- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Hackage and Free Software
by Mike Meyer 03 Mar '15

03 Mar '15

On Tue, Mar 3, 2015 at 3:19 AM, fr33domlover <fr33domlover(a)riseup.net> wrote: > On Mon, 2 Mar 2015 16:27:53 -0600 > Mike Meyer <mwm(a)mired.org> wrote: > > Until you've got an objective set of ethics - or a definition of "software > > freedom" - that everyone accepts, that's just a long-winded way of saying > > "arbitrary". > Indeed there is an objective clear definition: > http://www.gnu.org/philosophy/free-sw.html Applying that definition may be objective, but not everyone agrees that it's a correct definition of free software, which makes it a subjective definition. > > Not quite. "OtherLicense" is an accepted license tag, and I take it to mean > > I can use any license I want. If you're going to place a restriction on the > > license types beyond "use one of our tags" (and if you disallow the > > otherLicense tag, then I'd say that's an arbitrary restriction), then you > > should either define the terms in it, or choose terms that are well > > defined. "free software" is so ill defined that gnu.org has to explain what > > they mean by "free software" ( https://www.gnu.org/philosophy/free-sw.html) > > They even point out that there are open source software licenses that don't > > meet their definition of free ( > > https://www.gnu.org/philosophy/open-source-misses-the-point.html) Their > > definition of proprietary as "not free" makes software licensed under such > > licenses proprietary, though that's certainly not common usage. > "Open source misses the point" talks about the open source movement - it > doesn't say the BSD, MIT or Apache are not free software licenses. They are! > gnu.org provides a definition of free software, which makes it quite well > defined. There's even a list of licenses. While the BSD, MIT and Apache licenses are free, the GNU license list (https://www.gnu.org/philosophy/license-list.html#NonFreeSoftwareLicenses) provides a long list of open source licenses that aren't free. Some of them aren't free because they are truly noxious, some aren't free because they are poorly written, and some aren't free because the developers restrictions they feel are reasonable, but violate that letter if not the spirit of that definition. > There is nothing arbitrary about it - in the same way the law that puts > murderers in prison isn't arbitrary. It's based on ethics: the value of human > life. Free software is similarly based on the value people's freedom to control > their computing, know what they run and be able to adapt and spread it. Yes, but not everyone agrees to something you would think would be clearcut, like the value of a human life. For instance, some cultures feel that giving your own life for the good of your religion is the best thing you can do with it, and think nothing of taking away some non-believers life for that cause. And in other cultures, your life isn't yours but the states, and you don't have the right to end it yourself. Since there's such broad disagreement on the value of a human life, then it should be no surprise that people disagree on an idea as recent as "free software". For instance, I find it a bit ironic that a defintion of "free software" puts restrictions on derived works that the original author doesn't have to abid by. > > So just saying "only free software licenses" or "no proprietary software" > > would make matters worse, not better, because those terms have multiple > > meanings in common use. And that makes them not only arbitrary, but vague. > The FSF's definition is the only definition I know of. If people understand it > in a different way, this only strengthens my point: make it official and > explain the details and rules, so people do understand what free software is. > If hackage.haskell.org explains this, there will be nothing vague anymore. Pointing to an external definition is certainly an acceptable way to clear up the issue of the meaning of the phrase. However, I object to any definition that excludes open source licenses that would otherwise be useable under the otherLicnese tag for reasons I find capricious (i.e - any license with a "no commercial use" clause is excluded, and the lovely "must use for good" clause in the JSON license causes it to be excluded). The OSI's definition of open source doesn't have any of those problems. How about using it: http://opensource.org/definition <mike

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A Proposed Law for Foldable?
by Gershom B 02 Mar '15

02 Mar '15

For a long time, many people, including me, have said that "Foldable has no laws" (or Foldable only has free laws) -- this is true, as it stands, with the exception that Foldable has a non-free law in interaction with Traversable (namely that it act as a proper specialization of Traversable methods). However, I believe that there is a good law we can give for Foldable. I earlier explored this in a paper presented at IFL 2014 but (rightfully) rejected from the IFL post-proceedings. ( http://gbaz.github.io/slides/buildable2014.pdf) That paper got part of the way there, but I believe now have a better approach on the question of a Foldable law -- as sketched below. I think I now (unlike in the paper) can state a succinct law for Foldable that has desired properties: 1) It is not "free" -- it can be violated, and thus stating it adds semantic content. 2) We typically expect it to be true. 3) There are no places where I can see an argument for violating it. If it pans out, I intend to pursue this and write it up more formally, but given the current FTP discussion I thought it was worth documenting this earlier rather than later. For simplicity, I will state this property in terms of `toList` although that does not properly capture the infinite cases. Apologies for what may be nonstandard notation. Here is the law I think we should discuss requiring: * * * Given Foldable f, then forall (g :: forall a. f a -> Maybe a), (x :: f a). case g x of Just a --> a `elem` toList x * * * Since we do not require `a` to be of type `Eq`, note that the `elem` function given here is not internal to Haskell, but in the metalogic. Furthermore, note that the use of parametricity here lets us make an "end run" around the usual problem of giving laws to Foldable -- rather than providing an interaction with another class, we provide a claim about _all_ functions of a particular type. Also note that the functions `g` we intend to quantify over are functions that _can be written_ -- so we can respect the property of data structures to abstract over information. Consider data Funny a = Funny {hidden :: a, public :: [a]} instance Foldable Funny where foldMap f x = foldMap f (public x) Now, if it is truly impossible to ever "see" hidden (i.e. it is not exported, or only exported through a semantics-breaking "Internal" module), then the Foldable instance is legitimate. Otherwise, the Foldable instance is illegitimate by the law given above. I would suggest the law given is "morally" the right thing for Foldable -- a Foldable instance for `f` should suggest that it gives us "all the as in any `f a`", and so it is, in some particular restricted sense, initial among functions that extract as. I do not suggest we add this law right away. However, I would like to suggest considering it, and I believe it (or a cleaned-up variant) would help us to see Foldable as a more legitimately lawful class that not only provides conveniences but can be used to aid reasoning. Relating this to adjointness, as I do in the IFL preprint, remains future work. Cheers, Gershom

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Re: [Haskell-cafe] Skolems!
by evan＠evan-borden.com 02 Mar '15

02 Mar '15

An extension of the message Tom sent a little while back, we've discovered a more in depth example of this possible GHC bug. It is exacerbated by GADTs, but can be fixed with NoMonoLocalBinds. Without GADTs and just leveraging ExistentialQuanitification it works fine. We've included a pretty exhaustive set of examples. {-# LANGUAGE ExistentialQuantification, GADTs #-} {- removing MonoLocalBinds fixes all of these errors {-# LANGUAGE ExistentialQuantification, GADTs, NoMonoLocalBinds #-} -} module PossibleGHCBug where data SumType = SumFoo | SumBar class SomeClass a where someType :: a -> SumType data SomeExistential = forall a. SomeClass a => SomeExistential a noError :: String -> [SomeExistential] -> String noError n st = n ++ concatMap cname st where cname (SomeExistential p) = d p d p = c $ someType p c p = case p of SumFoo -> "foo" _ -> "asdf" noError2 :: String -> [SomeExistential] -> String noError2 n st = n ++ concatMap cname st where cname (SomeExistential p) = d p d p = c $ someType p c :: SumType -> String c p = case p of SumFoo -> "foo" _ -> "asdf" ++ n noError3 :: String -> [SomeExistential] -> String noError3 n st = n ++ concatMap cname st where cname (SomeExistential p) = d p d :: SomeClass a => a -> String d p = c $ someType p c p = case p of SumFoo -> "foo" _ -> "asdf" ++ n partialTypedError :: String -> [SomeExistential] -> String partialTypedError n st = n ++ concatMap cname st where cname :: SomeExistential -> String cname (SomeExistential p) = d p d p = c $ someType p c p = case p of SumFoo -> "foo" _ -> "asdf" ++ n fullError :: String -> [SomeExistential] -> String fullError n st = n ++ concatMap cname st where cname (SomeExistential p) = d p d p = c $ someType p c p = case p of SumFoo -> "foo" _ -> "asdf" ++ n justNError :: String -> [SomeExistential] -> String justNError n st = n ++ concatMap cname st where cname (SomeExistential p) = d p d p = c $ someType p c p = case p of SumFoo -> "foo" _ -> n ignoreNError :: String -> [SomeExistential] -> String ignoreNError n st = n ++ concatMap cname st where cname (SomeExistential p) = d p d p = c $ someType p c p = case p of SumFoo -> "foo" _ -> fst ("foo", n)

