encounters a zero somewhere in the input list. Zero? > > Does anyone know the definition of the product function? You can use Hoogle [1] to search for product [2]. The documentation page [3] has a link to the source code [4]. Depending on some flag, it is either product = foldl (*) 1 or an explicit loop with an accumulator. That means that even for a non-strict (*), the whole input list would be processed before a result could be returned. > Does anyone know how to define it to avoid that? You have to define a multiplication function which is non-strict in the second argument if the first is 0. mult 0 b = 0 mult a b = a * b Now we can foldr this multiplication function over a list, and evaluation will stop at the first 0. foldr mult 1 ([1..100] ++ [0 ..]) However, this solution seems not to run in constant space. We can write it with a strict accumulator to avoid this problem: product = product' 1 where product' acc [] = acc product' acc (0 : xs) = 0 product' acc (x : xs) = (product' $! acc * x) xs Tillmann [1] http://www.haskell.org/hoogle/ [2] http://www.haskell.org/hoogle/?hoogle=product [3] http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:produ… [4] http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-List.html#p… _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell] ANN: persistent-map-0.0.0
by Alberto G. Corona 21 Apr '09

21 Apr '09

Interesting. It seems similar to TCache. However the design is different. persistent-map store different finite maps whenever any of them are modified transactionally, while TCache handles a single hashTable of object of type "a" and from time to time store all the transactionally modified objects and rearange the hashTable size. 2009/4/19 Peter Robinson <thaldyron(a)gmail.com>: > The persistent-map package [1] provides a thread-safe (STM) frontend > for finite map types together with a backend interface for persistent > storage. The TMap data type is thread-safe, since all access functions > run inside an STM monad . Any type that is an instance of > Data.Edison.Assoc.FiniteMapX (see EdisonAPI) can be used as a map > type. > > If a TMap is modified within an STM transaction, a corresponding > backend IO-request is added using the onCommit hook (cf. stm-io-hooks > package). To ensure consistency, the (Adv)STM monad runs these > requests iff the transaction commits. Additional backends (e.g. HDBC) > can be added by instantiating class Backend. > > Cheers, > Peter > > [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/persistent-map > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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Re: [Haskell-cafe] General function to count list elements?
by michael rice 20 Apr '09

20 Apr '09

Though I haven't tried it out, it's trying to use my function to count functions. The first argument is the identity function. The second argument is a list of a different form of the identity function. Though the two identity functions, given the same input, would produce the same output, I doubt they would be equal. So my guess at an answer would be zero. Michael --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: From: Eugene Kirpichov <ekirpichov(a)gmail.com> Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, April 18, 2009, 11:03 AM What should count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined)) return? 2009/4/18 michael rice <nowgate(a)yahoo.com>: > Is there a general function to count list elements. I'm trying this > > count :: a -> [a] -> Int > count x ys = length (filter (== x) ys) > > with this error upon loading > > ============= > > [michael@localhost ~]$ ghci count > GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help > Loading package ghc-prim ... linking ... done. > Loading package integer ... linking ... done. > Loading package base ... linking ... done. > [1 of 1] Compiling Main ( count.hs, interpreted ) > > count.hs:2:29: > Could not deduce (Eq a) from the context () > arising from a use of `==' at count.hs:2:29-32 > Possible fix: > add (Eq a) to the context of the type signature for `count' > In the first argument of `filter', namely `(== x)' > In the first argument of `length', namely `(filter (== x) ys)' > In the expression: length (filter (== x) ys) > Failed, modules loaded: none. > Prelude> > > ============= > > Not sure what it's trying to tell me other than I need an (Eq a) somewhere. > > Michael > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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Re: [Haskell-cafe] CPS and the product function
by michael rice 20 Apr '09

20 Apr '09

This also seems to work: myprod l = prod l id where prod [] k = k 1 prod (x:xs) k = if x == 0 then 0 else prod xs (\ z -> k (x * z)) *Main Data.List> :load prod [1 of 1] Compiling Main ( prod.hs, interpreted ) Ok, modules loaded: Main. *Main Data.List> myprod [1,2,3,4,5,0,6,7,8,9] 0 *Main Data.List> myprod [1,2,3,4,5] 120 *Main Data.List> myprod [0..] 0 *Main Data.List> Michael --- On Mon, 4/20/09, Tillmann Rendel <rendel(a)cs.au.dk> wrote: From: Tillmann Rendel <rendel(a)cs.au.dk> Subject: Re: [Haskell-cafe] CPS and the product function To: haskell-cafe(a)haskell.org Date: Monday, April 20, 2009, 11:07 AM michael rice wrote: > I've been looking at CPS in Haskell and wondering how many multiplications the product function performs if it encounters a zero somewhere in the input list. Zero? > > Does anyone know the definition of the product function? You can use Hoogle [1] to search for product [2]. The documentation page [3] has a link to the source code [4]. Depending on some flag, it is either product = foldl (*) 1 or an explicit loop with an accumulator. That means that even for a non-strict (*), the whole input list would be processed before a result could be returned. > Does anyone know how to define it to avoid that? You have to define a multiplication function which is non-strict in the second argument if the first is 0. mult 0 b = 0 mult a b = a * b Now we can foldr this multiplication function over a list, and evaluation will stop at the first 0. foldr mult 1 ([1..100] ++ [0 ..]) However, this solution seems not to run in constant space. We can write it with a strict accumulator to avoid this problem: product = product' 1 where product' acc [] = acc product' acc (0 : xs) = 0 product' acc (x : xs) = (product' $! acc * x) xs Tillmann [1] http://www.haskell.org/hoogle/ [2] http://www.haskell.org/hoogle/?hoogle=product [3] http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:produ… [4] http://haskell.org/ghc/docs/latest/html/libraries/base/src/Data-List.html#p… _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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CPS and the product function
by michael rice 20 Apr '09

