- Haskell-Cafe - Haskell.org

(automatic) type classes context inference
by João Paulo 28 Aug '10

28 Aug '10

Hello everyone, I am developing a toolset in which I have several (multiparameter) type classes; It is often the case that I can only define a data-type X as an instance of one such class (say A), if X is an instance of another class (say B); The thing is that, while it is hard for me, because of all the type parameters that I have to deal with, to add 'X is an instance of B' to the context cxt_A in 'instance cxt_A => A X' ghc is always able to correctly infer all type parameters; In fact, I always get: 'Could not deduce (B X t1 ... tn) from the context cxt_A arising from ... Probable fix: add (B X t1 ... tn) to the context cxt_A ...' In my case, this is the fix that I always need: most of the times, I am just copy-pasting (B X t1 ... tn) to cxt_A! Is there a way, say a compilation option, to avoid this? can anyone please help me here? :) thank you very much -- João Paulo Fernandes Universidade do Minho www.di.uminho.pt/~jpaulo

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type classes and logic
by Patrick Browne 28 Aug '10

28 Aug '10

Hi, I am trying to understand type classes and sub-classes in purely logical terms From the literature I gather that: 1)a type class is a predicate over types (using type variables) 2)type sub-classing is a logical implication between these predicates. But I feel that 1) and 2) does not give enough detail and I am missing the logical significance of the function definitions in the class and their implementations in the instances. This following example outlines my current (mis?)understanding: data Person = Person Name deriving Show data Employee = Employee Name Position deriving Show class BEING being where The class BEING is logically a predicate with type variable being(i.e. BEING(being)) BEING(being) is true for types that are members of the BEING type class. Any types that instantiate the type class BEING will have all the BEING class functions defined on them. Such a type is a member of the type-class BEING Actual function signatures and/or default implementations omitted A proof for the predicate BEING(being) must show that there is an inhabited type (a type which has values) If we find a value for a type that is a proof that a type exists (it is inhabited) that is a member of the class instance BEING Person where All the functions from the class BEING must be defined for the type Person It is required the functions in the instance respect the types from the class and type from the instantiated parameter. t is required that when the functions are run they produce values of the required type. Implementations omitted With at least one instantiation existing (e.g. BEING(Person) = true) we can define a sub-class In the sub-class definition we can assume BEING(human) to be true (type variable name does not matter) And based on that assumption we can write a logical implication BEING(human) => HUMAN(human) class BEING human => HUMAN human where At this point there is no additional functionality is defined for the subclass HUMAN Sub-classing is logical implication BEING(human) => HUMAN(human) All types t that make BEING(t) = true also make HUMAN(t)=true In general for the HUMAN sub-class to be useful additional constraints are added after the where keyword. These are similar in purpose to those described in an ordinary class (not a sub-class) Another example from Bird[1.Intro. to Functional Prog. using Haskell] class Enum a where toEnum :: a -> Int fromEnum :: Int -> a A type is declared an instance of the class Enum by giving definitions toEnum and fromEnum, functions that convert between elements of the type a and the type Int. In instance declarations fromEnum should be a left inverse to toEnum That is for all x fromEnum(toEnum x) = x. This requirement cannot be expressed in Haskell My questions are: a) Is the above *logical view* of type classes correct? b) What is the logical role of the functions in the classes and instances. Are they constraints on the types in predicates? c) Apart from signature matching between class-to-instance is there any logical relation between the functions in a class and the functions in an instances. From Birds[1] example I suspect that while the types are subject to logical rules of 1) and 2) the actual functions in the instance do not have to obey any logical laws. Any instantiation that respects types is fine? Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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On to applicative
by michael rice 28 Aug '10

28 Aug '10

fmap seems oddly named because no "mapping" takes place, except in the fourth example, where the map is "passed in." Just sayin'. Michael 1) Prelude Control.Monad Control.Applicative> fmap (++ "abc") getLine xyz "xyzabc" 2) Prelude Control.Monad Control.Applicative Data.Char Data.String> fmap (splitAt 3) getLine qwertyuio ("qwe","rtyuio") 3) Prelude Control.Monad Control.Applicative Data.Char> fmap toUpper getLine <interactive>:1:13: Couldn't match expected type `Char' against inferred type `[Char]' Expected type: IO Char Inferred type: IO String In the second argument of `fmap', namely `getLine' In the expression: fmap toUpper getLine 4) Prelude Control.Monad Control.Applicative Data.Char Data.String> fmap (map toUpper) getLine qwertyuio "QWERTYUIO"

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FRP - style
by Maciej Piechotka 28 Aug '10

28 Aug '10

I wanted to learn something about FRP. I tried to use existing packages on hackage and write something in OpenGL but I failed - mainly due to problem of handling single occurrence of event[1]. I tried to write my own library (https://patch-tag.com/r/uzytkownik/reactive-event/home) and I'd like to ask if I'm doing it from right direction or do I miss some important aspects of FRP. I assume that the events are discrete and the value can change (along with action) only on event. For specific class of events there exists 'always' event associated with time [In practice it would mean that it is executed as fast as possible]. Regards [1] In retrospection - I should have rely more on immutable data to pass around information that I did handled event - but it seems for me an overkill.

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ANN: mute-unmute-0.2.0.0
by Krzysztof Skrzętnicki 28 Aug '10

28 Aug '10

* Hello! I'm pleased to announce a small utility program I wrote recently. Today I pushed version 0.2.0.0 of mute-unmute to Hackage, from with you can cabal-install it right now (following the usual 'update' step). What it does? It's simple: it will mute the sound when you lock your screen and unmute it back again when you unlock it. Why anyone would need such program? Well, this is a little bit more complicated. I work in "open space" kind of office. It can get quite noisy there. I often sit with headphones on, but when I walk away from my desk I put them down and lock my screen. Here comes uncomfortable moment: when I remove my headphones suddenly people around me who doesn't use their headphones now start to hear the music I listen to. And they don't always like it, so I manually mute the sound to make happy. But now, instead of doing this task by hand I have a program do it for me :-) Unfortunately this kind of programs tend to be pretty much platform specific. Current version supports the recent versions of Gnome and KDE, but needs ALSA. It's quite common setup, but if you happen to have a different one you can easily hack the code (it's very straightforward) and send me patches. Patches are always welcome. The source repo is on Github and released versions are on Hackage: Github: http://github.com/Tener/mute-unmute <http://github.com/Tener/mute-unmute#> Hackage: http://hackage.haskell.org/package/mute-unmute Example use: ~/ mute-unmute # first session, it will ask for configuration options mute-unmute: Creating config directory... mute-unmute: Mute config is missing... Mute sound card and press RETURN mute-unmute: Unmute config is missing... Unmute sound card and press RETURN mute-unmute: Session: UnixAbstract {addrPath = "/tmp/dbus-1tB1AEUcgg", addrGuid = Just "19ec85e3778816ad38a77bbf0000003f"} mute-unmute: Connected mute-unmute: Handler added: [MatchType Signal,MatchInterface "org.gnome.ScreenSaver",MatchMember "ActiveChanged",MatchPath "/org/gnome/ScreenSaver"] mute-unmute: Handler added: [MatchType Signal,MatchInterface "org.freedesktop.ScreenSaver",MatchMember "ActiveChanged",MatchPath "/ScreenSaver"] ^C ~/ mute-unmute # remembers previous options which can be changed by calling with --store switch mute-unmute: Session: UnixAbstract {addrPath = "/tmp/dbus-1tB1AEUcgg", addrGuid = Just "19ec85e3778816ad38a77bbf0000003f"} mute-unmute: Connected mute-unmute: Handler added: [MatchType Signal,MatchInterface "org.gnome.ScreenSaver",MatchMember "ActiveChanged",MatchPath "/org/gnome/ScreenSaver"] mute-unmute: Handler added: [MatchType Signal,MatchInterface "org.freedesktop.ScreenSaver",MatchMember "ActiveChanged",MatchPath "/ScreenSaver"] mute-unmute: Message {mType = Signal, mFlags = [NoReplyExpected], mSerial = 31, mPath = Just "/org/gnome/ScreenSaver", mInterface = Just "org.gnome.ScreenSaver", mMember = Just "ActiveChanged", mErrorName = Nothing, mReplySerial = Nothing, mDestination = Nothing, mSender = Just ":1.19", mBody = [Variant (True) {- b -}]} mute-unmute: Lock screen mute-unmute: Message {mType = Signal, mFlags = [NoReplyExpected], mSerial = 32, mPath = Just "/org/gnome/ScreenSaver", mInterface = Just "org.gnome.ScreenSaver", mMember = Just "ActiveChanged", mErrorName = Nothing, mReplySerial = Nothing, mDestination = Nothing, mSender = Just ":1.19", mBody = [Variant (False) {- b -}]} mute-unmute: Unlock screen ^C * You can get usage information with --help switch: *~/ mute-unmute --help *mute-unmute --store ask for mute and unmute configurations --store-mute ask for mute configuration --store-unmute ask for unmute configuration -d --daemon wait for screensaver state changes -h, -? --help show help There isn't much more I can say. I hope you find the program at least educational (as a simple example of DBus usage) if not useful. Best regards, Krzysztof Skrzętnicki