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New Functional Programming Job Opportunities
by Functional Jobs 02 Mar '15

02 Mar '15

Here are some functional programming job opportunities that were posted recently: Senior Software Engineer at McGraw-Hill Education http://functionaljobs.com/jobs/8787-senior-software-engineer-at-mcgraw-hill… Engineering Manager (OCaml or Haskell under Linux) at FireEye http://functionaljobs.com/jobs/8786-engineering-manager-ocaml-or-haskell-un… Cheers, Sean Murphy FunctionalJobs.com

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Would you use frozen-base
by Joachim Breitner 02 Mar '15

02 Mar '15

Hi, = Introduction (can be skipped) = Recently, I find myself often worried about stability of our ecosystem – may it be the FTP discussion, or the proposed¹ removal of functions you shouldn’t use, like fromJust. While all that makes our language and base libraries nicer, it also means that my chances that code from 3 or 4 years ago will compile without adjustment on newer compilers are quite low. Then I remember that a perl based webpage that I wrote 12 years ago just keeps working, even as I upgrade perl with every Debian stable release, and I’m happy about that. It makes me wonder if I am ever going to regret using Haskell because there, I’d have to decide whether I want to invest time to upgrade my code or simply stop using it. I definitely do not want to stop Haskell from evolving. Maybe we can make a more stable Haskell experience opt-in? This is one step in that direction, solving (as always) only parts of the problem. = The problem = One problem is that our central library "base" is both an implementation and an interface. As the implementation is tied to the compiler, and the interface comes with the implementation, you cannot upgrade one without the other. = My solution: frozen-base = My solution would be to provide a new library, let’s call it frozen-base, which decouples the interface (frozen-base) from the implementation (still base). We would start with one particular version of base, say, 4.6. With GHC-7.6 (which comes with base 4.6), frozen-base would simply re-export its modules². On newer compilers and newer versions of base, it would re-create the old interface using CPP. So if your program depends on frozen-base only, you can expect it to compile with newer versions of GHC – achievement unlocked. = How to get new features, then? = But does that mean that you do not get to use great new functionality in later versions of base, such as Data.Bool.bool? No: When a new version of base gets released, and a module gets changed, than frozen-base will ship a _new_ module, named Data.Bool1 that matches that interface. The next change in base causes yet another module to be created, i.e. Data.Bool2 So if you need to use Data.Bool.bool, in one of your modules, you simply change the import from "import Data.Bool" to "import Data.Bool2", adjust your code to the new interface, and are ready to go. Note that you only had to adjust to the changes in Data.Bool, nothing else. Note that you also had to touch only a single module in your program, the others still use whatever interface they were using. Your dependency on frozen-base would have to indicate a lower version bound, i.e. specify the lowest version that has all the Data.Bool<n> variants that you desire, but – by design – never an upper version.³ = When does it not work? = Of course, the promises by froze-base are not absolute: There are changes in base that frozen-base cannot protect you against: Data type changes, some type class changes, type class instances. I don’t have any great ideas here, but maybe they are rare enough so that frozen-base is still useful. = What about the Prelude? = I did not talk about the Prelude. There probably is not a good solution, and I’d simply leave the Prelude frozen. Maybe later GHC has a nice way to specifying which Prelude to use in a certain module, then you could use "Prelude1", "Prelude2" etc. in the same manner. = How does it related to base-compat? = The idea is similar to what base-compat provides, but the other way around: base-compat allows you to use the latest features of base on older compilers, while frozen-base would allow you to use the API of old base versions on newer systems. Frozen-base might subsume base-compat by providing, say Data.Bool2 with the API from base-4.8 also on systems with base-4.6. = How does it related to the split base proposal = Depending on how exactly base is being reconstructed, it might become a pure interface package on its own, or at least independent from compiler-specific bits. Then it might be feasible that, say, after the release of ghc-7.10 there is a new upload of base-4.6.0.n that provides the 4.6 API, but compiles on ghc-7.10. This way, if you depend on base == 4.6.* you could still expect your program to work. If that happens, and also cabal would learn that a plan with different versions of a interface only library like base are not a problem, frozen-base might be obsoleted. See https://ghc.haskell.org/trac/ghc/wiki/SplitBase for more on that. = No Conclusion and lots of future work = So what do you think? Would you be interested in using this? Do you have reasons to believe that this will not work as nicely as I think i could? Greetings, Joachim ¹ but rejected, it seems ² or a sensible subset; I could imagine not exporting certain .Internal modules and things like OldTypeable that are about compatibility themselves. ³ or just one on the very major version number (frozen-base < 2), which to allow for exceptional breaking changes – a redesign of the module naming scheme for example. -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata(a)joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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how to make this work recursive ?
by Roelof Wobben 02 Mar '15

02 Mar '15

Hello, I found out that for 1 item I can do this : node = Node leaf "msg 1" leaf And for 2 items I can do this : node = Node leaf "msg1" (Node leaf "msg2" leaf) But now I wonder how I can make this work with recursion. It seems that I keep the first 2 items and change the 3th one from leaf to another node Roelof

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ANN: clustering-0.1.0
by Kai Zhang 02 Mar '15

02 Mar '15

https://hackage.haskell.org/package/clustering This package provides optimal algorithms (O(N^2)) for hierarchical clustering, including single linkage, complete linkage, average linkage, weighted linkage, and Ward's method. Benchmarks of average linkage can be found here: https://github.com/kaizhang/clustering. I hope this would be useful in general. Motivation of this work: I found the "hierarchical-clustering" library cannot handle large input (>10^4) due to its suboptimal algorithms. Best, Kai

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What other prelude to cleanly convert text-files into json
by Bram Neijt 01 Mar '15

01 Mar '15

Dear reader, I wrote some tiny code to parse files into json (to automate escaping multi-line text). It quickly became a collection of "import qualified" and "T.unpack, T.pack" calls that made the whole thing look ugly [1]. What "alternative prelude" would help in getting clean code when combining "aeson" and reading text files? Greetings, Bram [1] Source can be read here: https://gist.github.com/bneijt/9bdb4b1759790a8463c9

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A Question about IO monad
by Zongzhe Yuan 01 Mar '15

01 Mar '15

Hi i have a question about IO monad. Sometimes IO do something and return something, i wonder if the return type, for example is IO Int means it will return an int, could i purely fetch the int? but i fail, i cannot construct a function, i use do notation but i found i must use something to pass the int to another monad. But i could write a function has the type of Maybe a -> a and it is easily. Maybe monad and IO monad both instance monad but why i can’t do that? Zongzhe Yuan 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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ANNOUNCE postgresql-query and postgresql-config
by Alexey Uimanov 01 Mar '15

