- Haskell-Cafe - Haskell.org

ghc-mod errors in Emacs
by Đorđe Grbić 21 Apr '16

21 Apr '16

Greetings dear Haskell community, I'm trying to use Spacemacs for Haskell development but as soon as I open a .hs file ghc-mod dumps errors: ghc-mod: <command line>: cannot satisfy -package-id base-4.6.0.1-8aa5d403c45ea59dcd2c39f123e27d57 (use -v for more information) <command line>: cannot satisfy -package-id base-4.6.0.1-8aa5d403c45ea59dcd2c39f123e27d57 (use -v for more information) <command line>: cannot satisfy -package-id base-4.6.0.1-8aa5d403c45ea59dcd2c39f123e27d57 (use -v for more information) I have ghc-mod version: 5.5.0.0 compiled by GHC 7.8.4. GHC version : 7.8.4 Emacs: 24.5.1 Spacemacs: 0.105.18 OS: Ubuntu 14.04.4 LTS Any help is appreciated, Djordje Grbic

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Re: [Haskell-cafe] Documenting extensions
by Doug McIlroy 21 Apr '16

21 Apr '16

>>> Later I discovered >>> that by default GHC rejected almost all my report-conforming (H98 >>> and H2010) code. >> >> Do you mean that GHC can't compile Haskell2010 conformant code? >> Can you please post (or point to) some examples? > > This page [1] lists the ways in which GHC diverges from the standard. > > Erik > > [1] https://downloads.haskell.org/~ghc/master/users-guide/bugs.html The main problem was that my code relied on constraint (Num a) implying (Eq a) per the standard. I also had context clauses in data declarations. Thus I had to make pervasive, albeit trivial, changes to get past GHC's nonconformance. (Presumably option -XHaskell2010 would have papered over the trouble, but I prefer to maximize code portability.) Doug McIlroy

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Hugs
by David Feuer 21 Apr '16

21 Apr '16

Apparently, Eric Meijer is now teaching a large class using Hugs, which leads to many of his students asking questions about it on StackOverflow. I therefore suggest that we change the status of Hugs on the website from "is no longer in development" to "has been maintained by Eric Meijer since 2015". I think we should also include on the website a statement to the effect that people seeking help using Hugs should contact Eric Meijer directly, and provide his email address to facilitate that. Obviously, he'll need full write access to the Hugs source repo to fulfill his duties as maintainer. Yes, this is all a bit tongue-in-cheek, but I really do think that pointing students to an unmaintained language implementation (regardless of the pedagogical reasons) has negative consequences for the functional programming community as a whole.

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[ANN] Stratosphere: AWS CloudFormation EDSL in Haskell
by David Reaver 20 Apr '16

20 Apr '16

Github: https://github.com/frontrowed/stratosphere Hackage: https://hackage.haskell.org/package/stratosphere CloudFormation is a system that provisions and updates Amazon Web Services (AWS) resources based on declarative templates. Common criticisms of CloudFormation include the use of JSON as the template language and limited error-checking, often only available in the form of run-time errors and stack rollbacks. By wrapping templates in Haskell, we are able to easily construct them and help ensure correctness. The goals of stratosphere are to: - Build a Haskell EDSL to specify CloudFormation templates. Since it is embedded in Haskell, it is type-checked and generally much easier to work with than raw JSON. - Have a simple checking/linting system outside of the types that can find common errors in templates. - Be able to also read valid CloudFormation JSON templates so they can be type-checked. This also gives us free integration tests by using the huge amount of example templates available in the AWS docs. Most of the commonly used CloudFormation resources are implemented, and adding new resources is very straightforward. (We created a web scraper that generates a JSON file from a given CloudFormation documentation page URL, and from that we generate a Haskell module.) So far, we have implemented resources we use at Front Row Education, and we will add more resources over time. The library is very much in a usable state as-is. However, we want to make any sweeping changes while it is still young. If you have any suggestions at all, you want us to add your favorite resource, or if you find a bug, please file an issue on Github! Also, we want to give a huge thanks to Brendan Hay, the author of amazonka, for his ideas and feedback on the project.

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Typeclass Constraint Causing Typechecker to Fail
by James M 19 Apr '16

19 Apr '16

Below I have a contrived example. Please do not take this to be real world code. f :: (Show a) => Int -> (Int -> a) -> Int -> IO a f i g x = do print i return $ g (x + i) foo :: Bool -> IO (Either [Int] [String]) foo b = do let helper = f 2 if b then Left <$> sequence (fmap (helper negate) [0,1]) else Right <$> sequence (fmap (helper show) [0,1]) The above will fail stating Example.hs:14:35: Couldn't match type ‘Int’ with ‘[Char]’ Expected type: Int -> IO String Actual type: Int -> IO Int In the first argument of ‘fmap’, namely ‘(helper show)’ In the first argument of ‘sequence’, namely ‘(fmap (helper show) [0, 1])’ Example.hs:14:42: Couldn't match type ‘[Char]’ with ‘Int’ Expected type: Int -> Int Actual type: Int -> String In the first argument of ‘helper’, namely ‘show’ In the first argument of ‘fmap’, namely ‘(helper show)’ However, if I simply change f's type signature to not have the typeclass constraint, the type checker is happy: f :: Int -> (Int -> a) -> Int -> IO a Another possibility to remove the problem is to remove the let statement and instead put the entire expression in. foo :: Bool -> IO (Either [Int] [String]) foo b = do if b then Left <$> sequence (fmap (f 2 negate) [0,1]) else Right <$> sequence (fmap (f 2 show) [0,1]) This seems to be a bug in the typechecker, and maybe it is a well known issue. Can someone please confirm that this is a bug and whether or not it is known? James

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Re: [Haskell-cafe] Is it possible to make lazy combinators for IO?
by David Feuer 19 Apr '16

19 Apr '16

lazyMap :: (a -> b) -> IO a -> IO b, hypothetically. Let me take a step back. There are two ways I know of to use the result of an IO action. The most common way, by far, is to use >>= to send it on to the next computation. The other notable way is to use fixIO. No part of the result of an IO action becomes available until that action "completes" (in the shady case of lazy IO, this may be immediate, but I'm ignoring lazy IO for this discussion). This causes the well-known inefficiency of mapM in IO: individual results cannot be consed until the entire computation is complete. It seems to cause much more painful effects for fixIO--lazy programming techniques may simply be unavailable. For example, it's possible for a pure computation using a queue in a single-threaded fashion to represent that queue as a list, tying the knot to pass the list of all enqueued elements to the beginning. If an IO computation wants to use a queue, this technique does not seem to be available. The knot can be tied with fixIO, but conses are delayed until it's too late--attempting to dequeue an element will block forever. I'm wondering if there's some way to make this sort of thing work, either with plain Haskell IO or some thin layer built on top. On Apr 18, 2016 5:12 PM, "David Turner" <dct25-561bs(a)mythic-beasts.com> wrote: > Sorry, you've lost me. I suspect you're trying to give a minimal example > of the problem you're having, but you've stripped away too much context. > What are lazyMap and f? At least, what are their types? > On 18 Apr 2016 22:03, "David Feuer" <david.feuer(a)gmail.com> wrote: > >> Consider the implementation of `second` for pairs: >> >> second f ~(a,b) = (a, f b) >> >> Now >> >> fix $ second (3 :) >> >> Will be (undefined, [3,3,....]) >> >> Translating this to IO, I'd want >> >> lazyMap f undefined >> >> to produce as much as possible of the result, although it cannot produce >> the final State# RealWorld token. >> On Apr 18, 2016 4:47 PM, "David Turner" <dct25-561bs(a)mythic-beasts.com> >> wrote: >> >>> You can't know that the final result of the computation (x `seq` >>> (3:...)) will begin with 3, because sometimes it doesn't! More >>> specifically, it diverges (without yielding the 3) if x diverges. >>> >>> I don't think this is anything special about mfix: (let x = x `seq` >>> 3:... in x) also diverges for the same reason. >>> >>> Hope that helps, >>> >>> David >>> On 18 Apr 2016 21:19, "David Feuer" <david.feuer(a)gmail.com> wrote: >>> >>>> If >>>> >>>> f :: a -> IO a >>>> >>>> for some a, and I want to use >>>> >>>> mfix f >>>> >>>> then f must not inspect its argument in any way, or the computation >>>> will get stuck. In some cases, this seems a bit harsh. For example, >>>> >>>> mfix (\x -> fmap (3 :) (x `seq` readLn)) >>>> >>>> looks perfectly reasonable. There is no need to inspect the return [] >>>> action to know that the final result of the computation will begin >>>> with 3:. Is there a lazy IO mapping function somewhere that can work >>>> such magic? >>>> _______________________________________________ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe(a)haskell.org >>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>>> >>>

