- Haskell-Cafe - Haskell.org

up-to-date and complete version of http://packdeps.haskellers.com/reverse ?
by Ömer Sinan Ağacan 24 Jan '15

24 Jan '15

Hi all, I was wondering if we have up-to-date and complete version of this http://packdeps.haskellers.com/reverse. Any ideas?

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Happstack: serveDirectory vs serveFile
by Semen Trygubenko / Семен Тригубен ко 24 Jan '15

24 Jan '15

Dear all, I've ran a simple benchmark where two serve functions were side-by-side in an msum and looked like dirs "hehe" $ serveDirectory DisableBrowsing [] "/hehe" dirs "favicon.ico" $ serveFile (asContentType "image/x-icon") "favicon.ico" "hehe" directory also contained file "favicon.ico". This is happstack server version 7.3.9. I've been hammering it with ab and confirmed this puzzling result: it seems that serveFile route is approximately two orders of magnitude slower than serveDirectory route... Will be grateful for any clues as to what I'm doing wrong! Thanks, S. -- Семен Тригубенко http://trygub.com

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Unacceptable argument type in foreign declaration: Ptr ()
by Henk-Jan van Tuyl 23 Jan '15

23 Jan '15

L.S., I am trying to compile a package that hasn't been updated since 2009 and I get a lot of messages like: Unacceptable argument type in foreign declaration: Ptr () When checking declaration: foreign import stdcall safe "dynamic" prim_System_Win32_Com_Automation_TypeLib_setGuid :: Ptr () -> Ptr () -> Ptr GUID -> IO Int32 How can I correct this? Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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cabal keeps relinking
by Semen Trygubenko / Семен Тригубен ко 23 Jan '15

23 Jan '15

Dear Haskell-Cafe, cabal seems to be re-linking every time I run cabal build Wait was not an issue for small projects but for big ones it is starting to get in the way… How could I persuade cabal to re-link only if there are changes in the source tree and only these exes that require it? Many thanks, Semen PS I googled around and found this https://www.haskell.org/pipermail/libraries/2005-August/004306.html which seems to imply relink is (was?) conditional on multiple exes being present in the project. However, for me it was always relinking regardless. -- Семен Тригубенко http://trygub.com

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Munich Haskell Meeting, 2015-01-26 @ 19:30
by Christian Neukirchen 23 Jan '15

23 Jan '15

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, January 26 at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates 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-jan-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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hs-fltk build fails
by Jeremy Henty 23 Jan '15

23 Jan '15

I am building with " cabal sandbox init ; cabal install hs-fltk ". Logs attached. Any advice? Regards, Jeremy Henty

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HDBC installation error
by Arunram A 23 Jan '15

23 Jan '15

Hi, I am trying to install HDBC, using the following command. cabal install hdbc and getting the following error. Preprocessing library HDBC-2.4.0.0... [1 of 8] Compiling Database.HDBC.Locale ( Database\HDBC\Locale.hs, dist\build\Database\HDBC\Locale.o ) [2 of 8] Compiling Database.HDBC.SqlValue ( Database\HDBC\SqlValue.hs, dist\build\Database\HDBC\SqlValue.o ) Database\HDBC\SqlValue.hs:265:29: Ambiguous occurrence `defaultTimeLocale' It could refer to either `Data.Time.defaultTimeLocale', imported from `Data.Time' at Database\HDBC\SqlValue.hs:20:1-16 (and originally defined in `time-1.5:Data.Time.Form at.Locale') or `Database.HDBC.Locale.defaultTimeLocale', imported from `Database.HDBC.Locale' at Database\HDBC\SqlValue.hs:22:30-46 (and originally defined in `System.Locale') Seems to be some kind of import conflict. But, I am not able to identify the problem and resolve the issue. May I get a clarity on what the problem could be and is there a known solution for this? Thanks! Regards, Arunram A.