01 Mar '15

Hi guys! Here is yet another some-level library to work with PostgreSQL DB. http://hackage.haskell.org/package/postgresql-query This library uses `postgresql-query` and is a list of helpers/workarounds to generate complex queries more safely and parse result. What it contains: 1. interplating quasiquote `sqlExp` which uses instances of `ToField` typeclass to paste values inside query. It looks like this: let val = "'hello'" :: Text q = [sqlExp|SELECT field1 FROM tbl WHERE field2 = #{val}|] where `val` is an arbitrary Haskell expression which type has instance of `ToField` Resulting query will be like: "SELECT field1 FROM tbl WHERE field2 = '''hello'''" Yes, proper string escaping is performed authomatically like using this '?'-like queries with parameters. Resulting type of quasiquote is `SqlBuilder` which uses bytestring builders inside and can be concatenated efficiently. There is also posibility to paste one query inside another like: let q2 = [sqlExp|SELECT field1 f FROM (^{q}) WHERE field1 is not null|] so `q2` will generate nested query. sqlExp also removes SQL comments and removes long sequences of space characters. It also properly handles string literals and quoted identifiers inside quasiquotes. 2. Typeclass `HasPostgres`, simple connection reader `PgMonadT` and functions like `pgQuery` to perform queries. http://hackage.haskell.org/package/postgresql-query-1.0.1/docs/Database-Pos… 3. TH functions to authomatically derive `FromRow` and `ToRow` instances (from postgresql-simple) http://hackage.haskell.org/package/postgresql-query-1.0.1/docs/Database-Pos… 4. Some kind of primitive pre-ORM which generates queries for CRUD-ing simple record types. It looks like: data User = User { userName :: !Text , userPasswordEncrypted :: !Text } deriving (Show, Eq, Ord, Typeable) type UserId = EntityId User $(deriveFromRow ''User) $(deriveToRow ''User) instance Entity User where newtype EntityId User = UserId { unUserId :: UUID } -- Note that you can use any type for id deriving (Show, Read, Ord, Eq, FromField, ToField, PathPiece) -- To use with Yesod tableName _ = "users" fieldNames _ = [ "name" , "password_encrypted" ] runPgMonadT con $ do uid <- pgInsertEntity $ User "name" "j27dna74ja784ha7" pgUpdateEntity uid $ MR [("name", mkValue "John")] pgDeleteEntity uid There is also package http://hackage.haskell.org/package/postgresql-config which contains trivial code to make your postgresql-simple connection pool easily configurable.

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Re: [Haskell-cafe] Hackage and Free Software
by Mike Meyer 01 Mar '15

01 Mar '15

On Sun, Mar 1, 2015 at 3:03 AM, fr33domlover <fr33domlover(a)riseup.net> wrote: > Hello Mike, > I think there's some confusion here. I wan't talking about GPL compatible > licenses, but about *any* free software license! According to Gershom's letter, it already does that. Or at least "cabal check" does. > It looks like Creative Commons licenses may apply too, in particular the > version 4 ones. CC by 4 is even GPL-compatible. > The same for EPL and CDDL! Check out this: > http://www.gnu.org/licenses/license-list.HTML > Both are free sofware licenses. GPL compatibility isn't the issue here :) That URL gets a 404 error. However, if you look at this page: http://www.gnu.org/licenses/license-list.HTML you'll find that the EPL and CDDL are listed as incompatible. The CC licenses are only mentioned as non-software licenses, and not all of them are listed. Nuts, it's even documented that the CC licenses aren't all compatible with each other! https://wiki.creativecommons.org/Wiki/cc_license_compatibility > > And what if I want something like the CDDL or the EPL? Those are both > > licenses that the OSI says are popular. > The FSF approves them as well, like I said. Well, I found them on a free license list. Of course, some of those include things like "We urge you not to use the CDDL" in them, which hardly sounds like approval to me. 5 > > You're addressing the nits, not the core issue I tried to raise: > > placing restrictions on what licenses (or lack thereof) are acceptable > > will discourage people from making software available via Hackage. > I don't think it will, because people are already making free software. Look at > other existing hosting services - they're *full* of free software! This is what > people are making anyway. All I suggest is to make it official, providing a > guarantee so people know each `cabal install` indeed installs only free > software. As long as you're willing to accept any license that the author thinks of as free, then it won't. But that's such a broad scope as to make this change effectively negligible, even from my nit-picking standpoint. > > Or I may not care. If I build a binary that uses one package that's > > GPL-licensed and one that uses an incompatible OSI-approved license, I > > can distribute my source under whatever terms I want, because my > > source doesn't include source from those packages. I can build and run > > binaries myself with no problems, and that may be fine. But I can't > > distribute binaries because I can't satisfy both licenses > > simultaneously, and that may not be acceptable. > That's true, but eventually you wouldn't want to do that. I mean, if you build > some program, you'd be happy to have it packaged for distros and make binary > releases for people who don't want to build from source. Sure, I may be happy to have the binaries build and packaged for all distros. Then again, I know there are people building software using Haskell - and hence hackage or stackage - that aren't planning on distributing the software in any form, so don't care about this issue. For me personally, the FreeBSD package system allows for adding things that have to be built from source for some reason or another, and make building them as easy as installing the binaries. I'm happy providing FreeBSD ports, but making it possible for people building packages for less flexible systems who actually check all the licenses involved isn't something I'm going to spend a lot of effort on. And you're proposed change - or possibly non-change, given what Gershom reported - won't change that effort. All the licenses involved are free. However, some of them aren't compatible with each other, so the only way to fix this is to use different packages. I suppose I could try and get some of the authors to change their license, but that's probably even more work. > This is essentially the question I'm asking the community: do you care about > the packages being free software, allowing legal distribution of binaries? > Specifically, would you make a step forward and make it official, build-in > into Hackage? Except you can't answer the question "does it allow legal distribution of binaries" by just looking at the license on the package. You also have to consider the licenses on the transitive closure of the libraries it uses, as they will be included in the binary, or at least dynamically linked to it, and thus may be considered a derivative work of the packages it includes. So your binary has to comply with the terms of all those licenses, which may not be possible even if all the licenses are approved by the OSI or on the FSF's list of free licenses. Which is also why even the relatively minor restriction "must allow free redistribution of binaries" isn't one I'd be happy with. Sure, it probably won't affect most people. But it equally won't help most people who want to distribute binaries, because it only removes a minor source of barriers to doing so. Unless, of course, by "must allow free redistribution of binaries", you mean "doesn't place any restrictions whatsoever on the distribution of binaries", which would eliminate the not only many of the CC licenses, but also the GPL and some of the BSD licenses. <mike

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Re: [Haskell-cafe] Hackage and Free Software
by Mike Meyer 01 Mar '15

01 Mar '15

On Sat, Feb 28, 2015 at 1:43 PM, fr33domlover <fr33domlover(a)riseup.net> wrote: On 2015-02-28 Mike Meyer <mwm(a)mired.org> wrote: > > Anything that excludes software that we could use, or otherwise > > discourages people from making software available to the community, is > > a bad idea. A restriction to OSF-approved licenses would exclude > > anything released under a Creative Commons license, since the OSF > > doesn't list those, which makes sense as they aren't "software > > licences" per se. And your restriction of "released under a license" > > would exclude public domain software - at least in countries that > > recognize such a thing. > > As to creative commons - indeed they aren't meant for software (maybe except > for CC0, which is basically like public domain). But I also don't think people > want to use them once they see the problem: If you want it permissive, you have > Apache for example (also BSD/MIT), and if you want copyleft you have GPL (also > AGPL, etc.). And what if I want something like the CDDL or the EPL? Those are both licenses that the OSI says are popular. > As to public domain: It's actually just fine! It's true than most countries > recognize lack of license as "all rights reserved", but I also think it's a > very small and trivial task for a maintainer to add a few license lines to > state that the software should be globally treated as public domain. > > So public domain software is just fine, free software. You're addressing the nits, not the core issue I tried to raise: placing restrictions on what licenses (or lack thereof) are acceptable will discourage people from making software available via Hackage. > > Yes, these are nits, but these are nits that could cause someone to > > decide not to put software that is otherwise perfectly acceptable on > > Hackage. > That depends on what "acceptable" means. In the worst case you can have > separation between free and nonfree software by tag, and make it possible to > turn nonfree software on/off using a cabal-install commandline option. > > This will allow people to upload first, and then think and understand the > licensing situation. Once they do, they can properly tag their project. Could > that work? I don't know. I suspect that if you do that, a lot of people would never bother tagging their packages. Would that work for you? You also talk like free/non-free was a binary decision, when it isn't. The OSI lists licenses that aren't compatible with the GPL - like the aforementioned EPL and CDDL. People releasing software under one of those will want to avoid GPL licensed software, whereas people releasing GPL licensed software will want to avoid those licenses, but they are all free. Or I may not care. If I build a binary that uses one package that's GPL-licensed and one that uses an incompatible OSI-approved license, I can distribute my source under whatever terms I want, because my source doesn't include source from those packages. I can build and run binaries myself with no problems, and that may be fine. But I can't distribute binaries because I can't satisfy both licenses simultaneously, and that may not be acceptable. <mike

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Re: [Haskell-cafe] Hackage and Free Software
by Francesco Ariis 01 Mar '15

01 Mar '15

On Sat, Feb 28, 2015 at 07:27:16PM +0200, fr33domlover wrote: > I'd like to make a suggestion: have Hackage accept only packages > released under free software licenses. This is probably true for > most/all packages there, but making it official will both send a > strong message to and from the community, and provide people with > the security and peace of mind, knowing their software is free as > in freedom. I was pretty sure there was a note somewhere on hackage to only upload software with open source licences, but alas I cannot find it now... (maybe I am confusing myself with another package archiver?). I agree having a "please use a licence approved by the OSI/FSF" could prevent problems and seems a sensible choice in any case.

