June 2010 - Haskell-Cafe

Proposal: Sum type branches as extended types (as Type!Constructor)
by Gabriel Riba 04 Jun '10

04 Jun '10

Extending sum types with data constructors would spare runtime errors or exception control, when applying functions to inappropriate branches, as in the example ... data List a = Nil | Cons a (List a) -- List!Nil and List!Cons -- as extended types * Actual system, with runtime errors (as in GHC Data.List head) or exception throwing hd :: List a -> a hd (Cons x _) -> x hd Nil -> error "error: hd: empty list" -- error or exception throwing * Proposed system extending types with constructors as Type!Constructor: User must do pattern matching before applying the constructor-specific type function. In ''var @ (Constructor _ _)'' the compiler should append the constructor to the type as a pair (Type, Constructor) as an extended type for ''var'' No need for runtime errors or exception control hd :: List!Cons a -> a hd (Cons x _) = x using it: headOf :: List a -> Maybe a headOf list = case list of li @ (Cons _ _) -> Just hd li -- extTypeOf li == ( 'List', 'Cons') -- should pass typechecker for List!Cons li @ Nil -> Just hd li -- compiler error !! -- extTypeOf ('List','Nil') don't match _ -> Just hd list -- compiler error !! -- extTypeOf ('List',Nothing) don't match Maybe we could take out importance on the number of _ wildcards (constructor arity) with a syntax like. li @ (Cons ...) li @ (Nil ...) Cheers! Gabriel Riba Faura.

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Cabal -j
by Louis Wasserman 04 Jun '10

04 Jun '10

What, if anything, stands in the way of parallelizing Cabal installs, make -j style? Louis Wasserman wasserman.louis(a)gmail.com http://profiles.google.com/wasserman.louis

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Re: [Haskell-cafe] Issue with "connectTo" function in "network" package
by aditya siram 04 Jun '10

04 Jun '10

Great! If there is a wiki or FAQ page for networking with Haskell it might be worth updating. -deech On 6/4/10, Roman Cheplyaka <roma(a)ro-che.info> wrote: > /etc/hosts is the file responsible for resolving 'localhost' hostname. > As you can see from it, there are two entries for localhost: one > contains IPv4 address (127.0.0.1) and another contains IPv6 address > (::1). > > The behaviour you observe is kinda bug in connectTo function. Instead of > trying all of the addresses, this function just tries the first one. > Until it is fixed (I'm gonna write and send a patch soon), can you live > with this temporary hack? If for some reason the answer is "no", you > have to use low-level functions from Network.Socket module for now. > > > * aditya siram <aditya.siram(a)gmail.com> [2010-06-04 14:15:47-0500] >> Doing (1) solves my problem. What did I do and why did it work? Thanks >> a lot for your help! >> -deech >> >> On 6/4/10, Roman Cheplyaka <roma(a)ro-che.info> wrote: >> > Confirm if any of the following solves the problem: >> > >> > 1. commenting out (by prepending putting hash sign '#' in the beginning >> > of the line) the following line in /etc/hosts: >> > >> > ::1 localhost ip6-localhost ip6-loopback >> > >> > 2. executing as root the following command: >> > >> > rmmod ipv6 >> > >> > * aditya siram <aditya.siram(a)gmail.com> [2010-06-04 13:51:29-0500] >> >> Here's my GHCI session (with my network card off): >> >> Prelude Network Network.Socket> connectTo "localhost" (PortNumber 3000) >> >> *** Exception: connect: does not exist (Connection refused) >> >> Prelude Network Network.Socket> connectTo "127.0.0.1" (PortNumber 3000) >> >> {handle: <socket: 7>} >> >> >> >> The OS is Ubuntu (Lucid) Linux. The contents of "/etc/hosts" and >> >> "/etc/nsswitch.conf" are: >> >> >> >> > cat /etc/nsswitch.conf >> >> # /etc/nsswitch.conf >> >> # >> >> # Example configuration of GNU Name Service Switch functionality. >> >> # If you have the `glibc-doc-reference' and `info' packages installed, >> >> try: >> >> # `info libc "Name Service Switch"' for information about this file. >> >> >> >> passwd: compat >> >> group: compat >> >> shadow: compat >> >> >> >> hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4 >> >> networks: files >> >> >> >> protocols: db files >> >> services: db files >> >> ethers: db files >> >> rpc: db files >> >> >> >> netgroup: nis >> >> >> >> > cat /etc/hosts >> >> 127.0.0.1 localhost >> >> 127.0.1.1 deech-mac-ubuntu >> >> >> >> # The following lines are desirable for IPv6 capable hosts >> >> ::1 localhost ip6-localhost ip6-loopback >> >> fe00::0 ip6-localnet >> >> ff00::0 ip6-mcastprefix >> >> ff02::1 ip6-allnodes >> >> ff02::2 ip6-allrouters >> >> ff02::3 ip6-allhosts >> >> >> >> >> >> -deech >> >> >> >> On 6/4/10, Roman Cheplyaka <roma(a)ro-che.info> wrote: >> >> > * aditya siram <aditya.siram(a)gmail.com> [2010-06-03 22:30:33-0500] >> >> >> Hi all, >> >> >> I had an issue where using the connectTo [1] function would fail to >> >> >> connect to "localhost" if my wireless card was turned off. The >> >> >> moment >> >> >> I turned on my wireless connection it worked. But if I use connectTo >> >> >> with "127.0.0.1" [2] with my network off it works. >> >> >> >> >> >> I confirmed that "localhost' resolves to "127.0.0.1" by pinging >> >> >> "localhost" from command line with the Internet off. >> >> >> >> >> >> Is this a bug in "connectTo"? >> >> >> >> >> >> -deech >> >> >> >> >> >> [1] connectTo "localhost" (PortNumber some_number) >> >> >> [2] connectTo "127.0.0.1" (PortNumber some_number) >> >> > >> >> > How exactly did it fail? Did it throw an exception? If so, what was >> >> > the >> >> > exception? >> >> > >> >> > Which operating system did you observe that under? >> >> > >> >> > If that was Linux, can you show the contents of the following files >> >> > on >> >> > that system: >> >> > >> >> > /etc/nsswitch.conf >> >> > /etc/hosts >> >> > >> >> > -- >> >> > Roman I. Cheplyaka :: http://ro-che.info/ >> >> > "Don't let school get in the way of your education." - Mark Twain >> >> > _______________________________________________ >> >> > Haskell-Cafe mailing list >> >> > Haskell-Cafe(a)haskell.org >> >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > >> > >> > -- >> > Roman I. Cheplyaka :: http://ro-che.info/ >> > "Don't let school get in the way of your education." - Mark Twain >> > > > -- > Roman I. Cheplyaka :: http://ro-che.info/ > "Don't let school get in the way of your education." - Mark Twain >

