- Haskell-Cafe - Haskell.org

haskell-ide repository name
by Alan & Kim Zimmerman 28 Oct '15

28 Oct '15

At the moment the repository/project is called haskell-ide This leads to the impression that it is an IDE. It is actually a backend/server to provide services for an IDE We are considering changing the name. The options are haskides / ghc-ide-daemon / ghc-ide-service / ghc-ide-engine Please cast your vote at http://strawpoll.me/5842105 Thanks Alan

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asked for a mentor for some math exercises
by Roelof Wobben 28 Oct '15

28 Oct '15

Hello, I try to learn Haskell by the Craft of Functional programming book. But on Chapter 4 Designing there are a lot of very math questions which I do not understand. Is there someone who wants to be a volunteerbasis a mentor for me for this sort of problems. Regards, Roelof

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Cabal depend on local package with foreign dependencies
by Richard Lewis 28 Oct '15

28 Oct '15

Hi there, I have two libraries that I'm working on, packageA and packageB. packageA was written some time ago and depends on a foreign C library, libfoo, which I also wrote and which is installed in a non-standard location. packageB depends on packageA and is new and under active development. I have the following things, then: - libfoo is installed in ~/.local/lib, with includes in ~/.local/include/foo - packageA is in ~/src/packageA - packageB is in ~/src/packageB packageA has a .cabal file which includes: library build-depends: base, ... exposed-modules: A include-dirs: /home/richard/.local/include extra-lib-dirs: /home/richard/.local/lib executable testA ... extra-lib-dirs: /home/richard/.local/lib extra-libraries: foo I've created a cabal sandbox in packageA and I can successfully build it. Now, packageB makes use of packageA. It has a .cabal file which looks like this: library build-depends: base, packageA, ... exposed-modules: B executable testB ... extra-lib-dirs: /home/richard/.local/lib extra-libraries: foo Again, I've created a cabal sandbox in packageB. In order to be able to use packageA (which is not installed anywhere) I believe I have to do: $ cabal add-source ~/src/packageA Next, when I try and configure, it says: cabal: At least the following dependencies are missing: packageA -any That's fine, I assume, because I can now just `cabal install --only-dependencies` to get packageA built into packageB's sandbox. But when I try and do this, I get: $ cabal install --only-dependencies Resolving dependencies... Configuring packageA-0.1... Building packageA-0.1... Preprocessing library packageA-0.1... In-place registering packageA-0.1... Preprocessing executable 'testA' for packageA-0.1... Foo.hsc:10:33: fatal error: foo/foo.h: No such file or directory compilation terminated. So a target of packageA which compiles fine when doing cabal build directly from packageA now fails to compile when I attempt to build it from packageB by installing packageB's dependencies. I'm not sure how to proceed. It feels like maybe I have to tell cabal--when running it from packageB--where the include files are for packageA, because packageA needs to get compiled (which involves linking against libfoo and using libfoo's includes). I know that packageA *does* compile (because I've done that already), but it won't compile inside packageB's sandbox. Any suggestions? Richard

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Let, Equations, and Monad
by PATRICK BROWNE 27 Oct '15

27 Oct '15

{- From Learn Haskell Fast and Hard : 4.3.1. Maybe is a monad http://yannesposito.com/Scratch/en/blog/Haskell-the-Hard-Way/#maybe-monad Concerning the code below I have the following questions: 1) Are eligibleLet and eligibleEquational operationally equivalent (i.e. perform the same operations) and/or semantically equivalent(i.e. Church-Rosser)? 2) Apart from the return type of Maybe instead of Bool how does eligibleMonad differ from eligibleLet? Regards, Pat -} deposit value account = account + value withdraw value account = account - value -- You are eligible for a bonus only if your sequence of transactions stays out of the red. eligibleLet :: (Num a,Ord a) => a -> Bool eligibleLet account = let account1 = deposit 100 account in if (account1 < 0) then False else let account2 = withdraw 200 account1 in if (account2 < 0) then False else let account3 = deposit 100 account2 in if (account3 < 0) then False else let account4 = withdraw 300 account3 in if (account4 < 0) then False else let account5 = deposit 1000 account4 in if (account5 < 0) then False else True eligibleEquational :: (Num a,Ord a) => a -> Bool eligibleEquational account = let account1 = deposit 100 account in if (account1 < 0) then False else let account2 = withdraw 200 account1 in if (account2 < 0) then False else let account3 = deposit 100 account2 in if (account3 < 0) then False else let account4 = withdraw 300 account3 in if (account4 < 0) then False else let account5 = deposit 1000 account4 in if (account5 < 0) then False else True -- Monadic version depositM :: (Num a) => a -> a -> Maybe a depositM value account = Just (account + value) withdrawM :: (Num a,Ord a) => a -> a -> Maybe a withdrawM value account = if (account < value) then Nothing else Just (account - value) eligibleMonad :: (Num a, Ord a) => a -> Maybe Bool eligibleMonad account = do account1 <- depositM 100 account account2 <- withdrawM 200 account1 account3 <- depositM 100 account2 account4 <- withdrawM 300 account3 account5 <- depositM 1000 account4 Just True main = do print $ eligibleLet 300 -- True print $ eligibleLet 299 -- Falsen print $ eligibleEquational 300 -- True print $ eligibleEquational 299 -- False print $ eligibleMonad 300 -- Just True print $ eligibleMonad 299 -- Nothing -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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SQL select for HDBC.ODBC MSSQL driver
by Anakreontas 27 Oct '15