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Existential quantification of config data types
by Charlie Durham 19 Apr '16

19 Apr '16

Hi, I'm running into a weird problem i've never had before. I want to define a datatype like: data Network = forall a. Layer a => Network [a] for which I have things that implement Layer, like data TypicalLayer = ... instance Layer TypicalLayer where ... with this, I want to be able to load a Network from a config file where Network can consist of different data types all implementing Layer. Is there a nice way to do this without making a data type that combines all of my layer types, something like: data LayerType = TypicalLayerType TypicalLayer | AnotherLayerType AnotherLayer ... deriving (Show,Read) Thanks for the help! Charlie Durham

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Re: [Haskell-cafe] Sending email
by David Escobar 19 Apr '16

19 Apr '16

No explicit mention is made anywhere in the documentation about TLS or even SSL, so perhaps not? Some libraries I've come across specifically mention that they don't support TLS or SSL. Being relatively new to this part of Haskell, what is the most standard library the community uses for email that supports modern protocols such as those used by GMail? Thanks. On Mon, Apr 18, 2016 at 2:49 PM, Alex Feldman-Crough <alex(a)fldcr.com> wrote: > Does the library support TLS? Does it have to be configured differently? > It sounds like a negotiation error to me. > On Mon, Apr 18, 2016 at 2:43 PM David Escobar <davidescobar1976(a)gmail.com> > wrote: > >> Hi everyone, >> I'm trying to use the *Network.Mail.SMTP* library to send email: >> >> *{-# LANGUAGE OverloadedStrings #-}* >> >> *module Main where* >> >> *import Control.Exception* >> >> *import qualified Data.Text as T* >> *import qualified Data.Text.Lazy as LT* >> *import Network.Mail.SMTP* >> >> *main :: IO ()* >> *main = do* >> * sendEmail (“Person sender”, “sender@somewhere.com >> <sender(a)somewhere.com>”)* >> * [(“Person recipient“, “recipient@somewhere.com >> <recipient(a)somewhere.com>”)]* >> * "Test email"* >> * "Some message goes here."* >> >> >> *sendEmail :: (T.Text, T.Text) -> [(T.Text, T.Text)] -> T.Text -> T.Text >> -> IO ()* >> *sendEmail (fromName, fromEmail) toAddresses subject' body' = do* >> * let toNameAddrs = map (\(toName, toEmail) -> Address (Just toName) >> toEmail) toAddresses* >> * msg = simpleMail (Address (Just fromName) fromEmail)* >> * toNameAddrs* >> * []* >> * []* >> * subject'* >> * [ plainTextPart $ LT.fromStrict body' ]* >> * result <- try $ sendMailWithLogin' "smtp.gmail.com >> <http://smtp.gmail.com>"* >> * 465 -- SSL port* >> * “sender_login”* >> * “sender_password”* >> * msg :: IO (Either SomeException ())* >> * case result of* >> * Left e -> putStrLn $ "Exception caught: " ++ (displayException e)* >> * Right _ -> putStrLn "Sent email successfully."* >> >> >> The program compiles, but when I run it, I get: >> >> *Exception caught: <socket: 49>: Data.ByteString.hGetLine: end of file* >> >> I tried using the TLS port of 587, but then I just get an authentication >> failure. Am I using the wrong library or is it just the wrong >> configuration. Thanks. >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > -- > Alex Feldman Crough > 602 573-9588

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diffs on HaskellWiki
by Patrick Pelletier 19 Apr '16

19 Apr '16

I've noticed that when viewing changes on HaskellWiki, the diffs are missing. We had a similar problem on the OpenSSL wiki (which, like HaskellWiki, is based on MediaWiki), and the sysadmin said he fixed the problem by switching from the external diff engine to the internal diff engine. Is there someone who could look into this and possibly make that change to the wiki's configuration? I looked around the HaskellWiki and couldn't find any mention of how to contact a maintainer. Thanks, --Patrick

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Is it possible to make lazy combinators for IO?
by David Feuer 18 Apr '16

18 Apr '16

If f :: a -> IO a for some a, and I want to use mfix f then f must not inspect its argument in any way, or the computation will get stuck. In some cases, this seems a bit harsh. For example, mfix (\x -> fmap (3 :) (x `seq` readLn)) looks perfectly reasonable. There is no need to inspect the return [] action to know that the final result of the computation will begin with 3:. Is there a lazy IO mapping function somewhere that can work such magic?

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[ANNOUNCE] GHC 8.0.1 release candidate 3 available
by Ben Gamari 18 Apr '16

18 Apr '16

Hello Haskellers, The GHC team is happy to announce the availability of third release candidate of the Glasgow Haskell Compiler's 8.0.1 release. Source and binary distributions can be found at, http://downloads.haskell.org/~ghc/8.0.1-rc3/ This is the third of what will be four candidates leading up to the 8.0.1 release. As there are a number of known issues with this candidate, we expect to produce the fourth and final release candidate shortly. While we considered not releasing this candidate at all and instead skipping to rc4, we decided that it would be helpful to make it available despite its issues to increase the likelihood of catching any remaining bugs. That being said, this candidate lacks the range of binary distributions which we usually offer, * We are unable to release Windows bindists due to a compatibility issue which we didn't catch until after preparing the source distribution. * Binaries targeting Redhat distributions are currently not available Regardless, users for whom a distribution is available are encouraged to try this candidate as it fixes dozens of issues present in release candidate two. These include, * A type-checker panic triggered by use of Typeable on a kind-polymorphic type constructor (#11334) * A type-checker explosion where -XTypeInType would gobble up massive amounts of memory when used in a data instance (#11407) * Various issues involving unexpected laziness resulting in exception handlers not being invoked (#11555) * GHC now fails more gracefully when used with an older cabal-install release (#11558) * TypeInType now has proper documentation in the users guide (#11614) * The story surrounding type `RuntimeRep`s (formerly known as `Levity`) is now far better developed, closing several doors to unsafe behavior that TypeInType previously opened (#11473, #11723) * A long-standing bug in the constant-folding rules for `mod` for the `Word` type has been resolved (#11702) * Various issues introduced by OverloadedRecordFields have been fixed (#11662, #11401) * A regression in the typechecker resulting in the rejection of code in the `free` and `microlens` packages has been fixed (#11608) * A bug in the LLVM code generator which caused some programs to emit unbuildable LLVM IR has been fixed (#11649) * A bug where pattern synonyms defined in terms of a pattern match on a record would be rejected if the fields weren't written in the same order as they were defined has been resolved (#11633) * A bug in the runtime system's treatment of weak references which could result in segmentation faults in some cases has been fixed (#11108) * a variety of optimizations improving compiler performance have been merged * and many others Mac OS X users should be aware that the recent XCode 7.3 release ships with a broken `nm` utility which breaks the GHC build (#11744, #11823). Unfortunately, this candidate does not notice this brokenness and odd behavior can result. OS X users running XCode 7.3 are encouraged to open a ticket with Apple and build GHC with the `--with-nm=nm-classic` configure option until a fix is available. As mentioned earlier, there are a number of known issues with this candidate which will be addressed in release candidate four. These issues include, * A number of typechecker bugs (#11811, #11797, #11813, #11814) * A build issue seen on OS X (#11828) * Template Haskell lacks the ability to produce instances with OVERLAPP{ING,ABLE,ED} pragmas * Autoconf has inadequate version checks for libdw (#11820) * Typeable and hs-boot files interact poorly (#11824) * The build system doesn't check for the broken Apple `nm` utility, leading to unpredictable results (#11823, #11744) We hope to have release candidate four available by the middle of next week. Thanks to everyone who has contributed code, testing, builds, and bug reports thusfar! Cheers, - Ben

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Pretty-printing haskell source code - could the line breaking be improved?
by David Fox 18 Apr '16