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Issue with "connectTo" function in "network" package
by aditya siram 04 Jun '10

04 Jun '10

Here's my GHCI session (with my network card off): Prelude Network Network.Socket> connectTo "localhost" (PortNumber 3000) *** Exception: connect: does not exist (Connection refused) Prelude Network Network.Socket> connectTo "127.0.0.1" (PortNumber 3000) {handle: <socket: 7>} The OS is Ubuntu (Lucid) Linux. The contents of "/etc/hosts" and "/etc/nsswitch.conf" are: > cat /etc/nsswitch.conf # /etc/nsswitch.conf # # Example configuration of GNU Name Service Switch functionality. # If you have the `glibc-doc-reference' and `info' packages installed, try: # `info libc "Name Service Switch"' for information about this file. passwd: compat group: compat shadow: compat hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4 networks: files protocols: db files services: db files ethers: db files rpc: db files netgroup: nis > cat /etc/hosts 127.0.0.1 localhost 127.0.1.1 deech-mac-ubuntu # The following lines are desirable for IPv6 capable hosts ::1 localhost ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters ff02::3 ip6-allhosts -deech On 6/4/10, Roman Cheplyaka <roma(a)ro-che.info> wrote: > * aditya siram <aditya.siram(a)gmail.com> [2010-06-03 22:30:33-0500] >> Hi all, >> I had an issue where using the connectTo [1] function would fail to >> connect to "localhost" if my wireless card was turned off. The moment >> I turned on my wireless connection it worked. But if I use connectTo >> with "127.0.0.1" [2] with my network off it works. >> >> I confirmed that "localhost' resolves to "127.0.0.1" by pinging >> "localhost" from command line with the Internet off. >> >> Is this a bug in "connectTo"? >> >> -deech >> >> [1] connectTo "localhost" (PortNumber some_number) >> [2] connectTo "127.0.0.1" (PortNumber some_number) > > How exactly did it fail? Did it throw an exception? If so, what was the > exception? > > Which operating system did you observe that under? > > If that was Linux, can you show the contents of the following files on > that system: > > /etc/nsswitch.conf > /etc/hosts > > -- > Roman I. Cheplyaka :: http://ro-che.info/ > "Don't let school get in the way of your education." - Mark Twain > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Tricks with GMap -- question about conflicts w/ indexed type families
by Ryan Newton 04 Jun '10

