November 2012 - Haskell-Cafe

Is there a tool like ri from ruby?
by Magicloud Magiclouds 26 Nov '12

26 Nov '12

RI is a very easy using tool that search and show the documents of ruby modules/functions, etc. Using RI, I could get help when programming with a few commands. Quick and simple. And with RI backend (libs), I could also simply integrate ri to my IDE (like one hotkey to show function summary). So I am wondering if there is such thing in Haskell world. I know haddock saves the .haddock files in document folder. But I do not know if there is any existing tools to index and view them. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Quasiquotation page on HaskellWiki needs updating
by Erik de Castro Lopo 26 Nov '12

26 Nov '12

Hi all, It seems the Quasiquotation page on HaskellWiki http://www.haskell.org/haskellwiki/Quasiquotation has fallen behind the actually Quasiquotation implementation that is in ghc-7.4.2 and later. Specifically, the QuasiQuoter constructor that the Wiki takes two parameters: data QuasiQuoter = QuasiQuoter { quoteExp :: String -> Q Exp , quotePat :: String -> Q Pat } while the one in ghc-7.4 and later takes four: data QuasiQuoter = QuasiQuoter { quoteExp :: String -> Q Exp , quotePat :: String -> Q Pat , quoteType :: String -> Q Type , quoteDec :: String -> Q [Dec] } I'm just starting out with quasquotation and am not yet qualified to update this page myself. Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Munich Haskell Meeting
by Heinrich Hördegen 25 Nov '12

25 Nov '12

Dear all, next Tuesday, 27. November 2012, will be our Haskell Meeting in Munich at 19h30 at Cafe Puck. Please go to http://www.haskell-munich.de/dates and hit the button, if you intend to join. Have a good start into your week, Heinrich -- -- Funktionale Programmierung Dr. Heinrich Hördegen Gutenbergstr. 26 80638 München FON: +49 (89) 12 59 79 49 FAX: +49 (89) 12 59 79 50 hoerdegen(a)funktional.info www.funktional.info --

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GHC 7.4.1 hangs and memory usage shoots ups to 100% when compiling with -O2
by magesh b 25 Nov '12

25 Nov '12

I'm facing this problem only when I compiled the code using -O2 Flag. I tried the following, * Compiled with -O1 Flag (7.4.1), It compiled fine * Compiled with ghc-7.0.4 with -O2, it's getting compiled successfully but with the Warning, SpecConstr Function `$wa{v s27vx} [lid]' has one call pattern, but the limit is ... [Module that is getting stuck is using heavy quasiquotation] Env: OS: Ubuntu 12.04 [x86_64] GHC: 7.4.1 Is this a bug? Anything else should I need to try?

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need help with understanding expression
by Daryoush Mehrtash 25 Nov '12

25 Nov '12

I am having hard time understanding how removing the outer parenthesis in (max.(+1)) 2 2 to max.(+1) 2 2 changes the meaning of expression. My expectation was that "max.(+1) takes two numbers and returns the max as defined in the type: :t max.(+1) max.(+1) :: (Ord b, Num b) => b -> b -> b With parenthesis it does what I expect it to: Prelude> :t (max.(+1)) 2 2 (max.(+1)) 2 2 :: (Ord b, Num b) => b Prelude> (max.(+1)) 2 2 3 But if I remove the parenthesis I get a beast that I have no idea what its type signature mean any more Prelude> :t (max.(+1)) 2 2 (max.(+1)) 2 2 :: (Ord b, Num b) => b Prelude> :t max.(+1) 2 2 max.(+1) 2 2 :: (Ord b, Num a1, Num (a1 -> a -> b)) => a -> b -> b How did removal of parenthesis changed the meaning? How do you interpret the type: "(Ord b, Num a1, Num (a1 -> a -> b)) => a -> b -> b"? Thanks -- Daryoush Weblog: http://onfp.blogspot.com/

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cabal isn't updating local doc index after local package upgrade
by Richard Cobbe 24 Nov '12

