July 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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Data constructor ‘Minus’ comes from an un-promotable type ‘Ints’
by Nicholls, Mark 15 Sep '16

15 Sep '16

Hello, I clearly don’t really know what I’m doing…but at least I know it…. Here we defined the Naturals…and then attempt to construct the Integers…. > {-# LANGUAGE DataKinds #-} > {-# LANGUAGE ExplicitForAll #-} > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE GADTs #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE PolyKinds #-} > {-# LANGUAGE StandaloneDeriving #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE UndecidableInstances #-} > {-# LANGUAGE ScopedTypeVariables #-} > import Prelude hiding (head, tail, (++), (+), replicate) > import qualified Prelude as P naturals > data Nat where > Z :: Nat > S :: Nat -> Nat I can now define + and * and prove things about them…1 * x == x etc….nice..but lets put that on one side. Borrow bits and bobs from singletons > data family Sing (a :: k) Borrow bits and bobs from singletons..i.e. the isomorphic values…my proofs in nat now map to SNat…double nice. > type SNat = (Sing :: Nat -> *) Create the integers by following my nose……(the integers are the equivalence class of pairs of naturals….) i.e. we have “positive” or “negative” or “zero”… > data Ints (a :: Nat) (b :: Nat) where > Minus :: SNat a -> Ints 'Z a > Zero :: Ints 'Z 'Z > Plus :: SNat a -> Ints a 'Z > Ok….this works as a set of values….but…. I can’t prove anything about these because the data constructors for my integers aren’t “promotable”…..so I cant do the same trick I did with Nat. “:k Zero” ….. Data constructor ‘Zero’ comes from an un-promotable type ‘Ints’ In a type in a GHCi command: Zero I’ve tried rejigging this in various futile and ignorant manners….but the bottom line is its…”un-promotable”…..look at the docs, and it says something vague about GADT’s…but that isnt the problem directly. Is there a nifty way around this log jam? (I can start proving things about ‘(Nat,Nat)….but this will soon become a bit clumsy….) Or do I just stop here and wrestle with getting agda installed and lose another few months in the agda-café (which appears to be a very nice place somewhere in finland….hmmmm…I think that’s something different). (I’ve already allegedly descended into cabal hell following my nose with an agda install… setup.exe: The program cpphs version >=1.18.6 && <1.19 is required but the version found at C:\Users\nichom\AppData\Roaming\cabal\bin\cpphs.exe is version 1.19). computer science would be good, if it wasn’t for the computers. CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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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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Haskell Tooling Infrastructure
by Alan & Kim Zimmerman 02 Oct '15

02 Oct '15

Now that I am finally turning back to HaRe, the question of infrastructure again comes up. Basically I woudl like to focus in the tool itself, but it can only be useful if it can deal with real world environments. This means properly managing all the different project and compiler environments, and being integrated into IDEs. The current version uses ghc-mod for the lower level interface, and a simple command line interface for the IDE integration. But I have a definite sense that as a community every tool maker is solving the same problems over and over, from ghc-mod to ide-backend to ghci-ng, and onwards. To start a discussion around some kind of architeccture that tool writers can develop agains, ide's can integrat against and ghc/cabal can eventually internalise, I have put some basic points down at https://github.com/RefactoringTools/HaRe/wiki/Requirements-for-IDE-GHC-inte… Regards Alan

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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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Re: [Haskell-cafe] Discussion: The CLOEXEC problem
by Donn Cave 31 Aug '15

31 Aug '15

Quoth David Turner <dave.c.turner(a)gmail.com>, > Could you be a bit more specific? Which bits of pre-existing software > didn't have a FD_CLOEXEC bit and would be broken by this proposal? Well, of course to be precise, the bit's always there, it's just normally not set - that's the normal environment that anything written up to now would expect. And of course, anything that depends on a GHC-opened file to stay open over an exec would be broken. I can't enumerate the software that meets that criterion. > Since Python recently decided to go through this exact transition, their > experience should be instructive. Do you know if there was negative fallout > from PEP 0466? I gave up on Python a long time ago and don't follow what goes on. If recently means less than a decade or so, though, it's not much to go on. If the problem addressed by the O_CLOEXEC proposal is obscure, the problems it may create are even more so - I'll certainly concede that - and it could take a lot of experience before those problems would be well known enough to show up if you went looking for them. > When thinking about FDs from outside the Haskell runtime (whether inherited > or simply opened in an external library), can you give an example of a case > where such a FD causes a problem if inherited and yet cannot be set as > FD_CLOEXEC at source? Sorry, I'm confused here. Files opened within GHC and externally have equal potential to work as intended or to cause problems, it seems to me. If we infer from the proposal that GHC-opened files with CLOEXEC unset may cause a problem, then it follows that other files with CLOEXEC unset also may cause the same problem. The proposal addresses only the former, and not the latter, and only for normal files - while the ordinary solution, as implemented in UNIX popen(3), deals with all - pipes, sockets, etc. Donn

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