04 Jun '10

GMaps -- families of map implementations indexed by the key type -- are an example on the wiki: http://www.haskell.org/haskellwiki/GHC/Type_families I've been using something like this myself. It sure would be nice to have a fully developed version on Hackage, and I may try to submit this myself if no one else has already done it. But I'm running into a problem in achieving a particular optimization that I want -- namely, small tuple keys packed into Ints. For example, with a key type (Int16,Int16) it's possible to pack the key into a plain Int and use Data.IntMap. In fact, in the rough/partial implementation here (http://people.csail.mit.edu/newton/gmap/GMap.hs) I have a class for things that fit in a word: class FitInWord v where toWord :: v -> Word fromWord :: Word -> v (Or fitInInt for that matter.) Which includes things like small tuples: instance FitInWord (Int16,Int16) where toWord (a,b) = shiftL (fromIntegral a) 16 + (fromIntegral b) fromWord n = (fromIntegral$ shiftR n 16, fromIntegral$ n .&. 0xFFFF) (If you know a better idea than using templates/program generation to generate every combination of small scalar tuples that fit in a word.... let me know.) What I would next *like* to do is something like the following: import qualified Data.IntMap as DI instance FitInWord t => GMapKey t where data GMap t v = GMapInt (DI.IntMap v) deriving Show empty = GMapInt DI.empty lookup k (GMapInt m) = DI.lookup (wordToInt$ toWord k) m insert k v (GMapInt m) = GMapInt (DI.insert (wordToInt$ toWord k) v m) alter fn k (GMapInt m) = GMapInt (DI.alter fn (wordToInt$ toWord k) m) toList (GMapInt m) = map (\ (i,v) -> (fromWord$ intToWord i, v)) $ DI.toList m The problem is that there's already a more general instance of GMapKey that handles pairs by representing them as nested GMaps: instance (GMapKey a, GMapKey b) => GMapKey (a, b) where data GMap (a, b) v = GMapPair (GMap a (GMap b v)) .... Ideally, I want both of these to coexist (and to prioritize the more specific one). With normal type classes, OverlappingInstances can handle this, but with type families I get an error like the following: Conflicting family instance declarations: data instance GMap t v -- Defined at Intel/GMap.hs:108:7-10 data instance GMap (a, b) v -- Defined at Intel/GMap.hs:225:7-10 Any fixes? Thanks, -Ryan

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alphanum in infix constructors (and possibly functions as well) - why not?
by Ozgur Akgun 04 Jun '10

04 Jun '10

Hi all, The constructor names in Haskell need to obey one of the two following rules: - either starts with a capital letter and contains alphanumeric characters afterwards, (including _ I guess) - or starts with a colon (:) and only contains symbols afterwards The first one is used Prefix by default, and the second ose is used infix by default. What stops us from allowing alphanum characters appear in the Infix version (after the colon)? Can't it be relaxed to only start woth a colon? So I want to be able to say something like: data Expr = Expr :< Expr -- checks for LT betwen two Expr's | Expr :<2 Expr -- a different implementation of the same thing maybe | Expr :<veryfast Expr -- and the veryfast implementation of it (just a silly example I could think of now.) Best, Ozgur Akgun

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Strange GHC messages?
by Liam O'Connor 04 Jun '10

04 Jun '10

Hi all, I've got some strange GHC output that doesn't seem to be affecting my program adversely but I thought I should post about it cos it's disquieting mkUsageInfo: internal name? r{tv aIR} mkUsageInfo: internal name? s{tv aIS} It seems to be complaining about this newtype declaration: > newtype ThreadStateM r s v = ThreadState { unbox :: StateT s (ReaderT r IO) v } > deriving (Monad, Functor, MonadIO, MonadState s, MonadReader r) (uses newtype deriving) I'm not sure what the problem here is? Cheers. ~Liam

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Re: [Haskell-cafe] Weird socket problem on Mac OS X
by Antoine Latter 04 Jun '10

04 Jun '10

On Fri, Jun 4, 2010 at 12:11 AM, Arnaud Bailly <arnaud.oqube(a)gmail.com> wrote: > Yes, I have also seen the other post on the same topic, but it seemed > to refer to the PortNumber in Network.Socket with a PortNum > constructor I think, not the one in Network (but I am probably > misleading myself as one is a reexportation of the other...). > But then I got. > > Slim\SlimClientIO.hs:26:51: > No instance for (Num PortID) > arising from a use of `fromInteger' > at Slim\SlimClientIO.hs:26:51-61 > Possible fix: add an instance declaration for (Num PortID) > In the first argument of `($)', namely `fromInteger' > In the expression: fromInteger $ slimport $ slimConfig st > In the definition of `port': > port = fromInteger $ slimport $ slimConfig st > Oops! I didn't read carefully enough. That's what I get for trying to program without haddocks or GHCi in front of me. I am glad you have something that works. Antoine