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A Question about IO
by Zongzhe Yuan 01 Mar '15

01 Mar '15

Hi i have a question about IO monad. Sometimes IO do something and return something, i wonder if the return type, for example is IO Int means it will return an int, could i purely fetch the int? but i fail, i cannot construct a function, i use do notation but i found i must use something to pass the int to another monad. But i could write a function has the type of Maybe a -> a and it is easily. Maybe monad and IO monad both instance monad but why i can’t do that? Zongzhe Yuan 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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Parallel numeric class hierarchy
by Clinton Mead 28 Feb '15

28 Feb '15

Let's say for arguments sake I want to make a new finer grained numeric class hierarchy. But, I also want to have the following occur: (1) Existing modules which have defined data types which have Num instances can be used with functions in my new hierarchy (2) New data types defined in the new hierarchy automatically get Num instances if appropriate so they can be used in existing modules. I know this is somewhat of a vague question but roughly how would I go about this?

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ANN: diagrams-hsqml - a Qt5, QtQuick Canvas backend
by Marcin Mrotek 28 Feb '15

28 Feb '15

Hello, I wrote a Diagrams backend that paints on QtQuick (Qt5) canvases through HsQML. It's somewhat half-baked at this point and I haven't fully tested it, but it's good enough for my puroposes (rendering charts through Chart-diagrams) and I have more urgent things to do right now, so I decided to release it as it is. http://hackage.haskell.org/package/diagrams-hsqml https://github.com/marcinmrotek/diagrams-hsqml https://github.com/marcinmrotek/diagrams-hsqml/issues There's one bug I already know of but have no idea how to fix; namely when rendering charts from Chart-diagrams all text is fully filled, with a properly rendered black tex laid on top of it. This can be worked around by setting the font colour to transparent. Bug reports and help in improving the backend by all means welcome. Best regards, Marcin Mrotek

1 0

Bounded STM Queues with load shedding
by Alex Hammel 28 Feb '15

28 Feb '15

Forwarding from Haskell-Beginners: Hi list, It would be handy for me to have a bounded STM Queue along the lines of TBQueue with the difference that when the queue is full, a call to writeTBQueue is a non-blocking no-op. This is in the context of an asynchronous logging system. If the load (somehow) gets heavy enough that the log message queue is taking up a lot of memory, It's better to lose a few log messages than to have processes block until the logger catches up. Are there any off-the-shelf solutions? Cheers, Alex

3 3

Hackage and Free Software
by fr33domlover 28 Feb '15

28 Feb '15

Hello haskellers! I would like to make a proposal regarding the license of software in Hackage. One of the major parts of the Haskell community infrastructure in the package database server Hackage. As far as I know - please correct me if I'm wrong - Hackage makes no restriction on the license of packages upload to it. But as a community working in cooperation to make good software, the Haskell community has embraced licenses like (L)GPL, MIT and BSD, which are free software licenses. Actually the all-rights-reserved tag in Hackage [1] has only two packages tagged by it - the dummy no-op project HNop, and another package whose COPYING file contains a broken link and whose README says "BSD style license". Software freedom is an ethical basis for collaboration on making software that's truly good and loayl to its users, and providing them control and freedom to access and use their computing resources. It seems to me that the Haskell community is already enbracing this ethical basis, but Hackage doesn't provide any guarantees and it means that you'd have to check each package to be sure. By having that all-rights-reserved tag it also in a way welcomes software that doesn't go by these rules - however it seems that no packages do that even in the presence of the possibility. I'd like to make a suggestion: have Hackage accept only packages released under free software licenses. This is probably true for most/all packages there, but making it official will both send a strong message to and from the community, and provide people with the security and peace of mind, knowing their software is free as in freedom. It is also possible that companies use Haskell to create proprietary software using permissive-licensed libraries and tools from Hackage. I hope this isn't true, but even if it is, this software isn't offered by Hackage and I hope its existence doesn't affect the community's use of Hackage and free software. Would you consider to embrace free software officially, including in Hackage? Thanks for reading, waiting to hear from the community and from haskell.org maintainers, fr33domlover [1] http://hackage.haskell.org/packages/tag/all-rights-reserved --- fr33domlover <http://www.rel4tion.org/people/fr33domlover> GPG key ID: 63E5E57D (size: 4096) GPG key fingerprint: 6FEE C222 7323 EF85 A49D 5487 5252 C5C8 63E5 E57D

1 0

Free monoids in Haskell
by Dr. Olaf Klinke 27 Feb '15

27 Feb '15

Dear Dan, you posted an entry on the Comonad.Reader blog about free monoids in Haskell. I commented about FM being a monad, and meanwhile verified that indeed every monoid is an FM-algebra. The proofs are analogous to proving that the continuation (_ -> r) -> r for fixed r is a monad and that r is an algebra for it, respectively. Moreover, the FM-algebra stucture is a monoid homomorphism w.r.t. the monoid instance you gave for (FM a). I'd like to ask what happens when one omits the Monoid constraint. That is, what are the elements of newtype Y a = Y { runY :: forall b. (a -> b) -> b } (Y a) is like (Control.ForFree.Yoneda Identity a), that is, elements of (Y a) should be natural transformations from the reader functor ((->) a) to the identity functor. If that is true, then the Yoneda lemma tells us that (Y a) is isomorphic to (Identity a), but at least operationally that seems not to be the case: u = runY (return undefined) has different semantics than u' = runY (Y undefined) as can be seen by applying both to const (). So return does not map ⊥ to (Y ⊥). I wonder whether one can exhibit other elements not equal to any return(x). Of course (Identity a) is actually the lifted a, since ⊥≠I dentiyt ⊥. Yet this does not serve as an explanation for u vs. u', as both u and u' are of the form (Y _), but one function evaluates its argument while the other does not. Olaf

2 1

Kind synonyms
by Tom Ellis 27 Feb '15

27 Feb '15

Is there an extension that will allow me to do type T = (*, *) so I can replace type family F (k :: (*, *)) with type family F (k :: T) ? Tom

2 1

Idiom Brackets for GHC (first full proposal)
by Oliver Charles 27 Feb '15

27 Feb '15

Hi all, A few days ago I made a post here to gauge interest in adding idiom brackets to GHC. Response was a bit more mixed than I was hoping, but no one seemed to drastically against the idea, so I've moved forward with a more detailed proposal. You can find the full proposal here: https://ocharles.org.uk/IdiomBrackets.html A particular difference in my proposal from existing solutions comes from my desire to lift almost *all* expressions - with the original syntax - into idiom brackets. This means normal function application and tuples, but also case expressions, let bindings, record construction, record update, infix notation, and so on. At first I was skeptical about this, but I am finding uses for this more and more. I really like how it lets me use the interesting data (that is, whatever is "under" the applicative functor) where it's most relevant - rather than having to build a function and thread that value back through. Examples of this can be seen in my proposal. To prove its use, I've been working with this Template Haskell expression: https://ocharles.org.uk/IdiomExp.hs It *almost* does exactly what I want, the only problem is I can't get Template Haskell to give me a difference between $(i [| const True False |]) and $(i [| (const True) False |]) which I was planning to be significant. The proposal has mostly grown into my own personal notebook, so I'm happy to clarify anything that is vague/contradictory/confusing. Look forward to hearing your thoughts! -- Ollie

2 3

RankNTypes question
by Nikolay Amiantov 26 Feb '15

26 Feb '15

Hi Cafe, I can't find how can I do this: {-# LANGUAGE RankNTypes #-} example :: forall a. (forall b. b) -> a example = undefined The first `forall` is for cosmetic reasons. If I understand correctly this is a safe and legit function (please correct me if I'm wrong!). I thought that I can just define `example = id` and GHC would do the rest, but alas, that didn't happen. Thanks! -- Nikolay.

