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Do all "fromList" functions redirect to List operations?
by martin 14 Apr '15

14 Apr '15

Hello all, I have a datatype like this: data Time = DPast | T Integer deriving (Eq, Show) data Temporal a = Temporal { at :: Time -> (a, Maybe Time) } and I wrote a fromList function to create one. The fromList function implements it own "at" function and wraps it in a Temporal. IIUC, this means that whenever I invoke "at" on a Temporal created from a List I actually operate on that List. Alternatively I can directly create a Temporal as in exNat = Temporal f where f DPast = (0, Just $ T 1) f (T t) = (t, Just $ T $ t+1) whose "at" function is a lot faster. Do all "fromList" functions behave this way, i.e. they redirect operations on the new type to List operations? Is there a way to make a Temporal, created via fromList "forget" its List heritage?

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Eta-reducing case branches
by Tom Ellis 14 Apr '15

14 Apr '15

Has anyone ever considered permitting case branches to be eta reduced? For example, it is often nice to rewrite foo x = bar baz x as foo = bar baz Likewise, I have often wanted to rewrite case m of Nothing -> n Just x -> quux x as case m of Nothing -> n Just -> quux Am I missing an obvious reason this wouldn't work? Tom

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Re: [Haskell-cafe] Help wanted: working on the GHC webpage
by Sergey Bushnyak 14 Apr '15

14 Apr '15

> Why would it be easier? What's difficult about publishing on > https://ghc.haskell.org/trac/ghc/blog? I'm actually don't know how it's published on track. From my standpoint as newcomer it's better to see what's happening from one place, with one design, have some shared git repo where people contribute in markdown. > Moreover, the GHC weekly news are intimately linked to Trac, as they > reference Trac-tickets and Git commits, which Trac is able to annotate > with meta-data (ticket-type, -status, and -title for Ticket references, > as well as part of the Git commit msg for Git-commit refs). Ok, it was just a suggestion. Maybe it's a bad idea, doesn't know about annotation. Anyway, still can help on updating ghc home page.

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Semantics of temporal data
by martin 14 Apr '15

14 Apr '15

Hello all, I wrote a Temporal datatype data Temporal a = Temporal { td :: a, -- "temporal default" tc :: [Change a] -- "temporal changes" } deriving (Eq, Show) data Change a = Chg { ct :: Time, -- "change time" cv :: a -- "change value" } deriving (Eq,Show) I got into semantic trouble when trying to write "join" for Temporal (Temporal a). If I have a Temporal Temporal, then each change's value is a Temporal. The question is: what significance does the temporal default have? In my implementation, when I look at this Chg t0 (Temporal default0 changes0) Chg t1 (Temporal default1 changes1) --<-- you are here Chg t2 (Temporal default2 changes2) I accept the changes1 which are >= t1 and less than t2. So far so good. But I also take the default1 and prepend it to the resuling list of changes with a change time of t1 (unless the first of the changes1 occurs also at t1). But by comparing results with a handwritten <*>, QuickCheck revealed that this is not in line with <*> = ap x >>= f = join $ fmap f x It appears to me that the defaults of the inner Temporal lose most of their meaning. The effective default is the change value of the last accepted change from the previous (t0) iteration. This would make some sense. If I have a summer schedule which lists the location of a single train over time, and it sais it is usually "nowhere", but at July 1st is shall be in Manchester. If I ask this schedule, where the train will be before July 1st it'll say "nowhere". Now if I prepend the summer schedule with a spring schedule, then there will be a last location of that train in the spring schedule (say Brighton), i.e. before the summer schedule becomes effective. Hence at the beginning of the summer, the train won't be "nowhere" but in Brighton. Does this make some sense? Please feel free to comment. BTW: I am really impressed that haskell pointed out this semantic issue to me. This could have gone unnoticed for months in other languages.

