April 2011 - Haskell-Cafe

ANNOUNCE: hplaylist-0.1
by Tim Chevalier 09 Apr '11

09 Apr '11

Hello, hplaylist is a very simple command-line utility, written in Haskell, for automatically syncing an iTunes playlist to a non-Apple-manufactured MP3 player (anything that mounts as a USB drive should work in principle). For example, if I have an iTunes playlist called "Songs about cheese", hplaylist allows me to type: $ hplaylist "Songs about cheese" which automatically copies the named playlist, and all music files included in the playlist, to my device. Download at: http://hackage.haskell.org/package/hplaylist-0.1 or https://github.com/catamorphism/hplaylist Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc/ * Often in error, never in doubt "an intelligent person fights for lost causes,realizing that others are merely effects" -- E.E. Cummings

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Testing Implementation vs Model - Records or Type Classes?
by Heinrich Apfelmus 09 Apr '11

09 Apr '11

Hello, I'm writing a small Haskell library for functional reactive programming. The core of the library consists of two data types and several primitives. However, I have programmed this core *twice*: once as a *model* that displays the intended semantics, and once as the actual *implementation* to be used in production code. Of course, I would like to use QuickCheck to test that the implementation gives the same results as the model. My problem is: how to organize this with the minimum amount of boilerplate? It appears that I can use *multiparameter type classes* to solve this, but I'm not sure I'm happy with this, in particular because it makes the generated Haddock less readable (see 3) below). On the other hand, I could also use *record wildcards* to conveniently reify a module into a record data structure. But that will give me problems with combinators that are derived from the core combinators (see 2) below). Haskell Café, what are your suggestions and ideas? In particular, I wish to: 1) Write examples to be QuickChecked only once. I imagine that my main function for testing looks like this test_equal example = forAll $ \input -> example model input == example implementation input where example an expression involving the combinators from my library. The point is that I don't want to write example twice, once for each version. 2) Use derived combinators. For reference, here the full signature of the core combinators: data Event a data Behavior a instance Functor Behavior instance Applicative Behavior instance Functor Event instance Monoid (Event a) filter :: (a -> Bool) -> Event a -> Event a apply :: Behavior (a -> b) -> Event a -> Event b accumB :: a -> Event (a -> a) -> Behavior a When writing tests, I also want to use common derived combinators, like, say filterJust :: Event (Maybe a) -> Event a filterJust = fmap fromJust . filter isJust without implementing them twice. 3) Obtain readable Haddock. I like the simplicity and readability of an ordinary module signature. In contrast, libraries with many type class scare me. Or is there a way to make this less scary? I'm not sure about the last one: 4) Make both model and implementation available to the user, so that he can QuickCheck his own programs as well. Since the implementation makes heavy use of IO, it is a bit harder to test automatically, so the model might be useful to have. Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

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ANNOUNCE: normaldistribution-1.1 – Minimum fuss normally distributed random values.
by Bjorn Buckwalter 09 Apr '11

09 Apr '11

Dear all, Thanks to enlightening advice from Tom Nielsen and Zoltan Novak normaldistribution now generates normally distributed values using the Box-Muller method instead of the Central Limit Theorem. This change provides a 5-10x speedup. The only API impact is that the random values must now be instances of Floating rather than just Fractional. (The pre-canned Random instances that satisfy Fractional, i.e. Double and Float, also satisfy Floating.) Cheers, Bjorn ---------- Forwarded message ---------- From: Bjorn Buckwalter <bjorn.buckwalter(a)gmail.com> Date: Sat, Apr 9, 2011 at 15:21 Subject: ANNOUNCE: normaldistribution-1.0 – Minimum fuss normally distributed random values. Dear all, I'm pleased to announce the immediate availability of the normaldistribution library on hackage: http://hackage.haskell.org/package/normaldistribution This purpose of this library is to have a simple API and no dependencies beyond Haskell 98 in order to let you produce normally distributed random values with a minimum of fuss. This library does not attempt to be blazingly fast nor to pass stringent tests of randomness. It attempts to be very easy to install and use while being "good enough" for many applications (simulations, games, etc.). The API builds upon and is largely analogous to that of the Haskell 98 Random module (more recently System.Random). Pure: > (sample,g) = normal myRandomGen -- using a Random.RandomGen > samples = normals myRandomGen -- infinite list > samples2 = mkNormals 10831452 -- infinite list using a seed In the IO monad: > sample <- normalIO > samples <- normalsIO -- infinite list With custom mean and standard deviation: > (sample,g) = normal' (mean,sigma) myRandomGen > samples = normals' (mean,sigma) myRandomGen > samples2 = mkNormals' (mean,sigma) 10831452 > sample <- normalIO' (mean,sigma) > samples <- normalsIO' (mean,sigma) Internally the library uses the Central Limit Theorem to approximate normally distributed values from multiple uniformly distributed random values. Enjoy, Bjorn

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ANNOUNCE: normaldistribution-1.0 – Minimum fuss normally distributed random values.
by Bjorn Buckwalter 09 Apr '11

