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ANN: hledger 0.13
by Simon Michael 07 Dec '10

07 Dec '10

I've released hledger 0.13, with readline editing and tab completion from Judah Jacobson, more ledger compatibility, a more robust and installable web interface, bugfixes, and a much-deliberated package split. Docs and mac and 64-bit linux binaries are at http://hledger.org , and of course you can cabal update and cabal install hledger, cabal install hledger-web and so on. I didn't manage to build new windows or linux 32-bit binaries today, but they'll appear when I do. Let it be noted that maintaining a non-trivial cross-platform haskell app is currently quite hard. Release notes: * move web, vty, chart commands into separate hledger-web, hledger- vty, hledger-chart packages. This both simplifies (no more build flags) and complicates (more room for dependency hassles), but I hope overall it will be easier and more scalable. * all packages but chart are now marked "beta", ie "not finished but suitable for everyday use" * parsing: ledger compatibility: support D default commodity directive * parsing: ledger compatibility: ignore metadata tags on transactions and postings * parsing: ledger compatibility: ignore cleared flags at the start of postings * parsing: ledger compatibility: ignore C commodity conversion directives * parsing: price precisions no longer affect commodities' display precisions * add: readline-style editing * add: tab-completion for account names * add: add the default commodity, if any, to commodity-less amounts (#26) * add: misc. commodity/precision/defaults-related bugfixes * chart: give a meaningful error message for empty journals * chart: update for current Chart lib (0.14) * web: support files now live in ./.hledger/web/ and will be auto- created at startup * web: page layout is more robust with wide content * web: allow editing of included files * web: handle multiple filter patterns correctly * web: allow single- or double-quoted filter patterns containing spaces * web: update for current yesod lib (0.6.*) * transaction balancing is now based on display precision (#23) * briefer, more informative usage error messages Stats: 91 days and 117 commits since last release, now at 5997 lines of code with 151 tests. hledger is at a point where real-world user and developer input is needed to focus it; participants welcome. Best, -Simon

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Help defining a Typeable polymorphic-state monad transformer
by Brandon Simmons 07 Dec '10

07 Dec '10

Hi all, I gave myself until this evening to figure this out on my own, and time is up! Hopefully this makes for a good discussion, though the idea could be dumb. What I'm trying to do is define a state monad in which the passed state can change type during the computation. The only constraint is that the state types must always be of the Typeable class (see: http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Typea… ). The idea is that the new monad would be something like 'StateT s Maybe a', but where the 's' type is not fixed (indeed is hidden in an existential type) and where any programmer errors in the chaining of the polymorphic state will be caught in the Maybe type (or really the 'fail' implementation of any monad). Here is how I imagine a computation might look: computation :: TypeableState Maybe String computation = do (c:cs) <- getTS putTS (length cs) return ("c" ++ " was the first letter of the string passed as initial state.") So TypeableState is very similar to StateT, except that the state type is not present as a type argument. In the example above 'Maybe' is the monad that catches Typeable errors, and String is the return type of the computation. getTS and putTS would be get and put functions that constrain their arguments to the Typeable class. Here is what I have so far (at least this is my most recent uncommented attempt): > {-# LANGUAGE ExistentialQuantification #-} > module Main > where > > import Control.Monad.State > import Data.Typeable > > -- we might have restricted our 'm' to MonadPlus and used the explicit > -- 'mzero', but decided instead to use Monad, with 'fail'. This is > -- more appropriate since we won't be using 'mplus'. See 'liftMaybe'. > data TypeableState m a = forall s0 sN. (Typeable s0, Typeable sN)=> > TypeableState (s0 -> m (a,sN)) > > -- this is probably one of the more non-sensical attempts I've made at > -- this... but I'm not sure: > runTypeableState :: (Monad m, Typeable s0, Typeable sN)=> TypeableState m a -> s0 -> m (a,sN) > runTypeableState (TypeableState st) s0 = (liftMaybe $ cast s0) >>= st > > -- copied from Control.Monad.StateT > instance (Monad m) => Monad (TypeableState m) where > return a = TypeableState $ \s -> return (a, s) > m >>= k = TypeableState $ \s -> do > ~(a, s') <- runTypeableState m s > runTypeableState (k a) s' > fail str = TypeableState $ \_ -> fail str > > -- I imagine using this with 'cast' to thread the type in our monad > -- transformer > liftMaybe :: (Monad m)=> Maybe a -> m a > liftMaybe = maybe (fail "Monadic failure") return So is this even feasible? Or do I not grok what we can and can't do with the Typeable class? Any thoughts on this are appreciated. Sincerely, Brandon Simmons http://coder.bsimmons.name

