August 2012 - Haskell-Cafe

PatternSignatures (rant)
by Johannes Waldmann 06 Apr '18

06 Apr '18

Dear all, (this is a rant - please ignore) why has "PatternSignatures" been renamed to "ScopedTypeVariables" (in ghc-6.10)? show me the alleged "type variable" in this code: do x :: Int <- [ 1 .. 10 ] ; return $ x^2 and why on earth do I need to use a language extension at all to get to the most basic thing of declarative programming: to declare the type of an identifier? This seriously hurts (me, at least) when teaching Haskell. Thanks - J.W.

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Commutative monads vs Applicative functors
by Ronald Guida 09 Jul '16

09 Jul '16

I have a few questions about commutative monads and applicative functors. >From what I have read about applicative functors, they are weaker than monads because with a monad, I can use the results of a computation to select between alternative future computations and their side effects, whereas with an applicative functor, I can only select between the results of computations, while the structure of those computations and their side effects are fixed in advance. But then there are commutative monads. I'm not exactly sure what a commutative monad is, but my understanding is that in a commutative monad the order of side effects does not matter. This leads me to wonder, are commutative monads still stronger than applicative functors, or are they equivalent? And by the way, what exactly is a commutative monad?

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ANNOUNCE: ioctl 0.0.1
by Maciej Piechotka 13 Sep '15

13 Sep '15

A package for type-safe I/O control. Currently only ioctl is supported. Currently simply a extract from my tuntap fork TODO: - Return the integer as well as structure (will break the API) - Port for Windows Network.Socket.IOCtl (as soon as I manage to setup some sane environment on this platform) - Wrapping around DeviceIoControl Example (in hsc): data NotRead = NotRead instance NotRead Int where ioctlReq _ = #const FIONREAD notRead s = ioctlsocket' s NotRead Regards

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Combining databases from packages installed with cabal install
by Marco Túlio Pimenta Gontijo 20 Jan '14

20 Jan '14

Hi. I'm configuring haddock via cabal install (see [0]) to build the hoogle database. The database is being installed in ~/.cabal/share/doc/$package-$version/html/$package.txt, but is not being combined with the default database. That is, if right after the installation I try to search with the hoogle command for some a function, it will not work. I wrote the following script, which I called cabal-install: #!/bin/sh set -e set -x cabal \ install \ --enable-documentation \ --enable-library-profiling \ --haddock-hyperlink-source \ --haddock-hoogle \ --haddock-html \ "$@" cd ~/.cabal/share/hoogle-4.2.13/databases/ for file in ~/.cabal/share/doc/*/html/*.txt do hoo=`echo $file | sed 's/.txt$/.hoo/;s#.*/##'` if [ ! -f $hoo ] then hoogle convert $file $hoo || true hoogle combine default.hoo $hoo -o /tmp/cabal-install-$$.hoo mv /tmp/cabal-install-$$.hoo default.hoo fi done Basically, it searches for .txt hoogle databases installed that were not combined yet with the default database, and combines them. I think it would be good if this was the default behaviour of cabal install when called with --haddock-hoogle. Is this a bug? Greetings. 0: http://hackage.haskell.org/trac/hackage/ticket/517 -- marcot http://marcot.eti.br/

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An APL library for Haskell
by Simon Peyton-Jones 16 Sep '13

16 Sep '13

Friends Many of you will know the array language APL<http://en.wikipedia.org/wiki/APL_%28programming_language%29>. It focuses on arrays and in particular has a rich, carefully-thought-out array algebra. An obvious idea is: what would a Haskell library that embodies APL's array algebra look like? In conversation with John Scholes and some of his colleagues in the APL community a group of us developed some ideas for a possible API, which you can find on the Haskell wiki here: http://www.haskell.org/haskellwiki/APL However, we have all gone our separate ways, and I think it's entirely possible that that the idea will go no further. So this message is to ask: * Is anyone interested in an APL-style array library in Haskell? * If so, would you like to lead the process of developing the API? I think there are quite a few people who would be willing to contribute, including some core gurus from the APL community: John Scholes, Arthur Whitney, and Roger Hui. Simon

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Re: [Haskell-cafe] [Haskell] Spam on the Haskell wiki
by Henk-Jan van Tuyl 03 Aug '13

03 Aug '13

On Mon, 16 Jul 2012 00:03:49 +0200, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> wrote: > > I am willing to do administrator tasks. > >> 4. ReCAPTCHA enabled for 'edits adding new, unrecognized external >> links' - which is all of the spam. > > This is already enabled. The HaskellWiki is still flooded with spam; we should take some measure to reduce the stream severely. Most spam seems to be created (semi-)automated; the pages do not contain links, the usernames end with two digits, most of the time. Some cures I have thought up: - Verify new wiki accounts, before granting them rights, based on e-mails in the Haskell mailing lists (or subscription of a Haskell mailing list) - Let new users only change pages, not create new pages - Block creation of usernames o ending with two or more digits o with more than one x or q o starting with "buy" o longer than 20 characters o with more than 4 consonants in a row - Block creation of pages with words in a certain list (Coach, Vuitton, Chanel, handbags, purses, outlet, luggage, "Nike Air Jordan") Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Proposal: TypeDirectedNameResolution
by Johannes Waldmann 03 Mar '13

