- Haskell-Cafe - Haskell.org

Reversing a fixed-length vector
by Cristiano Paris 09 Sep '14

09 Sep '14

Hi, I'm playing around with Type Families so I decided to implement a simple fixed-length Vect of Integers. Here is my straightforward implementation (ghc 7.8.3): https://gist.github.com/anonymous/d838e68ce6a02412859f As you can see, I'm trying to implement a reverse function for my vectors which guarantees that the output vector is the same size as the input one. I tried to express this constraint at the type level. The problem is that I can't have ghc to type check the reverse function in the non-trivial case: _________________ Could not deduce (Plus n1 (S m) ~ S (Plus n1 m)) from the context (n ~ S n1) bound by a pattern with constructor CV :: forall n. Int -> Vect n -> Vect (S n), in an equation for ‘vecReverse’ at vect3.hs:30:13-18 Expected type: Vect (Plus n m) Actual type: Vect (Plus n1 (S m)) _________________ Iit has to do with the fact that the type checker can't deduce that: Plus n1 (S m) ~ S (Plus n1 m) ~ Plus (S n1) m ~Plus n m I tried to insert the following instance to the family: Plus n (S m) = S (Plus n m) but to no avail. Any clue? Thanks. C. -- GPG Key: 4096R/C17E53C6 2010-02-22 Fingerprint = 4575 4FB5 DC8E 7641 D3D8 8EBE DF59 B4E9 C17E 53C6

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Warnings for unhandled exceptions
by Henk-Jan van Tuyl 09 Sep '14

09 Sep '14

L.S., I was reading about the Aardvark language [0] and the robustness they intend to implement in it by, amongst others, catching each possible exception. I wonder, is it possible to implement compiler warnings for unhandled exceptions in Haskell? One of the strong points of Haskell is the type checking and clearly identifying functions with side effects, but exceptions are not reflected by the type of a function. It would be useful if the compiler gave a warning for each type of exception, for each expression, that is not handled by the program. Regards, Henk-Jan van Tuyl [0] http://aardvarklanguage.sourceforge.net/ -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Specifying build flags in cabal depencies
by Pedro Vasconcelos 09 Sep '14

09 Sep '14

Hello, I've run across an annoyance with a Haskell project of mine that requires Snap built with SSL: I couldn't find a way to specify the flag "-fopensll" to pass when building the snap-server package. This means that building on a new machine requires either manually installing the right version of snap-server with -fopenssl or re-installing with after the Cabal build. Is there a better way to do this? Thanks, Pedro

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Re: [Haskell-cafe] Parallel interruptible computations
by Julian K. Arni 09 Sep '14

09 Sep '14

Are you looking for something like amb/unamb? [0] In particular, if you look at the comments in the source for 'race', it should be pretty intuitive how to go from there to "run x computations, kill the remaining ones after y return". You just need to fiddle with the MVar, possibly adding another thread and MVar that represents whether the calculation can already be completed. [0] https://hackage.haskell.org/package/unamb-0.2.5/docs/src/Data-Unamb.html#amb

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End of support for test-framework-smallcheck and test-framework-golden
by Roman Cheplyaka 08 Sep '14

08 Sep '14

I'm going to stop maintaining the test-framework-smallcheck and test-framework-golden packages. I recommend switching to tasty's tasty-smallcheck and tasty-golden, respectively. If you are interested in taking over these test-framework packages, let me know. Otherwise, I will continue supporting them for the next 3 months and abandon/deprecate them afterwards. Roman

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Can't create a static Linux executable when using hmatrix
by Kai Zhang 08 Sep '14