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Ed Lambda: Functional programming meetup in Edinburgh
by Ollie Saunders 07 Dec '10

07 Dec '10

It's that time again... When: Monday the 13th of December at 7:00pm Where: 12a The Outhouse, Broughton Street Lane, Edinburgh This month Chris Yocum will be talking about some functional fundamentals with his talk: The Loopless Loop: Perl and Ocaml as functional programming languages Phil Scott may also be talking, we're not sure yet. Also I'm not sure if I'm going to be able to make it but I've made provisions if I can't so the show will definitely be going on. As always there will be plenty of beer and friendly banter. I think everyone who comes enjoys themselves and learns something. If you missed out on last month then you can see my talk on Vimeo: <http://vimeo.com/17343181> (2nd part isn't there yet but is uploading right now).

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Categorical description of systems with dependent types
by Petr Pudlak 07 Dec '10

07 Dec '10

Hi, recently, I was studying how cartesian closed categories can be used to describe typed functional languages. Types are objects and morphisms are functions from one type to another. Since I'm also interested in systems with dependent types, I wonder if there is a categorical description of such systems. The problem (as I see it) is that the codomain of a function depends on a value passed to the function. I'd happy if someone could give me some pointers to some papers or other literature. Thanks, Petr

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need help with syntax...
by Daryoush Mehrtash 07 Dec '10

07 Dec '10

I am having hard time understanding the following code. The code is from Applicative Parser library: http://hackage.haskell.org/packages/archive/uu-parsinglib/2.5.5.2/doc/html/… instance (Show a, loc `IsLocationUpdatedBy` a) => Provides (Str a loc) (a -> Bool, String, a) a where splitState (p, msg, a) k (Str tts msgs pos del_ok) = let ins exp = (5, k a (Str tts (msgs ++ [Inserted (show a) pos exp]) pos False)) del exp = (5, splitState (p,msg, a) k (Str (tail tts) (msgs ++ [Deleted (show(head tts)) pos exp]) (advance pos (head tts)) True )) in case tts of (t:ts) -> if p t then show_symbol ("Accepting symbol: " ++ show t ++ " at position: " ++ show pos ++"\n") (Step 1 (k t (Str ts msgs (advance pos t) True))) else Fail [msg] (*ins:* if *del_ok* then *[del*] else []) [] -> Fail [msg] [*ins*] Specifically I am having hard time understanding how the "ins exp" and "del exp" in the "let" is related to "ins:" "del_ok", [del], [ins]. I don't understand how given the "let" expression the following expressions are expanded Fail [msg] (*ins:* if *del_ok* then *[del*] else []) and Fail [msg] [*ins*] Appreciate any help. Thanks Daryoush

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Bifold: a simultaneous foldr and foldl
by Noah Easterly 07 Dec '10

07 Dec '10

Somebody suggested I post this here if I wanted feedback. So I was thinking about the ReverseState monad I saw mentioned on r/haskell a couple days ago, and playing around with the concept of information flowing two directions when I came up with this function: bifold :: (l -> a -> r -> (r,l)) -> (l,r) -> [a] -> (r,l) bifold _ (l,r) [] = (r,l) bifold f (l,r) (a:as) = (ra,las) where (ras,las) = bifold f (la,r) as (ra,la) = f l a ras (I'm sure someone else has come up with this before, so I'll just say I discovered it, not invented it). Basically, it's a simultaneous left and right fold, passing one value from the start of the list toward the end, and one from the end toward the start. It lets you do some interesting stuff, like filter based on positionor other left-dependent information: evenIndexed :: [a] -> [a] evenIndexed = fst . bifold alternate (0,[]) where alternate 0 x xs = (x:xs, 1) alternate 1 _ xs = (xs, 0) maximums :: (Ord a) => [a] -> [a] maximums [] = [] maximums (a:as) = a : (fst $ bifold (\m a l -> if a > m then (a:l,a) else (l,m)) (a,[]) as) As long as you don't examine the left-to-right value, it can still work on infinite lists: ghci> take 20 $ evenIndexed [0..] [0,2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32,34,36,38] Also, it can be used for corecursive data (or, at least, doubly-linked lists): data DList a = Start { first :: DList a } | Entry { value :: a, next :: DList a, prev :: DList a } | End { last :: DList a } deriving (Eq) ofList :: [a] -> (DList a, DList a) ofList as = (start,end) where start = Start first end = End last (first,last) = bifold mkEntry (start,end) as mkEntry p v n = let e = Entry v n p in (e,e) It's just been running around my head all night, so I thought I'd share.