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Anyone interested in taking over network-uri?
by Johan Tibell 14 Apr '15

14 Apr '15

Hi! I find myself with less time to hack lately and I'm trying to reduce the number of libraries I maintain so I can invest time in new things. Would anyone be interested taking over maintenance of network-uri? It should not be much work at all. Bump a few dependencies and fix a bug or two. The package is very widely used so I suggest that the new maintainer should avoid breaking changes whenever possible*. * In other words, if you want to completely rethink network URI handling, this package is probably not the place for it. -- Johan

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Haskell Weekly News
by Kim-Ee Yeoh 14 Apr '15

14 Apr '15

*Top picks:* - Aditya Siram announces the first release of Haskell bindings <http://haskell.1045720.n5.nabble.com/Announcement-FLTKHS-Bindings-to-the-FL…> to the C++-based FLTK cross-platform GUI library. FLTK <http://www.fltk.org/> can be used to build a modern windows-and-widgets-based desktop app and comes with a UI builder called FLUID. - The startup called Helium releases a "homegrown Webmachine-inspired web framework in Haskell." <http://blog.helium.com/helium/2015/04/01/helium-webmachine-airship.html> Airship is very small (just under 1000 LoC) and extremely unopinionated: it works with any WAI-compatible web server and any templating language (including none at all!). Reddit discussion. <http://www.reddit.com/r/haskell/comments/312pk2/haskell_webmachine_airship/> - Adam Chlipala releases a new Ur/Web library for producing custom event-planning web apps <http://www.impredicative.com/pipermail/ur/2015-April/001928.html> quickly. All you need to do is "assemble highly parametrized components." - Abe Voelker <https://blog.abevoelker.com/you-got-haskell-in-my-ruby-cleaner-ruby-validat…> finds himself wanting algebraic data types and pattern matching when writing Ruby. He presents an evolution of a file upload validator, culminating in a finale that uses an Either monad provided by a Ruby gem called Kleisli. - Eitan Chatav shares a correct-by-construction JSON serializer/deserializer <http://lpaste.net/130064> using lens-json. - Wouldn't it be neat to get profiling info without stopping the program and pissing off your users? Mark Wotton <https://ghc.haskell.org/trac/ghc/ticket/10235> has filed exactly such a feature request. - Is Call Arity optimization to blame for your 7.10 compilation slowdowns? Joachim Breitner <http://ghc.haskell.org/trac/ghc/ticket/10293> investigates. - Ever felt Haskell on Windows is 2nd class, even though it's Tier-1 according to the Platform? Well, installing hmatrix on Win requires additional steps, as Redditor wrvn <http://www.reddit.com/r/haskell/comments/322iut/trying_to_install_hmatrix_o…> kindly explains. - Do you program in Haskell using emacs? You must be using haskell-mode then. Here's monthly news <https://github.com/haskell/haskell-mode/wiki/Month-in-haskell-mode-March-20…> straight from the haskell-mode development team. - Dominic Steinitz <http://haskell.1045720.n5.nabble.com/A-Random-Strawman-tp5768239.html> raises awareness about the brokenness of System.Random. Solution? Use tf-random for now. - Big number exponentiation segfaults, in this reddit discussion. <http://www.reddit.com/r/haskell/comments/31yajd/can_you_explain_this/> Turns out it's a bug <https://ghc.haskell.org/trac/ghc/ticket/10282> involving the GNU Multi-precision Library. Make sure you have the latest GMP version 6. - Devan Stormont <https://voyageintech.wordpress.com/2015/04/02/haskell-library-for-forecast-…> creates his first hackage library that obtains weather forecast data via a web-based API. He writes, "The really brilliant part is in being able to completely replace a core piece of an app within a single day and having complete confidence in the result. It’s moments like this that make you really happy to be working with such a powerful language as Haskell." - Carl <http://stackoverflow.com/questions/29455850/pretty-printing-with-free-monad…> reminds us that GADT can always be pattern-matched in a case expression. "It's let expressions that cause GHC to provide amusing messages about its brain exploding." - Zohaib Rauf <http://zohaib.me/yet-another-what-is-a-monad-post/> publishes a monad tutorial. He explains that the 'M' in 'M a' is "some metadata wrapped around 'a'." *Tweets of the week:* - John Carmack: <https://twitter.com/ID_AA_Carmack/status/585871417855516672> If I had to write software that my life depended on, I would seriously consider using Haskell. - shanelogsdon: <https://twitter.com/shanelogsdon/status/586145419639578625> tried the #haskell web framework http://www.spock.li/ last night with a meaningless micro benchmark. ~38k req/s is pretty quick - AlexanderKatt: <https://twitter.com/AlexanderKatt/status/583279982186786817> 'it is entirely unnecessary to understand category theory in order to understand monads in #haskell' said the guys who know category theory - justusadam_: <https://twitter.com/justusadam_/status/586939066748379136> I'm warming up to the idea of using #Haskell more. Wonderful language but the syntax and some of the concepts were difficult to understand - least_nathan: <https://twitter.com/least_nathan/status/581544451597709312> Algebraic Data Types Considered Harmful: once you use them, every language lacking them drives you to madness. #LangSec - chwthewke: <https://twitter.com/chwthewke/status/583385239138996224> #Haskell is a very hot programming language :) No, really, an hour of it and my laptop is on the verge of becoming a brown dwarf :D - robinbateboerop: <https://twitter.com/robinbateboerop/status/583396694047481857> Troll tries to get banned from #Haskell IRC channel, decides to learn Haskell <http://buff.ly/1NFKWiX> instead. - stephan_gfx: <https://twitter.com/stephan_gfx/status/583493468162400256> Cool, cabal will multithread installation/build if you run it with the -j option. cabal install -j <package_name> - Pythux: <https://twitter.com/Pythux/status/583535948408958976> One thing I learned at Haskell's meetup yesterday: it's possible to create a start-up with Haskell full stack! #Haskell #Awesome *Quote of the week:* - Lisp, Haskell and other FP are currently killing it in the quiet world of DSLs, especially Mining, Oil & Gas industries which need to parse petaflops of seismic data and reservoir simulations, need rapid prototyping, need formally verified drilling platform components that field engineers can interact with easily, and HPC algorithms for financial trading. -- Source <https://news.ycombinator.com/item?id=9317567#up_9318837> -- Kim-Ee

