- Haskell-Cafe - Haskell.org

who maintains the POSIX/Unix Unistd.hs?
by Galchin, Vasili 07 May '08

07 May '08

Hello, I am trying to find the definition for the data type "TimeSpec"/"CTimeSpec". thanks, Vasili

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Figuring out if an algebraic type is enumerated through Data.Generics?
by Alfonso Acosta 07 May '08

07 May '08

Hi all, <probably_unnnecesary_background> I'm writing a Hardware-oriented DSL deep-embedded in Haskell (similar to Lava, for those familiar with it). One of the goals of the language is to support polymorphic signals (i.e. we would like to allow signals to carry values of any type, including user-defined types) The embedded compiler has different backends: simulation, VHDL, graphical representation in GraphML ... The simulation backend manages to support polymorphic signals by forcing them to be Typeable. In that way, circuit functions, no matter what signals they process, can be transformed to Dynamic and included in the AST for later simulation. The situation is more complicated for the VHDL backend. The Typeable trick works just fine for translating a limited set of predefined types (e.g. tuples) but not for user-defined types, since the embedded-compiler doesn't have access to the user-defined type declarations. One possible solution to access the type-definition of signal values would be constraining them to instances of Data. Then, we could use dataTypeOf to get access to the type representation. (Another option would be using template Haskell's reify, but I would like to avoid that by now) It would certainly be difficult map any Haskell type to VHDL, so, by now we would be content to map enumerate algebraic types (i.e. algebraic types whose all data constructors have arity zero, e.g. data Colors = Green | Blue | Red) </probably_unnnecesary_background> So, the question is. Is there a way to figure out the arity of data constructors using Data.Generics ? I'm totally new to generics, but (tell me if I'm wrong) it seems that Constr doesn't hold any information about the data-constructor arguments. Why is it so? Do you think there is a workoaround for this problem? (maybe using some other function from Data.Generics different to dataTypeOf?) Thanks in advance, Alfonso Acosta

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Haddock and upload questions?
by Galchin, Vasili 07 May '08

07 May '08

Hello, 1) Can I set up Haddock to run everytime I do a build? 2) http://hackage.haskell.org/packages/upload.html .... - do I have to set up my .cabal in a special way to run "dist"? - once I checked that my Cabal package is "compliant" on this page, how do I get a username and password? Kind regards, Vasili

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license question
by Henrique Ferreiro García 07 May '08

07 May '08

Hello everybody, I have one question regarding a licensing issue. I am developing a library to parse and pretty-print the Core Erlang language and it is heavily based on the modules included in haskell-src. What I want to know is if I have to reproduce all of the LICENSE file included in that package with my name included in the copyright notice or what other thing do I have to do. Thanks in advance. -- Henrique Ferreiro García <hferreiro(a)udc.es>

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Re: [Haskell-cafe] Figuring out if an algebraic type is enumerated through Data.Generics?
by Edward Kmett 07 May '08

07 May '08

On Tue, May 6, 2008 at 12:34 PM, Alfonso Acosta <alfonso.acosta(a)gmail.com> wrote: | So, the question is. Is there a way to figure out the arity of data | constructors using Data.Generics ? | I'm totally new to generics, but (tell me if I'm wrong) it seems that | Constr doesn't hold any information about the data-constructor | arguments. Why is it so? Hmrmm, Playing around with it, I was able to abuse gunfold and the reader comonad to answer the problem : fst $ (gunfold (\(i,_) -> (i+1,undefined)) (\r -> (0,r)) (toConstr "Hello") :: (Int,String)) returns 2, the arity of (:), the outermost constructor in "Hello" A longer version which does not depend on undefined would be to take and define a functor that discarded its contents like: > module Args where > import Data.Generics > newtype Args a = Args { runArgs :: Int } deriving (Read,Show) > tick :: Args (b -> r) -> Args r > tick (Args i) = Args (i + 1) > tock = const (Args 0) > argsInCons = runArgs $ (gunfold tick tock (toConstr "Hello") :: (Args String) Basically all I do is rely on the fact that gunfold takes the 'tick' argument and calls it repeatedly for each argument after a 'tock' base case. The use of the reader comonad or functor is to give gunfold a 'functor-like' argument to meet its type signature. -Edward Kmett

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TimeSpec
by Galchin, Vasili 06 May '08

06 May '08

Hello, I checked Hoogle for the POSIX data type TimeSpec. Nada. Does anybody know where the definition is? Thanks, Vasili

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Still no joy with parsec
by Ross Boylan 06 May '08

06 May '08

I kept getting parse failures when I ran my little Parsec TeX snippet on a sample code. Seeing that ghc6.8 had debugging, I upgraded to it, only to discover that I can't even get the code to compile. $ ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :load g [1 of 1] Compiling Main ( g.hs, interpreted ) g.hs:11:19: Couldn't match expected type `t1 -> GenParser Char () t' against inferred type `CharParser st ()' In the expression: reserved "\\begin" 1 In a 'do' expression: reserved "\\begin" 1 In the expression: do reserved "\\begin" 1 braces (many1 letter) Failed, modules loaded: none. Do parsec and 6.8 just not get along? More generally, how can I go about diagnosing such problems? Since I can't load it, I can't debug it or get :info on the types. It looks as if maybe it's expecting a Monad, but getting a parser. But I don't know why that would have changed vs using 6.6. More questions about the error messages. Where is the expected type, and where is the inferred type, coming from? I'm guessing the expected type is from the function signature and the position inside a do (or perhaps from the argument following the ; in the do?) and the inferred type is what I would just call the type of reserved "begin". And what is the 1 that appears after 'reserved "\\begin"'? An indicator that all occurrences of the text refer to the same spot in the program? Nesting level? Thanks. Ross P.S. There have been some issues with the Debian packaging of ghc6.8, so it's possible I'm bumping into them. I thought/hoped the problems were limited to non i386 architectures. Also, I'm pretty sure that the parsec code used by ghc6.6, ghc6.8, and hugs is all in different files. So conceivably the parsec source differs. I have ghc6 6.8.2-5 and libghc6-parsec-dev 2.1.0.0-2. Source: import Text.ParserCombinators.Parsec import qualified Text.ParserCombinators.Parsec.Token as P import Text.ParserCombinators.Parsec.Language(haskell) reserved = P.reserved haskell braces = P.braces haskell -- TeX example envBegin :: Parser String envBegin = do{ reserved "\\begin" 1 ; braces (many1 letter) }