08 Sep '14

Hi Cafe, I am trying to build a static exec to be run on another machine, and I failed probably due to hmatrix. How can I fix this? Thanks! Test code: import Numeric.LinearAlgebra.Data main = print (fromLists [[2,3,3], [2,3,3]] :: Matrix Double) When complied with "ghc -O2 -static -optc-static -optl-static test.hs -optl-pthread", gave me lots of errors: /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgees.o): In function `dgees_': (.text+0xbc4): undefined reference to `dcopy_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgees.o): In function `dgees_': (.text+0xed9): undefined reference to `dswap_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgees.o): In function `dgees_': (.text+0x1304): undefined reference to `dswap_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgees.o): In function `dgees_': (.text+0x1377): undefined reference to `dswap_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0xf9a): undefined reference to `dnrm2_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0xfd4): undefined reference to `dnrm2_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x1022): undefined reference to `dscal_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x103b): undefined reference to `dscal_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x109c): undefined reference to `idamax_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x1125): undefined reference to `drot_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x12aa): undefined reference to `dnrm2_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x12e4): undefined reference to `dnrm2_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x1332): undefined reference to `dscal_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x134b): undefined reference to `dscal_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x13ac): undefined reference to `idamax_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x143d): undefined reference to `drot_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x148e): undefined reference to `dnrm2_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x14bc): undefined reference to `dscal_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x14fe): undefined reference to `dnrm2_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgeev.o): In function `dgeev_': (.text+0x152c): undefined reference to `dscal_' /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../lib/liblapack.a(dgehrd.o): In function `dgehrd_': (.text+0x65d): undefined reference to `dtrmm_' . . .

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ANNOUNCE: buildable-0.1.0.0
by Ivan Lazar Miljenovic 08 Sep '14

08 Sep '14

Have you ever wanted to deal with the builders for various data types in a polymorphic/overloaded fashion? I'm needing to do so and couldn't find any existing code that did so, so I decided to rectify this: http://hackage.haskell.org/package/buildable As a (very contrived) example: λ> build ((365 :: Dec Int) <| fromValue (Char7 ' ') |> (365 :: BigEndian Int16) |> (" omega=𝟂 " :: Utf8 String) |> Utf16 (LE ("hello" :: LT.Text))) :: SB.ByteString "365 \SOHm omega=\240\157\159\130 h\NULe\NULl\NULl\NULo\NUL" -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Re: [Haskell-cafe] Parallel interruptible computations
by Corentin Dupont 08 Sep '14

08 Sep '14

Hi guys, thanks for the nice answers! I'll give you a little bit more context: I'm designing an event engine. I have instances for Applicative, Alternative, Monad, MonadPlus. It's like that: -- | Composable events data Event a where SumEvent :: Event a -> Event a -> Event a -- The first event to fire will be returned. AppEvent :: Event (a -> b) -> Event a -> Event b -- Both events should fire, and then the result is returned. PureEvent :: a -> Event a -- Create a fake event. The result is useable with no delay. EmptyEvent :: Event a -- An event that is never fired. BindEvent :: Event a -> (a -> Event b) -> Event b -- The first event should fire, then a second event is created using the result. BaseEvent :: BaseEvent a -> Event a -- Embed a base event. ShortcutEvents :: [Event a] -> ([Maybe a] -> Maybe b) -> Event b -- The function is called each time an event fires, as soon as the result can be computed from the available data, it is returned, dismissing the events that haven't fired yet. instance Functor Event where fmap f a = pure f <*> a instance Applicative Event where pure = PureEvent (<*>) = AppEvent instance Alternative Event where (<|>) = SumEvent empty = EmptyEvent instance Monad Event where (>>=) = BindEvent return = PureEvent instance MonadPlus Event where mplus = SumEvent mzero = EmptyEvent The Applicative instance is good if you have two events and you want both of them to fire ("and"). The Alternative instance is good if you have two events and you need only one to fire ("or"). But what if you have several events, but you need only a part of them to fire in order to construct a final result? Say you have 10 events, but the 5 first to fire will give you enough data to construct a result. You cannot do that with Applicative/Alternative because with Applicative, you need *all* events results, with Alternative you need *only one*. That's why I added this primitive "ShortcutEvents" in my DSL, but I'm not convinced by it. So my questions are: 1. is ShortcutEvents expressible in term of Applicative/Alternative/Monad/MonadPlus? 2. if not is their a well known typeclass that covers this case? 3. if not is their a better way to write it? I especially don't like the list of Event, I'd prefer a more generic writing. What if I want a structure containing the events, instead of a list? What if I want event of various types (say a pair (Event a, Event b) for example)? Note that I'm not working with streams of events (like in traditional FRP frameworks): just with single events (the "BaseEvents") that I want to combine with each other. Those "BaseEvents" will fire only once. The final result of the combination of events will trigger a callback. Cheers, Corentin