2 1

Re: [Haskell-cafe] RankNTypes question
by Nikolay Amiantov 26 Feb '15

26 Feb '15

Ah, thanks! For some reason I haven't even thought of it -- I knew that `id` was wrong because of the type, but had no idea what else to try. Feeling stupid now. On 02/26/2015 09:46 PM, Marcin Mrotek wrote: > Well still, id is (a -> a). So given some function (forall b. b -> b) > it's result is a function (forall b. b -> b). This works: > > test :: (forall a. a -> a) -> Int -> Int > test f a = f a > > Kind regards, > Marcin Mrotek -- Nikolay.

1 0

Re: [Haskell-cafe] RankNTypes question
by Nikolay Amiantov 26 Feb '15

26 Feb '15

Hm, now that I look at it, I've given the wrong example which badly describes what I want -- sorry. Let me give you a more specific one: {-# LANGUAGE RankNTypes #-} test :: (forall a. (a -> a)) -> (Int -> Int) test = id-- something else should be here i.e., I want to "specialize" a polymorphic value -- in this case, a polymorphic function to the function on `Int`s. On 02/26/2015 09:24 PM, Marcin Mrotek wrote: > Hello, > > id has type a -> a, a function from some type to exactly the same > type. Your function would be from any type to any other (the same type > as given being a special case). It seem to me that > > example _ = undefined > > is the only legal definition. What do you exactly want the function to do? > > Kind regards, > Marcin Mrotek -- Nikolay.

1 0

safe ways how to get head/last of Seq (or Foldable in general)
by Petr Pudlák 26 Feb '15

26 Feb '15

Hi, today I was a bit surprised that apparently there is no easy way how to safely get the head element of `Seq` in a point-free way. Of course there is `viewl`, but it seems the data type has no folding function (something like 'foldViewL :: b -> (a -> Seq a -> b) -> b`. Is there any existing function like `Seq a -> Maybe a` to safely retrieve the head (or last) element? If not, I'd suggest to add headMaybe :: (Foldable t) => t a -> Maybe a headMaybe = getFirst . foldMap (First . Just) and similarly lastMaybe to Data.Foldable. Thanks, Petr

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[ANN] hoodle 0.4
by Ian-Woo Kim 26 Feb '15

26 Feb '15

Hi, Haskellers! I am happy to announce a new version of hoodle that I uploaded to hackage right now. The newest version is 0.4. Hoodle is a pen notetaking program developed in haskell using gtk2hs. You can find relevant information here: http://ianwookim.org/hoodle If all the dependencies are correctly installed (hoodle has rather big dependencies though), the installation of hoodle should be easily doable by cabal install hoodle. I am managing hoodle in nix package repository, so nix users can also easily install hoodle by nix command once after nix expression in nixpkgs is updated. The major change in this update is *tabbed* interface! Like modern web browsers, you can open multiple hoodle files in one hoodle window and navigate them using tabs. Internally, I improved its rendering engine quite much. Now it has a fully asynchronous renderer, which enables a faster change of zoom ratio using cached image and then update rendering later more precisely. This improves display performance from user's point of view a lot. I also made a support for synchronisation and shared documents in association with a web service which I am now preparing for. By default, it is turned off, so all network-related dependencies for that will not be required unless turned on by -fhub flag. In addition, I also made dyre (dynamic reloading as xmonad and yi) opt-in, so you need to use -fdyre if you want to have the feature. Now hoodle is being migrated to gtk3 though this version will not support gtk3 fully yet. I plan to migrate hoodle to gtk3 fully and use gtk3 exclusively in the next version, so this version will be the last gtk2 version. Once this is done, a multi-touch support will be enabled. (and also hoodle will be able to live in wayland, too!) Enjoy hoodle and please bug me if you have any issues. Thank you! best, Ian-Woo Kim

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SBLP 2015: 19th Brazilian Symposium on Programming Languages
by Simon Thompson 26 Feb '15

26 Feb '15

+++++++++++++++++++++++++++++++++++++++++++++++++++ CALL FOR PAPERS - SBLP 2015 19th Brazilian Symposium on Programming Languages 21-26 September 2015 Belo Horizonte, Brazil http://cbsoft.org/sblp2015 +++++++++++++++++++++++++++++++++++++++++++++++++++ IMPORTANT DATES Abstract submission: 20 April, 2015 Paper submission: 27 April, 2015 Author notification: 18 June, 2015 Camera ready deadline: 2 July 2015 INTRODUCTION 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 2015 is part of 6th Brazilian Conference on Software: Theory and Practice, CBSoft 2015, that will be held in Belo Horizonte, Minas Gerais, Brazil, from September 21st to September 26th, 2015. 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. INVITED SPEAKERS * Doaitse Swierstra, Utrecht University * TBA 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 5 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=sblp2015. As in previous editions, after the conference, authors of selected 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 Alberto Pardo, Universidad de la República, Uruguay Doaitse Swierstra, Utrecht University, The Netherlands PROGRAM COMMITTEE Alberto Pardo, Universidad de la República (co-chair) Alex Garcia, IME Alvaro Moreira, Federal University of Rio Grande do Sul Andre Rauber Du Bois, Federal University of Pelotas Carlos Camarão, Federal University of Minas Gerais Christiano Braga, Fluminense Federal University Doaitse Swierstra, Utrecht University (co-chair) Fabio Mascarenhas, Federal University of Rio de Janeiro Fernando Pereira, Federal University of Minas Gerais Fernando Castor, Federal University of Pernambuco Francisco Carvalho-Junior, Federal University of Ceara Hans-Wolfgang Loidl, Heriot-Watt University João Saraiva, University of Minho João F. Ferreira, Teesside University Louis-Noel Pouchet, University of California, Los Angeles Lucilia Figueiredo, Federal University of Ouro Preto Luis Barbosa, University of Minho Manuel A. Martins, University of Aveiro Marcelo Maia, Federal University of Uberlândia Marcelo D'Amorim, Federal University of Pernambuco Mariza Bigonha, Federal University of Minas Gerais Martin Musicante, Federal University of Rio Grande do Norte Noemi Rodriguez, PUC-Rio Peter Mosses, Swansea University Rafael Lins, Federal University of Pernambuco Renato Cerqueira, PUC-Rio Roberto Bigonha, Federal University of Minas Gerais Roberto Ierusalimschy PUC-Rio Rodrigo Geraldo, Federal University of Ouro Preto Sandro Rigo, State University of Campinas Sérgio Medeiros, Federal University of Rio Grande do Norte Simon Thompson, University of Kent Varmo Vene, University of Tartu Zongyan Qiu Peking University Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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Haskell Weekly News: Issue 318
by Semen Trygubenko / Семен Тригубен ко 26 Feb '15