18 Apr '16

When I pretty print a certain deleration (with default width) I get this: upeekRow _unv (_xconc@(ImageSize {})) = Node (upeekCons idPath Nothing) (concatMap (\f -> forestMap (liftPeek f) (map (\x' -> Node (upeekCons idPath (Just (u x' :: Univ)) :: UPeek Univ Dimension) []) (toListOf (toLens (f idPath) . ulens' (Proxy :: Proxy Univ)) _xconc :: [Dimension]))) [UPath_ImageSize_dim] ++ (concatMap (\f -> forestMap (liftPeek f) (map (\x' -> Node (upeekCons idPath (Just (u x' :: Univ)) :: UPeek Univ Double) []) (toListOf (toLens (f idPath) . ulens' (Proxy :: Proxy Univ)) _xconc :: [Double]))) [UPath_ImageSize_size] ++ (concatMap (\f -> forestMap (liftPeek f) (map (\x' -> Node (upeekCons idPath (Just (u x' :: Univ)) :: UPeek Univ Units) []) (toListOf (toLens (f idPath) . ulens' (Proxy :: Proxy Univ)) _xconc :: [Units]))) [UPath_ImageSize_units] ++ []))) However, what I would like to see is something closer to this: upeekRow _unv (_xconc@(ImageSize {})) = Node (upeekCons idPath Nothing) (concatMap (\f -> forestMap (liftPeek f) (map (\x' -> Node (upeekCons idPath (Just (u x' :: Univ)) :: UPeek Univ Dimension) []) (toListOf (toLens (f idPath) . ulens' (Proxy :: Proxy Univ)) _xconc :: [Dimension]))) [UPath_ImageSize_dim] ++ (concatMap (\f -> forestMap (liftPeek f) (map (\x' -> Node (upeekCons idPath (Just (u x' :: Univ)) :: UPeek Univ Double) []) (toListOf (toLens (f idPath) . ulens' (Proxy :: Proxy Univ)) _xconc :: [Double]))) [UPath_ImageSize_size] ++ (concatMap (\f -> forestMap (liftPeek f) (map (\x' -> Node (upeekCons idPath (Just (u x' :: Univ)) :: UPeek Univ Units) []) (toListOf (toLens (f idPath) . ulens' (Proxy :: Proxy Univ)) _xconc :: [Units]))) [UPath_ImageSize_units] ++ []))) So I guess what I'm seeing is that as it is traversing the syntax tree when it finds it is too far to the right it just inserts a line break - but since it is so far down the tree it only buys it a few spaces, whereas if it had backtracked a few levels up the tree the new position would be much further to the left. Could anyone point me towards the code that implements this decision making? I'm pretty sure it is in the display function Text.PrettyPrint.Annotated.HughesPJ, but the code there doesn't make much sense to me. Any thoughts would be appreciated. David Fox

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Re: [Haskell-cafe] Haskell on OpenWRT
by Patrick Pelletier 18 Apr '16

18 Apr '16

On 4/17/16 4:13 PM, Henk-Jan van Tuyl wrote: > Could you create a HaskellWiki page (called OpenWRT) describing how to > do this? Or send the text to me, so that I can put it in a HaskellWiki > page. Sure. I had written some detailed text about my cross-compilation adventure for my blog, so I went ahead and used that as the starting point for the wiki page: https://wiki.haskell.org/OpenWRT It's written in a much more narrative, first-person style than is really appropriate for a wiki, so of course please feel free to edit it to make it more appropriate for that context. I also haven't linked that page from anywhere else on the wiki yet, so feel free to do that, too. --Patrick

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Building shared library to be called from C
by Jack Hill 17 Apr '16

17 Apr '16

Hi, I'm attempting to build a shared library that can be called from C. My end goal is to build a PAM module, but for now I'm working with toy programs so that I can get a feel for building how to build the library. The Haskell part looks like this (in a file called hfact.hs) module Hfact where import Foreign import Foreign.C.Types foreign export ccall fact_hs :: CInt -> CInt fact :: Int -> Int fact x = product [1..x] fact_hs :: CInt -> CInt fact_hs = fromIntegral . fact . fromIntegral I can compile this into a so (I think successfully) with "ghc --make -shared -dynamic -fPIC -o libhfact.so hfact.hs". Now I would like to be able to call this from C. I have a C program (in a file called main.c) as follows: #include <stdio.h> #include "HsFFI.h" #include "hfact_stub.h" int main(int argc, char *argv[]) { int user, answer; scanf("%d", &user); hs_init(&argc, &argv); answer = fact_hs(user); hs_exit(); printf("Result: %d\n", answer); return 0; } Unfortunately, I'm at a loss for how to build and link this against my so. If I try "gcc main.c -lhfact", I get many undefined symbol errors like: /nfshomes/j/jackhill/.stack/programs/x86_64-linux/ghc-7.10.3/lib/ghc-7.10.3/base_HQfYBxpPvuw8OunzQu6JGM/libHSbase-4.8.2.0-HQfYBxpPvuw8OunzQu6JGM-ghc7.10.3.so: undefined reference to `__hscore_get_saved_termios' If I try "ghc maint.c -lhfact" I get: /tmp/ghc19146_0/ghc_4.o: In function `main': ghc_3.c:(.text+0x0): multiple definition of `main' maint.o:maint.c:(.text+0x0): first defined here /tmp/ghc19146_0/ghc_4.o: In function `main': ghc_3.c:(.text+0x61): undefined reference to `ZCMain_main_closure' collect2: error: ld returned 1 exit status How should I be calling Haskell from C? Best, Jack

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is monomorphism restriction necessary?
by Alexey Muranov 16 Apr '16

16 Apr '16

Sorry for the question that probably is already answered somewhere, but i do not get it from the basic documentation i have looked at so far. Is monomorphism restriction really necessary, or is it just a convenience? In the example from A History of Haskell genericLength :: Num a => [b] -> a f xs = (len, len) where len = genericLength xs can't the monomorphism restriction be replaced with just adding the signature f :: Num a => [b] -> (a, a) ? If yes, are there other cases where there is no way to get the desired behavior without enabling the monomorphism restriction? Alexey.

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[ANN] sarsi 0.0.2.0 - A universal quickfix toolkit and his protocol (nvim/vim)
by Alois Cochard 16 Apr '16

16 Apr '16

Hello Café, Here is a new tool which should basically improve the "quick fixing" experience in Haskell with vim/neovim. The aim is not to replace any of the great features provided by integrating the type checker in the editor, but to complement them with a different approach. I explain the motivation there: http://aloiscochard.blogspot.ch/2016/04/quickfix-all-things-with-sarsi.html If you feel like this might be a tool for you, you can jump straight to the README to find the install and usage instructions: https://github.com/aloiscochard/sarsi I did test it extensively with stack, let me know if you have issues using an other tool (I do only pipe stderr which is fine for stack, I suspect this might not be the case for others). Thanks -- *Λ\ois* http://twitter.com/aloiscochard http://github.com/aloiscochard

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Haskell on OpenWRT
by Patrick Pelletier 15 Apr '16

15 Apr '16

Has anyone had any success running Haskell programs on OpenWRT? Specifically, I'm running OpenWRT on x86_64, so processor architecture shouldn't be an issue. However, by default OpenWRT uses musl as its C library, so binaries from a "normal" Linux system wouldn't be compatible with OpenWRT. I attempted to get around this problem by building an OpenWRT image with glibc as the C library. In theory, that ought to solve the problem. In practice, my program (a simple hello-world type program, which runs fine on Ubuntu) hung, printing nothing, using nearly all of one core, and was unkillable by any signal other than SIGKILL. If left alone to run, it eventually used all available memory and then died. I took a look at ldd, to see if there were any clues there. On my Ubuntu 12.04 machine, where I compiled the program (using ghc 7.4.1), I get: ppelletier@patrick64:~/programming/haskell$ ldd contact linux-vdso.so.1 => (0x00007fff36f50000) libgmp.so.10 => /usr/lib/x86_64-linux-gnu/libgmp.so.10 (0x00007f8d49cf8000) libffi.so.6 => /usr/lib/x86_64-linux-gnu/libffi.so.6 (0x00007f8d49af0000) libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f8d497f3000) librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f8d495eb000) libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f8d493e7000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f8d49028000) libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f8d48e0b000) /lib64/ld-linux-x86-64.so.2 (0x00007f8d49f8d000) On my OpenWRT (trunk) box, I get: root@OpenWrt:~# ldd /tmp/contact linux-vdso.so.1 (0x00007ffd2afcc000) libgmp.so.10 => /usr/lib/libgmp.so.10 (0x00007f27a057a000) libffi.so.6 => /usr/lib/libffi.so.6 (0x00007f27a0373000) libm.so.6 => /lib/libm.so.6 (0x00007f27a0080000) librt.so.1 => /lib/librt.so.1 (0x00007f279fe79000) libdl.so.2 => /lib/libdl.so.2 (0x00007f279fc76000) libc.so.6 => /lib/libc.so.6 (0x00007f279f906000) libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x00007f279f6f4000) libpthread.so.0 => /lib/libpthread.so.0 (0x00007f279f4d9000) /lib64/ld-linux-x86-64.so.2 (0x00007f27a07dc000) This is almost the same, except it has "libgcc_s.so.1", while on the Ubuntu box it does not. I'd welcome any ideas on how to solve (or at least diagnose) this problem. I'd also be interested in any thoughts on the broader question of running Haskell programs on OpenWRT. Thanks, --Patrick

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Re: [Haskell-cafe] Hugs
by Johannes Waldmann 15 Apr '16

15 Apr '16

> the learning process for a student rarely > involves consuming a precise, coherent definition, But it should! I do make a point of embedding references to actual standards in my teaching, mostly in exercises like "what paragraph of the (e.g., Java) language spec. says that the following program is illegal". The intention is: students should a) get used to reading (semi-)formal specifications b) see that concepts from "theory" (lecture, papers) appear verbatim in actual "industrial" specs c) get scared of using software/language that has no specification > .. the nature of functional programming supported by a review of the > theory upon which it is founded: at the very least, the lambda > calculus, preferably along with some basic ideas from type theory. That. And I do start my FP teaching with term rewriting, as a model of first-order programming. (gives you terms as a model of data, and rewrite rule application as a model of pattern matching) - J.W.