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Trying out GHC 7
by Andrew Coppin 06 Dec '10

06 Dec '10

OK, so the other day I decided to take GHC 7.0.1 for a spin. Suffice it to say, it definitely compiles stuff. So far, I've only tried running it in a Windows XP virtual machine with a single core, so I haven't been able to test what multicore performance is like. One thing that did interest me is the conflicting information about whether or not dynamic linking is supported on Windows. Half the User Guide claims that this /is/ supported, and the other half still contains statements that it is /not/ supported. So I thought I'd just go physically try it and see if it works. Well... it compiles stuff. Hello World goes from being 518KB to being a piffling 13KB. Which is nice. I almost didn't bother actually trying to /run/ the resulting binary... But when I did, I discovered that it immediately crashes, whining that it can't find the DLL it wants. So, out of the box, the Windows setup is broken. (Obviously the Windows section of the User Guide makes no mention of how DLLs are located; it only explains the situation for Linux and Mac OS. I especially liked the bit where it says that there are "three modes", and then proceeds to list only two...) In the interests of science, I wanted to see if I could make it run by just putting the necessary DLLs into the same folder as the binary. But first I had to /find/ them. I had expected to find a folder somewhere with all the DLLs in it... but no, the installer appears to have sprinkled them around the filesystem more or less at random. Some of them are in folders named for the package, some of them just sit in the root folder. After about 20 minutes of searching, I found the RTS DLL. Running the program, it now instantly crashes because of a /different/ DLL. This one didn't take quite so long to find (now that I know roughly where to look). I fixed that, and now the binary crashes with yet another missing DLL. And so on and so forth. In the end, Hello World required 5 DLLs. Better still, together these totalled 8MB of data. (!) So the distribution package is now about 16x bigger than with static linking. Oh joy. Clearly if you just want to distribute one executable binary, you're going to use static linking. (E.g., darcs.exe will hopefully remain statically linked.) But I suppose if I had a whole heap of Haskell programs then the extra complexity of delivering half a dozen DLLs might may off in terms of space reductions. And of course, on /my/ development box, I don't need a zillion redundant copies of the RTS code, etc. (Or at least, I won't if I can figure out how to make the programs able to actually /find/ the necessary DLLs...) Actually, of all the DLLs, only "base" is actually large - it's approximately 6MB on its own. GHC-Prim is 0.5MB, and the others are all relatively tiny. It feels slightly irritating that I need to inclide the FFI DLL when I'm not doing and FFI - but of course, the entire Haskell I/O subsystem /is/ doing FFI. Similarly, I'm not using arbitrary-precision integers, but various I/O operations return file sizes as Integer, so GMP must also be included. Neither of these things are actually especially large. It's rather confusing that the User Guide is out of date in so many places. (And not just regarding dynamic linking on Windows. For example, one section claims that "+RTS -f" will tell you what options are supported, but that actually yields "unknown RTS opion: -f". It seems that "-?" is the correct option name.) If I can get a suitable Linux VM going, I might have a go at contributing some updates. Just for completeness, I tried asking for dynamic linking with an older version of GHC (6.10, IIRC) - one that supports it on Linux but not Windows. I was expecting to get either "dynamic linking is not supported on this platform", or at worst "unknown option -dynamic". In fact, what I got was "gcc: cannot find file Main.dyn_hi". Oh, that's graceful failure then. >_< I did also briefly see if the LLVM backend would work under Windows. I wasn't entirely sure if LLVM comes with the installer binary or you need to get it seperately. (You might say "obviously it's seperate", but the installer already contains the entire GCC system, for example.) Attempting to use this resulted in a summary error message ("cannot run: opt") which doesn't make it especially clear that the problem is that LLVM isn't installed. Perhaps there's no better way of detecting this condition, I don't know, but something more illuminating would be nice. I briefly had a look at how to install LLVM, but it appears that you have to build it from source. This is not my idea of a fun time, so given the minimal speed difference, I just won't bother. I haven't had time to play with any of the other new good stuff in GHC 7. (The multicore performance, the new GC stuff, etc.)

