March 2015 - Haskell-Cafe

Join a transformer
by Michael Snoyman 11 Apr '18

11 Apr '18

I'm wondering if this pattern exists and has a name. We have the concept of joining a Monad: join :: Monad m => m (m a) -> ma How about joining a monad transformer? joinT :: (Monad m, MonadTrans t) => t (t m) a -> t m a I believe implementing this in terms of MonadTransControl[1] might be possible, but I was wondering if there's an already existing idiom for this. Michael [1] http://haddocks.fpcomplete.com/fp/7.4.2/20130301-40/monad-control/Control-M…

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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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possible bug in latest hackage Elf (Elf-0.27)
by Baojun Wang 18 Feb '17

18 Feb '17

Hi list, with below test program: -- /* test1.c: build with -g -O0 */ include <stdio.h> static const char appmsg[] = "hello, world"; int main(int argc, char* argv[]) { fputs(appmsg, stdout); return 0; } --- elf-test1.hs module Main where import qualified Data.ByteString as B import qualified Data.ByteString.Char8 as C import Control.Monad import Data.Elf testelf = "/tmp/test1" testelfsym = C.pack "appmsg" lookupSymbol1 _ [] = Nothing lookupSymbol1 sym (t:ts) = case (snd (steName t)) of Nothing -> lookupSymbol1 sym ts Just sname -> if sname == sym then Just t else lookupSymbol1 sym ts lookupSymbol _ [] = Nothing lookupSymbol sym (t:ts) = case (lookupSymbol1 sym t) of Nothing -> lookupSymbol sym ts t1 -> t1 test1 elf symtab symbol = mapM_ (print) (elfSections elf) test2 elf symtab symbol = lookupSymbol symbol symtab test3 elf symtab symbol = lookupSymbol symbol symtab >>= \et -> findSymbolDefinition et mainloop elf symtab symbol = -- (test1 elf symtab symbol) >> print (test2 elf symtab symbol) >> print (test3 elf symtab symbol) >> return () main = do contents <- B.readFile testelf let elf = parseElf contents symtab = parseSymbolTables elf mainloop elf symtab testelfsym --- the latest Data.Elf doesn't geive correct output as expected: output will be: Just (EST {steName = (9,Just "appmsg"), steEnclosingSection = Just (ElfSection {elfSectionName = ".fini", elfSectionType = SHT_PROGBITS, elfSectionFlags = [SHF_EXECINSTR,SHF_ALLOC], elfSectionAddr = 4195908, elfSectionSize = 9, elfSectionLink = 0, elfSectionInfo = 0, elfSectionAddrAlign = 4, elfSectionEntSize = 0, elfSectionData = "H\131\236\bH\131\196\b\195"}), steType = STTObject, steBind = STBLocal, steOther = 0, steIndex = SHNIndex 14, steValue = 4195924, steSize = 13}) >From above, you can see the steEnclosingSection is wrong and offset by 1. The *correct* output should be: Just (EST {steName = (9,Just "appmsg"), steEnclosingSection = Just (ElfSection {elfSectionName = ".rodata", elfSectionType = SHT_PROGBITS, elfSectionFlags = [SHF_ALLOC], elfSectionAddr = 4195920, elfSectionSize = 17, elfSectionLink = 0, elfSectionInfo = 0, elfSectionAddrAlign = 4, elfSectionEntSize = 0, elfSectionData = "\SOH\NUL\STX\NULhello, world\NUL"}), steType = STTObject, steBind = STBLocal, steOther = 0, steIndex = SHNIndex 14, steValue = 4195924, steSize = 13}) Just "hello, world\NUL" After check Elf.hs, I found there could be two issues: 1) in sectionByIndex, (SHNIndex) should start from 0, not 1; this cause the steEnclosingSection from my exmaple offset by 1; 2) in findSymbolDefinition, start should substract the sectionAddr (base address). please see below diff for details: -- --- a/Elf.hs 2012-12-04 19:27:51.000000000 -0800 +++ b/Elf.hs 2014-05-14 22:57:01.014498515 -0700 @@ -1,5 +1,5 @@ -- | Data.Elf is a module for parsing a ByteString of an ELF file into an Elf record. -module Data.Elf ( parseElf +module Elf ( parseElf , parseSymbolTables , findSymbolDefinition , Elf(..) @@ -666,12 +666,12 @@ getSymbolTableEntries e s = -- If the size is zero, or the offset larger than the 'elfSectionData', -- then 'Nothing' is returned. findSymbolDefinition :: ElfSymbolTableEntry -> Maybe B.ByteString -findSymbolDefinition e = - let enclosingData = fmap elfSectionData (steEnclosingSection e) - start = fromIntegral (steValue e) +findSymbolDefinition e = steEnclosingSection e >>= \enclosingSection -> + let enclosingData = elfSectionData enclosingSection + start = ( (fromIntegral (steValue e)) - (fromIntegral (elfSectionAddr enclosingSection) ) ) len = fromIntegral (steSize e) - def = fmap (B.take len . B.drop start) enclosingData - in if def == Just B.empty then Nothing else def + def = (B.take len . B.drop start) enclosingData + in if def == B.empty then Nothing else Just def runGetMany :: Get a -> L.ByteString -> [a] runGetMany g bs @@ -712,7 +712,7 @@ getSymbolTableEntry e strtlb = return $ EST (nameIdx,name) sec typ bind other sTlbIdx symVal size sectionByIndex :: Elf -> ElfSectionIndex -> Maybe ElfSection -sectionByIndex e (SHNIndex i) = lookup i . zip [1..] $ (elfSections e) +sectionByIndex e (SHNIndex i) = lookup i . zip [0..] $ (elfSections e) sectionByIndex _ _ = Nothing infoToTypeAndBind :: Word8 -> (ElfSymbolType,ElfSymbolBinding) -- Would you please check above change and update Data.Elf hackage? I cannot find the author from http://hackage.haskell.org/package/elf-0.27 Thanks baojun