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warn-incomplete-patterns and GADTs
by Tom Nielsen 28 Aug '10

28 Aug '10

Hi, is warn-incomplete-patterns (in GHC 6.10.3) less clever than it could be? {-# OPTIONS_GHC -fglasgow-exts #-} {-# OPTIONS_GHC -fwarn-incomplete-patterns #-} module Vec where data Z data S a data Vec n a where VNil :: Vec Z a VCons :: a -> Vec m a -> Vec (S m) a instance Eq a => Eq (Vec n a) where VNil == VNil = True VCons x vx == VCons y vy = x==y && vx == vy give the warning: Warning: Pattern match(es) are non-exhaustive In the definition of `==': Patterns not matched: VNil (VCons _ _) (VCons _ _) VNil but of course VNil and VCons can never have the same type. Tom

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phantom types for inheritance between types
by Andrew U. Frank 28 Aug '10

28 Aug '10

there are packages which use phantom types to express inheritance (subtype) relations. unfortunately i cannot find a simple example and seem to misunderstand something. can somebody help with the following simple example? i have a most general type Dated, a subtype Note and a subsubtype Task. i have only instances for Note and Task, but operations for tasks and dated. my primary question is: why should i have the contexts in the classes (the code compiles without them)? they do not say more than what is already expressed in the instance declarations. i do also not think that i have used phantom types properly (eg. gtk2hs uses phantom types differently). what would be the proper solution? would it work for cases with multiple inheritance? thank you! andrew class CDated a class CDated a => CNotes a class CNotes a => CTasks a instance CDated Task instance CDated Note instance CNotes Note instance CNotes Task instance CTasks Task class Dated n where datedop :: n -> n class Notes1 n where noteop :: n -> n class Tasks1 n where taskop :: n -> n data Note = Note Int data Task = Task Int instance CTasks x => Tasks1 x where taskop n = n instance CDated x => Dated x where datedop n = n t:: Task = Task 1 n :: Note = Note 1 tt = taskop t -- nt = taskop n -- compiler error -- as expected! tx = datedop t nx = datedop n

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Re: [Haskell-cafe] Haskell and SciTE (was: Hackage on Linux)
by C. McCann 27 Aug '10

27 Aug '10

On Fri, Aug 27, 2010 at 12:57 PM, Andrew Coppin <andrewcoppin(a)btinternet.com> wrote: > C. McCann wrote: >> >> On Fri, Aug 27, 2010 at 7:40 AM, Andrew Coppin >> <andrewcoppin(a)btinternet.com> wrote: >> >>> >>> Unfortunately, I haven't found anything for Windows yet which has syntax >>> hilighting for Haskell. >>> >>> I use SciTE, which has hilighting for a bazillion languages (including >>> XML >>> and JSON), but not Haskell sadly. >>> >> >> Veering somewhat offtopic, but last time I checked, SciTE does have >> lexer support for Haskell, it just doesn't actually include (for >> unknown reasons) a language properties file to go with it. > > OK. Well maybe it's just the version I've got then? Or maybe, as you say, > because it's not enabled I don't know it's there. Going by the Scintilla release history, it's been in there for about five years, but you wouldn't find it just by using SciTE, since all the menu options and such are controlled by the .properties files, none of which mention Haskell... Anyway, if the version you have is less than five years old, you should be able to drop the file I linked to alongside the other language files, make a few global properties changes to add Haskell to the menus, probably a couple other minor things, and be good to go. I'd offer more detail, but I set mine up when I first started learning Haskell (about a year ago) and have since forgotten what needed to be done. > Ah yes. The reason I seldom update SciTE is that it then takes hours to put > all the configuration back to the way I like it. (Especially if option names > have changed or defaults are different now.) > > SciTE is a nice editor, but not especially well documented. (And, what, they > haven't made a configuration editor yet? :-P ) Yes, well. SciTE isn't so much an editor as a bit of trivial code to demonstrate the functionality of the Scintilla editor widget, which people (such as myself) insist on using as if it were a proper editor. If you want something friendlier, I'm afraid you're on your own. Unfortunately, there don't seem to be any Haskell bindings for Scintilla... - C.

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Re: philosophy of Haskell
by Patai Gergely 27 Aug '10

27 Aug '10

> I don't agree. A concurrent change is the effect of an IO action, not > of the thread. For example if a concurrent thread writes to an MVar, > then that change becomes the effect of the next takeMVar, which gets > executed. If a concurrent thread changes a file on disk, then that > changing becomes the effect of the next readFile, which reads the > changed file. But that's exactly what the model cannot handle. Look at the following snippet again: > let (x, world1) = getLine world0 > world2 = print (x+1) world1 This clearly says that the world returned by getLine and the world consumed by print is the same one, since the state monad model is pure, therefore world1 is immutable. However, this is not true, since someone else could have modified it in the meantime. The state monad can only describe a single thread, but that's a non-existent situation in the case of I/O, since the world keeps changing outside the program even if the program itself is single-threaded. Gergely -- http://www.fastmail.fm - IMAP accessible web-mail

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FW: [TYPES/announce] CUFP 2010 Call for Participation
by Simon Peyton-Jones 27 Aug '10

27 Aug '10

Dear Haskellers CUFP should be a lot of fun this year. Do consider going. Simon Commercial Users of Functional Programming Workshop (CUFP) 2010 Call for Participation Sponsored by SIGPLAN Co-located with ICFP 2010 _________________________________________________________ 1 - 2 October 2010 Baltimore (MD), USA http://cufp.org/conference/schedule/2010 Reservation will be available through ICFP's website: http://www.icfpconference.org/icfp2010 _________________________________________________________ Functional programming languages have been a hot topic of academic research for over 35 years, and have seen an ever larger practical impact in settings ranging from tech startups to financial firms to biomedical research labs. At the same time, a vigorous community of practically-minding functional programmers has come into existence. CUFP is designed to serve this community. The annual CUFP workshop is a place where people can see how others are using functional programming to solve real world problems; where practitioners meet and collaborate; where language designers and users can share ideas about the future of their favorite language; and where one can learn practical techniques and approaches for putting functional programming to work. CUFP 2010 will feature three hour Functional Programming tutorials given by language experts on the first day and Experience and Technical Talks on day two. Attendees may attend either or both days. Talks Program, October 2nd 2010 Luke Hoban (Microsoft) Keynote: F#: Embracing Functional Programming in "Visual Studio 2010" Sally A Browning (Galois Inc) Cryptol, a DSL for Cryptographic Algorithms Marius Eriksen (Twitter) Scaling Scala at Twitter Michael Fogus (Science Applications International Corporation) Naïveté vs. Experience - or, How We Thought We Could Use Scala and Clojure, and How We Actually Did It Neal Glew & Leaf Petersen (Intel) Functional Language Compiler Experiences at Intel Warren Harris (Metaweb) Functional Programming at Freebase Warren A. Hunt, Jr. (U. of Texas) Eating One’s Own Dog Food Rusty Klophaus (Basho Technologies) Riak Core: Building Distributed Applications Without Shared State Howard Mansell (Credit Suisse) Eden: An F#/WPF frameworok for building GUI tools Erik Meijer (Microsoft) Reactive Extensions (Rx): Curing Your Asynchronous Programming Blues Tutorial Program, October 1st 2010 Morning: Clojure (Aaron Bedra) Building robust servers with Erlang (Martin Logan) High Performance Haskell (Johan Tibell) Afternoon: F# 2.0 - A day at the beach (Rick Minerich) Implementing web sites with Scala and Lift (David Pollak) Camlp4 and Template Haskell (Nicolas Pouillard, Jake Donham) There will be no published proceedings, as the meeting is intended to be more a discussion forum than a technical interchange. For more information, for more information, including presentation abstracts and the most recent schedule information, visit http://cufp.org See you there!

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Warning: Module `Prelude' is deprecated:
by Michael Litchard 27 Aug '10

27 Aug '10

So lately when I use cabal to install something get Text/CSV.hs:1:0: Warning: Module `Prelude' is deprecated: You are using the old package `base' version 3.x. Future GHC versions will not support base version 3.x. You should update your code to use the new base version 4.x. how do I implement cabal's advice?