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FFI question
by Vasili I. Galchin 08 Sep '14

08 Sep '14

Hello, I need memory refreshing about FFI because I need to fix a bug (plus I got down a rat hole because of Ukraine war :-(). Following a snippet of something I wrote: -- | lock all of the memory space of a process lockAllMemory :: LockAllFlags -> IO () lockAllMemory flags = do throwErrnoIfMinus1 "lockAllMemory" (c_mlockall cflags) return () where cflags = case flags of CURRENT -> (#const MCL_CURRENT) FUTURE -> (#const MCL_FUTURE) CURRENTandFUTURE -> (#const MCL_CURRENTandFUTURE) it is a file called LockedMem.hsc and the third arm CURRENTandFUTURE I just added to fix the this bug .. MCL_CURRENTandFUTURE ends up in LockedMem_make.c(sp??) but I get a undeclared nasty message when I try to build LockedMem.hsc... my bad... . Please help so I can go back to help Ukraine. Kind regarsd, Vasya

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[ANN] haddock-2.15-0, haddock-api-2.15-0, haddock-library-1.1.1
by Mateusz Kowalczyk 07 Sep '14

07 Sep '14

Hello, I'd like to announce the release of Haddock 2.15.0. Before we dive in, I'd like to announce that Haddock development has moved out of the GHC tree and is now done against the latest compiler release. If in the past you wanted to contribute but were intimidated by having to make sure it works against GHC HEAD then here's your chance. Additionally, it now should also work with GHC 7.8.2 so if for whatever reason you're forced to use that, you can now get the latest Haddock features. If you want to contribute then please let us know, we're understaffed! Come chat at #haddock or send us an e-mail. If you have been using 2.14.x, you should be good to simply ‘cabal install haddock’. Now the changes: This is an API-breaking bugfix release so any tickets originally scheduled for 2.15.0 were pushed towards 2.16. Perhaps the biggest change is that ‘cabal install haddock’ will now only give you the Haddock binary. To use Haddock API, please depend on ‘haddock-api’ from now on. I have sent out an e-mail about this couple of weeks in advance to any reverse dependencies of ‘haddock’. This is to workaround cabal's inability to pass flags to dependencies from the cabal file. ‘haddock-library’ only got a minor bump, 1.1.0 → 1.1.1 improving its output. The Monoid instance for DocH is gone however as it was ill-formed to begin with. Changes in version 2.15.0 * Always read in prologue files as UTF8 (#286 and Cabal #1721) * parser: don't wrap headers in DocParagraph (#307) * parser: don't mangle append order for nested lists (pandoc #1346) * parser: preserve list ordering in certain scenarios (#313) * parser: update the attoparsec version used internally giving slight parsing performance boost. * Move development to be against latest GHC release and not GHC HEAD. * Further split up the package to separate the executable from the library, necessary by things like GHCJS. We now have ‘haddock-library’ which are the parts that don't use GHC API, ‘haddock-api’ which are (some of) the parts that do use GHC API and ‘haddock’ which merely provides the executable. * Export few extra functions in the API. * Add compatibility with GHC 7.8.2. * Omit unnecessary ‘forall’s (#315 and #86) * Remove some files which were really old or did not belong in the repository in the first place. Once again, we'd love to have more people on board be it for the HTML backend, refactoring, LaTeX/Hoogle backend, parser improvements, test writing, new feature implementation or bug fixing. Our issue tracker[1] is up to date if you are interested. Thanks! [1]: https://github.com/haskell/haddock/issues -- Mateusz K.

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