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Re: [Haskell-cafe] Hugs
by Doug McIlroy 15 Apr '16

15 Apr '16

> I really do think that pointing students to an unmaintained language > implementation (regardless of the pedagogical reasons) has negative > consequences for the functional programming community as a whole. If it works, maintenance doesn't matter. So, I assume the real concern is that Hugs isn't evolving. To put it in the worst light, this may be read as a complaint that Hugs doesn't keep up with some party line. I would hate to think that the "community as a whole" is that conformist. The disdain for "pedagogical reasons" brushes aside an implicit wakeup call to the community. Hugs is attractive because it is well described and bounded, whereas Haskell realized in GHC lacks a coherent description and presents a myriad of often inscrutable faces. To the extent that the community is defined by such an artifact, it has turned away from educators, not vice versa. Learning how to wrangle a marvelous, but cantankerous, beast should not be confounded with initiation to the insights of functional programming. Doug McIlroy

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Re: [Haskell-cafe] Hugs
by Johannes Waldmann 14 Apr '16

14 Apr '16

> https://twitter.com/headinthebox/status/652834731806052352 This mentions and :: Foldable t => t Bool -> Bool as an example of "abstract nonsense". Well, I will run into teaching this soon myself, though not to freshmen - my students had some Java before but not Haskell. With regards to the type of "and", I will first have them write "data List a = ..." and a concrete "and :: List Bool -> Bool" from scratch. Later, to explain the type of "the true and" I will claim that the corresponding Java method would have type "static Boolean and (Iterable<Boolean> xs)" where Iterable<> is an interface (that the students should know). That's not too bad: xs.iterator() corresponds to toList xs (a lazy stream). "toList" would even be a complete definition for the Foldable instance? (since foldr f z = foldr f z . toList ) So perhaps "Foldable \approx Iterable \approx Enumerable" is a useful guideline. (Yes, there is a difference between a one-argument type class and an interface. But they have some common use cases.) - J.W.

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Re: [Haskell-cafe] how ghc finds libgmp.so when linking
by Johannes Waldmann 14 Apr '16

14 Apr '16

more data: when I compile with -optl -L/usr/local/lib, then the correct library is found, when I put ("ld flags", "-L/usr/local/lib") in ghc-7.10.3/settings, then it's not, when I put ("C compiler link flags", "-L/usr/local/lib") in settings, then it is. So, I can leave it at that. But I still don't know how this came to be. I built this ghc-7.10.3 from source. - J.W.

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Haskell, stack and the Atom editor
by Thomas Koster 14 Apr '16

14 Apr '16

Hi friends, I want to switch to the Atom editor for my Haskell development. I have tried several times over the last few years and given up each time because hardly anything works. I can't stand Emacs so don't bother suggesting it. I am currently using Geany, which has served me well, mostly because it is dead simple so nothing can go wrong. In addition to the usual universal text editor functions, all it does is highlight syntax (albeit with an outdated grammar), runs custom commands to build, displays clickable errors in a separate pane and underlines them in the editor pane. But for larger projects, I want more (better autocompletion, better integration with the build system, better VCS support). But back to Atom, I am having extreme difficulty getting even the basics working. I am using GHC 7.10.2, Cabal 1.22.4.0, Stack 1.0.4. I installed Atom 1.6.1 from their deb package. I installed the "language-haskell" package. This works very well, thankfully, but is the *only* thing that I can get working. I avoided "ide-haskell", even though it looks great, because their readme says it can't build with stack and I rely heavily on stack. Instead, I installed the "build" and "linter" packages and created an ".atom-build" file to build my project with stack. It invokes stack correctly, but I can't get it to recognise any errors: linter always says "No issues". I must have tried twenty different regexes in "errorMatch" but none of them seem to work. I also installed "linter-hdevtools", hoping that that may let me see some errors, but I can't get that to work either. The package is loaded but it appears to do absolutely nothing. strace does show that Atom is trying to exec hdevtools through stack, but I get no output at all within Atom as evidence that something worked or failed. hdevtools appears to have its own problems. I had installed hdevtools (0.1.2.1) with stack: stack install hdevtools I ran it myself using: stack exec --no-ghc-package-path hdevtools -- check Main.hs and it gives me the infamous error: Cabal error: Use of GHC's environment variable GHC_PACKAGE_PATH is incompatible with Cabal. This is strange because "stack exec --no-ghc-package-path env" reports that no such environment variable is defined, as expected. At this point I am ready to give up yet again. Does anybody have any last-minute advice on using Atom to develop with Haskell and Stack? Thanks, Thomas Koster

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[ANN] Vivid 0.2
by amindfv＠gmail.com 14 Apr '16

14 Apr '16

After about a year of blood, sweat, and tears, the new Vivid is out! This gets us to a pretty polished state and updates will now be much more frequent. What is Vivid? It's a library to create music (and other sound) using the SuperCollider synth engine -- a really powerful audio "rendering engine." Vivid lets you create and alter music in real time, or render audio files faster than real time. It's been used in live performances (not only by me!), and I'm very happy with its sound, its power, and its expressivity. Without further ado: http://hackage.haskell.org/package/vivid Happy hacking! Tom

1 0

Lazy monadic queues
by David Feuer 13 Apr '16

13 Apr '16

I've come up with a very simple implementation of lazy monadic queues based loosely on ideas from Leon P. Smith's control-monad-queue package. This implementation ties a similar sort of lazy knot, but avoids the complexity and strictness of continuation-passing style. I'm curious whether something similar is already available on Hackage, and, if not, whether it would be useful enough to package it. The source code can currently be found at https://gist.github.com/treeowl/5c14a43869cf14a823473ec075788a74 David Feuer

1 0

Using existential types from TAPL book in Haskell
by Dominik Bollmann 13 Apr '16

13 Apr '16

Hi all, I'm currently reading Prof. Pierce's TAPL book. Reading the chapter on existential types, I've been wondering how to use existentials in Haskell. In particular, in the TAPL book there is a section on purely functional objects, which are encoded (e.g.) as follows: counter = {*Nat, { state = 5 , methods = { get = \x:Nat. x , inc = \x:Nat. succ x } } } as Counter where Counter = {∃X, { state : X , methods = { get : X -> Nat , inc : X -> X } } } That is, a Counter consists of some internal state as well as methods `get` to retrieve its state and `inc` to increment it. Internally, a Counter is just implemented with a natural number. However, from the outside, we hide this fact by making its state an existential type X. Thus, we can only modify a Counter instance (e.g., counter) using its exposed methods. For example, let {X, body} = counter in body.methods.get (body.state) (see Ch 24.2 on Existential Objects for further details) I've been wondering how to write such counters in Haskell. In particular, how to write a Counter instance like the above counter that uses a concrete representation type Nat, while exposing its state to the outside world abstractly with an existential type X. I've been trying to do this, but so far I haven't succeeded... If anyone could hint me at how to do this, would help me understanding existentials better, I think :-). Cheers, Dominik.