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On to applicative
by michael rice 26 Aug '10

26 Aug '10

From: http://en.wikibooks.org/wiki/Haskell/Applicative_Functors ============================= import Control.Applicative f :: (a -> b -> c) fmap :: Functor f => (d -> e) -> f d -> f e fmap f :: Functor f => f a -> f (b -> c) -- Identify d with a, and e with (b -> c) sumsqr :: Int -> Int -> Int -- my f sumsqr i j = i*i+j*j ============================= I'm trying to understand how the above works but... [michael@localhost ~]$ ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :l bozo.hs [1 of 1] Compiling Main ( bozo.hs, interpreted ) bozo.hs:5:0: Invalid type signature Failed, modules loaded: none. Prelude> Michael

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Chez What
by Jeff Rubard 26 Aug '10

26 Aug '10

Haskell CURRY? Curried potatoes? The lambda calculus? Historical actuality? SI!

2 2

Ordering TypeRep from Data.Typeable.
by Serguey Zefirov 26 Aug '10

26 Aug '10

I think, that TypeRep type from Data.Typeable needs Ord class instance. It is unnecessary, but is handy when needed. My use case follows. I try to create graph whose node and arc labels are differently typed. So I can add Int node, Float node and link them by Conversion arc. Right now I am quite good with GADTs that hide values of different types. But when I thinking about querying that structure, I start to want ordered TypeRep. I then can collect different arcs and nodes into different sets and speed up query processing dramatically.

1 0

non-blocking recv from UDP socket
by Vo Minh Thu 26 Aug '10

26 Aug '10

Hi, Is is possible to get Network.Socket.ByteString.recv to be non-blocking (i.e. return directly even if no data is available) ? I have tried ti use setSocketOption sock NoDelay 1 but then I get the following error: setSocketOption: unsupported operation (Protocol not available) Here is the code if it may help (I'm on linux btw). sock <- socket AF_INET Datagram 17 -- 17 is IPPROTO_UDP addr <- inet_addr "127.0.0.1" bindSocket sock (SockAddrInet 44004 addr) setSocketOption sock NoDelay 1 -- <-- doesn't seem to work addr <- inet_addr "127.0.0.1" forever $ do sent <- sendTo sock "hello" (SockAddrInet 44005 addr) dat <- recv sock 5 -- <-- I'd like it to be non-blocking B.putStrLn dat threadDelay 1000000 -- 1 second Thanks, Thu

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foreign function interface "Invalid type signature"
by Ken Takusagawa 26 Aug '10

26 Aug '10

What am I doing wrong? module Ffi2 where { import Foreign.C.Types; foo :: CInt -> CInt; foo x = x; foreign export ccall foo :: CInt -> CInt; } $ ghc -c Ffi2.hs Ffi2.hs:8:0: Invalid type signature --ken

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Re: Re : Re: Re : Error importing
by Luc TAESCH 26 Aug '10

26 Aug '10

btw, this could be helpful, if you have some time: http://en.wikipedia.org/wiki/Cabal_(software) http://www.haskell.org/cabal/ in there you maay find : http://www.haskell.org/cabal/FAQ.html in which you can see ( hey !): Hidden packages (a)<http://www.haskell.org/cabal/FAQ.html#hidden-packages-a> What is this hidden package? You're writing your own package and you get: Could not find module `Data.Map': it is a member of package containers-0.1.0.0, which is hidden. Hidden packages (b)<http://www.haskell.org/cabal/FAQ.html#hidden-packages-b> You're building some other package and you get: Could not find module `Data.Map': it is a member of package containers-0.1.0.0, which is hidden. this is insteresting but maybe too advaced for now, may side track you. however may give you the big picture http://www.haskell.org/haskellwiki/How_to_write_a_Haskell_program besides, Howard, leksah is a small audience list, you may find more support on the cafe list, as your question as less leksah specific, but more on the "haskell bootstrapping procees" Ill copy you here, in case some good sould know some fast track ( btw , do you know about rwh : http://book.realworldhaskell.org/read/ especially http://book.realworldhaskell.org/read/installing-ghc-and-haskell-libraries.… ) 2010/8/25 HowardBrazee <howard(a)brazee.net> > Boy, am I dense. > > I see two "depends" lines in Reading.cabal: > > pkgconfig-depends: > & > build-depends: base -any > > I uncommented out > import qualified Data.Map as M > > and looked at Lekah's message: > src\Main.hs:19:17: > Could not find module `Data.Map': > It is a member of the hidden package `containers-0.3.0.0'. > Perhaps you need to add `containers' to the build-depends in > your .cabal file. > Use -v to see a list of the files searched for. > > So I put "containers" after one of the depends: > build-depends: containers base -any > > Leksah said: > Setup: .\Reading.cabal has been changed, please re-configure. > ========================================= > Setup: Reading.cabal:9: Parse of field 'build-depends' failed. > ========================================= > Can't activate package C:\Documents and Settings\brazee\My Documents > \Code\Windows\Reading\Reading.cabal: 9: user error (Parse of field > 'build-depends' failed.) > Can't read package file > > So I undid that change and did this instead: > pkgconfig-depends: containers > > I got the following: > Can't read package file > Setup: .\Reading.cabal has been changed, please re-configure. > ========================================= > Configuring Reading-0.0.1... > Setup: pkg-config version >=0.9.0 is required but it could not be > found. > ========================================= > The job now runs with these unused imports. > > I never found "build-depends" in my cabal file. > > I edited this with ultra-edit as I know how to get to it. > > I'm still clueless about what I did, despite being told by two > different people what to do and not being able to follow the > directions, but doing trial and error instead. Even after I > finished, I don't understand the directions. >

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How to generate dependend values with QuickCheck
by Jürgen Nicklisch-Franken 26 Aug '10

26 Aug '10

I want to generate values, so that i have some arbitrary object, which has a certain set of signals, and each set of signals has a certain set of sensor value pairs, were the values are arbitrary. However to demonstrate my problem I have an easy example, were I want to generate an instance of data QCExample = QCExample Int Int deriving Show and the second value should always be the double of the first. So I tried this: instance Arbitrary QCExample where arbitrary = let i1 = arbitrary i2 = fmap (* 2) i1 in liftM2 QCExample i1 i2 but showing some of the generated test cases in ghci does not give me what I expected: let gen :: Gen (QCExample) = arbitrary Test.QuickCheck.Gen.sample gen > QCExample (-2) 0 > QCExample (-4) (-6) > QCExample 3 30 > ... I know that I can filter, but this would be to inefficient. Jürgen

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Low level problem with Data.Text.IO
by Pieter Laeremans 26 Aug '10

26 Aug '10

Hello, I have a strange (low level) problem with the Data.Text library. Running the simple program below on a certain text file causes a low level error. runghc ./ReadFiles.hs testfile ghc(16402,0xb0103000) malloc: *** error for object 0x2501710: pointer being freed was not allocated *** set a breakpoint in malloc_error_break to debug ghc(16402,0xb0103000) malloc: *** error for object 0x2501710: pointer being freed was not allocated *** set a breakpoint in malloc_error_break to debug ReadFiles.hs: testfile: hGetContents: invalid argument (Illegal byte sequence) module Main where import qualified Data.Text.IO as TI import qualified Data.Text as T import System main = do args <- getArgs let fileName:_ = args txt <- TI.readFile fileName putStrLn $ show txt Unfortunately I can 't post the specific data file. But according to the file program it is a text file with : Non-ISO extended-ASCII text, with very long lines, with CRLF, LF line terminators encoding. How can I debug this problem ? What would you guys do ? Trying gdb ? thanks in advance, Pieter -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

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ANN: Bugfix release for timezone-olson
by Yitzchak Gale 26 Aug '10

26 Aug '10

Version 0.1.1 of the timezone-olson package is now available on Hackage. http://hackage.haskell.org/package/timezone-olson The timezone-olson package provides a parser and renderer for binary Olson timezone files whose format is specified by the tzfile(5) man page on Unix-like systems. These files are often found in /usr/share/zoneinfo on Unix-like systems. Functions are provided for converting the parsed data into TimeZoneSeries objects from the timezone-series package. Now that "Version 2 format" Olson files are beginning to appear in the wild, we can see the full details of how the specification in the tzfile(5) man page is applied in real life. This release fixes several bugs related to parsing and rendering Version 2 format Olson files, as well as a few other small issues. Updating is recommended. A new version of the timezone-series package, also version 0.1.1, is also now available on Hackage. http://hackage.haskell.org/package/timezone-olson The timezone-series package endows Data.Time, from the time package, with several data types and functions for enhanced processing of timezones. Among them is TimeZoneSeries,which represents all of the clock settings that occur in a time zone, and functions to convert between UTCTime and LocalTime without knowing in advance what clock setting is in effect at the given moment. This release of timezone-series only adds a few type signatures and more documentation for isValidLocalTime and isRedundantLocalTime and friends. Aristid Breitkreuz provided a proof for the basic fact about time change gaps and overlaps that is used to implement them. The proof can be seen here: http://projects.haskell.org/time-ng/gaps_and_overlaps.html Many thanks to Aristid who, in addition to providing the above proof, assisted in finding bugs and preparing for the release of these two updates. Regards, Yitz