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Fwd: [Haskell-cafe] Weird socket problem on Mac OS X
by Arnaud Bailly 04 Jun '10

04 Jun '10

---------- Forwarded message ---------- From: Arnaud Bailly <arnaud.oqube(a)gmail.com> Date: Fri, Jun 4, 2010 at 11:59 AM Subject: Re: [Haskell-cafe] Weird socket problem on Mac OS X To: Antoine Latter <aslatter(a)gmail.com> Hello, I managed to solve the problem using Network.Socket instead of Network. Is this a bug or a feature ? Arnaud On Fri, Jun 4, 2010 at 7:11 AM, Arnaud Bailly <arnaud.oqube(a)gmail.com> wrote: > Yes, I have also seen the other post on the same topic, but it seemed > to refer to the PortNumber in Network.Socket with a PortNum > constructor I think, not the one in Network (but I am probably > misleading myself as one is a reexportation of the other...). > But then I got. > > Slim\SlimClientIO.hs:26:51: > No instance for (Num PortID) > arising from a use of `fromInteger' > at Slim\SlimClientIO.hs:26:51-61 > Possible fix: add an instance declaration for (Num PortID) > In the first argument of `($)', namely `fromInteger' > In the expression: fromInteger $ slimport $ slimConfig st > In the definition of `port': > port = fromInteger $ slimport $ slimConfig st > > :-( > Thanks > > Arnaud > > On Fri, Jun 4, 2010 at 1:46 AM, Antoine Latter <aslatter(a)gmail.com> wrote: >> Hi Arnaud, >> >> One thing you might want to try is to stop using the PortNumber data >> constructor, and instead rely on 'fromInteger' to do the right thing. >> >> The data constructor assumes that it's argument is in network byte order, >> which won't always be the case. >> >> It's not obvious that the constructor should be avoided - this is the second >> time today I've seen this! >> >> Take care, >> Antoine >> >> On Jun 3, 2010 5:13 PM, "Arnaud Bailly" <arnaud.oqube(a)gmail.com> wrote: >> >> Hello, >> I have the following code which works ok on Linux and Windows XP, but >> fails on Mac OS X with error message: >> >> Connect: does not exist (connection refused) >> >> The server: >> >> doStartstate = do pr <- liftIO $ runProcess ... >> liftIO $ threadDelay 500000 >> st <- get >> put st { slimHandle = Just pr } >> >> >> The client: >> >> fetchAnswers st msgs = liftIO $! do let port = PortNumber $ >> fromInteger $ port $ config st >> cnx <- connectTo "localhost" port >> >> Help most welcome, thanks >> >> Arnaud >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

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Haskell opportunity at investment bank
by Pearse, Chris 04 Jun '10

04 Jun '10

Hello, I'm looking for a developer with expertise in Haskell for an investment bank in London. The role will involve working on a multi-asset, multi-region, pricing, risk and trade lifecycle solution for the wholesale banking division. Solutions are developed using Java, C#/WPF, C++ and make heavy use of Haskell. The requirement is for an experienced developer with in-depth knowledge of functional programming in Haskell - ideally in a commercial environment. If interested, please feel free to send your CV through or call me to discuss. Thanks, Chris Pearse 0207 469 5050 http://www.linkedin.com/pub/3/9a1/142 ------------------------------------------------------------------------------- Huxley Associates - Professional Recruitment Solutions Established in 1995, Huxley Associates is an international recruitment consultancy. We provide bespoke recruitment services to the Accountancy & Finance, Banking Technology, Energy, Engineering, Global Markets, Human Resources, IT, Oil & Gas, Sales & Marketing, and Supply Chain sectors. With over 400 specialist recruitment professionals, we are consistently able to meet the high recruitment needs of a diverse range of companies across the UK, Europe, Asia and the United States. This email message may contain confidential and/or privileged information, and is intended solely for the addressee(s). If you have received this email in error, please notify Huxley Associates immediately. Unauthorised copying, disclosure or distribution of the material in this email is forbidden. If you do not want to receive emails from us about our services in future, or if you want to query or qualify how your personal data is held by us (for details on this see our privacy statement http://www.huxley.co.uk/privacy_policy.html ) please e-mail audit.data(a)huxley.com.

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