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Type synonyms considered harmful?
by Julian Ospald 23 Jan '15

23 Jan '15

Hi, I've recently had a discussion about type synonyms and when to use them in haskell. On the one hand, they make reading type signatures easier and tell you what is meant, not just what is inside. On the other hand they also sort of hide what is inside, although you will probably need to know exactly that when using them. This might make reading code for new collaborators more difficult if they have to memorize all type synonyms first. So there are basically a few questions: * What do you think is a good policy for when and how to use them? Or would you say just not use them at all and put equivalent information in parameter documentation? * What are the upsides, downsides, pitfalls and also alternatives? (for completeness, also since https://www.haskell.org/haskellwiki/Type_synonym is a bit sparse) * Can we do something to improve the downsides? Or is there already something? (e.g. editor/IDE that can tell me the underlying type, error messages etc.) -- Regards, Julian Ospald

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[ANNOUNCE] New release of SBV
by Levent Erkok 23 Jan '15

23 Jan '15

I'm happy to announce a new release of SBV (v4.0); a library for seamlessly integrating SMT solvers with Haskell. https://hackage.haskell.org/package/sbv Most of the changes in this release are due to Brian Huffman of Galois; essentially adding capabilities so end-users can define symbolic bit-vector types that are not natively supported by SBV. For instance, a user can define `SWord17` for representing 17-bit words and use them just like other symbolic types natively supported by SBV. This feature allows SBV to take advantage of arbitrary bit-size decision procedures for bit-vector logics as found in modern SMT solvers, which are becoming increasingly more valuable in (semi-)automated verification of software artifacts. (Note that SBV already supports other basic types such as unbounded Integers, IEEE Floats and Doubles, Rationals, Algebraic reals, and traditional machine word sizes such as Word8/Int8; 16; 32; and 64 symbolically.) We plan to also add automatic support for SWordN/SIntN for arbitrary N, once GHC gets proper support for type-level naturals. Thanks to Brian Huffman for his contributions to this release. Bug reports/comments are always welcome. -Levent.

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Overlapping instances with "instance F a => G a"
by Bryan Gardiner 22 Jan '15

22 Jan '15

Hi cafe! Happy New Year. I'm writing a version of the Binary typeclass that encodes values with host endianness, and I have the code at the bottom of the message. HostBinary provides the encoding/decoding interface, and with HostBinaryNum I want to be able to write only e.g. "instance HostBinaryNum Int32" for numeric types. I had the manual HostBinary Word8 and HostBinary Int8 instances before I wrote HostBinaryNum. What I can't see is why I get this error at all: Binary.hs:21:29: Overlapping instances for HostBinary Word8 arising from a use of ‘hget’ Matching instances: instance HostBinary Word8 -- Defined at Binary.hs:16:10 instance HostBinaryNum a => HostBinary a -- Defined at Binary.hs:26:10 In the second argument of ‘fmap’, namely ‘(hget :: Get Word8)’ In the expression: fmap fromIntegral (hget :: Get Word8) In an equation for ‘hget’: hget = fmap fromIntegral (hget :: Get Word8) and also why commenting out either the HostBinary Int8 or the HostBinaryNum a => HostBinary a instances fixes the problem; and yet, the HostBinaryNum Word8 is accepted! Doesn't "HostBinaryNum a => HostBinary a" create a HostBinary instance for all instances of HostBinaryNum only? So why would it cause problems with an Int8 instance, and why isn't the HostBinaryNum Word8 instance needed to trigger a collision with the explicit HostBinary Word8? GHC 7.8.3, if it matters. Thanks for any clarifiation you can provide, Bryan Binary.hs: {-# LANGUAGE FlexibleInstances, UndecidableInstances #-} module Binary where import Data.Binary.Get import Data.Binary.Put import Data.Bits import Data.Int import Data.Word import Foreign.Storable class HostBinary a where hget :: Get a hput :: a -> Put instance HostBinary Word8 where hget = getWord8 hput = putWord8 instance HostBinary Int8 where hget = fmap fromIntegral (hget :: Get Word8) hput = hput . (fromIntegral :: Int8 -> Word8) class (Bits a, Integral a, Storable a) => HostBinaryNum a instance HostBinaryNum a => HostBinary a where hget = getNum hput = putNum --instance HostBinaryNum Word8 getNum :: (Bits a, Integral a, Storable a) => Get a getNum = undefined putNum :: (Bits a, Integral a, Storable a) => a -> Put putNum = undefined

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