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Type families - how to resolve ambiguities?
by DavidA 26 Aug '10

26 Aug '10

Hi, The code below defines a type synonym family: {-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-} {-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-} data Vect k b = V [(b,k)] deriving (Eq,Show) data TensorBasis a b = T a b deriving (Eq, Ord, Show) type family Tensor k u v :: * type instance Tensor k (Vect k a) (Vect k b) = Vect k (TensorBasis a b) class Algebra k v where -- "v is a k-algebra" unit :: k -> v mult :: Tensor k v v -> v instance Algebra Integer (Vect Integer [Int]) where unit 0 = V [] unit x = V [([],x)] mult (V ts) = V [(g++h,x) | (T g h, x) <- ts] Naively I think of the type instance declaration as saying that the two types are synonyms, so I should be able to use one in a context where the other is expected. However, when I try to use the code, I don't seem to be able to get the type inferencer to recognise an object as being of both "types" at the same time. For example: *> mult $ (V [(T [1] [2],3)] :: Vect Integer (TensorBasis [Int] [Int])) <interactive>:1:8: Couldn't match expected type `Tensor k v v' against inferred type `Vect Integer (TensorBasis [Int] [Int])' NB: `Tensor' is a type function, and may not be injective In the second argument of `($)', namely `(V [(T [1] [2], 3)] :: Vect Integer (TensorBasis [Int] [Int]))' In the expression: mult $ (V [(T [1] [2], 3)] :: Vect Integer (TensorBasis [Int] [Int])) In the definition of `it': it = mult $ (V [(T [1] [2], 3)] :: Vect Integer (TensorBasis [Int] [Int])) *> mult $ (V [(T [1] [2],3)] :: Tensor Integer (Vect Integer [Int]) (Vect Integer [Int])) <interactive>:1:8: Couldn't match expected type `Tensor k v v' against inferred type `Vect Integer (TensorBasis [Int] [Int])' NB: `Tensor' is a type function, and may not be injective In the second argument of `($)', namely `(V [(T [1] [2], 3)] :: Tensor Integer (Vect Integer [Int]) (Vect Integer [Int]))' In the expression: mult $ (V [(T [1] [2], 3)] :: Tensor Integer (Vect Integer [Int]) (Vect Integer [Int])) In the definition of `it': it = mult $ (V [(T [1] [2], 3)] :: Tensor Integer (Vect Integer [Int]) (Vect Integer [Int])) Are type families the right mechanism to express what I'm trying to express? If so, what am I doing wrong, and how do I fix it?

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Re: [Haskell-cafe] Hackage on Linux
by Mathew de Detrich 25 Aug '10

25 Aug '10

I used to use archlinux packages however it became a pain for the following reasons - packages on archlinux don't auto update when cabal does. This becomes really annoying when package X gets updated on cabal but not on arch and causes conflicts with other packages - in some situations doing a general update with arch (through clyde or packer) breaks ghc (last time it happened packer tried to uninstall/update arch packages which failed because those packages had dependencies. The files got removed but since unregister failed ghc thought they still existed) Apart from base/required packages, unless your linux distro has proper metapackages its in my opinion just better off using cabal install (and only use arch packages for binaries) On 23/08/2010 2:21 AM, "Brandon S Allbery KF8NH" <allbery(a)ece.cmu.edu> wrote: -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 08/22/2010 07:19 AM, Jonas Almström Duregård wrote: >> Now, you say it's preferable to use the na... No, the Windows and OSX installers are just that. They provide no facilities for finding packages, identifying the package a given file came from, or dependency tracking. OSX's installer doesn't even have uninstall support; it records the installed files, but provides no mechanism for undoing configuration changes such as removing package-installed kernel modules. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla -... iEYEARECAAYFAkxxTmoACgkQIn7hlCsL25VNsQCfZKJIz+KONa4yWAI97QYyttGU ITMAnjoAhcj3kMrWxnsSxWb5jraFBu1r =ERlI -----END PGP SIGNATURE----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org ...

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Function signatures and type class constraints
by Oscar Finnsson 25 Aug '10

25 Aug '10

Hi, I'm wondering why I have to repeat the class constraints at every function. If I got the data type > data (Eq a, Show a, Data a, Eq b, Show b, Data b, Eq c, Show c, Data c) => Foo a b c = Foo a b c and then a function from Foo to String I have to supply the signature > bar :: (Eq a, Show a, Data a, Eq b, Show b, Data b, Eq c, Show c, Data c) => Foo a b c -> String even though it should be clear that a, b and c *must* fulfill the constraints already so I should be able to just supply the signature > bar :: Foo a b c -> String Another related problem I got is that even though I can create the type > type B = (Eq a, Show a, Data a, Eq b, Show b, Data b, Eq c, Show c, Data c) => Foo a b c I cannot use it like > bar :: B -> String so my type class constraints got a tendency become huge! It is possible to work around this somehow? I'm in a situation at the moment where I got a data type with four fields each with three constraints (Show, Eq, Data), so I have to repeat 12 constraints at every function signature... :( Finally is there some way to bundle type class constraints, something like > data {Eq, Show, Data} {a, b, c} => Foo a b c = Foo a b c -- make believe syntax so I don't have to repeat every constraint and variable all the time? -- Oscar

3 4

Code that writes code
by Andrew Coppin 25 Aug '10

25 Aug '10

I'm working on a small Haskell package. One module in particular contains so much boilerplate that rather than write the code myself, I wrote a small Haskell program that autogenerates it for me. What's the best way to package this for Cabal? Just stick the generated file in there? Or is there some (easy) way to tell Cabal how to recreate this file itself?

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Re: ANNOUNCE: darcs 2.5 beta 4
by Benjamin Franksen 25 Aug '10

25 Aug '10

Benjamin Franksen wrote: > I have trouble building this latest beta. I found that the build uses ghc-6.8.3 which, I think, is no longer supported for building darcs, IIRC. Sorry for the noise. Cheers Ben

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Re: [Haskell-cafe] Fwd: Semantics of iteratees, enumerators, enumeratees?
by John Lato 25 Aug '10

25 Aug '10

From: John Millikin <jmillikin(a)gmail.com> > > Here's my (uneducated, half-baked) two cents: > > There's really no need for an "Iteratee" type at all, aside from the > utility of defining Functor/Monad/etc instances for it. The core type > is "step", which one can define (ignoring errors) as: > > data Step a b = Continue (a -> Step a b) > | Yield b [a] > > Input chunking is simply an implementation detail, but it's important > that the "yield" case be allowed to contain (>= 0) inputs. This allows > steps to consume multiple values before deciding what to generate. > > In this representation, enumerators are functions from a Continue to a > Step. > > type Enumerator a b = (a -> Step a b) -> Step a b > > I'll leave off discussion of enumeratees, since they're just a > specialised type of enumerator. > > ------------- > > Things become a bit more complicated when error handling is added. > Specifically, steps must have some response to EOF: > > data Step a b = Continue (a -> Step a b) (Result a b) > | Result a b > > data Result a b = Yield b [a] > | Error String > > In this representation, "Continue" has two branches. One for receiving > more data, and another to be returned if there is no more input. This > avoids the "divergent iteratee" problem, since it's not possible for > Continue to be returned in response to EOF. > Is this really true? Consider iteratees that don't have a sensible default value (e.g. head) and an empty stream. You could argue that they should really return a Maybe, but then they wouldn't be divergent in other formulations either. Although I do find it interesting that EOF is no longer part of the stream at all. That may open up some possibilities. Also, I found this confusing because you're using Result as a data constructor for the Step type, but also as a separate type constructor. I expect this could lead to very confusing error messages ("What do you mean 'Result b a' doesn't have type 'Result'?") > > Enumerators are similarly modified, except they are allowed to return > "Continue" when their inner data source runs out. Therefore, both the > "continue" and "eof" parameters are Step. > > type Enumerator a b = (a -> Step a b) -> Step a b -> Step a b > I find this unclear as well, because you've unpacked the continue parameter but not the eof. I would prefer to see this as: type Enumerator a b = (a -> Step a b) -> Result a b -> Step a b However, is it useful to do so? That is, would there ever be a case where you would want to use branches from separate iteratees? If not, then why bother unpacking instead of just using type Enumerator a b = Step a b -> Step a John

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[CUFP 2010] Birds of a Feather sessions
by Anil Madhavapeddy 25 Aug '10