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ANNOUNCE: fast-tags 1.0
by Evan Laforge 13 Apr '15

13 Apr '15

fast-tags makes vi (and now emacs) tags from haskell source. It does so quickly and incrementally. I have it bound to vim's BufWrite to keep tags always updated, and it's basically instant. It works with hsc, literate haskell, and mid-edit files with syntax errors. I've announced this before, but I'm doing it again because it's been a long time and there was a major update. I bumped to 1.0 to reflect that it's not dead, it just does everything I need it to do, so further development is unlikely unless someone finds a bug. That's just for me of course, if someone else wants to add features they are welcome to do so. The major update was almost entirely thanks to Sergey Vinokurov and a few others who contributed a bunch of patches. Details and changelog are at http://hackage.haskell.org/package/fast-tags-1.0 Source is at https://github.com/elaforge/fast-tags

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[Announcement] FLTKHS - Bindings to the FLTK GUI Toolkit
by aditya siram 13 Apr '15

13 Apr '15

I'm pleased to announce the first release of Haskell bindings [1] to the FLTK GUI [2] toolkit. It now works smoothly on Windows (64-bit), Linux and Mac allowing you to create truly cross-platform native GUI applications in pure Haskell and deploy statically linked executables with no dependencies. Most of the FLTK API is covered except for a few minor widgets which I plan to get to in the next release. Motivation behind the package and installation instructions are found in the Haddocks [3]. And to get you started it ships with a number of demos. If you have any issues please report them on the Github [4] page. I'd also love any other feedback so feel free to comment here or email me at the address listed on the Hackage [5] page. Hope you enjoy! [1] https://hackage.haskell.org/package/fltkhs-0.1.0.2 [2] http://fltk.org [3] https://hackage.haskell.org/package/fltkhs-0.1.0.2/docs/Graphics-UI-FLTK-Lo… [4] http://github.com/deech/fltkhs [5] https://hackage.haskell.org/package/fltkhs-0.1.0.2

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Static executables in minimal Docker containers
by Michael Snoyman 13 Apr '15

