January 2017 - 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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What is this applicative functor?
by Joachim Breitner 20 Feb '17

20 Feb '17

Hi, I recently wrote this applicative functor: data OneStep a = OneStep a [a] instance Functor OneStep where fmap f (OneStep o s) = OneStep (f o) (map f s) instance Applicative OneStep where pure x = OneStep x [] OneStep f fs <*> OneStep x xs = OneStep (f x) (map ($x) fs ++ map f xs) takeOneStep :: OneStep t -> [t] takeOneStep (OneStep _ xs) = xs This was useful in the context of writing a shrink for QuickCheck, as discussed at http://stackoverflow.com/a/41944525/946226. Now I wonder: Does this functor have a proper name? Does it already exist in the libraries somewhere? Should it? Greetings, Joachim -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • https://www.joachim-breitner.de/ XMPP: nomeata(a)joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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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] Monad transformer performance - Request to review benchmarking code + results
by Saurabh Nanda 07 Feb '17

07 Feb '17

Thanks for digging deeper, David. What exactly did you inline? Also, am I the only one losing my mind over this? It's such a straightforward use of available code structuring tools in Haskell. How come the compiler is not being smart about this OOB? -- Saurabh. On 29 Jan 2017 9:42 pm, "David Turner" <dct25-561bs(a)mythic-beasts.com> wrote: Here's the profiling summary that I got: COST CENTRE MODULE %time %alloc getOverhead Criterion.Monad 41.3 0.0 >>= Lucid.Base 19.2 41.6 makeElement.\.\ Lucid.Base 11.4 23.4 fromHtmlEscapedString Blaze.ByteString.Builder.Html.Utf8 7.9 14.9 >>= Data.Vector.Fusion.Util 2.3 1.7 return Lucid.Base 1.4 2.1 runBenchmark.loop Criterion.Measurement 1.2 0.0 with.\ Lucid.Base 1.0 2.1 foldlMapWithKey Lucid.Base 0.5 2.6 streamDecodeUtf8With.decodeChunk Data.Text.Encoding 0.0 1.7 As expected, HtmlT's bind is the expensive bit. However I've been unable to encourage it to go away using INLINE pragmas. On 29 January 2017 at 15:45, Oliver Charles <ollie(a)ocharles.org.uk> wrote: > I would start by inlining operations in the Functor, Applicative and Monad > classes for your monad and all the layers in the stack (such as HtmlT). An > un-inlining monadic bind can end up allocating a lot (as it's such a common > operation) > > On Sun, 29 Jan 2017, 3:32 pm Saurabh Nanda, <saurabhnanda(a)gmail.com> > wrote: > >> Please tell me what to INLINE. I'll update the benchmarks. >> >> Also, shouldn't this be treated as a GHC bug then? Using monad >> transformers as intended should not result in a severe performance penalty! >> Either monad transformers themselves are a problem or GHC is not doing the >> right thing. >> >> -- Saurabh. >> >> On 29 Jan 2017 7:50 pm, "Oliver Charles" <ollie(a)ocharles.org.uk> wrote: >> >> I would wager a guess that this can be solved with INLINE pragmas. We >> recently added INLINE to just about everything in transformers and got a >> significant speed up. >> >> On Sun, 29 Jan 2017, 11:18 am David Turner, < >> dct25-561bs(a)mythic-beasts.com> wrote: >> >> I would guess that the issue lies within HtmlT, which looks vaguely >> similar to a WriterT transformer but without much in the way of >> optimisation (e.g. INLINE pragmas). But that's just a guess after about 30 >> sec of glancing at https://hackage.haskell.org >> /package/lucid-2.9.7/docs/src/Lucid-Base.html so don't take it as gospel. >> >> My machine is apparently an i7-4770 of a similar vintage to yours, >> running Ubuntu in a VirtualBox VM hosted on Windows. 4GB of RAM in the VM, >> 16 in the host FWIW. >> >> >> On 29 Jan 2017 10:26, "Saurabh Nanda" <saurabhnanda(a)gmail.com> wrote: >> >> Thank you for the PR. Does your research suggest something is wrong with >> HtmlT when combined with any MonadIO, not necessarily ActionT? Is this an >> mtl issue or a lucid issue in that case? >> >> Curiously, what's your machine config? I'm on a late 2011 macbook pro >> with 10G ram and some old i5. >> >> -- Saurabh. >> >> On 29 Jan 2017 3:05 pm, "David Turner" <dct25-561bs(a)mythic-beasts.com> >> wrote: >> >> The methodology does look reasonable, although I think you should wait >> for all the scotty threads to start before starting the benchmarks, as I >> see this interleaved output: >> >> Setting phasers to stun... (port 3002) (ctrl-c to quit) >> Setting phasers to stun... (port 3003) (ctrl-c to quit) >> Setting phasers to stun... (port 3001) (ctrl-c to quit) >> benchmarking bareScotty >> Setting phasers to stun... (port 3000) (ctrl-c to quit) >> >> Your numbers are wayyy slower than the ones I see on my dev machine: >> >> benchmarking bareScotty >> Setting phasers to stun... (port 3000) (ctrl-c to quit) >> time 10.94 ms (10.36 ms .. 11.52 ms) >> 0.979 R² (0.961 R² .. 0.989 R²) >> mean 12.53 ms (11.98 ms .. 13.28 ms) >> std dev 1.702 ms (1.187 ms .. 2.589 ms) >> variance introduced by outliers: 66% (severely inflated) >> >> benchmarking bareScottyBareLucid >> time 12.95 ms (12.28 ms .. 13.95 ms) >> 0.972 R² (0.951 R² .. 0.989 R²) >> mean 12.20 ms (11.75 ms .. 12.69 ms) >> std dev 1.236 ms (991.3 μs .. 1.601 ms) >> variance introduced by outliers: 50% (severely inflated) >> >> benchmarking transScottyBareLucid >> time 12.05 ms (11.70 ms .. 12.39 ms) >> 0.992 R² (0.982 R² .. 0.996 R²) >> mean 12.43 ms (12.06 ms .. 13.01 ms) >> std dev 1.320 ms (880.5 μs .. 2.071 ms) >> variance introduced by outliers: 54% (severely inflated) >> >> benchmarking transScottyTransLucid >> time 39.73 ms (32.16 ms .. 49.45 ms) >> 0.668 R² (0.303 R² .. 0.969 R²) >> mean 42.59 ms (36.69 ms .. 54.38 ms) >> std dev 16.52 ms (8.456 ms .. 25.96 ms) >> variance introduced by outliers: 92% (severely inflated) >> >> benchmarking bareScotty >> time 11.46 ms (10.89 ms .. 12.07 ms) >> 0.986 R² (0.975 R² .. 0.994 R²) >> mean 11.73 ms (11.45 ms .. 12.07 ms) >> std dev 800.6 μs (636.8 μs .. 975.3 μs) >> variance introduced by outliers: 34% (moderately inflated) >> >> but nonetheless I do also see the one using renderTextT to be >> substantially slower than the one without. >> >> I've sent you a PR [1] that isolates Lucid from Scotty and shows that >> renderTextT is twice as slow over IO than it is over Identity, and it's >> ~10% slower over Reader too: >> >> benchmarking renderText >> time 5.529 ms (5.328 ms .. 5.709 ms) >> 0.990 R² (0.983 R² .. 0.995 R²) >> mean 5.645 ms (5.472 ms .. 5.888 ms) >> std dev 593.0 μs (352.5 μs .. 908.2 μs) >> variance introduced by outliers: 63% (severely inflated) >> >> benchmarking renderTextT Id >> time 5.439 ms (5.243 ms .. 5.640 ms) >> 0.991 R² (0.985 R² .. 0.996 R²) >> mean 5.498 ms (5.367 ms .. 5.631 ms) >> std dev 408.8 μs (323.8 μs .. 552.9 μs) >> variance introduced by outliers: 45% (moderately inflated) >> >> benchmarking renderTextT Rd >> time 6.173 ms (5.983 ms .. 6.396 ms) >> 0.990 R² (0.983 R² .. 0.995 R²) >> mean 6.284 ms (6.127 ms .. 6.527 ms) >> std dev 581.6 μs (422.9 μs .. 773.0 μs) >> variance introduced by outliers: 55% (severely inflated) >> >> benchmarking renderTextT IO >> time 12.35 ms (11.84 ms .. 12.84 ms) >> 0.989 R² (0.982 R² .. 0.995 R²) >> mean 12.22 ms (11.85 ms .. 12.76 ms) >> std dev 1.159 ms (729.5 μs .. 1.683 ms) >> variance introduced by outliers: 50% (severely inflated) >> >> I tried replacing >> >> forM [1..10000] (\_ -> div_ "hello world!") >> >> with >> >> replicateM_ 10000 (div_ "hello world!") >> >> which discards the list of 10,000 () values that the forM thing >> generates, but this made very little difference. >> >> Hope this helps, >> >> David >> >> >> [1] https://github.com/vacationlabs/monad-transformer-benchmark/pull/2 >> >> >> >> On 29 January 2017 at 07:26, Saurabh Nanda <saurabhnanda(a)gmail.com> >> wrote: >> >> Hi, >> >> I was noticing severe drop in performance when Lucid's HtmlT was being >> combined with Scotty's ActionT. I've tried putting together a minimal repro >> at https://github.com/vacationlabs/monad-transformer-benchmark Request >> someone with better knowledge of benchmarking to check if the benchmarking >> methodology is correct. >> >> Is my reading of 200ms performance penalty correct? >> >> -- Saurabh. >> >> >> _______________________________________________ >> 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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ghci, running :load on filename provided by variable
by Dennis Raddle 04 Feb '17