25 Aug '10

===================================================================== Birds of a Feather sessions (BOFs) Commercial Uses of Functional Programming Workshop (CUFP 2010) http://cufp.org/bofs-2010 Baltimore, Maryland, September 30 - October 1 ===================================================================== This year, the Commercial Uses of Function Programming (CUFP) workshop has a new feature to bring industry and academics together to promote real-world uses of functional programming. Birds of a Feature (BoF) sessions provide a place for our community to gather informally at ICFP and reach consensus on matters of importance. Any CUFP attendee can propose a BoF session, and grab one of the spare rooms in the evening slots below. Attendance in the evening is open to all. BoF sessions facilitate ad-hoc discussions and provide a place to gather and start off the chat, before moving on during the evening to a local Baltimore restaurant or pub. We particularly encourage "cross-language" discussions that might not otherwise happen at one of the bigger workshops. Please see http://cufp.org/bofs-2010 for details on how to propose a slot or confirm attendance at one of them. There are a couple of sessions already which may be of interest to Haskell developers: Thu 30th Sep 6pm-8pm, Haskell for the Real World, Bryan O'Sullivan and Johan Tibell http://cufp.org/bofs-2010#haskellrw Fri 1st Oct 6pm-8pm, Cross Language Serialisation, Anton Lavrik http://cufp.org/bofs-2010#xlangserial =====================================================================

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Fwd: Semantics of iteratees, enumerators, enumeratees?
by Conal Elliott 25 Aug '10

25 Aug '10

For anyone interested in iteratees (etc) and not yet on the iteratees mailing list. I'm asking about what iteratees *mean* (denote), independent of the various implementations. My original note (also at the end below): With the encouragement & help of Conrad Parker, I've been looking at > iteratees, enumerators, enumeratees. I can find plenty written about them, > but only about benefits and implementation. In sifting through chunks, > error/control messages, and continuations, I find myself longing for a > precise semantic basis. I keep wondering: what simpler & precise semantic > notions do these mechanisms implement? Has anyone worked out a denotational > semantics for iteratees, enumerators, enumeratees -- something that > simplifies away the performance advantages & complexities? I've worked out > something tentative, but perhaps I'm covering old ground. > - Conal ---------- Forwarded message ---------- From: Conal Elliott <conal(a)conal.net> Date: Sun, Aug 22, 2010 at 11:02 PM Subject: Re: Semantics of iteratees, enumerators, enumeratees? To: John Lato <jwlato(a)gmail.com> Hi John, I just remembered: Luke Palmer wrote an accessible & helpful description of this approach to library design. See his post *Semantic Design<http://lukepalmer.wordpress.com/2008/07/18/semantic-design/> *. (I've switched to using the more specific term "denotational design", since I specifically intend denotational semantics, not operational.) Denotational design is at the heart of most of what I do, particularly including the three pieces of work you mentioned: functional images (Pan), FRP, and automatic differentiation. My main goal is simplicity with precision. Without the precision, I can't tell whether the simplicity is real or illusory. We functional programmers have a strong advantage over imperative programming (including OO) in that we can achieve semantic precision. I want to see us exploit this advantage! And not settle for being a power tool for generating semantically inscrutable imperative computations. Regards, - Conal On Sun, Aug 22, 2010 at 10:43 PM, John Lato <jwlato(a)gmail.com> wrote: > Hi Conal, > > I've always regarded your work in essentially the same category as Edward > Kmett's (and most of Oleg's): stuff that's incredible powerful and concise, > but I can't understand at all what it means. I've admired a lot of your > work, particularly on Pan, FRP, and automatic differentiation, but most of > the rest I couldn't understand at all. > > I'll take a look at your *Denotational Design* paper again; maybe now that > I have a lot more experience I'll be able to make sense of it. > > John > > > On Sun, Aug 22, 2010 at 8:18 AM, Conal Elliott <conal(a)conal.net> wrote: > >> Hi John, >> >> Thanks for the reply. A denotational semantics would be independent of >> any implementation, so it would apply to any of them, as long as they have >> the same programming interface. The purpose is to simply & precisely say >> what the types and their building blocks (API) mean by providing a precise, >> implementation-independent, and simple-as-possible math model. Such a >> semantics can be used to prove properties and to define correctness of any >> implementation. It also gives clear feedback on how elegant or inelegant a >> library design is. >> >> For instance, given a type, Map k v, of finite maps, we might say the >> meaning is the type of partial functions from k to v, either k -> v (where >> absent is _|_) or k -> Maybe v (where absent is Nothing). Then we'd give >> the meaning of each Map operation as a function of the meanings of its >> arguments. This example and several others are given in the paper *Denotational >> design with type class morphisms<http://conal.net/papers/type-class-morphisms/> >> *. >> >> Regards, - Conal >> >> On Sun, Aug 22, 2010 at 8:31 PM, John Lato <jwlato(a)gmail.com> wrote: >> >>> Hi Conal, >>> >>> To my knowledge, nobody has attempted this. Oleg may have some ideas, >>> but I don't think he's written about it. I really don't know anything about >>> denotational semantics, so I couldn't do this myself. For some time I've >>> thought it would be good if somebody were able to put together a formal >>> semantics for iteratees, so I'd be very interested if you'd share what you >>> have so far. >>> >>> Would a denotational semantics apply equally to multiple implementations, >>> or would it be tied to a specific implementation? >>> >>> John >>> >>> On Sun, Aug 22, 2010 at 3:47 AM, Conal Elliott <conal(a)conal.net> wrote: >>> >>>> With the encouragement & help of Conrad Parker, I've been looking at >>>> iteratees, enumerators, enumeratees. I can find plenty written about them, >>>> but only about benefits and implementation. In sifting through chunks, >>>> error/control messages, and continuations, I find myself longing for a >>>> precise semantic basis. I keep wondering: what simpler & precise semantic >>>> notions do these mechanisms implement? Has anyone worked out a denotational >>>> semantics for iteratees, enumerators, enumeratees -- something that >>>> simplifies away the performance advantages & complexities? I've worked out >>>> something tentative, but perhaps I'm covering old ground. >>>> >>>> - Conal >>>> >>>> _______________________________________________ >>>> Iteratee mailing list >>>> Iteratee(a)projects.haskell.org >>>> http://projects.haskell.org/cgi-bin/mailman/listinfo/iteratee >>>> >>>> >>> >> >

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Re: [Haskell-cafe] Fwd: Semantics of iteratees, enumerators, enumeratees?
by John Lato 25 Aug '10

25 Aug '10

> > From: "C. McCann" <cam(a)uptoisomorphism.net> > > What sets an iteratee-style design apart from something conventional > based on a State monad is that the iteratee conceals its internal > state completely (in fact, there's no reason an iteratee even has to > be the "same" function step-to-step, or have a single consistent > "state" type--almost has an existential flavor, really), but is at > another function's mercy when it comes to actually doing anything. > > All of which doesn't really shed too much light on the denotation of > these things, I suppose, as there's barely anything there to talk > about; the iteratee automaton itself is a terribly simple construct, > relying on an underlying monad to perform actions, on an external > "push" data source to recurse, and being given only bite-size chunks > of data at each step. It's little more than foldl with a "pause" > button attached. > This is how I think of them. I particularly your description of them as a foldl with a "pause" button. Maybe it would be helpful to consider iteratees along with delimited continuations? John

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Re: [Haskell-cafe] Hackage on Linux
by Mathew de Detrich 25 Aug '10