20 Apr '09

I've been looking at CPS in Haskell and wondering how many multiplications the product function performs if it encounters a zero somewhere in the input list. Zero? Does anyone know the definition of the product function? Michael

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Re: [Haskell-cafe] CPS and the product function
by michael rice 20 Apr '09

20 Apr '09

Hi Eugene, Clever solution. It didn't come right back so I interrupted it. Guess it would do all those multiplies by zero. Does anyone know how to define it to avoid that? Thanks. Michael --- On Mon, 4/20/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: From: Eugene Kirpichov <ekirpichov(a)gmail.com> Subject: Re: [Haskell-cafe] CPS and the product function To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Monday, April 20, 2009, 9:40 AM Check that by experiment in ghci or by looking at the source: Prelude*> product [0..] Or search "product" at http://holumbus.fh-wedel.de/hayoo/hayoo.html and click on one of the Source links. 2009/4/20 michael rice <nowgate(a)yahoo.com>: > I've been looking at CPS in Haskell and wondering how many multiplications > the product function performs if it encounters a zero somewhere in the input > list. Zero? > > Does anyone know the definition of the product function? > > Michael > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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Code Golf
by Sebastian Fischer 20 Apr '09

20 Apr '09

Fancy some Codegolf? I wrote the following function for list diagonalization: > diag l = foldr (.) id ((sel l . flip sel) ((:[]).(:))) [] > where > sel = foldr (\a b c -> id : mrg (a c) (b c)) (const []) . map (flip id) > > mrg [] ys = ys > mrg xs [] = xs > mrg (x:xs) (y:ys) = (x.y) : mrg xs ys Self explanatory, isn't it? Here is a test case: *Main> take 10 $ diag [[ (m,n) | n <- [1..]] | m <- [1..]] [(1,1),(1,2),(2,1),(1,3),(2,2),(3,1),(1,4),(2,3),(3,2),(4,1)] I was trying to golf it down [^1] but my brain explodes. If you succeed in reducing keystrokes, I'd be happy to know! Cheers, Sebastian [^1]: http://codegolf.com/

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Re: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
by Niemeijer, R.A. 20 Apr '09

20 Apr '09

> data EShow = forall a. Show a => EShow a > > data E t = forall a. E (a->t) a > > smallPrint_ t = concatMap (\f-> f t) [show . foo, show . bar, show . baz] Yeah, I am aware of these solutions, but like Dan says: > but first-class existentials are still desirable because introducing a new > type for every existential is annoying. It's comparable to having to write a > new class for every combination of argument and result types to mimic first > class functions in Java (aside from first class functions being more > ubiquitous in their usefulness, although perhaps it only appears that way > because we don't have easy use of existential types). I've personally always looked at the extra data type or repeated functions as ugly hacks around the fact that GHC doesn't have real existential typing. Since Haskell is otherwise virtually free of ugly hacks (at least at the level I work at, which doesn't require things like unsafePerformIO and unboxed arrays) this has always annoyed me a bit. So I figured that since we now have a working implementation it would be worth a shot to ask if this language wart can be removed from GHC.

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Re: [Haskell-cafe] General function to count list elements?
by michael rice 20 Apr '09

