December 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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Why is Megaparsec treating these two operators differently?
by Jeffrey Brown 14 May '17

14 May '17

I used Text.Megaparsec.Expr to write a minimal (56 lines) 2-operator recursive parser. It's on github[1]. It outputs a binary tree of the following form: data AExpr = Var String | Pair AExpr AExpr deriving (Show) The operator table supplied to makeExprParser defines two operators, # and ##. ## binds after #, but both of them refer to the same function, the Pair constructor of the AExpr data type: aOperators :: [[Operator Parser AExpr]] aOperators = [ [ InfixN # symbol "#" *> pure (Pair) ] , [ InfixN # symbol "##" *> pure (Pair) ] ] The # operator works in isolation: > parseMaybe aExpr "a # b" Just (Pair (Var "a") (Var "b")) Parentheses work with the # operator: > parseMaybe aExpr "(a # b) # (c # d)" Just (Pair (Pair (Var "a") (Var "b")) -- whitespace added by hand (Pair (Var "c") (Var "d"))) And the # and ## operators work together as intended: > parseMaybe aExpr "a # b ## c # d" Just (Pair (Pair (Var "a") (Var "b")) -- whitespace added by hand (Pair (Var "c") (Var "d"))) But the ## operator in isolation does not parse! > parseMaybe aExpr "a ## b" Nothing [1] https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/master/howt… -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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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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Re: [Haskell-cafe] Datomic-like db for Haskell?
by Ovidiu Deac 24 Jan '17

24 Jan '17

I'll do some research to evaluate my options. So far I have: Homegrown implementation on top of some DB (Riak/Postgres/Mongo...) https://github.com/agentm/project-m36 https://hackage.haskell.org/package/filestore Thanks for the answers! On Fri, Dec 23, 2016 at 8:07 PM, Ertugrul Söylemez <esz(a)posteo.de> wrote: > >> * Another option is to run an insertion-only relational model on > >> really any database system you want like PostgreSQL. There is > >> nothing about preserving history that needs special handling to be > >> efficient. > > > > I assume that some sort of structural sharing is needed once the > > collections grow above a certain size. Also some support for caching > > would be nice. > > Yeah, sharing could improve disk usage and cache utilisation. I'm not > aware of any DBMS-side sharing features in systems I have used > (PostgreSQL and SQLite mainly). The usual way to get sharing in > relational systems is to normalise the schema. That also allows you to > use domain-specific knowledge to get even better sharing. > > Caching is of course supported, at least by PostgreSQL. It aims to keep > as much of the database in memory as possible (or as you tell it to). >

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is it normal for docs to be pending for 24 hours?
by Patrick Pelletier 24 Jan '17

24 Jan '17

This is only my second time uploading a package to Hackage, so I don't yet have a feel for how it's supposed to go. I uploaded normalization-insensitive-2.0.0.1 about 24 hours ago: https://hackage.haskell.org/package/normalization-insensitive-2.0.0.1 Under "Status", it says "Docs pending". (And the module names are all non-clickable.) Is it normal to take this long to build the docs? Is there some way to find out where in the queue my job is? Is this an indication that something has gone wrong? How do I fix it? Thanks, --Patrick

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Build base with stack
by Robin Palotai 06 Jan '17

06 Jan '17

Hello Cafe, Do you know a way to force a rebuild of base with stack? I don't want to modify it, just want to feed it through a customized ghc pipeline. Using stack is desirable for consistency with how other packages are run through that pipeline. When I download the appropriate base package and put it in a multi project stack directory, I see all the dependent packages are rebuilt, but not base. Thanks! Robin

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threadDelay delays less time than expected (Windows)
by Lian Hung Hon 05 Jan '17

05 Jan '17

Dear Haskellers, I am baffled by a strange bug (?) with threadDelay on Windows. It delays for less time than expected (500 seconds vs expected 24 hours), and is consistently reproducible on my system. Can someone have a look (need some patience, ~8 minutes waiting), just to be sure? I glanced around trac but didn't find any related issues. How to reproduce: module Main where import Data.Time import Control.Concurrent import Control.Monad main :: IO () main = do _ <- forkIO thread getLine >>= print -- Just to keep the main thread running thread :: IO () thread = forever $ do now <- getCurrentTime print now threadDelay 86400000000 print "done waiting" Expected result: 2016-12-28 13:07:49.5113098 UTC ... (next timing should be the next day 2016-12-29) Actual result: 2016-12-28 13:07:49.5113098 UTC "done waiting" 2016-12-28 13:16:10.2159485 UTC "done waiting" 2016-12-28 13:24:30.8735845 UTC "done waiting" 2016-12-28 13:32:51.5292203 UTC ... System: Windows 7 Enterprise Service Pack 1, ghc 7.10.3 This seems specific to Windows, it doesn't appear on Debian 8. Help is appreciated! Regards, Hon Rant: It has cost me significant debugging time! Please tell me I made a rookie mistake or misread the docs somewhere!

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How to call python from haskell
by jun zhang 01 Jan '17

01 Jan '17

Dear cafe I had try to use the http://hackage.haskell.org/package/cpython <http://hackage.haskell.org/package/cpython> package. I can call python function once by the package, but when I call function second time it will occur a "Segmentation fault”. Does there any stable package support call python from haskell? Thanks. Julian

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