January 2015 - Haskell-Cafe

Multi-pattern LambdaCase?
by Tom Ellis 27 Jan '15

27 Jan '15

Is there a particular reason that LambdaCase does not support muliple patterns? Specifically I am suggesting that \case { pA1 pA2 pA3 -> eA, ... } be sugar for \freshName1 freshName2 freshName3 -> case freshName1 freshName2 freshName3 of { pA1 pA2 pA3 -> eA, ... } Is there some technical reason that prevents it? Tom

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Could cabal-install shows dependencies as dot or some format?
by Magicloud Magiclouds 26 Jan '15

26 Jan '15

Hi, I know ghc-pkg could do that. But that is for packages already installed. I'd like cabal-install to do that when it is trying to resolve dependencies. The reason is that, now I got a "Could not resolve dependencies" error. But I could not simply figure out why based on the output of cabal-install. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Skolems!
by Tom Murphy 26 Jan '15

26 Jan '15

Like any good mystery, let's start with the body: repro.hs:22:5: Couldn't match type `y0' with `y' because type variable `y' would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: Foo x y -> Bool The following variables have types that mention y0 b :: Foo x0 y0 -> Bool (bound at repro.hs:18:9) Expected type: Foo x y -> Bool Actual type: Foo x0 y0 -> Bool In the first argument of `h', namely `b' In a stmt of a 'do' block: h b In the expression: do { xs <- return ["test"] :: IO [String]; let a = ... b = ...; h b } Failed, modules loaded: none. So it's the "(rigid, skolem)" error you sometimes happen across. The code that's causing it, though, is pretty unsuspicious: {-# LANGUAGE RankNTypes #-} -- remove this pragma and it typechecks (1): {-# LANGUAGE GADTs #-} data Foo x y = Foo x y main :: IO () main = do xs <- return ["test"] :: IO [String] let -- typechecks (2): -- a = Just "test" :: Maybe String -- typechecks (3): -- a = f ["test"] a = f xs :: Maybe String b = g (a :: Maybe String) :: forall x y. Foo x y -> Bool -- typechecks if it's inlined (4): -- h (g (a :: Maybe String) :: forall x y. Foo x y -> Bool) h b f :: [a] -> Maybe a f _ = Nothing -- doesnt affect typechecking: -- g :: Maybe String -> (Foo x y -> Bool) g :: Maybe String -> (forall x y. Foo x y -> Bool) g (Just _) _ = False g Nothing _ = True -- typechecks (5): -- h :: (Foo x y -> Bool) -> IO () h :: (forall x y. Foo x y -> Bool) -> IO () h = undefined This has been reproduced in ghc 7.6.3, 7.8.3, 7.8.4, and 7.10.1 RC1 Any idea what's going on? Thanks! Tom

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NumberSieves status/maintenance
by David Feuer 25 Jan '15

25 Jan '15

This package was last updated in 2012, and the linked darcs repository is inaccessible. Do you intend to continue to maintain this package? If not, I would be happy to take over. Thanks, David Feuer

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ANN: deepseq-bounded 0.5 -> 0.6
by Andrew Seniuk 25 Jan '15

25 Jan '15

This may affect depending code. Please consult the distro changelog for specific information about how your code might be affected, or better yet, refer to http://www.fremissant.net/deepseq-bounded/transition-5-6-7.html and http://www.fremissant.net/deepseq-bounded/grammar.html This is a transitional version of deepseq-bounded, with stability of the grammar expected in early March for version 0.7. This also causes major version bumps to the seqaid and leaky packages. To update everything, issue cabal update cabal install --reinstall --force-reinstalls seqaid which reinstalls the latest deepseq-bounded in passing. And also if you were to run "seqaid demo" it will use the updated version of leaky, so everything's up to date. The homepage for these projects starts at http://www.fremissant.net/deepseq-bounded-seqaid-leaky.html Kind Regards, Andrew

