June 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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The 13-line example in Text.Megaparsec.Expr
by Jeffrey Brown 24 Sep '16

24 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 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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Resumable applicative parsing from tail end?
by martin 07 Jul '16

07 Jul '16

Hello all, in my attempt to write a simulation program, I am facing the following problem: as the simulation progresses, I collect "primary" log entries, each one associated with a timestamp. These entries carry the information I am interested in (and not so much the final state). Currently I am using a simple list to collect log entries. Since (:) is so much easier than (++) I prepend new entries to the head of the list. So my log runs backwards in time with the oldest entries at the tail end. Because the log-entries are typically too fine-grained to be useful, I want to aggregate several of them into a single log entry in a secondary log. I want to do this "as I go" and discard primary log entries which are already aggregated. Thus I can keep the primary log reasonably small. I thought that aggregation is just a special case of parsing. And indeed, I could write an applicative parser PrimaryLog -> (PrimaryLog, SecondaryLog) which does the trick, except I had to reverse the ordering of the primary log. This is because the parser/aggregator must parse from past to future, because there may be new log entries coming up in the future. I need to be able to resume parsing where I left off whenever new primary log entries are entered, becuase I may then have just enough unconsumed primary log entries to create another secondary entry. Currently I'm trying Data.Sequence to let the primary log grow to the left and let parsing to begin at the far-right. But I have a bad feeling about it. Particularly, I don't have any hard reasons why a simple List wouldn't be sufficient. But I just wasn't able to write the code. What do you think? How would you parse a list from the tail end? Or how else would you deal with this problem?

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Packaging a library with a separate shared object
by Bryan Gardiner 05 Jul '16

05 Jul '16

Hi folks! I'm working on packaging Qtah (my Qt bindings) for upload to Hackage. This is somewhat complicated, as I have some C++ glue code that I build into a shared library that the Haskell library needs. Currently, my build hooks are as follows, in: https://gitlab.com/khumba/qtah/blob/packaging/qtah/Setup.hs - postConf: Generate Haskell code in src/ and C++ code in cpp/. - preBuild: Add cpp/ to HookedBuildInfo.extraLibDirs because the build phase needs this needs this. - buildHook: Run make to build the C++ code into libqtah.so before the Haskell code builds. - copyHook and instHook: Copy libqtah.so into InstallDirs.libdir (~/.cabal/lib/.../qtah-*/). This installs successfully, but ultimately doesn't work because the Haskell libHSqtah*.so that gets built can't find libqtah.so. The installation directory ~/cabal/lib/.../qtah-*/ isn't on libHSqtah*.so's RUNPATH (which makes sense, it's not known at link time). I could create a separate package for the C++ bits, but I'd rather not do that unless necessary (there's already a qtah-generator package as well). I can't see how I can avoid doing this though. Are there any Cabal experts that could point me in the right direction, or confirm that I'll need to split the package? Many thanks, Bryan

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Type-class predicates head-normal forms
by sheng chen 01 Jul '16

01 Jul '16

Hello, I got a question about what the form of types in Haskell are allowed. Specifically, what are the requirements of the context of a type? The paper Typing Haskell in Haskell required the type to be in the form P => t, where P to be in head-normal form or in other words shouldn't be something like Num Bool. Now what about multi-parameter classes? The paper Type classes: an exploration of the design space listed several potential choices but didn't say exactly what. The Haskell 2010 report covered single-parameter type classes only. Given the following definition. class Collects ce e | ce -> e where empty :: ce insert :: e -> ce -> ce insert2 c = insert True (insert False c) The function insert2 has the following type insert2 :: Collects ce Bool => ce -> ce This shows that the definition of head-normal forms for multi-parameter class differs from that for single-parameter classes. Is there any definite specification about the context of Haskell types? Thank you, Sheng

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