26 Feb '15

New Releases A very fast BufferBuilder-based JSON encoder for Aeson. Encode JSON 2-5x faster than Aeson. https://hackage.haskell.org/package/buffer-builder-0.2.0.3 https://hackage.haskell.org/package/buffer-builder-aeson-0.2.0.3 http://www.reddit.com/r/haskell/comments/2wwbuj/announcing_bufferbuilder_en… FLTK GUI bindings - Alpha release This library aims to make it easy for users to build native apps that work portably across platforms. http://hackage.haskell.org/package/fltkhs-0.1.0.0/docs/Graphics-UI-FLTK-Low… Calls for Participation Primitive Haskell Originally this is a chapter in Mezzo Haskell, covering primitive operations, evaluation order and mutation. Michael Snoyman wants feedback on style and teaching approach. https://www.fpcomplete.com/blog/2015/02/primitive-haskell https://github.com/fpco/mezzohaskell Intermediate Haskell documentation proposal Michael Snoyman points out that Haskell community lacks middle-weight documentation beyond the levels of RWH and LYaH, or it is redundant, of varying quality and hard to find. He proposes to create conditions that will help Haskell community to write this documentation collectively. Part of Commercial Haskell effort. https://github.com/commercialhaskell/commercialhaskell/blob/master/proposal… https://github.com/commercialhaskell/commercialhaskell https://groups.google.com/forum/#!topic/commercialhaskell/Ou00AvRdDTU Discussion GHC 7.10 will embody Plan FTP In the 11 day window ~800 people participated in the survey and 82% were in favour of Foldable/Traversable Proposal. https://mail.haskell.org/pipermail/libraries/2015-February/025009.html http://www.reddit.com/r/haskell/comments/2wx64g/ghc_710_will_use_plan_ftp/ Web development using Haskell A discussion of (micro)frameworks, active and (seemingly) dormant, and the ultimate minimalism. http://www.reddit.com/r/haskell/comments/2wfap0/web_development_using_haske… Free Monoids in Haskell Lists aren't! http://comonad.com/reader/2015/free-monoids-in-haskell/ https://www.reddit.com/r/haskell/comments/2wqxqv/the_comonadreader_dan_doel… Let's teach GHC idiom brackets! (| pureFunction (fetchWith some params) (fetchMore withOthers) |) vs pureFunction <$> fetchWith some params <*> fetchMore withOthers https://mail.haskell.org/pipermail/haskell-cafe/2015-February/118228.html http://www.reddit.com/r/haskell/comments/2wclln/haskellcafe_lets_teach_ghc_… Is working with numeric formulas on Haskell a pain for anyone else? (/) and quot have different semantics, and this makes it hard to implement formulas. http://www.reddit.com/r/haskell/comments/2ws786/is_working_with_numeric_for… Monads in a non Functional Programming language X Haskell Fluid interfaces and Monads have little in common, although it might look like a lot. http://www.reddit.com/r/haskell/comments/2wefft/monads_in_a_non_functional_… Quotes of the Week "would require parens" in haskell world is like saying "would require taking lives" (chrisdone) "Functional programming beats all GOF Gang of Four patterns." (eniacsparc2xyz) http://www.reddit.com/r/haskell/comments/2wjtq5/haskell_actually_does_chang… "… pure functional programming and HTTP are a match made in heaven (type Application = Request -> Response - it doesn't get any more sensible than that ;)) …" (tdammers) http://www.reddit.com/r/haskell/comments/2wfap0/web_development_using_haske…

1 0

Typed label
by Konstantine Rybnikov 25 Feb '15

25 Feb '15

Hi! I have a problem that seems very basic and fundamental, but at the same time I keep somehow skipping "the right way" to solve it. So I decided to give a try and describe it here. The task itself is very common. You have some data, which has some property inside of it. Often, you want to write a function that works only on an object with specific property being equal to something, but you can't move it out to type-parameter, because then you get all sorts of problems like you can't use regular lists/vectors anymore and other similar things. Let's take a look at real-world example to get a better understanding of a problem: Given a `Vegetable` type with a type that describes it's "type" (saying which concretely veg is it), you need to write a function `embraceOnion`, which would only work on onions (because it doesn't make sense to embrace any other veg). https://gist.github.com/k-bx/32b3f6a770ad81330f51 ``` {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} module Main where main :: IO () main = putStrLn "Hi!" data Vegetable = Vegetable { vegName :: String , vetType :: VegType } deriving (Show, Eq) data VegType = Potato | Tomato | Cucumber | Onion deriving (Show, Eq) newtype TypedVegetable (t::VegType) = TypedVegetable Vegetable unType :: TypedVegetable (t::VegType) -> Vegetable unType (TypedVegetable v) = v -- | bad embraceOnion :: Vegetable -> IO () embraceOnion veg = putStrLn ("Yay for onion! It's name is: " ++ vegName veg) -- | good, but little ugly because of boilerplate embraceOnion2 :: TypedVegetable Onion -> IO () embraceOnion2 onion = putStrLn ("Yay for onion! It's name is: " ++ vegName (unType onion)) -- | doesn't work, need to specify kind of `t` newtype TypedLabel1 t a = TypedLabel1 a unTyped1 :: TypedLabel1 t a -> a unTyped1 (TypedLabel1 a) = a embraceOnion3 :: TypedLabel1 Onion Vegetable -> IO () embraceOnion3 = undefined ``` Currently, I'm sticking to solution `embraceOnion2` in my code. `embraceOnion3` doesn't work because of: ``` /Users/kb/workspace/typelabels/typelabels.hs The first argument of ‘Main.TypedLabel1’ should have kind ‘*’, but ‘Main.Onion’ has kind ‘Main.VegType’ In the type signature for ‘Main.embraceOnion3’: Main.embraceOnion3 :: Main.TypedLabel1 Main.Onion Main.Vegetable -> GHC.Types.IO () ``` What are the other options and "state of the art"? Thank you!

2 2

PowerPC variants (GHC 7.8.3)
by Jon Schneider 25 Feb '15

25 Feb '15

Good morning, We have a product with an MPC8544E we might want to target. Also known as PowerQUICC and e500v2. The ABI is gnuspe rather than gnueabi. I have built a powerpc---ghc cross compiler but the thing stopping "hello world" is a SIGILL happening in StgCRun.c where the stfd and lfd instructions are used to stash registers. The e500 has different instructions but that cannot be dropped in trivially because of offset encoding Commenting these out fixes "hello world" though this surely only scratches the surface of what would need doing. I notice that whereas ARMv5, v6 and v7 appear to be catered for along with various knobs and whistles there appears to be no such thing for PowerPC. Is anybody else out there working on this area ? Jon

1 0

Trouble with forkProcess
by Rob Leslie 25 Feb '15

25 Feb '15

Hi folks, I’d love to get a sense of the prevailing wisdom with respect to forkProcess from System.Posix.Process. I’m building a multithreaded (STM) application with potentially many threads simultaneously reading and writing to pieces of (an otherwise rather large) shared state. Occasionally I would like to record a whole snapshot of that state, and it seems to me a natural way to accomplish this is to fork a process (eliminating all the other threads) and perform a read-only dump of the state. This dump can be time-consuming, which is why I’d rather not have contention with other threads that may be trying to modify the state while I’m dumping. My experience with forkProcess so far is that sometimes it works brilliantly, and other times it just deadlocks. I’m at a loss to understand what the problem is, but the deadlock seems to occur before the new process gets any control -- certainly before it has started to access any of the shared state. I’m aware of the giant warning in the forkProcess documentation, but I’m not sure if or how it could explain the behavior I’m seeing. Has anyone else used forkProcess successfully? It’s possible too that I’m running into something related to the way I’m setting up a pipe to receive messages from the forked process. Here is what I’m doing: > runInForkedProcess :: ((Text -> IO ()) -> IO ()) -> > Consumer Text IO () -> IO Bool > runInForkedProcess iof consumer = do > process <- newEmptyMVar > result <- newEmptyMVar > > let handler = Catch $ do > status <- getProcessStatus False False =<< readMVar process > case status of > Just (Exited code) -> void $ tryPutMVar result (code == ExitSuccess) > Just Terminated{} -> void $ tryPutMVar result False > _ -> return () > > bracket (installHandler processStatusChanged handler Nothing) > (\former -> installHandler processStatusChanged former Nothing) $ \_ -> do > > (input, output) <- createPipe > > processId <- forkProcess $ do > closeFd input > bracket (fdToHandle output) hClose $ \outHandle -> do > hSetBuffering outHandle NoBuffering > iof (hPutStrLn outHandle) > putMVar process processId > > closeFd output > > bracket (fdToHandle input) hClose $ \inHandle -> do > hSetBuffering inHandle NoBuffering > runEffect $ fromHandle inHandle >-> for cat (yield . T.pack) >-> consumer > > takeMVar result Does anyone have any advice? Many thanks, -- Rob Leslie rob(a)mars.org

4 3

Should foldl' be added to the post-"Plan FTP" Prelude?
by Bob Ippolito 24 Feb '15

24 Feb '15

I would love to have foldl' as part of the Prelude. I use it quite frequently and it seems like a beginner trap to have foldl but not foldl' (which is almost always the better choice). I see that foldMap has been added (in addition to all of Monoid and Traversable), why was foldl' left out? It might be good measure to add foldr' as well, although I use that much less frequently. -bob

3 2

Proposal: Infix expression keyword: -XInfixExpressions
by Christopher Done 24 Feb '15

24 Feb '15

Ahoy, The idiom discussion brought back to mind a general problem (well, for me) in Haskell syntax which is there is no syntactic sugar for interspersing operators to many arguments. Regarding a solution for this, I wrote up a wee proposal here: https://gist.github.com/chrisdone/d9d33e4770a2fef19ad1 If I go ahead and implement this in GHC as -XInfixExpressions or something (better names welcome), would it be likely to be accepted? I could first do an implementation in haskell-src-exts to demonstrate the concept. Ciao!

