July 2012 - Haskell-Cafe

Re: [Haskell-cafe] COBOL-85 parser, anyone?
by Richard O'Keefe 24 Jul '12

24 Jul '12

On 25/07/2012, at 1:47 AM, S D Swierstra wrote: > You can find a CFG at: http://www.cs.vu.nl/~x/grammars/vs-cobol-ii/index.html Thank you. I see it's closer to COBOL-85 than I realised.

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Which ghc binary does ghc-mod use?
by Peter Simons 24 Jul '12

24 Jul '12

Hi, I am a happy user of Emacs with ghc-mod for Haskell programming. There is just one issue I've run into: I have multiple versions of GHC installed on my machine. Now, ghc-mod seems to use the GHC binary that was used to compile ghc-mod itself, but that is not the version I want it to use for syntax checking, etc. In fact, I want to be able to switch ghc-mod between different GHC binaries depending on which project I'm working on, but I have no idea how to do that. Is there maybe some Elisp guru reading this list who can help me out? Can I somehow configure which GHC binary ghc-mod uses? Take care, Peter

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Two-day Haskell tutorial, Copenhagen, September 13-14
by Simon Thompson 24 Jul '12

24 Jul '12

This year CUFP is putting on a series of longer tutorials to introduce functional programming languages in depth, and in particular the organisers are delighted to announce a TWO DAY Haskell tutorial, taught by leading experts and educators Jeremy Gibbons (Oxford) and Simon Marlow (Microsoft Research). This two-day tutorial is designed to introduce you to Haskell without assuming that you know anything about the language in advance, and will introduce you to the underlying ideas and concepts of Haskell as well as giving you hands-on experience of programming in Haskell. So, if you're interested we do hope that you are able to attend, or if not, that you'll pass this information on to colleagues and friends who might be interested. There's more information and registration instructions at http://cufp.org/conference/2012 Hoping to see you there! Duncan Coutts, Simon Thompson, Michael Sperber CUFP tutorials organisers More information … Abstract Pure functional programming is about programming with expressions rather than actions, data rather than control. Many programs are more naturally expressed in this way, and functional programs are often an order of magnitude shorter than equivalent imperative programs - just as programs in traditional high-level languages like C and Java are typically much shorter and clearer than their equivalents in assembly language. Many exciting ideas in mainstream programming arose first in FP, including garbage collection, generics, closures, and list comprehensions. FP is a fertile laboratory for language innovation, and familiarity with FP is good preparation for future developments in programming. In particular, multicore chips and cloud computing architectures are forcing us all into dealing with concurrency, and there is good evidence that the breakthroughs here will come from FP, with its emphasis on immutability and independence of evaluation order. Content This two-day tutorial introduces the essentials of Haskell, a pure lazy functional language with a mature compiler, a wealth of libraries, and an active community. We will focus particularly on concurrent programming - handling multiple interactions at the same time - which is increasingly important in a wide range of applications today, from GUI programming to server-side web development. There will be a balance between lectures and exercises, so that you will be exposed to both the theory and the practice of FP. Day 1 will focus on the basics of FP with Haskell, introducing the concepts necessary to get you writing simple programs, including an introduction to monads. On Day 2 you will get a little taste of the joys of concurrent programming in Haskell. By the end, you will be writing some simple (but real!) Concurrent Haskell programs. We will also cover some of the practical aspects of day-to-day programming in Haskell, such as working with third-party libraries. Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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Re: [Haskell-cafe] Is (==) commutative?
by Tristan Seligmann 24 Jul '12

24 Jul '12

Forgot to copy the list. Sorry for the duplicates :( On Jul 24, 2012 1:35 PM, "Tristan Seligmann" <mithrandi(a)mithrandi.net> wrote: > On Jul 24, 2012 12:32 PM, "Twan van Laarhoven" <twanvl(a)gmail.com> wrote: > > > > On 2012-07-24 10:10, Christian Sternagel wrote: > >> > >> Dear all, > >> > >> with respect to formal verification of Haskell code I was wondering > whether (==) > >> of the Eq class is intended to be commutative (for many classes such > >> requirements are informally stated in their description, since Eq does > not have > >> such a statement, I'm asking here). Or are there any known cases where > >> commutativity of (==) is violated (due to strictness issues)? > > > > > > Strictness plays no role for Eq, since to test for equality both sides > will have to be fully evaluated. > > I don't think this is necessarily true. For example: > > (==) :: (Eq a) => Maybe a -> Maybe a -> Bool > Nothing == Nothing = True > Nothing == Just _ = False > Just _ == Nothing = False > Just x == Just y = x == y > > This particular example is still commutative, however (at least I think it > is). >

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TLS 0.9.6, question about session resumption.
by C Gosch 23 Jul '12

23 Jul '12

Hi Cafe, I am trying to use the TLS package from hackage, and it works fine so far -- except when a client wants to do session resumption (note I am not an expert in TLS, so it might be something quite simple). In that case, I get an alert, "unexpected message", during handshake. The handshake goes like this: ClientHello (with a SessionID) ServerHello (with the same SessionID) ServerHelloDone and then the server says (AlertLevel_Fatal,UnexpectedMessage) I'm not sure whether the "ServerHelloDone" should happen when resuming. Does anyone have a hint what may be going wrong? I am using TLS10 and the tls package with version 0.9.6. Thanks, Christian

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Re: [Haskell-cafe] Cabal install fails due to recent HUnit
by Simon Hengel 23 Jul '12

23 Jul '12

On Mon, Jul 23, 2012 at 12:51:32PM -0500, Stephen Paul Weber wrote: > > Currently you would have to do the upgrade manually, as `cabal-install > > cabal-install` won't work (or alternatively edit your local > > ~/.cabl/packages/hackage.haskell.org/00-index.tar). > > Pending a fix on hackage (hopefully) I've been using `cabal copy && cabal > register` to grab specific packages today. > > How would I go about fixing my 00-index.tar? I tride de-tarring, deleting > the recent versions of HUnit, and re-tarring, and when I copy that file in > cabal starts telling me no packages exist...? E.g. with vim, you can edit files in the tar file (in-place). That is what I did, and it worked fine. Cheers, Simon

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 23 Jul '12

23 Jul '12

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 23 Jul '12

23 Jul '12

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 23 Jul '12

23 Jul '12

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Need help with learning Parsec
by C K Kashyap 23 Jul '12

23 Jul '12

Dear gentle Haskellers, I was trying to whet my Haskell by trying out Parsec today to try and parse out XML. Here's the code I cam up with - I wanted some help with the "gettext" parser that I've written. I had to do a dummy "char ' ') in there just to satisfy the "many" used in the xml parser. I'd appreciate it very much if someone could give me some feedback. data XML = Node String [XML] | Body String deriving Show gettext = do x <- many (letter <|> digit ) if (length x) > 0 then return (Body x) else (char ' ' >> (return $ Body "")) xml :: Parser XML xml = do { name <- openTag ; innerXML <- many innerXML ; endTag name ; return (Node name innerXML) } innerXML = do x <- (try xml <|> gettext) return x openTag :: Parser String openTag = do char '<' content <- many (noneOf ">") char '>' return content endTag :: String -> Parser String endTag str = do char '<' char '/' string str char '>' return str h1 = parse xml "" "<a>A</a>" h2 = parse xml "" "<a>A</a>" h3 = parse xml "" "<a><c></c></a>" h4 = parse xml "" "<a><c></c></a>" Regards, Kashyap

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