27 Oct '15

Hello. I obtain a connection and try to execute a query on MSSQL server with HDBC c <- connectODBC "Driver={SQL Server};Server=.;Database=Pure_Local;Trusted_Connection=yes;" stm <- quickQuery c "select count(*) as cnt from X__FD" [] HDBC obtains the connection but the SQL query raises an exception with the following message: SqlError {seState = "[\"25000\"]", seNativeError = -1, seErrorMsg = "disconnect: [\"0: [Microsoft][ODBC SQL Server Driver]Invalid transaction state\"]"} No matter which statement I tried, I always get the "Invalid transaction state" message.

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Haskell Report Infelicity?
by Dominic Steinitz 27 Oct '15

27 Oct '15

I have probably misunderstood but is the following expected? floor, round, truncate and ceiling give similar results (all methods of the RealFrac class). properFraction :: (Integral b, RealFrac a) => a -> (b, a) *Girsanov> properFraction (-1 / 0) (-17976931348623159077293051907890247336179769789423065727343008115773267 5805500963132708477322407536021120113879871393357658789768814416622492847 4306394741243777678934248654852763022196012460941194530829520850057688381 5068234246288147391311054082723716335051068458629823994724593847971630483 5356329624224137216,0.0) *Girsanov> properFraction (1 / 0) (179769313486231590772930519078902473361797697894230657273430081157732675 8055009631327084773224075360211201138798713933576587897688144166224928474 3063947412437776789342486548527630221960124609411945308295208500576883815 0682342462881473913110540827237163350510684586298239947245938479716304835 356329624224137216,0.0) *Girsanov> properFraction (0 / 0) (-26965397022934738615939577861835371004269654684134598591014512173659901 3708251444699062715983611304031680170819807090036488184653221624933739271 1459592111865666518401372982279144533294018691411791796244281275086532572 2602351369432221086966581124085574502576602687944735992086890771957445725 3034494436336205824,0.0) The Haskell Report "6.4.6 Coercions and Component Extraction" is silent on what to expect with NaN, Infinity and -Infinity so I think it should be amended. Alternatively, one could expect implementations to throw a runtime error? In ghc, the problem is here https://phabricator.haskell.org/D160a /* This is expected to replace uses of __decodeDouble_2Int() in the long run */ StgInt __decodeDouble_Int64 (StgInt64 *const mantissa, const StgDouble dbl) { if (dbl) { int exp = 0; *mantissa = (StgInt64)scalbn(frexp(dbl, &exp), DBL_MANT_DIG); return exp-DBL_MANT_DIG; } else { *mantissa = 0; return 0; } } I believe frexp [1] and scalbn [2] both pass through NaN, Infinity and -Infinity and I think type coercion in C is undefined for these values. Furthermore exp is undefined for these values [1]. 1. http://en.cppreference.com/w/cpp/numeric/math/frexp 2. http://en.cppreference.com/w/cpp/numeric/math/scalbn Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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Melbourne Haskell Meetup, 2015-10-29 @ 18:30
by Lyndon Maydwell 27 Oct '15

27 Oct '15

Hi All, This week, the monthly Melbourne Haskell Meetup will take place on Thursday, October 29 at the Silverpond offices on Hardware Lane. For details see here: http://www.meetup.com/Melbourne-Haskell-Users-Group/events/225596759/ There will be a talk on Constraint Programming in Haskell by David Overton, as well as the usual free discussion about all things Haskell. Please come along if this interests you :-) - Lyndon ( Thanks to Christian Neukirchen's Munich Haskell announcement for reminding me to tell the Café about our meetup from time to time. )

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Munich Haskell Meeting, 2015-10-29 @ 19:30
by Christian Neukirchen 27 Oct '15

27 Oct '15

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, October 29 at Cafe Puck at 19h30. For details see here: http://chneukirchen.github.io/haskell-munich.de/dates (http://www.haskell-munich.de/ is currently out of order, but will be restored soon! I hope...) If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-oct-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Hackage feature request: show types of names listed in index page
by Ömer Sinan Ağacan 26 Oct '15

26 Oct '15

I pretty much said all I wanted to say in the title, but just to rephrase, I think it'd be super helpful if Hackage listed types of names listed in index pages. The reason is because most of the time I'm using index pages for searching things, but if I wanted to list all functions that somehow use a type, for example, index pages don't help. With type information in the index pages I can just search for a type and my browser would highlight all functions that somehow use the type.

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Five more days: Call for Contributions: BOB 2016 - Berlin, Feb 19, 2016
by Matthias Fischmann 26 Oct '15

26 Oct '15

The submission deadline for is five days away. We are happy with the submissions so far, but you can always have more. So if you always wanted to look into some library that everybody is fussing about and then tell people what you learned, or if you have this trick or you wrote that piece of code that you are fond of, please take a minute to check out our submission guidelines this week. http://bobkonf.de/2016/cfp (We have speaker grants for under-represented groups.) cheers, m.

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