- Haskell-Cafe - Haskell.org

2 line finite (monadic) state machine
by Izaak Meckler 13 Oct '12

13 Oct '12

Hi all, I just wrote some nice code: A finite state machine with monadic state, allowing both DFAs and NFAs to be expressed with the same functions. I've only used it in [], Set, and Maybe, but I think it would be interesting in several others (a probability monad comes to mind). Also, if anyone has any sensible interpretation of this in the continuation monad, let me know. import Control.Monad import Data.Maybe type State = String type Map a b = [(a, b)] -- In Map State (Map Char (m State)), the monad m determines the kind of FSM that is being run. -- If m = [] (or Set), these functions work as a NFA. If m = Maybe, we essentially have a DFA. transition :: (MonadPlus m) => Map State (Map Char (m State)) -> State -> Char -> m State transition transMap q c = fromMaybe mzero $ lookup q transMap >>= lookup c toFSM :: (MonadPlus m) => Map State (Map Char (m State)) -> State -> (String -> m State) toFSM transMap q0 = foldM (transition transMap) q0 egMachine = toFSM [("p", [ ('0', ["p"]) , ('1', ["p", "q"])])] "p"

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character encoding problems with hxt (I think)
by nadine.and.henry＠pobox.com 13 Oct '12

13 Oct '12

Dear Haskellers, I'm trying to write some code that grabs countries and provinces from the iso_3166 files on Linux systems. I seem to be running into some kind of character encoding problem. file says iso_3166_2.xml is a utf8 file, and isutf8 agrees, but when I run the following code, it crashes. uft8Copy makes a byte for byte copy as expected. noCrash read and writes the document without crashing, but the accented characters in the strings show up garbled. Just search for "DE" and you'll see what I mean. crash (on my system, (Debian testing)) produces the error message below. Can anyone enlighten me on what is going on? Thanks in advance. Henry Laxen ------------------------------------------------------------------------ {-# LANGUAGE Arrows #-} import Text.XML.HXT.Core import Data.List import qualified System.IO.UTF8 as U isoFile = "/usr/share/xml/iso-codes/iso_3166_2.xml" countZerosInLines = length . filter (\x -> x == '0') . concat utf8Copy = do xml <- U.readFile isoFile let doc = readString [withValidate no] xml U.writeFile "iso_written.xml" xml putStrLn "Ran utf8Copy" noCrash :: IO () noCrash = do lines <- runX (readDocument [withValidate no] isoFile >>> writeDocumentToString [withShowTree yes] ) -- mapM_ putStrLn lines print $ countZerosInLines lines putStrLn "Ran noCrash" crash :: IO () crash = do xml <- U.readFile isoFile let doc = readString [withValidate no] xml src <- runX $ doc //> writeDocumentToString [withShowTree yes] print $ countZerosInLines src putStrLn "Ran crash (didn't)" {- Ran utf8Copy 1236 Ran noCrash error: UTF-8 encoding error at input position 2640: InvalidLaterByte 1 error: UTF-8 encoding error at input position 2646: InvalidLaterByte 1 *** Exception: Enum.toEnum{Word8}: tag (363) is outside of bounds (0,255) -} main = do utf8Copy noCrash crash ------------------------------------------------------------------------

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Type of scramblings
by Roman Cheplyaka 12 Oct '12

12 Oct '12

I am reading through Oleg's "Eliminating translucent existentials"[1]. [1]: http://okmij.org/ftp/Computation/Existentials.html#eliminating-translucent He draws a distinction between forall a . [a] -> [a] and forall a . [a]^n -> [a] as types of "scramblings". This is something I'm struggling to understand. First of all, I think here we're talking about total functions, otherwise there's no point in introducing dependent types. There are of course more total functions of type `[a]^n -> [a]` than of type `[a] -> [a]`, in the sense that any term of the latter type can be assigned the former type. But, on the other hand, any total function `f :: [a]^n -> [a]` has an "equivalent" total function g :: [a] -> [a] g xs | length xs == n = f xs | otherwise = xs (The condition `length xs == n` can be replaced by a similar condition that also works for infinite lists.) The functions `f` and `g` are equivalent in the sense that for any list `xs` of length `n` `f xs === g xs`. Thus, even though it seems that we allow more total functions by replacing `[a]` with `[a]^n`, that doesn't buy us any additional expressiveness. What am I missing? Roman

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Using CatchIO on StateT
by Dmitry Vyal 12 Oct '12

12 Oct '12

Greetings, I found The MonadCatchIO-mtl package while looking for a way to catch exceptions in my custom StateT s (ReaderT r IO) monad. CatchIO worked flawlessly when handlers used the environment r from Reader, but things broken when I tried to use state. According to source, the handler is run with the same state which was passed to runStateT. instance MonadCatchIO m => MonadCatchIO (StateT s m) where m `catch` f = StateT $ \s -> (runStateT m s) `Control.Monad.CatchIO.catch` (\e -> runStateT (f e) s) And my finalizing action depends on the current state. Can you give an advice how to get the behavior I need? Best regards, Dmitry Vyal

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Hackage Documentation issue
by Jonas Almström Duregård 12 Oct '12

