September 2016 - 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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Source code location in IO?
by Tom Ellis 30 Jun '17

30 Jun '17

Is it compatible with the semantics of Haskell to have a function sourceLocation :: IO String which when run returns the source file location at which it is used? For example, suppose Main.hs is module Main where main = do putStrLn =<< sourceLocation putStrLn "Hello" putStrLn =<< sourceLocation It would print the following when run Main.hs:4 Hello Main.hs:6 and module Main where main = do let s = sourceLocation putStrLn =<< s putStrLn "Hello" putStrLn =<< s It would print the following when run Main.hs:4 Hello Main.hs:4 If this is not compatible with the semantics of Haskell, why not? I agree that the two programs must have the same denotation, but is there anything that requires them to have the same output when run? Tom

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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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Announcing binary-parsers
by 韩冬(基础平台部) 11 Oct '16

11 Oct '16

Hi all, I am happy to announce binary-parsers. A ByteString parsing library built on binary. I borrowed lots of design/tests/document from attoparsec so that i can build its shape very quickly, thank you bos! And thanks to binary's excellent design, the codebase is very small(<500 loc). From my benchmark, it’s insanely fast, it outperforms attoparsec by 10%~30% in aeson benchmark. it’s also slightly faster than scanner(a non-backtracking parser designed for speed) in http request benchmark. I’d like to ask you to give it a shot if you need super fast ByteString parsing. These parsers are just binary's Get monads, so you can seamlessly combine them with combinators from binary package. You can now write more complex Binary instances using comprehensive combinators, with serialisation packages like blaze-texual. all these goodies come for free! Happy hacking! Github: https://github.com/winterland1989/binary-parsers Hackage: http://hackage.haskell.org/package/binary-parsers Winterland

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Re: [Haskell-cafe] Batteries included (Was: GHC is a monopoly compiler)
by Olaf Klinke 03 Oct '16

03 Oct '16

I did not intend to start a Python-bashing thread. Most people on this list will agree on which language is superior. I did not intend to suggest that Haskell is still mainly a research language, nor that it should remain to be. I'd love to see more problems to be tackled with lenses, FRP or GADTs, just to show that it can be done this way. Maybe one day an idiom will prove superior to its competitors and take its place in whatever is the equivalent of base then. I am happy to have packages coexist that use either of String, Text, ByteString.Lazy or ByteString.Strict. I am a fan of the Unix philosophy: Do one thing, and do it well. There are modules in base that are very good examples of this: Data.Map provides just one data structure with a very easy interface, no newtypes, no typeclasses. The user needs no knowledge of how it is implemented, but the internals can be exposed if necessary. Control.DeepSeq provides one single typeclass to provide one single functionality. In Unix the glue between programs are streams of bytes. I feel that the glue between Haskell libraries should be lambda calculus and algebraic datatypes. So when you write a library that processes HTTP traffic as conduits: Provide a wrapper function whose type does not mention the conduit classes. When you write another efficient text library, provide fromString and toString functions, even if they are inefficient. As to what kind of batteries should be included: * I feel that Haskell already has an excellent set of containers for all sorts of tasks. * It has excellent parsers. * The web stuff seems less uniform, and my complaints about idioms apply. * To compete with other recent teaching languages, a really simple GUI library to use within ghci would be fine. (diagrams and GTK?) Quick Basic included "line" and "circle" commands to play with, even though the rest of the language did not prepare me for more advanced languages. * I know too little about the state of Haskell's number crunching libraries, but it seems that Vector is reasonably efficient nowadays. But what should I use to solve A*x = b? This is a problem so basic that is should be solved by a basic library. I like Peter's suggestion of an official wiki highlighting the state-of-the-art packages for common tasks. As to the versioning and backward compatibility issues of a standard library: In my eyes the whole concept of version numbers is broken. Dependencies should be stated in terms of type signatures. Olaf

