February 2016 - Haskell-Cafe

Join a transformer
by Michael Snoyman 12 Apr '18

12 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 07 Apr '18

07 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 19 Feb '17

19 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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The 13-line example in Text.Megaparsec.Expr
by Jeffrey Brown 25 Sep '16

25 Sep '16

At the bottom of the Hackage documentation for Text.Megaparsec.Expr [1] is a 13-line demonstration program. It includes no import statements. I added the ones I could deduce, which produced this: import Text.Megaparsec import Text.Megaparsec.Expr import Text.Megaparsec.Lexer (symbol,integer) parens = between (symbol "(") (symbol ")") expr = makeExprParser term table <?> "expression" term = parens expr <|> integer <?> "term" table = [ [ prefix "-" negate , prefix "+" id ] , [ postfix "++" (+1) ] , [ binary "*" (*) , binary "/" div ] , [ binary "+" (+) , binary "-" (-) ] ] binary name f = InfixL (reservedOp name >> return f) prefix name f = Prefix (reservedOp name >> return f) postfix name f = Postfix (reservedOp name >> return f) That still won't compile, because GHC does not know what reservedOp means. Does reservedOp refer to something that no longer exists, or have I just not found it? [1] https://hackage.haskell.org/package/megaparsec-4.4.0/docs/Text-Megaparsec-E… -- Jeffrey Benjamin Brown

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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 10 Jul '16

10 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 Weekly News
by Kim-Ee Yeoh 11 May '16

11 May '16

*Top Picks:* - Oskar Wickström rewrites the Oden-to-Go transpiler from Racket to Haskell <http://oden-lang.org/blog/compiler/2016/01/18/the-haskell-rewrite.html>. Oden is an FP language comprising an ML type system and LISP syntax. Oskar explains that he made the migration because of several advantages that Haskell offered over Racket: exhaustive pattern-match checking, type-guided refactoring, monad transformers, and faster execution times. Apropos, the convo over at lobste.rs <https://lobste.rs/s/8hidko/the_haskell_rewrite_the_motivation_for_moving_aw…> links to this claim by Gabriel Gonzalez: "Haskell is an amazing language for writing your own compiler. If you are writing a compiler in another language you should genuinely consider switching." - Reviewing 2015 work month-by-month, Gracjan Polak tells the story of how he decided to lead the development of Haskell Mode <https://github.com/haskell/haskell-mode/wiki/Haskell-Mode-2015-retrospective>, "a bunch of Emacs major and minor modes put together in a single package." Discussion over at /r/haskell <https://www.reddit.com/r/haskell/comments/425gn4/haskell_mode_2015_retrospe…> . - Jared Tobin presents monadic versions of five recursion-schemas <http://jtobin.ca/monadic-recursion-schemes/>, namely: cata-, ana-, para-, apo-, and hylomorphisms. *Quotes of the Week:* - Tim Kellogg: <https://lobste.rs/s/erxkun/redundancy_vs_dependencies_which_is_worse/commen…> I’ve known a few old programmers nearing retirement that have a long list of very impressive accomplishments. The older and more accomplished they get, the more they prefer redundancy over dependency. The oldest and most accomplished will write their own load balancers, TCP stacks, loggers, everything if need be. Are they on to something? - From HN: <https://news.ycombinator.com/item?id=10972051#up_10979314> If you have the time, I'd advise you to learn Haskell, in order to stretch your mind and become an excellent OCaml developer, the way learning Latin makes you a better French or Italian writer. - HN markov chain parody site headline: <http://news.ycombniator.com/comments/comments_419.html> $690 for an hour minimum wage for state management in haskell *Recorded Talk of the Week:* - On Dec 17 last year <http://www.meetup.com/haskellhackersathackerdojo/events/226870399/>, Andrew Gibiansky demoed IHaskell, a Mathematica-like "capable graphical alternative" to the ghci REPL at the NorCal Hacker Dojo. Thanks go to Joe "begriffs" Nelson who recorded the talk and summarized it into bullet points <http://begriffs.com/posts/2016-01-20-ihaskell-notebook.html>. Joe's page was well-received at both Hacker News <https://news.ycombinator.com/item?id=10942969> and Haskell Reddit <https://www.reddit.com/r/haskell/comments/41xnhw/demo_of_ihaskell_notebook_…> . p.s. There will be no News next week. HWN will resume the week after. -- Kim-Ee

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Haskell-related PhD Studentship at Kent
by Meng Wang 23 Mar '16

23 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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Foldable/Traversable and Applicative/Monoid?
by David Banas 15 Mar '16

15 Mar '16

Hi all, I don't understand why Foldable is a necessary super-class of Traversable, and I suspect that the Applicative/Monoid duality, which I've just begun discovering in the literature, has something to do with why that is so. Can anyone give me a hint, without giving me the answer? Thanks! -db

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Local mirrors of hackage
by Victor Miller 04 Mar '16

04 Mar '16

I have a computer that I work on which isn't directly connected to the internet, that I want to use Haskell on. In order to do that I want a local version of hackage. I was previously able to do this using mirror_hackage.py, but since hackage 2 came around this no longer works. Doing this would involve running something on a computer that *is* attached to the internet, and then transferring a bunch of files over to the other computer. Can someone point me to a recipe for doing this? Victor

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