12 24

Composition and type magic
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 24 Feb '15

24 Feb '15

Hi everyone, I defined a function for discrete convolution, i.e conv xs = sum . zipWith (*) xs . reverse And I was wondering, can it be done without explicitly passing in the xs (eta-reduction)? I've seen people play with (.).(.) and foldl foldl foldl and was thinking that maybe something similar might be used here. -- Regards Sumit Sahrawat

3 5

7 PhD studentships in Nottingham
by Graham Hutton 24 Feb '15

24 Feb '15

Dear all, The School of Computer Science in Nottingham is advertising 7 fully-funded PhD studentships for "home" students (or EU students who have been ordinarily resident in the UK for 3 years prior to the start of the programme). Applicants in the area of the Functional Programming lab (fp.cs.nott.ac.uk) are encouraged! If you are interested in applying, please contact a potential supervisor in the FP lab prior to submitting your application: Venanzio Capretta - type theory, mathematical logic, corecursive structures, proof assistants, dependently-typed programming. Henrik Nilsson - functional reactive programming, modelling and simulation, domain-specific languages, probabilistic languages. Thorsten Altenkirch and Graham Hutton - not planning to take on any further new PhD students this year. Best wishes, Graham +-----------------------------------------------------------+ 7 Fully-Funded PhD Studentships School of Computer Science University of Nottingham, UK http://www.nottingham.ac.uk/jobs/currentvacancies/ref/SCI1433 Applications are invited for up to 7 fully-funded PhD studentships for "home" students (or EU students who have been ordinarily resident in the UK for 3 years prior to the start of the programme) in the School of Computer Science at the University of Nottingham, starting on 1st October 2015. The topics for the studentships are open, but should relate to the interests of one of the School's research groups: Agents Lab; Automated Scheduling, Optimisation and Planning; Computer Vision Lab; Functional Programming Lab; Intelligent Modelling and Analysis; Mixed Reality Lab; Networked Systems. The studentships are for three years, include a stipend of Â£13,863 per year and tuition fees, and are available to students who qualify as "home" applicants in terms of their fee status. Applicants are normally expected to have a first-class Masters or Bachelors degree in Computer Science or a related discipline, and must obtain the support of a potential supervisor in the School prior to submitting their application. Initial contact with supervisors should be made at least two weeks prior to the closing date for applications. Applications should be submitted via the postgraduate applications website: http://www.nottingham.ac.uk/pgstudy/apply/apply-online.aspx. As part of your personal statement, please include an outline research proposal (max 1 page). Informal enquiries may be addressed to Christine.Fletcher(a)nottingham.ac.uk. Closing date for applications: Tuesday 31st March 2015 +-----------------------------------------------------------+ 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.

1 0

how can I do this the best
by Roelof Wobben 24 Feb '15

24 Feb '15

Hello, Im busy with a programm which translates log files to another format. I also have to look if the lines in the old one have the rigtht format. Schould I check the format and also if another field contains a I, E or W. Or schould I do the checking in one clause and make another case on the rest ? Roelof

7 23

The Future of Community.Haskell.Org
by Gershom B 23 Feb '15

23 Feb '15

This message is intended to kick off a discussion on the state of the Community.Haskell.Org server and possible future plans. Included below is the text of a blog post on the infra blog ( https://blog.haskell.org/post/the_future_of_community_haskell_org/) We would especially like input and feedback from those who use or rely on any community.haskell.org services or accounts. We don't want to leave you in the lurch, and want to make sure that you feel your needs can be taken care of smoothly, even as we look to wind things down. Feedback is welcome via email (to admin@h.o) as a post on the blog, or via the reddit discussion ( http://www.reddit.com/r/haskell/comments/2wwc42/the_future_of_communityhask… ) - - - Community.haskell.org is a server in our ecosystem that comparatively few know about these days. It actually was, to my knowledge, a key part of how the whole haskell.org community infrastructure got set up way back when. The sparse homepage still even says: "This server is run by a mysterious group of Haskell hackers who do not wish to be known as a Cabal, and is funded from money earned by haskell.org mentors in the Google Summer-of-Code programme." At a certain point after this server was created, it ceased to be run by a "mysterious group of Haskell hackers" and instead became managed officially by the haskell.org Committee that we know today. You can see the original announcement email in the archives <https://mail.haskell.org/pipermail/haskell/2010-November/022375.html>. The community server, first set up in 2007 <https://mail.haskell.org/pipermail/haskell/2007-June/019592.html> played a key role back before the current set of cloud-based services we know today was around. It provided a shared host which could provide many of the services a software project needs -- VCS hosting, public webspace for documentation, issue trackers, mailing lists, and soforth. Today, the server is somewhat of a relic of another time. People prefer to host projects in places like github, bitbucket, or darcs hub <http://hub.darcs.net/>. Issue trackers likewise tend to be associated with these hosts, and there are other free, hosted issue trackers around as well. When folks want a mailing list, they tend to reach for google groups. Meanwhile, managing a big box full of shell account has become a much more difficult, riskier proposition. Every shell account is a security vulnerability waiting to happen, and there are more adversarial "scriptkiddie" hackers than ever looking to claim new boxes to spam and otherwise operate from. Managing a mailman installation is likewise more difficult. There are more spammers out there, with better methods, and unmaintained lists quickly can turn into ghost towns filled with pages of linkspam and nothing but. The same sad fate falls on unmaintained tracs. As a whole, the internet is a more adversarial world for small, self-hosted services, especially those whose domain names have some "google juice". We think it would be good to, to the extent possible, get out of the business of doing this sort of hosting. And indeed, very few individuals tend to request accounts, since there are now so many nicer, better ways of getting the benefits that community.haskell.org once was rare in providing. So what next? Well, we want to "end of life" most of community.haskell.org, but in as painless a way as possible. This means finding what few tracs, if any, are still active, and helping their owners migrate. Similarly for mailing lists. Of course we will find a way to continue to host their archives for historical purposes. Similarly, we will attempt to keep source repositories accessible for historical purposes, but would very much like to encourage owners to move to more well supported code hosting. One purpose that, until recently, was hard to serve elsewhere was in hosting of private darcs repositories with shared access -- such as academics might use to collaborate on a work in project. However, that capability is now also provided on http://hub.darcs.net. At this point, we can't think of anything in this regard that is not better provided elsewhere -- but if you can, please let us know. On webspace, it may be the case that a little more leniency is in order. For one, it is possible to provide restricted accounts that are able to control web-accessible files but have no other rights. For another, while many open source projects now host documentation through github pages or similar, and there are likewise many services for personal home pages, nonetheless it seems a nice thing to allow projects to host their resources on a system that is not under the control of a for-profit third party that, ultimately is responsible to its bottom line and not its users. But all this is open for discussion! Community.haskell.org was put together to serve the open source community of Haskell developers, and its direction needs to be determined based on feedback regarding current needs. What do you think? What would you like to see continued to be provided? What do you feel is less important? Are there other good hosted services that should be mentioned as alternatives? And, of course, are you interested in rolling up your sleeves to help with any of the changes discussed? This could mean simply helping out with sorting out the mailman and trac situation, inventorying the active elements and collaborating with their owners. Or, it could mean a more sustained technical involvement. Whatever you have to offer, we will likely have a use for it. As always, you can emailadmin@h.o or hop on the #haskell-infrastructure freenode channel to get involved directly. Cheers, Gershom