13 Apr '15

I'm trying to put together a minimal Docker container consisting of nothing but GHC-compiled static executables. Below you can see a full interaction I've had with GHC and Docker. The high level summary is that: * When compiled statically, the executable runs just fine in both my host OS (Ubuntu 14.04) and an Ubuntu 14.04 Docker image * That same executable run from a busybox (or a "scratch" image, not shown here since it's slightly longer to set up) hangs and then runs out of memory I've watched the process in top, and it uses up a huge amount of CPU and memory. I get the same behavior whether I compiled with or without optimizations or the multithreaded runtime. I also get identical behavior with both GHC 7.8.4 and 7.10.1. I'm not sure how best to proceed with trying to debug this, any suggestions? vagrant@vagrant-ubuntu-trusty-64:~/Desktop$ cat > hello.hs <<EOF main :: IO () main = putStrLn "Static Hello" EOF vagrant@vagrant-ubuntu-trusty-64:~/Desktop$ ghc -optl-static -optl-pthread hello.hs -static [1 of 1] Compiling Main ( hello.hs, hello.o ) Linking hello ... /opt/ghc/7.8.4/lib/ghc-7.8.4/rts-1.0/libHSrts.a(Linker.o): In function `internal_dlopen': (.text+0x5c): warning: Using 'dlopen' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking vagrant@vagrant-ubuntu-trusty-64:~/Desktop$ ./hello Static Hello vagrant@vagrant-ubuntu-trusty-64:~/Desktop$ docker run --rm -v $(pwd)/hello:/usr/local/bin/hello:ro ubuntu:14.04 /usr/local/bin/hello Static Hello vagrant@vagrant-ubuntu-trusty-64:~/Desktop$ docker run --rm -v $(pwd)/hello:/usr/local/bin/hello:ro busybox /usr/local/bin/hello hello: out of memory (requested 1048576 bytes) vagrant@vagrant-ubuntu-trusty-64:~/Desktop$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.8.4

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Re: [Haskell-cafe] Generalized Newtype Deriving not allowed in Safe Haskell
by Simon Peyton Jones 13 Apr '15