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Re: [Haskell-cafe] Batteries included (Was: GHC is a monopoly compiler)
by Olaf Klinke 03 Oct '16

03 Oct '16

Can someone please define what exactly a "batteries included" standard library is? IMHO that Python-Haskell comparison is unfair. Although both claim to be general-purpose languages, the focus in Haskell certainly has been on language research for most of its life. I recently hacked together a web client in python, my first project in that language. Documentation is excellent. Yet I am still horrified I had to use a language that provides so few static guarantees to control megawatt machines. What puts me off Haskell nowadays is the direct result of Haskell's roots in language research: Often when I come across a package that does what I need, it uses the conduit, lens or another idiom, which are like a language in a language to learn. In milder ways Python seems to suffer the same problem. So please, developers: Write more batteries, but make them expose a neat lambda calculus interface if possible that can be combined freely with other batteries. Regards, Olaf

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Fwd: Alternative: you can fool many people some time, and some people many time, but...
by Corentin Dupont 01 Oct '16

01 Oct '16

Hi guys, I'm playing with the mysterious "some" and "many" from Control.Applicative. If I try: many $ Just 1 It just loops, I understand why: http://stackoverflow.com/questions/18108608/what-are-alterna tives-some-and-many-useful-for It seems that some and many are normally used in a context where something is consumed, and can be depleted, so the loop ends. But why doesn't this terminates? take 3 $ many $ Just 1 It's a recursive call, but the construction of the result should be lazy... -- ===================================== Corentin Dupont CREATE-NET OpenIoT Research Area - Senior Researcher Via alla Cascata 56/D - 38123 Povo, Trento - Italy e-mail: corentin.dupont(a)create-net.org Skype: corentin.dupont1 www.create-net.org ====================================== -- ===================================== Corentin Dupont CREATE-NET OpenIoT Research Area - Senior Researcher Via alla Cascata 56/D - 38123 Povo, Trento - Italy e-mail: corentin.dupont(a)create-net.org Skype: corentin.dupont1 www.create-net.org ======================================

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Re: [Haskell-cafe] [Haskell] Branch Streaming (was GHC is a monopoly compiler)
by Tony Day 30 Sep '16

30 Sep '16

I've actually looked down that particular rabbit hole. Here's a link to a monoid for pipes that represents a branch: https://github.com/tonyday567/pipes-extended/blob/master/src/Pipes/Monoid.h… For flavour, the computation involves hoisting and lifting back and forth over several layers, as the various meta-physical wires cross. That was years ago, but I haven't seen anything since. It looks like Pipes is the wrong abstraction point to stream with branching. I wonder what is? On Wed, Sep 28, 2016 at 9:59 PM, Carter Schonwald < carter.schonwald(a)gmail.com> wrote: > Nope :) > > And I think we are still learning how to do stream. Have have streaming > in a line down pat, but streaming with a DAG or general graphs wendontnhave > yet ;) > > > On Wednesday, September 28, 2016, Tom Ellis <tom-lists-haskell-cafe-2013@ > jaguarpaw.co.uk> wrote: > >> On Mon, Sep 26, 2016 at 08:56:55PM -0500, Christopher Allen wrote: >> > Streaming is a good example here. It's not "obvious" how to do many >> > things in Haskell that are "obvious" in other languages. Partly >> > because industry hasn't used languages like Haskell much, partly >> > because the canvas we work with permits more structure. >> >> Is it obvious how to do streaming in other languages? >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. > > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. >

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Question regarding Candidate Package builds
by Guru Devanla 30 Sep '16

30 Sep '16

Is there any information on what kinds of checks happen on candidate packages that are uploaded. There was a cabal error on a published build. So, I have fixed that and have uploaded a candidate package. I was hoping the same build/checks happen on the candidate package, but I have not seen that happen. This link on the HOME page is very old and does not give any definitive information on what is currently working: https://github.com/haskell/hackage-server/issues/41#issuecomment-25005344 Thanks Guru

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