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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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Haskell-related PhD Studentship at Kent
by Meng Wang 22 Mar '16

22 Mar '16

Dear Haskellers, I am seeking a PhD student on a Haskell-related project: http://www.cs.kent.ac.uk/research/studyingforaphd/phd-wang.html This would be a good opportunity for someone who is interested in applying Haskell ideas to real problems. The funding covers maintenance, EU student fees and research related expenses. But non-EU students are welcome to apply too. For Sep 2015 starting, the application deadline is 17 April 2015. If you are interested, please contact me at m.w.wang(a)kent.ac.uk<mailto:m.w.wang@kent.ac.uk>. Best wishes, Meng — Dr Meng Wang School of Computing University of Kent http://www.cs.kent.ac.uk/people/staff/mw516/

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repa parallelization results
by Anatoly Yakovenko 14 Jan '16

14 Jan '16

https://gist.github.com/aeyakovenko/bf558697a0b3f377f9e8 so i am seeing basically results with N4 that are as good as using sequential computation on my macbook for the matrix multiply algorithm. any idea why? Thanks, Anatoly

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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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Where is Haskell Weekly News syndicated?
by Jeremy 06 Aug '15

06 Aug '15

Haskell Weekly News used to be syndicated on http://contemplatecode.blogspot.com/ and http://sequence.complete.org/hwn. sequence only has HWN up to March 2010, while contemplatecode goes up to the beginning of February. Is there any way that I can subscribe to HWN (preferably as RSS/Atom), and not other stuff that doesn't interest me? -- View this message in context: http://haskell.1045720.n5.nabble.com/Where-is-Haskell-Weekly-News-syndicate… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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References on haskellwiki
by Alexis Praga 07 Jul '15

07 Jul '15

Hi, I've begun to do some formatting on old Monad Readers editions on Haskell Wiki. Unfortunately, there is no support for proper references support. Can the admin add the "Cite" extension ? It would help a lot. Thanks -- Alexis Praga, PhD Student (CERFACS) GPG key : AD4A AF6D BB5C 042F 9422 1223 06E1 C1BF E287 65D0

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Haskell Weekly News: Issue 322
by Semen Trygubenko / Семен Тригубен ко 21 May '15