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Simple Interpreter with State Monad
by Jake 13 Apr '16

13 Apr '16

I'm currently in a graphics class where, in order to provide a standard interface to all of our graphics libraries, we have to process small scripts that look like this: line 0 0 0 1 1 1 circle 0 0 10 scale 0 0 3 save pic.png I successfully wrote a parser with attoparsec that parses the file into a list of Commands. Now I'm trying to process that list to produce an IO action, and I thought the State monad would be useful to keep track of the persistent state of the shapes to draw and the transformations on them. I'm confused about how exactly to do this though. What should the type of my State be? Right now I have an execute function that is execute :: Command -> State ParseState (IO ()) where ParseState is a tuple of stuff. ParseState also includes an IO () because I wanted to be able to create multiple pictures one after another, and I couldn't figure out how to access the previous result value from State to add on to it in the next one. So can anyone offer advice on my specific situation, or maybe a simpler example of how to go about writing an interpreter with the State monad? Thanks, Jake

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how ghc finds libgmp.so when linking
by Johannes Waldmann 13 Apr '16

13 Apr '16

Hi. I have some obsolete libgmp in /usr/lib, and the correct one in /usr/local/lib, but ghc does not find it, resp., finds the /usr/lib one earlier, and this is missing some symbols /usr/local/lib/ghc-7.10.3/integ_2aU3IZNMF9a7mQ0OzsZ0dS/libHSinteger-gmp-1.0.0.0-2aU3IZNMF9a7mQ0OzsZ0dS.a(wrappers.o): In function `integer_gmp_mpn_xor_n': wrappers.c:(.text+0x1761): undefined reference to `__gmpn_xor_n' when I put ghc ... -optl -Wl,--verbose I see that: SEARCH_DIR("/usr/i486-linux-gnu/lib32"); SEARCH_DIR("/usr/local/lib32"); SEARCH_DIR("/lib32"); SEARCH_DIR("/usr/lib32"); SEARCH_DIR("/usr/i486-linux-gnu/lib"); SEARCH_DIR("/usr/local/lib"); SEARCH_DIR("/lib"); SEARCH_DIR("/usr/lib"); but that's apparently not what's happening. Is ghc changing this, and how can I work around it? - J.W.

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Finding way around libraries (was parsec reorganization)
by Rustom Mody 13 Apr '16

13 Apr '16

On Wed, Mar 30, 2016 at 10:06 PM, Joe Hillenbrand <joehillen(a)gmail.com> wrote: > Parsec isn't as maintained as Megaparsec > <https://mrkkrp.github.io/megaparsec/>. > > I recommend checking out the tutorials here > <https://mrkkrp.github.io/megaparsec/tutorials.html>. > So under the advice to use megaparsec instead of parsec, I tried the examples here: https://mrkkrp.github.io/megaparsec/tutorials/fun-with-the-recovery-feature… Got a page full of errors starting: fpl.hs:42:20: Not in scope: ‘<$>’ … Perhaps you meant one of these: On a hunch I changed import Control.Applicative (empty) to import Control.Applicative and then it started working. I can only expect that if megaparsec's own official tutorials are thus out of sync with current haskell, then parsec's will be even more so. Also I find no megaparsec on hoogle/hayoo So how does one go about handling the dilemma: - megaparsec : in development but undocumented on hayoo/hoohle - parsec : documented but bitrotten What/how do haskell pros go about handling such Thanks Rusi

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LumiGuide is hiring
by Bas van Dijk 13 Apr '16

13 Apr '16

Dear Haskellers, We're hiring a Haskell engineer to join our software engineering team. LumiGuide develops parking guidance systems for cars and bicycles. Our technology is based on optical detection techniques (Computer Vision). A nice example of one of our projects is the P-route Bicycle <https://bicycledutch.wordpress.com/2015/06/09/bicycle-parking-guidance-syst…> in Utrecht City. Functional Programming is used extensively at LumiGuide. Haskell, GHCJS and the Nix stack (Nix, NixOS, hydra and nixops) are some of the technologies we're using. I recently gave a talk <http://foswiki.cs.uu.nl/foswiki/pub/NlFpDay2016/WebHome/LumiGuide_FP-Dag-20…> at the Dutch Functional Programming Day that highlights some of the products and tools we're developing. We're looking for an experienced Haskell developer who wants to join our team full-time and will work from our office in Nijmegen, The Netherlands. As one of the first engineers you'll have a big impact on the culture and the technological direction of the company. If you're interested, please send a mail to bas(a)lumiguide.nl and tell me a bit about your experience (previous jobs, open source projects, things that you made that you're proud of, etc.). Best Regards, Bas van Dijk CTO LumiGuide

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Re: [Haskell-cafe] Proposal: Add functions to get consecutive elements to Data.List
by David Turner 13 Apr '16

13 Apr '16

Hi there, The idiom I normally use for this is 'zip xs (drop 1 xs)'. By using 'drop 1' instead of 'tail' it handles the empty case without partiality or needing an extra equation. Does that work for you? Cheers, David On 12 Apr 2016 21:55, "Johan Holmquist" <holmisen(a)gmail.com> wrote: I propose adding two new functions to Data.List: zipConsecutives :: [a] -> [(a,a)] zipConsecutives xs = zip xs (tail xs) zipConsecutivesWith :: (a -> a -> b) -> [a] -> [b] zipConsecutivesWith f xs = zipWith f xs (tail xs) (with possibly more efficient implementations) The Trac feature request is at https://ghc.haskell.org/trac/ghc/ticket/11815 . Why These functions are useful when one wants to process consecutive pairs of elements in a sequence, which is not uncommon. The ticket lists some examples, reiterated here: -- diff consecutive elements: diffs = zipConsecutivesWith (flip (-)) -- determine if list is ascending (similar for descending and strict): isAscending = and . zipConsecutivesWith (<=) -- an old friend of ours: fibs = 1 : 1 : zipConsecutivesWith (+) fibs -- get the edges of a closed path defined by points (ps): edges ps = zipConsecutivesWith makeEdge (ps ++ take 1 ps) Why add to Data.List (and base) The definition must either use an extra equation for the empty list case: zipConsecutives [] = [] zipConsecutives xs = zip xs (tail xs) which makes it non practical to define inline at each use site. Or one may omit the empty list equation, which is safe (thanks to laziness), but that may not be immediately obvious. (It wasn't to me anyway.) The tail function is generally considered unsafe so it is not desirable to force the user to use it. The proposed functions would offer an alternative. The Data.List module is often imported unqualified, so new identifiers may cause collissions. I do find it rather unlikely that the proposed names would cause such problems, however. Deadline for discussion: 2016-05-31. _______________________________________________ Libraries mailing list Libraries(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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Are libraries expected to enforce safe usage of thread-unsafe functions?
by Thomas Koster 13 Apr '16

13 Apr '16

Hi friends, I am writing a library that has Handle-like values for resources that are not thread-safe (foreign state from a non-thread-safe C library). In other words, a resource can only be "used" by one Haskell thread at a time. Unless I take steps, if one shares the resource between concurrent Haskell threads, bad things will happen at runtime (even with a single-threaded runtime). Naturally, I want consumers to use my library safely, so I am considering making it safe for all by pessimistically enforcing safe usage with MVars. Is it common/expected that Haskell libraries enforce safe access like this? MVars are not free from cost, but are they cheap (i.e. almost as fast as no forced synchronization) as long as no blocking/rescheduling occurs? Thanks, -- Thomas Koster

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Is it safe to postulate () has one inhabitant?
by David Feuer 13 Apr '16

13 Apr '16

The () kind has infinitely many inhabitants: the type '() and any stuck or looping types with appropriate kinds. However, I've not been able to find a way to break type safety with the postulate unitsEqual :: (a :: ()) :~: (b :: ()) in GHC 7.10.3 (it was possible using a now-rejected data family in 7.8.3). Is there some way I haven't found yet, or is this safe? If it's *not* safe, is the equivalent postulate using Void instead of () safe? For my purposes, () would be nicer, but it feels riskier.

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Emacs (was stack and the Atom editor)
by Rustom Mody 12 Apr '16

12 Apr '16

On Sun, Apr 10, 2016 at 7:29 AM, Thomas Koster <tkoster(a)gmail.com> wrote: > > If I can't make Atom work for me, I will drop it just as fast as I > dropped Emacs. Spacemacs was suggested to me off-list; if it addresses > my complaints about Emacs, I might give it a chance. But Atom is the > topic of this thread, so lest this thread descend into a text editor > flame war, I suggest we stop talking about Emacs (or Vim, or Eclipse, > or anything else). > > [Note the changed subject line :-) ] Ive been having a great deal of confusion/trouble with emacs haskell-mode of late. No this is not a gripe about haskell-mode Just that I find myself confused over 1. The old comint interface and new λ one -- when does which kick in 2. Basic setups -- is it called haskell-mode/haskell-emacs or what? 3. I dont quite know when things stopped working/being comfortable/familiar but I remember it as having nothing to do with haskell -- updating org mode or some such Just that I am supposed to be (at least out here!!) an emacs and haskell expert and while showing others how to get started I am often finding my own setups embarrassingly broken Any tips for an old emacs-geezer?