03 Mar '13

While browsing Haskell-Prime I found this: http://hackage.haskell.org/trac/haskell-prime/wiki/TypeDirectedNameResoluti… This is not some April Fool's day hoax? Because, it might actually turn Haskell into a somewhat usable (and marketable) language ... well, you know what I mean. Is there 'ghc -XTypeDirectedNameResolution' yet? - J.W.

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XCode Dependency for HP on Mac
by Tom Murphy 21 Jan '13

21 Jan '13

This may sound ignorant because, well, it is ignorant: I know very little about the underlying mechanics here. Installing the Haskell Platform currently requires XCode developer tools. To get XCode on my 10.6 machine, I... [*** begin ranty details (skippable) ... was told I could get a free version by registering as an Apple Developer. So I lie on the forms (phone number and address, for example, are _required_ fields!), and lie on the _required_ 2-3 page survey. I put in a code that they sent to my email (couldn't lie on that!), and log in. The page tells me, in the exact box that told me if I registered I could get XCode for free, that I... *** end ranty details] ...have to either pay to upgrade to their newer OS (10.7: Lion), or pay $99/year for a Mac OS Developer Membership. Is there a way to install HP without XCode? Could there be in the future? I'm tired of dealing with Apple's constant upgrade requirements, registration requirements, etc., and it seems like a small function that XCode actually performs in the Haskell development "toolchain." Again, I'm ignorant of the details and I'm sorry if this is ranty, but I'd love to hear your reactions. Thanks! Tom

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arr considered harmful
by Ryan Ingram 21 Dec '12

21 Dec '12

I know it's a bit of an 'intentionally provocative' title, but with the recent discussions on Arrows I thought it timely to bring this up. Most of the conversion from arrow syntax into arrows uses 'arr' to move components around. However, arr is totally opaque to the arrow itself, and prevents describing some very useful objects as arrows. For example, I would love to be able to use the arrow syntax to define objects of this type: data Circuit a b where Const :: Bool -> Circuit () Bool Wire :: Circuit a a Delay :: Circuit a a And :: Circuit (Bool,Bool) Bool Or :: Circuit (Bool,Bool) Bool Not :: Circuit Bool Bool Then :: Circuit a b -> Circuit b c -> Circuit a c Pair :: Circuit a c -> Circuit b d -> Circuit (a,b) (c,d) First :: Circuit a b -> Circuit (a,c) (b,c) Swap :: Circuit (a,b) (b,a) AssocL :: Circuit ((a,b),c) (a,(b,c)) AssocR :: Circuit (a,(b,c)) ((a,b),c) Loop :: Circuit (a,b) (a,c) -> Circuit b c etc. Then we can have code that examines this concrete data representation, converts it to VHDL, optimizes it, etc. However, due to the presence of the opaque 'arr', there's no way to make this type an arrow without adding an 'escape hatch' Arr :: (a -> b) -> Circuit a b which breaks the abstraction: circuit is supposed to represent an actual boolean circuit; (Arr not) is not a valid circuit because we've lost the information about the existence of a 'Not' gate. The arrow syntax translation uses arr to do plumbing of variables. I think a promising project would be to figure out exactly what plumbing is needed, and add those functions to a sort of 'PrimitiveArrow' class. All of these plumbing functions are trivially implemented in terms of 'arr', when it exists, but if it doesn't, it should be possible to use the arrow syntax regardless. -- ryan

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Possible bug in Criterion or Statistics package
by Till Berger 02 Oct '12

02 Oct '12

Dear all, I may have stumbled upon a bug in the Criterion package. When running the attached Haskell program (Benchmark.hs, a simple test case) on multiple cores (with +RTS -N, +RTS -N2, +RTS -N3 etc.) it sooner or later crashes with the following exception: Benchmark: thread blocked indefinitely in an MVar operation With profiling support enabled and run with the xc flag I get the following output before the crash: *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main, called from Main.CAF --> evaluated by: Main.main, called from Main.CAF *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main, called from Main.CAF I have tested this with GHC versions 7.0.4 and 7.4.2 and Criterion 0.6.0.1. So I am not sure if this is a bug in Criterion itself, the Statistics package or any dependency or if I am doing something obviously wrong. I would be grateful if someone could look into this as it is holding me back from using Criterion for benchmarking my code. Regards, Till Berger

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