09 Apr '11

Dear all, I'm pleased to announce the immediate availability of the normaldistribution library on hackage: http://hackage.haskell.org/package/normaldistribution This purpose of this library is to have a simple API and no dependencies beyond Haskell 98 in order to let you produce normally distributed random values with a minimum of fuss. This library does not attempt to be blazingly fast nor to pass stringent tests of randomness. It attempts to be very easy to install and use while being "good enough" for many applications (simulations, games, etc.). The API builds upon and is largely analogous to that of the Haskell 98 Random module (more recently System.Random). Pure: > (sample,g) = normal myRandomGen -- using a Random.RandomGen > samples = normals myRandomGen -- infinite list > samples2 = mkNormals 10831452 -- infinite list using a seed In the IO monad: > sample <- normalIO > samples <- normalsIO -- infinite list With custom mean and standard deviation: > (sample,g) = normal' (mean,sigma) myRandomGen > samples = normals' (mean,sigma) myRandomGen > samples2 = mkNormals' (mean,sigma) 10831452 > sample <- normalIO' (mean,sigma) > samples <- normalsIO' (mean,sigma) Internally the library uses the Central Limit Theorem to approximate normally distributed values from multiple uniformly distributed random values. Enjoy, Bjorn

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Is there a way to find out the type inferred for a local function inside another function? :)
by KC 09 Apr '11

09 Apr '11

-- -- Regards, KC

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Patch: adding CTR, CMAC and SIV modes to the Crypto API
by klondike 08 Apr '11

08 Apr '11

Hi, After a few weeks of hard work I have managed to add the CTR, CMAC and SIV modes of operation. The implemententation passes all the KATS I could find and is made as efficiently as possible without forgetting the risk of timing attacks over them. The patch is attached here and you can free it along with your library with a BSD-3 license as long as you recognize the authorship. Francisco Blas Izquierdo Riera (klondike) PS: Attached is also a set of commands I used with ghci to check quickly the implementation and which contain parts of the KATS I used, feel free to use them.

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erratic behavior for System.Time.diffClockTimes
by Warren Harris 08 Apr '11

08 Apr '11

I'm trying out GHC-7.0.3-x86_64 for Mac OS X and see what seems to be a bug in System.Time.diffClockTimes. The TimeDiff tdPicosec field returns values that seem to jump around erratically: test = do startTime <- System.Time.getClockTime endTime <- System.Time.getClockTime let dt = diffClockTimes endTime startTime putStrLn $ show dt Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = 0} Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = 3000000} Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = -3329012246138323150000000} Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = 3000000} Prelude Test> test TimeDiff {tdYear = 0, tdMonth = 0, tdDay = 0, tdHour = 0, tdMin = 0, tdSec = 0, tdPicosec = -3329012246138322851000000} Prelude Test> I can file a bug, unless I'm overlooking something here. Warren

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fmap and LaTeX
by Mitar 08 Apr '11

08 Apr '11

Hi! Is there for <$> (fmap) operator some nice looking symbol in mathematics, LaTeX? I am looking here: http://www.soi.city.ac.uk/~ross/papers/Applicative.html http://www.haskell.org/ghc/docs/6.12.1/html/libraries/base/Control-Applicat… but only for other operators there are nice symbols, and <$> is left as it is. I have also tried \mathbin{<\$>} but it adds space around $ (at least in lhs2tex), so it looks ugly. Mitar

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FFI for a beginner
by Andrew Pennebaker 08 Apr '11

08 Apr '11

ncurses is proving too difficult to setup, so I'm working on a new library called charm. The C code works by itself, but I can't compile a Haskell wrapper for it. While the tutorials at HaskellWiki<http://www.haskell.org/haskellwiki/FFI_complete_examples>are helpful, they're outdated. Argh! The docs<http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgo…>say that -#include pragmas no longer work, but fail to explain how to load code without them. Suffice to say I have no recourse but trial and error. GitHub: charm <https://github.com/mcandre/charm> and hscharm<https://github.com/mcandre/hscharm> <https://github.com/mcandre/charm>$ make cp /usr/include/charm.c . ghc --make -fforce-recomp -o hellocharm hellocharm.hs charm.hs charm.c -I/usr/include -dylib-install-name /usr/lib/libcharm.dynlib [1 of 2] Compiling Charm ( charm.hs, charm.o ) [2 of 2] Compiling Main ( hellocharm.hs, hellocharm.o ) Linking hellocharm ... ld: duplicate symbol _Charm_getWidth_info in charm.o and charm.o collect2: ld returned 1 exit status make: *** [hellocharm] Error 1 Cheers, Andrew Pennebaker www.yellosoft.us

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cab 0.1.2
by Kazu Yamamoto 07 Apr '11

07 Apr '11

Hello cafe, Let me announce "cab version 0.1.2". This version integrates "cabal-dev" as well as "cabal/ghc-pkg". So, you can use a sandbox for your development. http://www.mew.org/~kazu/proj/cab/en/ Here is the short explanation from the page above: ---- "cab" is a MacPorts-like maintenance command of Haskell cabal packages. Some parts of this program are a wrapper to "ghc-pkg", "cabal", and "cabal-dev". If you are always confused due to inconsistency of "ghc-pkg" and "cabal", or if you want a way to check all outdated packages, or if you want a way to remove outdated packages recursively, this command helps you. "cab" is designed to maintain user packages only assuming that users use a Haskell Platform and don't want to touch the global packages. ---- --Kazu

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