13 Apr '15

David If you would like to lead a debate, and drive it to a conclusion, that would be most helpful. Usually it's constructive to write a wiki page that sets out the design choices, with examples to illustrate their consequences, to set the terms of the debate. Otherwise you risk misunderstandings, with red herrings being discussed repeatedly. Thanks Simon | -----Original Message----- | From: davidterei(a)gmail.com [mailto:davidterei@gmail.com] On Behalf Of | David Terei | Sent: 12 April 2015 09:52 | To: Simon Peyton Jones | Cc: Omari Norman; ghc-devs(a)haskell.org; haskell Cafe | Subject: Re: Generalized Newtype Deriving not allowed in Safe Haskell | | On 10 April 2015 at 01:48, Simon Peyton Jones <simonpj(a)microsoft.com> | wrote: | > | prefer, such as only exporting the Coerce instance if all the | > | constructors are exported, it seems that the ship sailed on these | > | > Coercible is relatively recent; I don't think we should regard it as | cast in stone. | > | > But yes, the Coerbible instance of a newtype is only available when | the data constructor for the newtype is lexically in scope. | | Yes, so as you point out in the paper, this is done to preserve | abstractions, but the same rule isn't applied to data types since some | types like IORef don't even have constructors that can be in scope. | | Ideally I'd like to find a way forward that works for everyone and | isn't just a Safe Haskell mode setting. | | I think the first question is, are there situations where you'd want to | use `coerce` internally to a module but disallow it externally? The | role mechanism is a little awkward as it doesn't allow this (although | it does for newtype's). If yes, then I think we should start there. | | If it seems we don't need external vs internal control, then we could | simply change the default to be that GHC sets referential type | parameters to nominal and allows them to be weakened to referential | through role annotations. We could use hackage to test how much | breakage this would cause. | | The third option is something Safe Haskell specific, so probably | applying the newtype constructor rule to data types. | | > | > Simon | > | > | -----Original Message----- | > | From: davidterei(a)gmail.com [mailto:davidterei@gmail.com] On Behalf | > | Of David Terei | > | Sent: 10 April 2015 09:38 | > | To: Simon Peyton Jones | > | Cc: Omari Norman; haskell Cafe; ghc-devs(a)haskell.org | > | Subject: Re: Generalized Newtype Deriving not allowed in Safe | > | Haskell | > | | > | I'll prepare a patch for the userguide soon. | > | | > | As for something better, yes I think we can and should. It's on my | > | todo list :) Basically, the new-GND design has all the mechanisms | > | to be safe, but sadly the defaults are rather worrying. Without | > | explicit annotations from the user, module abstractions are | broken. | > | This is why we left GND out of Safe Haskell for the moment as it | is | > | a subtle and easy mistake to make. | > | | > | If the module contained explicit role annotations then it could be | > | allowed. The discussion in | > | https://ghc.haskell.org/trac/ghc/ticket/8827 has other solutions | > | that I prefer, such as only exporting the Coerce instance if all | > | the constructors are exported, it seems that the ship sailed on | > | these bigger changes sadly. | > | | > | Cheers, | > | David | > | | > | On 9 April 2015 at 00:56, Simon Peyton Jones | > | <simonpj(a)microsoft.com> | > | wrote: | > | > There is a long discussion on | > | > https://ghc.haskell.org/trac/ghc/ticket/8827 | > | > about whether the new Coercible story makes GND ok for Safe | Haskell. | > | > At a type-soundness level, definitely yes. But there are other | > | > less-clear-cut issues like “breaking abstractions” to consider. | > | The > decision on the ticket > (comment:36) seems to be: GND | stays | > | out of Safe Haskell for now, but > there is room for a better | > | proposal. | > | > | > | > | > | > | > | > I don’t have an opinion myself. David Terei and David Mazieres | > | are in > the driving seat, but I’m sure they’ll be responsive to | user input. | > | > | > | > | > | > | > | > However, I think the user manual may not have kept up with | #8827. | > | The | > | > sentence “GeneralizedNewtypeDeriving — It can be used to violate | > | > constructor access control, by allowing untrusted code to | > | manipulate > protected data types in ways the data type author did | > | not intend, > breaking invariants they have established.” | vanished | > | from the 7.8 > user manual (links below). Maybe it should be | restored. | > | > | > | > | > | > | > | > Safe Haskell aficionados, would you like to offer a patch for | the | > | manual? | > | > And maybe also a less drastic remedy than omitting GND | altogether? | > | > | > | > | > | > | > | > Simon | > | > | > | > | > | > | > | > From: Omari Norman [mailto:omari@smileystation.com] > Sent: 09 | > | April 2015 02:44 > To: haskell Cafe > Subject: Generalized | Newtype | > | Deriving not allowed in Safe Haskell > > > > When compiling | code | > | with Generalized Newtype Deriving and the > -fwarn-unsafe flag, I | > | get > > > > -XGeneralizedNewtypeDeriving is not allowed in Safe | > | Haskell > > > > This happens both in GHC 7.8 and GHC 7.10. | > | > | > | > | > | > | > | > I thought I remembered reading somewhere that GNTD is now part | of | > | the > safe language? The GHC manual used to state that GNTD is | not | > | allowed > in Safe > Haskell: | > | > | > | > | > | > | > | > | > | | https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/safe- | > | ha | > | > skell.html#safe-language | > | > | > | > | > | > | > | > But this language on GNTD not being part of the safe language | was | > | > removed in the 7.8 manual: | > | > | > | > | > | > | > | > | > | | https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_guide/safe- | > | ha | > | > skell.html#safe-language | > | > | > | > | > | > | > | > The GHC release notes don't say anything about this one way or | > | the other. | > | > Thoughts? | > | > | > | > | > | > _______________________________________________ | > | > ghc-devs mailing list | > | > ghc-devs(a)haskell.org | > | > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs | > | > | > _______________________________________________ | > ghc-devs mailing list | > ghc-devs(a)haskell.org | > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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