25 Aug '10

In all honesty I think the issue is that archlinux AUR packages are being generated in a very naive way (and not a Archlinux philosophy in general). The whole point of AUR is that its supposed to make it very easy for people to upload packages/libraries. In my opinion putting such packages in AUR in the first place (apart from the 'required' and 'basic' packages) was a mistake in the first place. I had numerous issues with AUR haskell libraries before (because those packages being in AUR kind of gives the impression they are supposed to be used over cabal) The thing is, libraries at least should be automated. There are just way too many packages in Hackage for someone to go through and manually check if something goes wrong, and you get the issues described previously in conflicts of latest version and outdated versions of libraries installed through AUR/cabal (and in my case having to manually go through and use cabal2arch to fix things was taking up way too much time). C/C++ libraries can be checked by hand because there aren't many of them (as mentioned earlier) where as the 'Haskell' way of doing things is using lots of smaller modularized libraries. If people just wanted an auto udpate version of cabal that works through arch's package management, then there should have just been a pacman wrapper which when you install/update haskell libraries/packages, it creates a local package through cabal2arch instead of using AUR. You either have everything properly updated (assuming it doesn't fail building for w/e reason) or if you want to 'clone' cabal onto AUR, then only do it for the necessary/base/required packages which is easy to keep track of On Wed, Aug 25, 2010 at 3:01 PM, Ivan Lazar Miljenovic < ivan.miljenovic(a)gmail.com> wrote: > On 25 August 2010 14:56, Mathew de Detrich <deteego(a)gmail.com> wrote: > > Thanks for clearing the stuff up. The issue is Ivan, that the archlinux > aur > > packages are auto generated (even the "official ones"). The only packages > > that I believe were ever done by hand was gt2hs (old gtk version) and > since > > its changed to gtk that isn't the case anymore > > Which I've argued with Don about before. Automation is good, but you > should then double-check what it spits out. > > (Note also that in Gentoo, we abstract away the actual interaction > with Cabal so for simple libraries we just have to specify things like > dependencies, download location, etc.). > > > If there is some issue (like you explained with the --flag=-have-gio > flag) > > archlinux aur would have the same problem. To make things more clear, > here > > is an example of the pkgbuild for gtk (from AUR) > > http://aur.archlinux.org/packages/haskell-gtk/haskell-gtk/PKGBUILD > > As you can see, all the script does is invoke runhaskell > > configure/build/haddock etc etc and then registers it with GHC. Thats the > > exact same with every other AUR package, and it isn't any different from > > doing a cabal install (which invokes configure/build/haddock anyways). > That > > pkgbuild generates the actual package you install through pacman, only > > difference being the library is already built (but it still registers > > through GHC). Likewise the uninstalling is the same story > > Yup: I think Don's approach of "oh, if we want to change something > we'll just re-generate them all" rather short-minded (then again, that > seems to be the whole point of Arch, one reason why if I give up > Gentoo I'm more likely to go to Fedora than Arch). > > > So the issues you stated would also be the case with AUR, which means I > am > > not worse off using cabal-install > > Except if you help out with AUR, you can try to make these things better... > ;-) > > And that uninstallation with AUR should be easier/better than what > cabal-install can do, and the ability to specify non-Haskell deps. > > -- > Ivan Lazar Miljenovic > Ivan.Miljenovic(a)gmail.com > IvanMiljenovic.wordpress.com >

4 4

bitmap examples
by Raluca-Elena Podiuc 25 Aug '10

25 Aug '10

Hi, I am trying to build a mini framework for image processing but I can find any examples about that (bitmap processing ). Does anybody know were can I find some tutorials or samples about that ? Regards, Raluca-Elena

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the overlapping instance that wasn't?
by Michael Vanier 25 Aug '10

25 Aug '10

Hi everyone, Here's some code that's giving me an error message I don't understand: {-# LANGUAGE EmptyDataDecls, MultiParamTypeClasses, UndecidableInstances, FlexibleInstances #-} data Z data S n class Nat n where toInt :: n -> Int instance Nat Z where toInt _ = 0 instance (Nat n) => Nat (S n) where toInt _ = 1 + toInt (undefined :: n) instance (Nat n) => Show n where show _ = show $ toInt (undefined :: n) -- end of code sample When I run this through ghci, I get this: test.hs:19:11: Overlapping instances for Show Int arising from a use of `show' at test.hs:19:11-14 Matching instances: instance Show Int -- Defined in GHC.Show instance (Nat n) => Show n -- Defined at test.hs:18:9-25 In the first argument of `($)', namely `show' In the expression: show $ toInt (undefined :: n) In the definition of `show': show _ = show $ toInt (undefined :: n) Adding OverlappingInstances to the language pragmas fixes the problem. My question is: why is this an overlapping instance? It would make sense if Int was an instance of Nat, but it isn't. Is this just a limitation in the way overlapping instances are identified? Mike

3 2

and from standard Prelude
by Oleg Lobachev 25 Aug '10

25 Aug '10

Hello all, the and function, and :: [Bool] -> Bool is defined in two different ways in the latest Prelude. I would expect it to be > and = foldr (&&) True However, there is a further recursive definition, and it is the one used! See http://haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/src/GHC-List… > #ifdef USE_REPORT_PRELUDE > and = foldr (&&) True > or = foldr (||) False > #else > and [] = True > and (x:xs) = x && and xs > or [] = False > or (x:xs) = x || or xs > > {-# RULES > > "and/build" forall (g::forall b.(Bool->b->b)->b->b) . > > and (build g) = g (&&) True > > "or/build" forall (g::forall b.(Bool->b->b)->b->b) . > > or (build g) = g (||) False > > #-} > #endif Is there any reason for that besides some clever optimizations? I am particularly interested in the behavior of and for infinite lists and these two versions are equivalent in this aspect. Greetings, Oleg

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lazy skip list?
by Evan Laforge 25 Aug '10

25 Aug '10

Is there a data type that's spine lazy like a list, but can be seeked (sought?) efficiently? IntMap and Sequence are spine strict so if you take the head element all the elements are forced even if their contents are not. E.g., 'Sequence.index (Sequence.fromList [0..]) 0' will diverge, but obviously 'head [0..]' won't. I can get what I want from a list of lists where the outer list is is 'iterate (drop n)' on the inner one... this is a kind of skip list, right? It seems like it should be possible to generalize this to multiple levels with increasing skip amounts, then I can seek relatively efficiently and won't force the spine beyond where I seek. Effectively, this is nested list like [[[a]]], where each level above the bottom is something like 'iterate (drop n) level_below'. Alternately, it should also be possible to do something fancy like a self-modifying data structure that constructs its index as you force bits of it, but this would require some hairy unsafePerformIO, I think. Has anyone heard of a data structure like this, either the plain list of lists or the self modifying magic one? Surely efficiently indexing a lazy list is something someone else has thought about? It looks like Luke Palmer's data-inttrie might be doing this, though it's not clear from the documentation. It looks like it lacks emptiness, so it doesn't apply to finite structures. lazyarray looks like it might be doing the magic self-modifying thing, though it wants a a static bounds right off the bat, which isn't much help when the whole point is to avoid having to figure out how long the input is.

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Re: [Haskell-cafe] the overlapping instance that wasn't?
by Michael Vanier 25 Aug '10

25 Aug '10

On 8/24/10 1:54 PM, Bartek Æwik³owski wrote: > Hello Michael, > > This is because instance selection is solely based on instance heads, > it doesn't consider contexts. There's a nice explanation available > here: http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap > > The fix in this case is very easy because there are only two cases > (and you don't want to provide new Nat class instances): > > instance Show Z where > show _ = "0" > > instance Nat n => Show (S n) where > show _ = show $ toInt (undefined :: n) + 1 > > regards, > Bartek Æwik³owski OK, thanks! Mike

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Re: [Haskell-cafe] Fwd: Semantics of iteratees, enumerators, enumeratees?
by John Lato 25 Aug '10

25 Aug '10

> > From: Stephen Tetley <stephen.tetley(a)gmail.com> > > On 24 August 2010 13:00, John Lato <jwlato(a)gmail.com> wrote: > > > This is how I think of them. I particularly your description of them as > a > > foldl with a "pause" button. > > Maybe it would be helpful to consider iteratees along with delimited > > continuations? > > Aren't they closer - in implementation and by supported operations - > to resumptions monads? > > See many papers by William Harrison here: > http://www.cs.missouri.edu/~harrisonwl/abstracts.html > > I'm not familiar with resumption monads. I'll have to read some of the papers and get back to you. John

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ANNOUNCE: Sifflet visual programming language, release 1.1
by gdweber＠iue.edu 24 Aug '10

24 Aug '10

Sifflet-lib 1.1 and sifflet 1.1 are now available on Hackage. Sifflet is a visual, functional programming language and support system for students learning about recursion. Sifflet programmers define functions by drawing diagrams, and the Sifflet interpreter uses diagrams to show how the function calls are evaluated. Sifflet-lib is the library containing many modules of the sifflet application. New in This Release ------------------- - Relaxed versioned dependencies on cairo, gtk, glib (== 0.11.* instead of == 0.11.0) should make it possible to build with slightly newer versions of these libraries (and pango). - Sifflet export to Haskell now removes most unnecessary parentheses. - The module Sifflet.Data.Number is now Data.Number.Sifflet. - Many undocumented API changes in sifflet-lib. (I really hope nobody is using this library for development but me! But if you are, contact me, and I'll send a partial list of the changes.) About Sifflet ------------- Sifflet is a visual, functional programming language intended as an aid for learning about recursion. * A picture explains Sifflet better than words: please see the screenshot showing how to evaluate 3!: http://mypage.iu.edu/~gdweber/software/sifflet/home.html * Features: - Visual editor. - Visual tracer/debugger which shows how recursive and other function calls are evaluated. To support active learning and avoid screen clutter, Sifflet displays only as much of the computation as the user requests. - A carefully crafted tutorial with 31 pictures and about as many pages. - Number, string, and list data types. - A small collection of primitive functions. - Runnable examples of compound functions. - Sifflet functions can be "exported" to Scheme, Python 3, and Haskell. * Download: http://hackage.haskell.org/package/sifflet-lib http://hackage.haskell.org/package/sifflet * Home page: http://mypage.iu.edu/~gdweber/software/sifflet/home.html * Tutorial: http://mypage.iu.edu/~gdweber/software/sifflet/doc/tutorial.html -- ___ ___ __ _ / _ \ / _ \| | | | Gregory D. Weber, Associate Professor / /_\// / | | | /\ | | Indiana University East / /_\\/ /__| | |/ \| | http://mypage.iu.edu/~gdweber/ \____/\_____/\___/\__/ Tel. (765) 973-8420; FAX (765) 973-8550