20 Apr '09

I know functions can be compared in Scheme Welcome to DrScheme, version 4.1 [3m]. Language: Swindle; memory limit: 128 megabytes. > (equal? equal? equal?) #t > but apparently not in Haskell [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> (==) (==) (==) <interactive>:1:0: No instance for (Eq (a -> a -> Bool)) arising from a use of `==' at <interactive>:1:0-13 Possible fix: add an instance declaration for (Eq (a -> a -> Bool)) In the expression: (==) (==) (==) In the definition of `it': it = (==) (==) (==) Prelude> though I'm new at Haskell and may not be posing the question properly. I would think a language with 1st-class support for functions would certainly include comparing them. To compare two functions in C, I would compare their machine addresses. Michael --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: From: Eugene Kirpichov <ekirpichov(a)gmail.com> Subject: Re: [Haskell-cafe] General function to count list elements? To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, April 18, 2009, 11:39 AM Could you then provide an example of two functions that *are* equal, or, even better, a definition of equality for arbitrary functions? Since Haskell may be compiled into C, this must be a definition that is implementable in C. 2009/4/18 michael rice <nowgate(a)yahoo.com>: > Though I haven't tried it out, it's trying to use my function to count > functions. > > The first argument is the identity function. > > The second argument is a list of a different form of the identity function. > > Though the two identity functions, given the same input, would produce the > same output, I doubt they would be equal. > > So my guess at an answer would be zero. > > Michael > > --- On Sat, 4/18/09, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: > > From: Eugene Kirpichov <ekirpichov(a)gmail.com> > Subject: Re: [Haskell-cafe] General function to count list elements? > To: "michael rice" <nowgate(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Saturday, April 18, 2009, 11:03 AM > > What should > > count (\x -> x) (replicate 10 (\y -> if 1==1 then y else undefined)) > > return? > > 2009/4/18 michael rice <nowgate(a)yahoo.com>: >> Is there a general function to count list elements. I'm trying this >> >> count :: a -> [a] -> Int >> count x ys = length (filter (== x) ys) >> >> with this error upon loading >> >> ============= >> >> [michael@localhost ~]$ ghci count >> GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help >> Loading package ghc-prim ... linking ... done. >> Loading package integer ... linking ... done. >> Loading package base ... linking ... done. >> [1 of 1] Compiling Main ( count.hs, interpreted ) >> >> count.hs:2:29: >> Could not deduce (Eq a) from the context () >> arising from a use of `==' at count.hs:2:29-32 >> Possible fix: >> add (Eq a) to the context of the type signature for `count' >> In the first argument of `filter', namely `(== x)' >> In the first argument of `length', namely `(filter (== x) ys)' >> In the expression: length (filter (== x) ys) >> Failed, modules loaded: none. >> Prelude> >> >> ============= >> >> Not sure what it's trying to tell me other than I need an (Eq a) >> somewhere. >> >> Michael >> >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > > > > -- > Eugene Kirpichov > Web IR developer, market.yandex.ru > > -- Eugene Kirpichov Web IR developer, market.yandex.ru

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Re: [Haskell-cafe] ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
by Niemeijer, R.A. 20 Apr '09

20 Apr '09

> * Experimental language extensions, some of which have not been > implemented before. Does anybody know if there are any plans to incorporate some of these extensions into GHC - specifically the existential typing ? I would love to be able to use existential typing without having to give up the robustness of GHC.

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Re: Matrices
by Patrick Perry 20 Apr '09

20 Apr '09

Hi Cetin, This is probably the easiest way: > (m - es)^2/es listMatrix (2,2) [0.6163270413689804,0.600918865334756,0.33217626255600896,0.3238718559921087 ] This will create 2 temporary arrays. Alternatively, > let res = listMatrix (shape m) [ (o-e)^2 / e | o <- colElems m | e <- colElems es ] where colElems = concatMap elems . cols > res listMatrix (2,2) [0.6163270413689804,0.600918865334756,0.33217626255600896,0.3238718559921087 ] In a later version of the library, "colElems" will probably be built- in. This version has the disadvantage that it doesn't use BLAS calls. If you *really* want to get tricky and eliminate temporary arrays: > runSTMatrix $ do res <- newCopyMatrix m subMatrix res es modifyWith (^2) res divMatrix res es return res Now, to get the sum: > sumAbs $ vectorFromMatrix res 1.8732940252518542 or > sum $ elems res 1.8732940252518542 The first version will usually be more efficient, since it calls the BLAS1 function "dasum". Patrick

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Looking for the fastest Haskell primes algorithm
by Niemeijer, R.A. 20 Apr '09

20 Apr '09

Today I happened to need a large list of prime numbers. Obviously this is a well-known problem, so I figured there would be something on Hackage that I could use. Surprisingly, there isn't, or if there is it's not easy to find. Searching for prime or primes on Hackage reveals nothing. Searching for primes on Hayoo gives Codec.Encryption.RSA.NumberTheory, but that uses the inefficient one-liner implementation. The HaskellWiki article on primes (http://www.haskell.org/haskellwiki/Prime_numbers) has a number of implementations, but the faster they get, the longer and uglier they become. Since it's such a common problem I'd say it would be a good idea to add a package to Hackage that exports primes :: [Integer] and hides the ugly implementation details. Data.Numbers.Primes seems a logical choice for the namespace, but I'm open to suggestions. The trick then is to find the most efficient implementation of primes. The Haskell wiki article mentions ONeillPrimes.hs as one of the fastest ones, but maybe there's a faster version. So my question is: does anybody know what the fastest Haskell algorithm for generating primes is?

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