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Induction (help!)
by PR Stanley 06 May '08

06 May '08

Hi I don't know what it is that I'm not getting where mathematical induction is concerned. This is relevant to Haskell so I wonder if any of you gents could explain in unambiguous terms the concept please. The wikipedia article offers perhaps the least obfuscated definition I've found so far but I would still like more clarity. The idea is to move onto inductive proof in Haskell. First, however, I need to understand the general mathematical concept. Top marks for clarity and explanation of technical terms. Thanks Paul

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ANNOUNCE: category-extras 0.44.2
by Edward Kmett 06 May '08

06 May '08

Dan Doel asked me to roll category-extras into my nascent comonad transformer library, and the result is category-extras 0.44.2! So since Dan's release a couple of weeks ago ( http://www.haskell.org/pipermail/haskell-cafe/2008-April/042240.html) we have added: * Comonad Transformers. Context/ContextT, Reader/ReaderT. * A suite of Bifunctors and combinators in Control.Bifunctor.* -- we attach most of the logic usually associated with a monoidal/comonoidal category to the individual Bifunctor since Hask is a rich category to begin with. * A suite of Functors and combinators in Control.Functor.* * Pointed and copointed functors. * Control.Recursion has been broken out into a Control.Morphism.* and recoded to use a simpler distributive law. * Type Indexed versions of Applicatives, Monads, and Comonads (including Diatchki's Indexed State and Wadler's Delimited Continuation Parameterized Monad) * Parameterized Monads a la Ghani and Johann's paper ICFP 07 paper, and their Applicative and comonadic dual. * Higher-order hylo-, cata- and ana- morphisms. * Higher-order Monads a la Ghani and Johann and their comonadic equivalents. * Kan extensions. * BiKleisli arrows as seen in Uustalu and Vene's Signals and Comonads and SIGFPE's recent posts * The Pointer comonad * Grabbed Iavor Diatchki's value-supply and rolled it in as Control.Comonad.Supply to make it clearer that it is a comonad, and pave the way towards a Supply comonad transformer * A richer set of compositions to allow for construction of comonads and monads not only from adjunctions, but also from pre-composition or post-composition of a monad with a pointed functor, and similarly pre-composition and post-composition of a comonad with a copointed functor. * Generic functor zapping, zipping, unzipping, and cozipping as mentioned recently on http://comonad.com/reader There is still a lot to do in terms of adding back a lot of the documentation from the original, documenting the extensions and fleshing out all of the definable instances as the concepts have grown exceptionally fine-grained. I definitely welcome feedback and additions. In particular if you were using a feature that was supported by the old library or is unnatural to program in the current one, let me know. My goal is to gather a lot of this esoterica into one place and integrate it into something cohesive. HEAD: haddock http://comonad.com/haskell/category-extras/dist/doc/html/category-extras/ darcs http://comonad.com/haskell/category-extras/ Release: package http://hackage.haskell.org/cgi-bin/hackage-scripts/package/category-extras-… -Edward Kmett

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ANNOUNCE: Sessions 2008.5.2
by Matthew Sackman 06 May '08

06 May '08

Howdy, I'm pleased to announce the general availability of Session Types for Haskell, version 2008.5.2. It is available from my website[0], Hackage[1][2] and I've just updated the online tutorial[3] to take into account the recent changes and new features. [0] http://wellquite.org/non-blog/sessions-2008.5.2.tar.gz [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sessions-2008.5.2 [2] http://hackage.haskell.org/ [3] http://wellquite.org/sessions/tutorial_1.html Session types are a means of describing communication between multiple threads, and statically verifying that the communication being performed is safe and conforms to the specification. The library supports multiple, concurrent channels being open and actions upon those channels being interleaved; forking new threads; the communication of process IDs, allowing threads to establish new channels between each other; higher-order channels, allowing an established channel to be sent over a channel to another process; and many other features common to session type systems. The significant new changes in this version are: a) an entirely new means to specify session types. This removes the old absolute indexing, is composeable and far more flexible and powerful than the old system. b) support for higher-order channels and session types allowing channels to be sent and received. This permits additional communication patterns such as two threads which are otherwise unaware of each other being able to communicate with one another by sending and receiving a channel via a common third party - delegation. Tests[4] and an example application[5] are available which should, in combination with the tutorial[6], explain how session types can be used. As ever, any feedback is very gratefully received. [4] http://wellquite.org/non-blog/sessions-browseable/Control/Concurrent/Sessio… [5] http://wellquite.org/non-blog/sessions-browseable/Control/Concurrent/Sessio… [6] http://wellquite.org/sessions/tutorial_1.html Matthew

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