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Re: ANNOUNCE: enumerator, an alternative iteratee package
by John Lato 24 Aug '10

24 Aug '10

> > From: Magnus Therning <magnus(a)therning.org> > > On 24/08/10 03:47, John Millikin wrote: > [...] > > I would like to avoid hard-coding the error type to SomeException, > because > > it forces libraries to use unsafe/unportable language features (dynamic > > typing and casting). However, given the apparent practical requirement > that > > all iteratees have the same error type, it seems like there's no other > > choice. > > I haven't worked enough with iteratees to have an informed opinion on this, > but I wonder what the pros and cons are of having an error state in the > iteratees at all. In other words, why would this > > data Step a m b > = Continue (Stream a -> Iteratee a m b) > | Yield b (Stream a) > | Error E.SomeException > > be preferred over this > > data Step a m b > = Continue (Stream a -> Iteratee a m b) > | Yield b (Stream a) > > (Maybe with the restriction that m is a MonadError.) > Oleg included the error state to enable short-circuiting of computation, and I guess everyone just left it in. Recently I've been wondering if it should be removed, though, in favor requiring explicit (i.e. explicit in the type sig) exceptions for everything. I'm not sure if that would be more or less complicated. John

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Re: [Haskell-cafe] Re: Fwd: Semantics of iteratees, enumerators, enumeratees?
by John Lato 24 Aug '10

24 Aug '10

I think the big problem with chunking is that many useful iteratees need to be able to inspect the length of the chunk. The most obvious is "drop", but there are many others. Or if not inspect the length, have a new function on the stream "dropReport :: Int -> s -> (s, Int)" which reports how much was dropped. Either way, chunking adds a deal of implementation burden. I suspect that the proper vocabulary for iteratees wouldn't include chunks at all, only single elements. This discussion has prompted me to consider the implications of such an implementation, as it would be much simpler. I have one idea that I think will at least maintain performance for many operations, although there will be performance hits too. If the drawbacks are in areas that aren't particularly useful, though, it may be acceptable. John > From: Conal Elliott <conal(a)conal.net> > > Here's a way I've been tinkering with to think about iteratees clearly. > > For simplicity, I'll stick with pure, error-free iteratees for now, and > take > chunks to be strings. Define a function that runs the iteratee: > > > runIter :: Iteratee a -> [String] -> (a, [String]) > > Note that chunking is explicit here. > > Next, a relation that an iteratee implements a given specification, defined > by a state transformer: > > > sat :: Iteratee a -> State String a -> Bool > > Define sat in terms of concatenating chunks: > > > sat it st = > > second concat . runIter it == runState st . second concat > > where the RHS equality is between functions (pointwise/extensionally), and > runState uses the representation of State directly > > > runState :: State s a -> s -> (a,s) > > (I think this sat definition is what Conrad was alluding to.) > > Now use sat to specify and verify operations on iteratees and to > *synthesize* those operations from their specifications. Some iteratees > might not satisfy *any* (State-based) specification. For instance, an > iteratee could look at the lengths or number of its chunks and produce > results accordingly. I think of such iteratees as abstraction leaks. Can > the iteratee vocabulary be honed to make only well-behaved (specifiable) > iteratees possible to express? If so, can we preserve performance > benefits? > > If indeed the abstraction leaks can be fixed, I expect there will be a > simpler & more conventional semantics than sat above. > > - Conal > > > On Tue, Aug 24, 2010 at 2:55 PM, Conrad Parker <conrad(a)metadecks.org> > wrote: > > > On 24 August 2010 14:47, Jason Dagit <dagit(a)codersbase.com> wrote: > > > > > > > > > On Mon, Aug 23, 2010 at 10:37 PM, Conrad Parker <conrad(a)metadecks.org> > > > wrote: > > >> > > >> On 24 August 2010 14:14, Jason Dagit <dagit(a)codersbase.com> wrote: > > >> > I'm not a semanticist, so I apologize right now if I say something > > >> > stupid or > > >> > incorrect. > > >> > > > >> > On Mon, Aug 23, 2010 at 9:57 PM, Conal Elliott <conal(a)conal.net> > > wrote: > > >> >>> > > >> >>> So perhaps this could be a reasonable semantics? > > >> >>> > > >> >>> Iteratee a = [Char] -> Maybe (a, [Char]) > > >> >> > > >> >> I've been tinkering with this model as well. > > >> >> > > >> >> However, it doesn't really correspond to the iteratee interfaces > I've > > >> >> seen, since those interfaces allow an iteratee to notice size and > > >> >> number of > > >> >> chunks. I suspect this ability is an accidental abstraction leak, > > >> >> which > > >> >> raises the question of how to patch the leak. > > >> > > > >> > From a purely practical viewpoint I feel that treating the chunking > as > > >> > an > > >> > abstraction leak might be missing the point. If you said, you > wanted > > >> > the > > >> > semantics to acknowledge the chunking but be invariant under the > size > > or > > >> > number of the chunks then I would be happier. > > >> > > >> I think that's the point, ie. to specify what the invariants should > > >> be. For example (to paraphrase, very poorly, something Conal wrote on > > >> the whiteboard behind me): > > >> > > >> run [concat [chunk]] == run [chunk] > > >> > > >> ie. the (a, [Char]) you maybe get from running an iteratee over any > > >> partitioning of chunks should be the same, ie. the same as from > > >> running it over the concatenation of all chunks, which is the whole > > >> input [Char]. > > > > > > I find this notation foreign. I get [Char], that's the Haskell String > > > type, but what is [chunk]? I doubt you mean a list of one element. > > > > sorry, that was just my way of writing "the list of chunks" or perhaps > > "the stream of chunks that represents the input". > > > > Conrad. > > > > > > > >> > > >> > I use iteratees when I need to be explicit about chunking and when I > > >> > don't > > >> > want the resources to "leak outside" of the stream processing. If > you > > >> > took > > >> > those properties away, I wouldn't want to use it anymore because > then > > it > > >> > would just be an inelegant way to do things. > > >> > > >> Then I suppose the model for Enumerators is different than that for > > >> Iteratees; part of the point of an Enumerator is to control the size > > >> of the chunks, so that needs to be part of the model. An Iteratee, on > > >> the other hand, should not have to know the size of its chunks. So you > > >> don't want to be able to know the length of a chunk (ie. a part of the > > >> stream), but you do want to be able to, say, fold over it, and to be > > >> able to stop the computation at any time (these being the main point > > >> of iteratees ...). > > > > > > I think I agree with that. > > > Jason > >

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Content Centric Networking
by Tom Hawkins 24 Aug '10

24 Aug '10

Anyone in the Haskell community interested in content-centric networking? Van Jacobson has done a couple of great presentations to introduce CCN [1, 2]. Personally, I find it fascinating what kind of doors could open if we got away from TCP/IP, especially for wireless ad-hoc networking. Parc has open-sourced their experimental CCN protocol, CCNx [3]. I thought it'd be cool to have a Haskell implementation of CCNx, so I started a little hackage project [4]. -Tom [1] http://video.google.com/videoplay?docid=-6972678839686672840# [2] http://www.parc.com/work/focus-area/networking/ [3] http://www.ccnx.org/ [4] http://hackage.haskell.org/package/ccnx/

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Higher-order algorithms
by Max Rabkin 24 Aug '10

24 Aug '10

(Accidentally sent off-list, resending) On Mon, Aug 23, 2010 at 15:03, Eugene Kirpichov <ekirpichov(a)gmail.com> wrote: > * Difference lists > I mean that not only higher-order facilities are used, but the essence > of the algorithm is some non-trivial higher-order manipulation. If I'm not mistaken, we can "defunctionalize" difference lists like this: data DList a = Chunk [a] | Concat (DList a) (DList a) fromList = Chunk (<>) = Concat singleton = Chunk . (:[]) empty = Chunk [] toList dl = dl `fold` [] where infixr `fold` fold :: DList a -> [a] -> [a] fold (Concat l r) ys = l `fold` r `fold` ys fold (Chunk xs) ys = xs ++ ys (This implementation has only been lightly tested) And of course, we knew this was possible, since we can compile DLists to first-order machines. I agree that the functional, higher-order presentation is clear and elegant. But is it essential? Also, I'm curious about how this performs relative to the functional version. --Max

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Re: [haskell-cafe] How to substitute FD like a -> b c by type families?
by Vladimir Matveev 24 Aug '10