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Re: [Haskell-cafe] Emacs (was stack and the Atom editor)
by Johannes Waldmann 12 Apr '16

12 Apr '16

> ... failed experiments like code folding So, what *is* the recommended method for folding (in emacs)? Yes, ideally we don't need this, because we do not write deeply nested code. But then we have a lot of top-level declarations, which I still want to fold. Yes, if there are too many declarations in one module, then this should be split. But still, that whole mess of declarations needs to be navigated, no matter how it's mapped to files (modules). The question is how it's mapped to the screen real estate. - J.W.

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[TFP 2016] extended deadline, april 25 2016, final call for papers
by Peter Achten 12 Apr '16

12 Apr '16

TFP 2016 has extended its deadline for draft papers by two weeks (now April 25). Although all draft papers accepted to TFP 2016 will be invited to submit to the post-symposium formal proceedings, authors are reminded that they are not obligated to do so; we welcome works in progress that may not be destined for the TFP proceedings. Thanks, David Van Horn ----------------------------- C A L L F O R P A P E R S ----------------------------- ======== TFP 2016 =========== 17th Symposium on Trends in Functional Programming June 8-10, 2016 University of Maryland, College Park Near Washington, DC http://tfp2016.org/ The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions (see below). Authors of draft papers will be invited to submit revised papers based on the feedback receive at the symposium. A post-symposium refereeing process will then select a subset of these articles for formal publication. TFP 2016 will be the main event of a pair of functional programming events. TFP 2016 will be accompanied by the International Workshop on Trends in Functional Programming in Education (TFPIE), which will take place on June 7nd. == INVITED SPEAKERS == TFP 2016 is pleased to announce keynote talks by the following two invited speakers: * Ronald Garcia, University of British Columbia: "Static and Dynamic Type Checking: A Synopsis" * Steve Zdancewic, University of Pennsylvania: "Type- and Example-Driven Program Synthesis" == HISTORY == The TFP symposium is the heir of the successful series of Scottish Functional Programming Workshops. Previous TFP symposia were held in * Edinburgh (Scotland) in 2003; * Munich (Germany) in 2004; * Tallinn (Estonia) in 2005; * Nottingham (UK) in 2006; * New York (USA) in 2007; * Nijmegen (The Netherlands) in 2008; * Komarno (Slovakia) in 2009; * Oklahoma (USA) in 2010; * Madrid (Spain) in 2011; * St. Andrews (UK) in 2012; * Provo (Utah, USA) in 2013; * Soesterberg (The Netherlands) in 2014; * and Inria Sophia-Antipolis (France) in 2015. For further general information about TFP please see the TFP homepage. (http://www.tifp.org/) == SCOPE == The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles: leading-edge, previously unpublished research work Position Articles: on what new trends should or should not be Project Articles: descriptions of recently started new projects Evaluation Articles: what lessons can be drawn from a finished project Overview Articles: summarizing work with respect to a trendy subject Articles must be original and not simultaneously submitted for publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include, but are not limited to: Functional programming and multicore/manycore computing Functional programming in the cloud High performance functional computing Extra-functional (behavioural) properties of functional programs Dependently typed functional programming Validation and verification of functional programs Debugging and profiling for functional languages Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. Interoperability with imperative programming languages Novel memory management techniques Program analysis and transformation techniques Empirical performance studies Abstract/virtual machines and compilers for functional languages (Embedded) domain specific languages New implementation strategies Any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2016 program chair, David Van Horn. == BEST PAPER AWARDS == To reward excellent contributions, TFP awards a prize for the best paper accepted for the formal proceedings. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are listed as first authors, and a student would present the paper. A prize for the best student paper is awarded each year. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, that paper will then receive both prizes. == SPONSORS == TFP is financially supported by CyberPoint, Galois, Trail of Bits, and the University of Maryland Computer Science Department. == PAPER SUBMISSIONS == Acceptance of articles for presentation at the symposium is based on a lightweight peer review process of extended abstracts (4 to 10 pages in length) or full papers (20 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate which authors are research students, and whether the main author(s) are students. A draft paper for which ALL authors are students will receive additional feedback by one of the PC members shortly after the symposium has taken place. We use EasyChair for the refereeing process. Papers must be submitted at: https://easychair.org/conferences/?conf=tfp2016 Papers must be written in English, and written using the LNCS style. For more information about formatting please consult the Springer LNCS web site: http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0 == IMPORTANT DATES == Submission of draft papers: April 25, 2016 Notification: May 2, 2016 Registration: May 13, 2016 TFP Symposium: June 8-10, 2016 Student papers feedback: June 14, 2016 Submission for formal review: July 14, 2016 Notification of acceptance: September 14, 2016 Camera ready paper: October 14, 2016 == PROGRAM COMMITTEE == Amal Ahmed Northeastern University (US) Nada Amin École Polytechnique Fédérale de Lausanne (CH) Kenichi Asai Ochanomizu University (JP) Małgorzata Biernacka University of Wroclaw (PL) Laura Castro University of A Coruña (ES) Ravi Chugh University of Chicago (US) Silvia Ghilezan University of Novi Sad (SR) Clemens Grelck University of Amsterdam (NL) John Hughes Chalmers University of Technology (SE) Suresh Jagannathan Purdue University (US) Pieter Koopman Radboud University Nijmegen (NL) Geoffrey Mainland Drexel University (US) Chris Martens University of California, Santa Cruz (US) Jay McCarthy University of Massachusetts, Lowell (US) Heather Miller École Polytechnique Fédérale de Lausanne (CH) Manuel Serrano INRIA, Sophia-Antipolis (FR) Scott Smith Johns Hopkins University (US) Éric Tanter University of Chile (CL) David Van Horn (Chair) University of Maryland (US) Niki Vazou University of California, San Diego (US) Stephanie Weirich University of Pennsylvania (US)

1 0

Haskell DockerHub Official Repository: Needs Maintainer TLC
by Peter Salvatore 12 Apr '16

12 Apr '16

Greetings, I have a confession to make: I've been a neglectful maintainer of the "haskell" Official Repository on DockerHub. I'm hoping to find a proper maintainer who is both a Docker nerd and a Haskell enthusiast to ensure that it is kept up to date and improved over time. Bonus points for someone involved in GHC development, or at an organization that is heavily involved in the Haskell community. I'm happy to help you get up to speed with where the image is at today and how to effectively submit future iterations after you've had a chance to review the DockerHub Official Repositories documentation at https://docs.docker.com/docker-hub/official_repos/. The current Dockerfiles are located at https://github.com/freebroccolo/docker-haskell and there are two other maintainers who also contribute occasionally. Thanks,

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Re: [Haskell-cafe] Is it safe to postulate () has one inhabitant?
by David Feuer 12 Apr '16

12 Apr '16

Great. My aim is to convert a poly-kinded implementation, s, of type-aligned sequences into an implementation of regular sequences by using newtype AsUnitLoop a (x :: ()) (y :: ()) = UL a newtype Lower s a = Lower' (s (AsUnitLoop a) '() '()) Using the postulate, I can "prove" that s (AsUnitLoop a) x y ~ s (AsUnitLoop a) '() '() So when I take the tail/init of a sequence I will again get a sequence, and two sequences holding values of the same type can be concatenated. The original reflection-without-remorse code uses data AsUnitLoop a (x :: *) (y :: *) where UL : a -> AsUnitLoop a () () which adds an extra constructor to each cell. With the postulate, I get what I want for free. I'm *hoping* it's possible to use a pattern synonym, Lower, that converts s (AsUnitLoop a) '() '() (requiring '()) to Lower s a but that provides only exists x y . s (AsUnitLoop a) x y when matching. This way, the postulate won't "leak" to other code unless it imports a Postulate module. Unfortunately, the pattern synonym documentation is a bit vague on types. On Apr 11, 2016 8:49 PM, "Richard Eisenberg" <eir(a)cis.upenn.edu> wrote: I believe this is safe, yes. Saying the kind () has infinitely many types in it is somewhat like saying the type () has infinitely many terms in it, like (), undefined, undefined 1, undefined False, ... With the change to type families in the past few years, it should be impossible to match on any inhabitant of the kind () except the type '(). That makes the postulate you postulate quite sensible. Richard On Apr 11, 2016, at 4:07 PM, David Feuer <david.feuer(a)gmail.com> wrote: The () kind has infinitely many inhabitants: the type '() and any stuck or looping types with appropriate kinds. However, I've not been able to find a way to break type safety with the postulate unitsEqual :: (a :: ()) :~: (b :: ()) in GHC 7.10.3 (it was possible using a now-rejected data family in 7.8.3). Is there some way I haven't found yet, or is this safe? If it's *not* safe, is the equivalent postulate using Void instead of () safe? For my purposes, () would be nicer, but it feels riskier. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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Differences between QuasiQuotes and TemplateHaskell
by Dominik Bollmann 11 Apr '16