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ANNOUNCE: Haddock 2.9.0
by David Waern 06 Dec '10

06 Dec '10

-------------------------------------------- -- Haddock 2.9.0 -------------------------------------------- A new version of Haddock, the Haskell documentation tool, is out! This version is compatible with .haddock files produced by the version of Haddock that comes with GHC 7.0.1. -------------------------------------------- -- Changes in version 2.9.0 -------------------------------------------- Changes in version 2.9.0 * Drop support for ghc < 7 * New flag --qual for qualification of names * Print doc coverage information to stdout when generating docs * Include an 'All' option in the A-Z subdivided index * Make TOC group header identifiers validate * Minor changes to the API -------------------------------------------- -- Links -------------------------------------------- Homepage: http://www.haskell.org/haddock Hackage page: http://hackage.haskell.org/package/haddock-2.9.0 Bugtracker and wiki: http://trac.haskell.org/haddock Mailing list: haddock(a)projects.haskell.org Code repository: http://code.haskell.org/haddock -------------------------------------------- -- Contributors -------------------------------------------- The following people contributed patches to this release: Tobias Brandt Mark Lentczner Ian Lynagh Simon Marlow Simon Michael Ryan Newton David Waern -------------------------------------------- -- Get Involved -------------------------------------------- We would be very happy to get more contributors. To get involved, start by grabbing the code: http://code.haskell.org/haddock Then take a look at the bug and feature tracker for things to work on: http://trac.haskell.org/haddock

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include modules in DLL? "undefined reference"
by Gerold Meisinger 06 Dec '10

06 Dec '10

Hello! I'm trying to compile a Windows DLL and need to include an additional module (specifically FRP.Yampa). I was successful with a Hello World type of DLL but when I use Yampa I get: $ ghc -shared -o hs.dll DLLTest.o DLLTest_stub.o StartEnd.o Creating library file: hs.dll.a DLLTest.o:fake:(.text+0x46): undefined reference to `Yampazm0zi9zi2zi3_FRPziYamp a_integral_closure' DLLTest.o:fake:(.text+0x4d): undefined reference to `Yampazm0zi9zi2zi3_FRPziYamp aziVectorSpace_zdfVectorSpaceDoubleDouble_closure' DLLTest.o:fake:(.text+0x194): undefined reference to `Yampazm0zi9zi2zi3_FRPziYam pa_embed_closure' DLLTest.o:fake:(.text+0x381): undefined reference to `__stginit_Yampazm0zi9zi2zi 3_FRPziYampa_' DLLTest.o:fake:(.data+0x0): undefined reference to `Yampazm0zi9zi2zi3_FRPziYampa _integral_closure' DLLTest.o:fake:(.data+0x4): undefined reference to `Yampazm0zi9zi2zi3_FRPziYampa ziVectorSpace_zdfVectorSpaceDoubleDouble_closure' DLLTest.o:fake:(.data+0x38): undefined reference to `Yampazm0zi9zi2zi3_FRPziYamp a_embed_closure' collect2: ld returned 1 exit status How can I define the references of Yampa in the DLL? I also tried to reinstall Yampa shared with: $ cabal install --reinstall --enable-shared yampa Could not find module `Data.IORef': Perhaps you haven't installed the "dyn" libraries for package `base-3.0.3. 2'? Use -v to see a list of the files searched for. $ cabal install --reinstall --enable-shared base # :) Resolving dependencies... cabal: internal error: impossible ---- (StartEnd is taken from: http://www.haskell.org/ghc//docs/latest/html/users_guide/win32-dlls.html ) DLLTest.hs: {-# LANGUAGE ForeignFunctionInterface, Arrows #-} module DLLTest embeddedSF :: Double -> Double -> Double -> IO Double embeddedSF v0 v1 v2 = return . last $ embed integral (v0, [(1.0, Just v1), (1.0, Just v2)]) foreign export ccall embeddedSF :: Double -> Double -> Double -> IO Double

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Storables and Ptrs
by Tyler Pirtle 06 Dec '10

06 Dec '10

Hi cafe, I'm just getting into Foreign.Storable and friends and I'm confused about the class storable. For GHC, there are instances of storable for all kinds of basic types (bool, int, etc) - but I can't find the actual declaration of those instances. I'm confused that it seems that all Storable instances operate on a Ptr, yet none of these types allow access to an underlying Ptr. I noticed that it's possible via Foreign.Marshal.Utils to call 'new' and get a datatype wrapped by a Ptr, but this isn't memory managed - I'd have to explicitly free it? Is that my only choice? Is there a way that given just simply an Int I could obtain a Ptr from it, and then invoke the storable functions on it? Or for that matter, if I go and create some new data type, is there some generic underlying thing (ghc-only or otherwise) that would let me have a Ptr of it? Thanks, Tyler

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