24 Aug '10

Thanks, that was helpful :) here is the working code if someone interested: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=29406#a29406 2010/8/24 wren ng thornton <wren(a)freegeek.org>: > Vladimir Matveev wrote: >> >> I'm trying to implement the "Advanced Example : Type-Level Quicksort" >> from HaskellWiki using type families instead of functional >> dependencies. My code is at [1]. I substituted all 'many to one' >> functional dependencies like xs ys -> zs by explicit type families, >> but I don't know how to replace 'many to many' dependencies by type >> families only, so I've used associated types. But for some unknown to >> me reason the typechecker hangs if I try to get listQuickSort type >> signature in ghci. >> So I have 2 questions: what is the correct replacement of FDs in this >> case and where is an error in my code? I assume that the correct >> replacement exists (though it may not be very pretty) because "type >> families and functional dependencies are equivalent in >> expressiveness" [2]. > > I'm no guru on TF/ATs, but it seems to me that the simplest translation from > fundeps would be to use multiple different type families for getting each of > the result types. Thus, > > class Foo a b c ... x y z | a b c ... -> ... x y z > > is first interpreted as: > > class Foo a b c ... x y z > | ... > , a b c ... -> x > , a b c ... -> y > , a b c ... -> z > > which then becomes: > > ... > type family FooX a b c ... > type family FooY a b c ... > type family FooZ a b c ... > > This is equivalent to converting a function of type (A,B,C,...)->(...,X,Y,Z) > into multiple functions for determining each member of the output tuple > separately. Your version using ATs to shove all the results into the Pick > and Split classes is equivalent to constructing the output tuple directly. > Apparently that doesn't work out for some reason. > > -- > Live well, > ~wren > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Changing my Profile
by David Webster 24 Aug '10

24 Aug '10

Is there any way to change my profile or signup options so that I don't get CC'ed on every posting to Haskell-Cafe? David

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Re: [Haskell-cafe] Fast Integer Input
by 200901104＠daiict.ac.in 24 Aug '10

24 Aug '10

> From: "Daniel Fischer" <daniel.is.fischer(a)web.de> > To: 200901104(a)daiict.ac.in > Sent: Monday, August 23, 2010 11:14:55 PM > Subject: Re: [Haskell-cafe] Fast Integer Input > On Monday 23 August 2010 17:06:02 you wrote: > > >>Does the length of those numbers happen to be fixed? It they are > > >>all > > >>exactly 13000 decimals then it should be possible to create a more > > >>optimised parser. > > > > Well actually they can have any number of digits less than 13000. > > But the only post processing of the numbers is calculating the > > binary logarithm of the number. Does that help? > > > > That, time limit, SPOJ? (If so, which one?) Yes. The time limit is 4 sec. The question has more parts than just this though. http://www.spoj.pl/problems/VGCD/. (Note: Spoiler at the bottom) > > You can calculate the binary logarithm to high accuracy without > parsing the > entire number. Just parse the first k digits and get the count of > digits, > from that the logarithm is quickly calculated. I will try this out. > > > >>It would also help if you could post a working example of your > > >>code > > >>and some test data somewhere so people can run it and test if for > > >>themselves. > > > > I have a slow Internet connection. So I will attach the script I > > used to generate the cases instead.(Note: It will take a few minutes > > to complete) Spoiler: The given numbers are Fibonacci. Wikipedia has said some interesting properties, which makes the question into finding where the number is in the Fibonacci sequence.

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Re: [Haskell] Re: [Haskell-cafe] ANNOUNCE: enumerator, an alternative iteratee package
by John Millikin 24 Aug '10

24 Aug '10

On Sat, Aug 21, 2010 at 23:14, Paulo Tanimoto <ptanimoto(a)gmail.com> wrote: > One question: enumFile has type > > enumFile :: FilePath -> Enumerator SomeException ByteString IO b > > and iterParser has type > > iterParser :: Monad m => Parser a -> Iteratee ParseError ByteString m a > > How do we use both together? Something in these lines won't type-check > > E.run (E.enumFile "file" E.$$ (E.iterParser p)) > > because the error types are different. Forgot to mention that -- use the "mapError" function from enumerator-0.2.1 thusly: http://ianen.org/haskell/enumerator/api-docs/Data-Enumerator.html#v%3AmapEr… -------------------------------------------------------------------------------- parser :: Parser Foo toExc :: Show a => a -> E.SomeException toExc = E.SomeException . E.ErrorCall . show main :: IO () main = do run (enumFile "parsetest.txt" $$ mapError toExc $$ iterParser parser) >>= print -------------------------------------------------------------------------------- You don't have to map to SomeException -- any type will do. For example, in a complex pipeline with real error handling at the other end, you might want a custom error type so you'll know at what stage the error occurred.

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How to substitute FD like a -> b c by type families?
by Vladimir Matveev 24 Aug '10

24 Aug '10

I'm trying to implement the "Advanced Example : Type-Level Quicksort" from HaskellWiki using type families instead of functional dependencies. My code is at [1]. I substituted all 'many to one' functional dependencies like xs ys -> zs by explicit type families, but I don't know how to replace 'many to many' dependencies by type families only, so I've used associated types. But for some unknown to me reason the typechecker hangs if I try to get listQuickSort type signature in ghci. So I have 2 questions: what is the correct replacement of FDs in this case and where is an error in my code? I assume that the correct replacement exists (though it may not be very pretty) because "type families and functional dependencies are equivalent in expressiveness" [2]. Regards, Vladimir [1] http://hpaste.org/fastcgi/hpaste.fcgi/view?id=29380#a29380 [2] http://www.mail-archive.com/haskell-cafe@haskell.org/msg54024.html

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feasability of implementing an awk interpreter.
by Michael Litchard 24 Aug '10

24 Aug '10

I'd like the community to give me feedback on the difficulty level of implementing an awk interpreter. What language features would be required? Specifically I'm hoping that TH is not necessary because I'm nowhere near that skill level. An outline of a possible approach would be appreciated. I am using http://www.math.utah.edu/docs/info/gawk_toc.html as a guide to the language description.

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Filename encoding error (was: Perform a research a la Unix 'find')
by Yves Parès 24 Aug '10

24 Aug '10

In fact the encoding problem is more general. When I simply do 'readFile "bar/fooé"', then I'm told: *** Exception: bar/fooé: openFile: does not exist (No such file or directory) How am I supposed to read files whose names contains non-ASCII characters? (I use GHC 6.12.3 under Ubuntu 10.04 32bits) ---------- Forwarded message ---------- From: Yves Parès <limestrael(a)gmail.com> Date: 2010/8/22 Subject: Re: [Haskell-cafe] Perform a research a la Unix 'find' To: Magnus Therning <magnus(a)therning.org> Cc: haskell-cafe(a)haskell.org I looked at both, and I have encoding issues with both. My locale is fr_FR.utf8 For instance, with HSH: I have a 'bar' directory, containing a file 'fooé' run $ "find bar" :: IO [String] returns me : ["bar", "bar/foo*\233*"] and run $ "find bar -name fooé" returns [] When I provoke an error by running: run $ "find fooé" it says : find: "foo*\351*": No file or directory So it is not the same encoding!

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ANNOUNCE: ipatch, the interactive patch editor
by Joachim Breitner 24 Aug '10

24 Aug '10

Dear Haskell and Darcs community, I have written a tool to interactively edit patches, which under the hood uses the Darcs API. A more catchy introduction, including a screencast, can be found on https://www.joachim-breitner.de/blog/archives/425-ipatch,-the-interactive-p… which ends (and this is where you come in) with „So this is an invitation to join me and make ipatch a great tool. This invitation goes especially to the Darcs developers: Please have a look how the code uses the Darcs API and help to improve the collaboration here. I think we can use the darcs-users mailing list until there is need for a dedicated mailing list.“ The code is on http://darcs.nomeata.de/ipatch/. Enjoy! Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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ANNOUNCE: darcs 2.5 beta 4
by Reinier Lamers 23 Aug '10

23 Aug '10

The darcs team would like to announce the immediate availability of darcs 2.5 beta 4 (also known as darcs 2.4.98.4 due to Cabal restrictions). Important changes since darcs 2.4.4 are: * trackdown can now do binary search with the --bisect option * darcs always stores patch metadata encoded with UTF-8 * diff now supports the --index option * amend-record now supports the --ask-deps option * apply now supports the --match option * amend-record has a new --keep-date option * inventory-changing commands (like record and pull) now operate in constant time with respect to the number of patches in the repository * the push, pull, send and fetch commands no longer set the default repository by default * the --edit-description option is now on by default for the send command Changes since the last beta release are: * Fix issue 1898: warn users about changes to command line flag defaults * Fix issue 1913: sort changes in treeDiff (fixes failure in record) * Fix issue 1815: make the tarball with the right permissions If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.4.98.4.tar.gz and build it by hand as explained in the README file. Kind Regards, the darcs release manager, Reinier Lamers

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