21 May '15

New Releases 99 Haskell: A web service by Bram Hoskin Solve live Haskell coding problems based on H-99 in the browser to strengthen your understanding of the language. http://www.99haskell.org/ https://github.com/bramgg/99haskell/ Magic Cookies A commercial Android game is released that is written in Haskell using SDL2 for multimedia and the Arrowized Functional Reactive Programming DSL Yampa. The authors had to "escape their functional comfort zones and come up with smarter abstractions that mutable reality and performance demand". The game consists of 2K lines of code, of which 1K is game specific and 400 are Yampa code. The most complex parts were certain Yampa constructs (arrow-based, with lots of tupling/untupling). http://keera.co.uk/blog/2015/03/19/magic-cookies-released-google-play/ https://play.google.com/store/apps/details?id=uk.co.keera.games.magiccookies https://github.com/keera-studios/keera-hails https://github.com/ivanperez-keera/Yampa Discussion Finding a GHC bug by Neil Mitchell A write up of the hunt for bug #10176 http://neilmitchell.blogspot.it/2015/03/finding-ghc-bug.html https://ghc.haskell.org/trac/ghc/ticket/10176 What are your most persuasive examples of using Quickcheck? Quickcheck helped many people in a number of areas — compiler optimisations, date/time validation, regular expressions, encoding/decoding, topological sort, fuzzing HTTP APIs and even exhibiting classical voting paradox! Apart from the obvious benefit of helping to find big juicy bugs as well as valid but potentially harmless tiny bugs in edge cases that no-one cares about, Quickcheck can help attain enlightenment in the sense that spelling out Quickcheck properties in itself can be rewarding because this can reveal assumptions that have been made without realizing it. Roman Cheplyaka reminds us about SmallCheck and that it should be used instead of Quickcheck when one has a good idea about what depth is needed and when exhaustive search at that depth is affordable. https://www.reddit.com/r/haskell/comments/308ps6/what_are_your_most_persuas… http://stackoverflow.com/questions/20092191/how-much-should-one-control-the… What is the difference between free monads and free monoids? A comment by Edward Kmett. https://www.reddit.com/r/haskell/comments/2znhjk/what_is_the_difference_bet… Type-Checked Pseudo-Code Tom Ashworth argues that Haskell is an excellent tool for fleshing out ideas and prototyping solutions, and that it makes one generalize and shake out many conceptual bugs before concepts become code. https://phuu.net/2015/03/24/type-checked-pseudo-code.html Where does GHC spend most of it's time during compilation? Optimisation and codegen, it seems. Also, GHC compilation times went up substantially from version 7.6 to 7.8. https://www.reddit.com/r/haskell/comments/309430/where_does_ghc_spend_most_… Quotes of the Week "Haskell doesn't feel like code. It feels like a language for thinking in: it's expressive, terse and simple, especially as type-checked pseudo-code." (Tom Ashworth) https://phuu.net/2015/03/24/type-checked-pseudo-code.html "Well that's not in the spec. If you want to change the requirements you have to renegotiate the contract =P" (sccrstud92) https://www.reddit.com/r/haskell/comments/2zxekg/while_learning_haskell_i_m… "Unit tests are double entry bookkeeping" (EvanDaniel) https://www.reddit.com/r/haskell/comments/308ps6/what_are_your_most_persuas… augustss> "Well, type checking is worst case exponential…" barsoap> "Yep. That's also the reason why I don't really get the insistence of dependently-typed languages to have to be total at the type level: I don't care whether type-checking takes ten or infinitely many years, both figures are too large." https://www.reddit.com/r/haskell/comments/309430/where_does_ghc_spend_most_… "Our prophet Djistra said that a testing shows the presence, not the absence of bugs … <snip> … Today Djistra would have been stoned under a myriad of inutile unit tests that people perform for a sense of false security, in a sort of superstitious sacrificial ritual, a waste of time to convince himself and others that his software is right." (agocorona) https://www.reddit.com/r/haskell/comments/308ps6/what_are_your_most_persuas… "You might also consider these sorts of infelicities as teachable moments" (ericpashman) https://www.reddit.com/r/haskell/comments/2zxekg/while_learning_haskell_i_m… "Nice gradual progression there. Big O notion, smoothly divided into 3 easy parts, then bam, Coyoneda. =) I approve. [Yes, I realize it is a bag of topics, not a course outline, but it still grabbed me.]" (edwardkmett) https://www.reddit.com/r/haskell/comments/2zqyes/we_released_a_video_librar…

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