24 Nov '12

Haskell Platform 2012 v2.0.0, MacOS 64-bit. (MacOS 10.8.2.) I just used cabal to upgrade the installation of a local package I'm writing, and I'm still seeing the old version of the documentation in ~/Library/Haskell/doc/index.html. How can I fix this? In more detail: this machine had greek-1.0.1 installed, but I was working on another package that needed greek-1.1.0. So I went into the directory where I keep the source for the greek package, made sure it was up-to-date, and ran cabal clean && cabal configure && cabal build && cabal install as my normal user, not as root. As far as I can tell, the rebuild was successful; cabal's output finished with Installing library in /Users/cobbe/Library/Haskell/ghc-7.4.1/lib/greek-1.1.0/lib Registering greek-1.1.0... Updating documentation index /Users/cobbe/Library/Haskell/doc/index.html However, when I open ~/Library/Haskell/doc/index.html in my browser, I still see the documentation for greek-1.0. The docs for the new version are present, in ~/Library/Haskell/ghc-7.4.1/lib/greek-1.1.0/doc, but they don't appear in the main index. Am I missing a step in the process, or is this a bug in cabal? Is there a workaround? I do have a couple of older versions of the greek package insatlled, because there doesn't seem to be an easy way to remove obsolete packages. Could they be causing problems? If so, what's the best way to delete them? (I don't particularly mind having the older versions hanging around, as long as they're actually harmless.) Thanks much, Richard

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Survey: What are the more common Haskell IDEs in use ?
by Dan 24 Nov '12

24 Nov '12

Because I see there are many preferences on what IDE to use for Haskell I've created a quick survey on this topic. Please click here and select your choices from the lists. http://kwiksurveys.com/s.asp?sid=oqr42h4jc8h0nbc53652 Any comments/suggestions are welcome. (if any is missing, etc) Apologies for the add they show after you select the preferences - it is a free survey tool - i guess they have to live somehow too :)

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class Bytestringable or ToBytestring
by Vincent Hanquez 24 Nov '12

24 Nov '12

Hi cafe, I've been adding lots of types recently that looks more or less like: newtype A = A ByteString data B = B ByteString This is great for extra type safety and letting the compiler do its job, however getting the bytestring back requires boiler plate. At the moment either you give access to the constructor, which is not always wanted, or you use the record syntax to create a function to extract just the bytestring. The latter is fine for 1 or 2 types, but the scheme fall apart when having many of those types and do pollute namespace. I'm basically after something that looks like this: class ToByteString a where toByteString :: a -> ByteString Before anyone suggest the Serialize interface from cereal or the Binary interface from binary which both looks quite similar (from far away): - serialize work in the put monad, and you have to define a get instance: which is something that is not required or possible sometime. - binary works with lazy bytestrings and got the same problem as cereal. - a serialize instance that just do a single putByteString is really slow: 12 ns to 329 ns (26x time slower) on the same exact data on one isolated bench) - neither of those packages are in the platform. If that doesn't exists, could it be a worthy addition to bytestring ? is this a good idea in general ? is there any other way ? Thanks, -- Vincent

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ANN: hledger-web 0.19.3
by Simon Michael 24 Nov '12

24 Nov '12

hledger-web now supports GHC 7.6 properly (no more "Prelude.read: no parse" error). Release notes: * fix "Prelude.read: no parse" errors with GHC >= 7.6 * web & lib refactoring, easier scripting/developing Thanks! -Simon hledger is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. In addition to reporting, it can also help you record new transactions, or convert CSV data from your bank. Add-on packages include hledger-web, a web interface. hledger is inspired by and compatible with John Wiegley's Ledger. For more, see http://hledger.org . Install it: cabal update; cabal install hledger-web For more installation help, see http://hledger.org/MANUAL.html#installing . Or, sponsor a ready-to-run binary for your platform: http://hledger.org/DOWNLOAD.html .

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Re: [Haskell-cafe] Cabal failures...
by Gregory Guthrie 24 Nov '12

24 Nov '12

No; the first sentence says that someone else had reported that testing on Windows was hard to do because of (a perceived) lack of access to Windows by Haskell developers... The implication is that Haskell developers (only/mainly) use *nix. I commented that if true this lack of Windows testing could limit the availability of Haskell to the largest market share of users. ------------------------------------------- > Subject: Re: [Haskell-cafe] Cabal failures... > To: haskell-cafe(a)haskell.org > On 12-11-20 08:48 AM, Gregory Guthrie wrote: > > It was also interesting to note a comment that most developers don't > > have access to a Windows machine for testing. With Windows at >90% of > > the computing market (Linux = 1.6%), this seems like a problem which > > might limit growth of Haskell usage. Just an observation. :-) > > There is a paradox in that sentence. > > The first sentence says, most developers don't have access to Windows machines for > testing. But they have access to Linux machines. Then Windows machines must be a scarcity > compared to Linux machines, no? So scarce, you even have difficulty borrowing or renting. > > Then the next sentence says, the scarcity is the other way round, Linux machines are scarce, > Windows machines are abundant. OK, so why is it so hard to access something abundant, and > so easy to access something scarce? > ------------------------------

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