12 Oct '12

Hi, Short question: can I or someone else make the Hackage build bot rebuild a package with a new dependency (even though the initial build was successful)? Long question: Does anyone know what would cause the documentation of a successful Hackage build not to appear on the web? Specifically I'm trying to get docs working for: http://hackage.haskell.org/package/BNFC-meta-0.4 Doing cabal haddock works well for me, and I managed to find the log of the Hackage build: http://hackage.haskell.org/packages/archive/BNFC-meta/0.4/logs/success/ghc-… The problem seems to be related to one of the dependencies (Alex-meta) which had a similar problem: the documentation step failed (but not the build). I was able to fix it (or at least work around it) so Alex-meta has working documentation now. Can I or someone else prompt the BNFC-meta documentation to rebuild with the new dependency (even though the initial build was successful)? I wouldn't want to upload a new version just to get docs rebuilt, and i'm not even sure it solves the issue. Regards, Jonas

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Ordering of BigFloats in numbers-3000.0.0.0
by Michael Orlitzky 11 Oct '12

11 Oct '12

I'm trying to use, http://hackage.haskell.org/package/numbers-3000.0.0.0 to get better precision "for free" out of some numerical code. I ran into an issue pretty quickly, though. In Data.Number.BigFloat, we have, data BigFloat e = BF (Fixed e) Integer deriving (Eq, Ord) and the derived Ord is obviously incorrect: Prelude Data.Number.BigFloat> let x = 0.1 :: BigFloat Prec50 Prelude Data.Number.BigFloat> let y = 0.02 :: BigFloat Prec50 Prelude Data.Number.BigFloat> x < y True But, I'm not sure who I should contact as the maintainer. The LICENSE file has, Copyright (c) 2007-2012 Lennart Augustsson, Russell O'Connor, Richard Smith, Daniel Wagner, Dan Burton The cabal file says (no email address), Maintainer: Lennart Augustsson And the repo is at, https://github.com/DanBurton/numbers Does anyone know who I should bug for a fix?

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London Haskell User Group is back
by Derek Wright 11 Oct '12

11 Oct '12

[Sorry if you have already seen / heard this already, this really is the last announcement...] After 3 years away the London Haskell User Group is back. Our first meeting is in a couple of weeks. Wednesday 24-Oct-2012 6-30pm City University, College Building, St John Street, London. EC1V 4PB http://www.meetup.com/London-HUG/events/85763942/ Why Do Monads Matter? Speaker: Derek Wright A quick tour through the why rather than the how of monads. Trying to give an intuition about why these concepts matter in the first place. From The Four Horsemen of the Catapocalypse (Failure, Dependence, Uncertainty and Destruction) through Kleisli arrows and ending up with monads and categorical programming. See why these abstract concepts should be part of every Haskell programmers toolkit. Join us for the first talk of London Haskell 2.0, then participate in a discussion about the future meetings and shape the vision! We are using Meetup to organise everything and the main page for London Haskell is: http://www.meetup.com/London-HUG/ There has already been a lot interest and 35 (out of a possible 40) have signed up already! Hope to see you there. Yours, Derek.

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64-bit vs 32-bit haskell platform on Mac: misleading notice on Platform website?
by Eugene Kirpichov 11 Oct '12

11 Oct '12

Hi, I installed Haskell Platform 32-bit on a fresh 64-bit mac, because the page http://www.haskell.org/platform/mac.html says: "Pick the 32-bit vesion, unless you have a specific reason to use the 64-bit version. The 32-bit one is slightly faster for most programs." Then, during the installation of a package, the following happeed: Loading package cairo-0.12.3.1 ... <command line>: can't load .so/.DLL for: /opt/local/lib/libz.dylib (dlopen(/opt/local/lib/libz.dylib, 9): no suitable image found. Did find: /opt/local/lib/libz.dylib: mach-o, but wrong architecture) That libz.dylib is a 64-bit library and it can't be used by 32-bit Haskell platform. QUESTION: It seems to me that most people, at least most who care about "slightly faster" programs, are likely to run into something like this - using native 64-bit libraries. Compatibility exists only in the opposite direction. Wouldn't it be appropriate to remove this notice and ask people to use the 64-bit version unless they have a specific reason not to? -- Eugene Kirpichov http://www.linkedin.com/in/eugenekirpichov We're hiring! http://tinyurl.com/mirantis-openstack-engineer

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Safe Haskell and instance coherence
by Mikhail Glushenkov 11 Oct '12

11 Oct '12

Hello, It's a relatively well-known fact that GHC allows for multiple type class instances for the same type to coexist in a single program. This can be used, for example, to construct values of the type Data.Set.Set that violate the data structure invariant. I was mildly surprised to find out that this works even when Safe Haskell is turned on: https://gist.github.com/3854294 Note that the warnings tell us that both instances are "[safe]" which gives a false sense of security. I couldn't find anything on the interplay between orphan instances and Safe Haskell both in the Haskell'12 paper and online. Is this something that the authors of Safe Haskell are aware of/are intending to fix? -- () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments

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ANN: data-dword: Long binary words from short ones
by Mikhail Vorozhtsov 11 Oct '12

11 Oct '12

Hi. I'm pleased to announce my new little library, data-dword[1]. It provides Template Haskell utilities for defining binary word data types from low and high halves, e.g. data Word96 = Word96 Word32 Word64 -- strictness is configurable data Int96 = Int96 Int32 Word64 -- All instances are fully implemented (including `quotRem`, etc) instance Bounded, Enum, Eq, Integral, Num, Ord, Read, Real, Show, Ix, Bits, Hashable -- Extra bit-manipulating functions, unwrapped addition and -- multiplication, etc. instance BinaryWord, DoubleWord -- Rewrite rules for converting to/from the standard integral types {-# RULES "fromIntegral/..." ... #-} The library comes with a pretty thorough test suite (that ATM has some failures on x86-32 due to bug #7233[2] in the base library). [1] http://hackage.haskell.org/package/data-dword [2] http://hackage.haskell.org/trac/ghc/ticket/7233

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