04 Feb '17

How do I load a file in ghci where the filename comes from a variable? So like, let f = "myfile.hs" :load f D

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Ensuring all values of an ADT are explicitly handled OR finding all occurrences of type X in my app
by Saurabh Nanda 31 Jan '17

31 Jan '17

Hi, If I have the following ADT data BookingState = Confirmed | Cancelled which had a very high chance of being expanded in the future to have more values. How do I ensure that every pattern match on BookingState matches each value explicitly. Basically prevent the '_' matcher ? If that is not possible, is the following possible instead: evolve BookingState and then find all possible occurrences of values of BookingState throughout my app? I'm basically trying to make sure that any a newly added state, which may require special-case handling is not automatically handled by the '_' branch. -- Saurabh.

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Galois is hiring again!
by Iavor Diatchki 31 Jan '17

31 Jan '17

Hello, We have more openings at Galois, for a variety of positions, including researchers, principle investigators, software engineers, project leads, and some others. The full list is available at: http://galois.com/careers/#careers-list We are looking for people with strong background in one or more of the following areas: functional programming, formal methods, machine learning, embedded systems, computer security, and networking. Galois has two offices, one on the west coast of the US, in Portland, Oregon, and one on the east coast, in Arlington, Virginia. There are positions available at both offices. Generally, we are looking for candidates who are allowed to work in the US, and are interested in working locally at one of the two offices. If this sounds exciting, please consider sending us your resume. The descriptions of the positions each contains an application link. Cheers, -Iavor

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