11 Apr '16

Hi, Originally, I thought that QuasiQuotes build on top of TemplateHaskell, since they just provide a way of parsing a string into a Template Haskell AST. That is, a quasi-quoter is defined by four parsers of types String -> Q Exp String -> Q Pat String -> Q Type String -> Q Dec that parse EDSLs into corresponding Haskell (abstract) syntax. However, while playing around with the Shakespeare package's quasiquoter hamlet, I noticed that (e.g.) a snippet html = [hamlet| <html> <body> <h1>Hello World |] () has type Html and not (as I would expect) Q Exp. Hence, it seems as if quasiquotes are spliced in implicitly? This confuses me since in Template Haskell in general a quote (e.g.,) [e| \x -> x |] has type Q Exp, which then needs to be explicitly spliced in order to produce the id function \x -> x. So why do quasiquotes not have types Q Exp, but rather concrete types? If anyone could clarify my confusion, this would be great! Dominik.

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MPTC and quantification in GADTs vs. regular ADTs
by Kosyrev Serge 11 Apr '16

11 Apr '16

Good day folks! What is the explanation for the difference in GHC reaction towards the two variants of the following piece of code: > {-# LANGUAGE GADTs #-} > {-# LANGUAGE RankNTypes #-} > {-# LANGUAGE RecordWildCards #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE UnicodeSyntax #-} > > import Prelude.Unicode > > class MPTC row col where > class TC a where > method ∷ a → a > 1 data T row col where 1 T ∷ ∀ row col. MPTC row col ⇒ 1 { col ∷ col 1 , row ∷ row } → T row col 2 data T row col 2 = ∀ row col. MPTC row col ⇒ 2 T { col ∷ col 2 , row ∷ row } > > instance MPTC row col ⇒ TC (T row col) where > method w@T{..} = > seq (recover (toCol w) row) w > > toCol ∷ MPTC row col ⇒ T row col → col > toCol = (⊥) > > recover ∷ MPTC row col ⇒ col → row → T row col > recover = T Note the missing > instance MPTC row col Nevertheless, the former typechecks, while the latter is rejected with: > mptc-data-vs-gadt.hs:23:14-34: error: … > • Could not deduce (MPTC row1 col) arising from a use of ‘recover’ > from the context: MPTC row col > bound by the instance declaration > at mptc-data-vs-gadt.hs:21:10-38 > or from: MPTC row1 col1 > bound by a pattern with constructor: > T :: forall row col row col. > MPTC row col => > col -> row -> T row col, > in an equation for ‘method’ > at mptc-data-vs-gadt.hs:22:14-18 > Possible fix: > add (MPTC row1 col) to the context of the data constructor ‘T’ > • In the expression: (recover (toCol w) row) > In the expression: seq (recover (toCol w) row) w > In an equation for ‘method’: > method w@(T {..}) = seq (recover (toCol w) row) w I'm sure this must have something to do with GADTs carrying type equality constraints, but still I don't quite understand the difference in interpretation of the forall quantification. To my understanding, a value bound by the pattern 'w@T{..}' ought to be treated as sufficient evidence for 'MPTC row col', no matter whether it is a GADT or not. ..but it isn't. -- с уважениeм / respectfully, Косырев Сергей

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C2HS - marchalling arrays
by Alexey Raga 10 Apr '16

10 Apr '16

Hi, I am using c2hs and I want to create bindings for a C function with the following signature: my_function(my_struct_t*, int* values, size_t num_values); Ideally I'd like to have a function that accepts [CInt], not a pair (pointer, size). What is the easiest way to do it? Can c2hs help me with this or I need to do it manually? Regards, Alexey.

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SBLP 2016 Deadline Extension
by Simon Thompson 09 Apr '16

09 Apr '16

We'd like to let you know that, due to multiple requests, we have extended the deadline for both abstract and paper submissions to SBLP. The new dates are the following: Abstract submission (extended!): April 22nd 2016 Paper submission (extended!): April 29th 2016 Author notification: June 10th 2016 Camera ready deadline: June 24th 2016 Please distribute the attached call for papers (with the extended deadlines) wherever you may find appropriate. Do not hesitate to contact us if you have any questions. Kind regards, Fernando Castor Yu David Liu The Brazilian Symposium on Programming Languages is a well-established symposium which provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. SBLP 2016 will be held in Maringá, in the Southern region of Brazil, and will be the 20th edition of the symposium. SBLP is part of the 7th edition of CBSoft, the Brazilian Congress on Software: Theory and Practice. More information is available at http://cbsoft.org/sblp2016/xx-brazilian-symposium-on-programming-languages. IMPORTANT DATES Abstract submission: April 22nd 2016 Paper submission: April 29th 2016 Author notification: June 10th 2016 Camera ready deadline: June 24th 2016 Symposium dates: September 22nd and 23rd Authors are invited to submit original research on any relevant topic which can be either in the form of regular or short papers. TOPICS Topics of interest include, but are not limited to: - Program generation and transformation, including domain-specific languages, and model-driven development in the context of programming languages. - Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. - Formal semantics and theoretical foundations, including denotational, operational, algebraic, and categorical. - Program analysis and verification, including type systems, static analysis, and abstract interpretation. - Programming language design and implementation, including new programming models, programming language environments, compilation, and interpretation techniques. SUBMISSION AND PUBLICATION All submissions will be peer-reviewed and judged on the basis of its originality, contribution to the field, technical and presentation quality, and relevance to the symposium. Contributions should be written in Portuguese or English. Papers should fall into one of two different categories: regular papers, which can be up to 15 pages long in LNCS format, or short papers, with up to 6 pages in LNCS format. Short papers can discuss new ideas which are at an early stage of development and which have not yet been thoroughly evaluated. We encourage the submission of short papers reporting partial results of on-going master dissertations or doctoral theses. Accepted papers written in English will be published in a volume of Lecture Notes in Computer Science (LNCS), by Springer. Both regular and short papers must be prepared using the LNCS format, available at http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0. Papers must be submitted electronically (in PDF format) via the Easychair System: http://www.easychair.org/conferences/?conf=sblp2016. As in previous editions, after the conference, authors of selected regular papers will be invited to submit an extended version of their work to be considered for publication in a journal special issue. Since 2009, selected papers of each SBPL edition are being published in a special issue of Science of Computer Programming, by Elsevier. PROGRAM CHAIRS Fernando Castor, Federal University of Pernambuco Yu David Liu, State University of New York, Binghamton PROGRAM COMMITTEE Luis Barbosa, University of Minho Thiago Tonelli Bartolomei, LogicBlox Mariza Bigonha, Federal University of Minas Gerais Roberto Bigonha, Federal University of Minas Gerais Andre Rauber Du Bois, Federal University of Pelotas Christiano Braga, Fluminense Federal University Carlos Camarão, Federal University of Minas Gerais Francisco Carvalho-Junior, Federal University of Ceara Fernando Castor, Federal University of Pernambuco Marcelo D'Amorim, Federal University of Pernambuco João Paulo Fernandes, University of Beira Interior João Ferreira, Teesside University Lucilia Figueiredo, Federal University of Ouro Preto Ismael Figueroa, Pontificia Universidad Católica de Valparaiso Alex Garcia, IME Rodrigo Geraldo, Federal University of Ouro Preto Roberto Ierusalimschy, PUC-Rio Rafael Lins, Federal University of Pernambuco Yu David Liu, State University of New York at Binghamton Hans-Wolfgang Loidl, Heriot-Watt University Marcelo Maia, Federal University of Uberlândia Manuel-A. Martins, University of Aveiro Fabio Mascarenhas, Federal University of Rio de Janeiro Sérgio Medeiros, Federal University of Rio Grande do Norte Ana Milanova, Rensselaer Polytechnic Institute Alvaro Moreira, Federal University of Rio Grande do Sul Martin Musicante, Federal University of Rio Grande do Norte Bruno Oliveira, The University of Hong Kong Zachary Palmer, Swarthmore College Alberto Pardo, Universidad de la República Fernando Pereira, Federal University of Minas Gerais Gustavo Pinto, Federal Institute of Science and Technology of Para Louis-Noel Pouchet, University of California Zongyan Qiu, Peking University Sandro Rigo, State University of Campinas Noemi Rodriguez, PUC-Rio João Saraiva, University of Minho Doaitse Swierstra, Utrecht University Leopoldo Teixeira, Federal University of Pernambuco Simon Thompson, University of Kent Varmo Vene, University of Tartu