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Monad layering and DSL
by Gautier DI FOLCO 25 Jan '15

25 Jan '15

Hello all, I'm trying to make an IMAP DSL and I'm face to an annoying issue (all the examples after are selected pieces of this: https://github.com/blackheaven/smoothmail) Here we are, IMAP provide a way to statefully interact with a server. For example: select "A" -- Select folder 'A' searchAll -- Get the list of UID (message identifiers) [1, 2, 3] But select "B" searchAll -- Get [4, 5] I decided to use the Free Monad because it seems well suite for what I'm doing and I end-up with something like that: data ImapF next = Select DirectoryName (Maybe DirectoryDescription -> next) | Noop (DirectoryDescription -> next) | Search MailSearch (Maybe [UID] -> next) -- Functor instance type Imap = Free ImapF searchAll :: Imap (Maybe [UID]) searchAll = liftF $ Search undefined id select :: DirectoryName -> Imap (Maybe DirectoryDescription) select directory = liftF $ Select directory id My main problem is the following: if I do a select of an unknown directory, I should stop the computation. My first thought was Monad Transformers. So here are the types: newtype FreeT f m a = FreeT { runFreeT :: m (FreeF f a (FreeT f m a)) } Such that f is a functor and m a monad. If I do a type Imap = FreeT ImapF Maybe I'll end up with some Maybe (ImapF a) which not expresses that the evaluation can fails but that the expression construction can fails. Then I thought harder and ends up with that: type Imap = FreeT Maybe (Free ImapF) Ok, that seems better, but all the available operations can't fail. I also have had a look at extensible effects and I think I'm stuck with the same issue of my first attempt. My goal is to provide flexibility for those who want to continue the evaluation, but to provide an API with less boilerplate in the default case. If you have any ideas/hints/comments, I'll be happy to read them. If you have any questions, I'll do my best to answer them and to make disappear all doubt and ambiguity. Thanks in advance for your help. Regards.

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Haskell wiki
by Ronald Legere 24 Jan '15

24 Jan '15

Greetings, Does anyone know how to get an account to help contribute to the haskell wiki? Registration has been closed for the last 3 years and there is link to email anyone that I could find, hence this "Broadcast" message. There are dead links and such that I would love to help clean up. Cheers! -- Ron Legere (rjljr2(a)gmail.com) legere et discere

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up-to-date and complete version of http://packdeps.haskellers.com/reverse ?
by Ömer Sinan Ağacan 24 Jan '15

24 Jan '15

Hi all, I was wondering if we have up-to-date and complete version of this http://packdeps.haskellers.com/reverse. Any ideas?

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Happstack: serveDirectory vs serveFile
by Semen Trygubenko / Семен Тригубен ко 23 Jan '15

23 Jan '15

Dear all, I've ran a simple benchmark where two serve functions were side-by-side in an msum and looked like dirs "hehe" $ serveDirectory DisableBrowsing [] "/hehe" dirs "favicon.ico" $ serveFile (asContentType "image/x-icon") "favicon.ico" "hehe" directory also contained file "favicon.ico". This is happstack server version 7.3.9. I've been hammering it with ab and confirmed this puzzling result: it seems that serveFile route is approximately two orders of magnitude slower than serveDirectory route... Will be grateful for any clues as to what I'm doing wrong! Thanks, S. -- Семен Тригубенко http://trygub.com

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Unacceptable argument type in foreign declaration: Ptr ()
by Henk-Jan van Tuyl 23 Jan '15

23 Jan '15

L.S., I am trying to compile a package that hasn't been updated since 2009 and I get a lot of messages like: Unacceptable argument type in foreign declaration: Ptr () When checking declaration: foreign import stdcall safe "dynamic" prim_System_Win32_Com_Automation_TypeLib_setGuid :: Ptr () -> Ptr () -> Ptr GUID -> IO Int32 How can I correct this? Regards, Henk-Jan van Tuyl -- 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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