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Extending Compiler With Authorized Types
by Jonn Mostovoy 07 Apr '16

07 Apr '16

Dear friends, we are thinking about implementing the paper Authenticated Data Structures, Generically[1], but we did not have any prior experience with extending Haskell. We are informed about compiler plugins[2] and hope that writing a compiler plugin will be enough to provide machinery required by the paper, but we are not sure about it. If anyone has an opinion on bringing Authenticated Data Structures to Haskell, please share. If there is some interest in our work, we will keep you up to date in this thread. - - - References: [1]: http://f.nn.lv/o5/a7/yk/GPADS-A_Miller-Authenticated_Data_Structures_Generi… [2]: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/compiler-pl… — Kindest regards, Jonn Mostovoy (IOHK / Serokell)

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Text.Regex.matchRegex: Success is empty?
by Jeffrey Brown 05 Apr '16

05 Apr '16

Esteemed Haskellers, It is working for me, but I am puzzled by the matchRegex function: > import Text.Regex > :i matchRegex matchRegex :: Regex -> String -> Maybe [String] -- Defined in ‘Text.Regex’ Searches that fail return Nothing: > matchRegex (mkRegex "z") "abracadabra" Nothing which makes sense. But why do searches that succeed return an empty list of Strings? > matchRegex (mkRegex "a") "abracadabra" Just [] > -- Jeffrey Benjamin Brown

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accessing the RTS -M flag value from GHC.RTS.Flags
by Adam Flott 05 Apr '16

05 Apr '16

Given, import GHC.RTS.Flags main = do gcf <- getRTSFlags print (maxHeapSize (gcFlags gcf)) And running on x86-64 in GHC 7.10.3, $ ./ghcflags 0 $ ./ghcflags +RTS -M1M 256 $ ./ghcflags +RTS -M1024K 256 $ ./ghcflags +RTS -M4096K 1024 Is there anyway I can get access to the value stored in the -M flag and depend on that value being correct? I was not able find it in the user's guide or anywhere else for that matter. Thanks, Adam

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[ANN] protobuf-simple: Protocol Buffers library
by Martijn Rijkeboer 05 Apr '16

05 Apr '16

Hello, I've just released protobuf-simple, an Haskell implementation of Google's Protocol Buffers with an emphasis on simplicity. This implementation consists of a library for the encoding and decoding of data and an executable for generating Haskell types from proto files. The code can be found at https://github.com/sru-systems/protobuf-simple Kind regards, Martijn Rijkeboer

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Re: [Haskell-cafe] [ANN] protobuf-simple: Protocol Buffers library
by Martijn Rijkeboer 05 Apr '16

05 Apr '16

Hi Alain, > What version of Protocol Buffer are you supporting (I did not find this > info, sorry) ? Are you planning on supporting the V3 if you don't > already? The protobuf-simple library only supports V2. The version wasn't specified anywhere, my bad. I've just added it to the readme. Currently I have no plans supporting V3, since I have no use for it. However, pull requests are always welcome, providing that the library remains "simple". Kind regards, Martijn Rijkeboer

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Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 05 Apr '16

05 Apr '16

If you or one of your students recently completed a PhD in the area of functional programming, please submit the dissertation abstract for publication in JFP: simple process, no refereeing, deadline 30th April 2016. Best wishes, Graham ============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 30th April 2016 http://tinyurl.com/jfp-phd-abstracts ============================================================ PREAMBLE: Many students complete PhDs in functional programming each year. As a service to the community, the Journal of Functional Programming publishes the abstracts from PhD dissertations completed during the previous year. The abstracts are made freely available on the JFP website, i.e. not behind any paywall. They do not require any transfer of copyright, merely a license from the author. A dissertation is eligible for inclusion if parts of it have or could have appeared in JFP, that is, if it is in the general area of functional programming. The abstracts are not reviewed. Please submit dissertation abstracts according to the instructions below. We welcome submissions from both the PhD student and PhD advisor/supervisor although we encourage them to coordinate. ============================================================ SUBMISSION: Please submit the following information to Graham Hutton <graham.hutton(a)nottingham.ac.uk> by 30th April 2016. o Dissertation title: (including any subtitle) o Student: (full name) o Awarding institution: (full name and country) o Date of PhD award: (month and year; depending on the institution, this may be the date of the viva, corrections being approved, graduation ceremony, or otherwise) o Advisor/supervisor: (full names) o Dissertation URL: (please provide a permanently accessible link to the dissertation if you have one, such as to an institutional repository or other public archive; links to personal web pages should be considered a last resort) o Dissertation abstract: (plain text, maximum 1000 words; you may use \emph{...} for emphasis, but we prefer no other markup or formatting in the abstract, but do get in touch if this causes significant problems) Please do not submit a copy of the dissertation itself, as this is not required. JFP reserves the right to decline to publish abstracts that are not deemed appropriate. ============================================================ PHD ABSTRACT EDITOR: Graham Hutton School of Computer Science University of Nottingham Nottingham NG8 1BB United Kingdom ============================================================ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Darcs+Pijul Hacking Sprint #11 (May 6th-8th, Helsinki)
by Ganesh Sittampalam 04 Apr '16

04 Apr '16

Dear Hackers, The 11th Darcs Sprint, organised by and jointly with the Pijul team, will be in Helsinki, on Friday May 6th-Sunday May 8th, at Aalto University's startup sauna in the Otaniemi Campus. Please check the details at http://pijul.org/2016/04/02/sprint.html Many thanks to Pierre-Étienne Meunier for organising it. The Darcs planning page is at: http://darcs.net/Sprints/2016-05 Here are three things to know: 1. Everybody is welcome to join us. We'd love to have you, whatever your hacking experience with Pijul or Darcs. Also, if you've got a wacky idea for the future of version control, or a cool use for the Darcs or Pijul libraries, you should join us too. 2. If you're attending please add your name to http://wiki.darcs.net/Sprints/2016-05 3. We may be able to reimburse travel costs (within reason!). Let us know if you'd like a reimbursement, and save your receipts. Many thanks to everybody who participated in our fundraising drives or who gave money on the side. Thanks also to the Software Freedom Conservancy for making fundraising and reimbursements so painless! If you can't join us in person, but you'd like to cheer us on, say hello at http://darcs.net/Donations ! See you next month! Ganesh

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Benchmarking harnesses for a more modern nofib?
by Ryan Newton 04 Apr '16

04 Apr '16

Hi all, Is anyone currently working in, or interested in helping with, a new benchmark suite for Haskell? Perhaps, packaging up existing apps and app benchmarks into a new benchmark suite that gives a broad picture of Haskell application performance today? Background: We run nofib, and we run the shootout benchmarks. But when we want to evaluate basic changes to GHC optimizations or data representation, these really don't give us a full picture of whether a change is beneficial. A few years ago, fibon <https://hackage.haskell.org/package/fibon> tried to gather some Hackage benchmarks. This may work even better with Stackage, where there are 180 benchmark suites among the 1770 packages currently. Also, these days companies are building substantial apps in Hackage. Which substantial apps could or should go into a benchmark suite? I see Warp and other web server benchmarks <http://www.infoq.com/news/2015/04/web-frameworks-benchmark-2015?utm_source=…> all over the web. But is there a harness that can time some of this code while running inside a single-machine, easy-setup benchmark suite? Best, -Ryan

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[ANN] bond-haskell: yet another data serialization library
by Andrey Sverdlichenko 04 Apr '16

04 Apr '16

bond-haskell and bond-haskell-compiler is a pair of packages adding Haskell support to bond serialization framework. Bond is an open source, cross-platform framework for working with schematized data. It supports cross-language serialization/deserialization and powerful generic mechanisms for efficiently manipulating data. It is broadly used at Microsoft in high scale services. For a discussion how Bond compares to similar frameworks (most notably protobufs) see https://microsoft.github.io/bond/why_bond.html Microsoft's bond package or other foreign code is not needed for building or using bond-haskell, all dependencies are published to hackage. For those interested, Microsoft's code is published at https://github.com/Microsoft/bond/

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