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GHC 7.10 will use Plan FTP
by Simon Peyton Jones 23 Feb '15

23 Feb '15

Colleagues You will remember (see this email<https://mail.haskell.org/pipermail/libraries/2015-February/024925.html>) that the Haskell community has been engaged in a debate about proposed changes to the Haskell Prelude, the Plan-List or Plan-FTP debate<https://ghc.haskell.org/trac/ghc/wiki/Prelude710>. We decided to hold an open survey to get feedback from the community. Simon Marlow and Simon PJ were asked to make a decision in the light of that feedback. The survey closed on 21 Feb, so it's time to decide. Bottom line: we recommend Plan FTP. There are strong arguments in both directions. For the record, our own personal instincts were both in the conservative, Plan List direction. But the survey gave some pretty compelling feedback, and was far more unequivocal than we expected. Some details * Over 800 people participated in the survey. That's a lot for an 11-day window. Many wrote several paragraphs of supporting information. Thank you for participating so actively! * Overall, 82% of respondents were in favour of FTP, a 4:1 majority. Simon and I found this level of unanimity quite surprising - and it made our task much easier. We clearly under-estimated the Haskell community's appetite for absorbing change when they approve of the direction of travel. * Herbert helped us by doing a rough categorisation into hobbyist/non-hobbyist responses. o The first interesting fact is that there were a lot of non-hobbyist responses: in fact a majority (58%) of the responses were from non-hobbyists. So increasingly people are using Haskell for real work. o The support for Plan FTP among hobbyists was overwhelming (over 87%). But it was still very strong indeed among non-hobbyists (over 79%). Caveat: time was short, so the hobby/non-hobby categorisation is extremely approximate. But since the results are so unambiguous, even the crude results are helpful. * We tried filtering out responses that were blank in the name field and both text boxes. The 82% majority did not budge. What happens now * GHC 7.10 will embody Plan FTP. * The Core Libraries committee, freshly energised and informed by this episode, is drafting new guidelines to clarify what it does, and how it works; and to help avoid a repetition of the recent drama. * The responses. In the survey we said "We won't publish either textual responses or personally-identifying information without your consent. The full responses will be made available only to a limited few." However, the English-language comments are a remarkable snapshot of the Haskell community, and it would be a shame to discard them without deeper study. We propose to make them available to the Core Libraries Committee, to inform their future choices. If anyone else wants to digest the comments into a summary report, we think that would be a service to the community, and (in our view, provided they are a reputable person etc) would fall under the "available to a limited few" rubric. If that appeals to you, write to us. If you object even to such limited sharing, write to us too. This wasn't an entirely easy process, especially because it happened so late in the release cycle. But although many people felt quite strongly about List vs FTP, the whole debate has involved light rather than heat; people have concentrated on the technical issues, rather than yelling. Overall, it's been very encouraging to see the resilience, transparency, and constructive tone of the community. We really appreciate that - thank you. Simon Marlow and Simon Peyton Jones

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Re: [Haskell-cafe] how can I do this the best
by Roelof Wobben 23 Feb '15

23 Feb '15

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Re: [Haskell-cafe] how can I do this the best
by Roelof Wobben 23 Feb '15

23 Feb '15

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Haskell implementation of longest path algorithm
by Jeremy 23 Feb '15

23 Feb '15

Compared to the Nim code [https://github.com/logicchains/LPATHBench/blob/master/nim.nim] for a longest path algorithm, Haskell [https://github.com/logicchains/LPATHBench/blob/master/hs.hs] looks horrendously verbose and ugly, even if you ignore the pragmas and imports. Is this idiomatic Haskell style? Could it be clearer, but has to be written that way for performance - although it still takes 3.7x as long as the Nim version [https://github.com/logicchains/LPATHBench/blob/master/writeup.md]? I posted this to the beginners group last week, but no reply, so trying again here. -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-implementation-of-longest-path… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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ANN: Lambdabot 5.0
by Bertram Felgenhauer 22 Feb '15

22 Feb '15

Dear Haskell users, after a long hiatus, I'm pleased to announce that we have just released Lambdabot 5.0 on hackage. Lambdabot is an IRC bot that can also be used offline; it provides tools for querying information about and even for producing Haskell code. To install it, use the following command: cabal install lambdabot-haskell-plugins lambdabot (cabal install lambdabot *should* work, but instead seems to send cabal's dependency solver onto a wild goose chase from which it fails to return.) * What's new Lambdabot is now maintained by a team, currently consisting of James Cook and me. The repository and bugtracker have moved to https://github.com/lambdabot/lambdabot The most significant change compared to lambdabot 4.3 is that lambdabot has been refactored and split into multiple packages. As a result, lambdabot has become easier to customize: simply edit the source files (Main.hs and Modules.hs) in the lambdabot package, and optionally add your own plugins. Other notable changes include - The code snippets provided by @src have been updated. (thanks to Artyom) - Support for IRC server authentication. - Pinging the IRC server regularly in order to detect one-sided disconnects. There have also been many minor tweaks and bug fixes. * What's next There are still a number of open issues on the bugtracker, so you can expect another release soon. Have fun with lambdabot, Bertram and James

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Looking for a new release manager for cabal
by Johan Tibell 22 Feb '15

22 Feb '15

(bcc: haskell-cafe) Hi, After about 3 years of cabal releases I'm looking for someone else to take over the responsibility as cabal release manager. As a release manager I try to keep on top of pull requests, make releases, and make sure bugs get triaged and bugfixes get release. Anyone interested? -- Johan

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Munich Haskell Meeting, 2015-02-24 @ 19:30
by Christian Neukirchen 21 Feb '15

21 Feb '15

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Tuesday, February 24 at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-feb-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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FLTK GUI Bindings - Alpha Release
by aditya siram 21 Feb '15

21 Feb '15

I'm pleased to announce the alpha release of Haskell bindings [1] to the FLTK [2] GUI library. On Linux and Mac this package and FLTK 1.3.3 [3] should install pretty smoothly. The library covers about 80+% of API and has 60+ widgets that can be used transparently from Haskell. You can read more about the motivation behind the library, installation instructions and quick start documentation here [4]. The package also comes with 14 demos to show you how to get started. There still isn't any support for Windows because I don't have access to a Windows machine, and don't have any experience with the Windows C/C++ build tools. The code itself should be fully portable. I could use some help here. Please report any issues on the Github page [5]. Hope you enjoy and I'd love any feedback on the install and development process. Thanks! -deech [1] http://hackage.haskell.org/package/fltkhs-0.1.0.0 [2] http://fltk.org [3] http://www.fltk.org/software.php?VERSION=1.3.3&FILE=fltk/1.3.3/fltk-1.3.3-s… [4] http://hackage.haskell.org/package/fltkhs-0.1.0.0/docs/Graphics-UI-FLTK-Low… [5] https://github.com/deech/fltkhs

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Type systems preventing laziness-related memory leaks?
by Eugene Kirpichov 20 Feb '15

20 Feb '15

Hello haskell-cafe, Let me repost here a question I posted to cstheory stackexchange - in hopes that there are more type theory experts here. http://cstheory.stackexchange.com/questions/29518/type-systems-preventing-l… Perhaps the main source of performance problems in Haskell is when a program inadvertently builds up a thunk of unbounded depth - this causes both a memory leak and a potential stack overflow when evaluating. The classic example is defining sum = foldr (+) 0 in Haskell. Are there any type systems which statically enforce lack of such thunks in a program using a lazy language? Seems like this should be on the same order of difficulty as proving other static program properties using type system extensions, e.g. some flavors of thread safety or memory safety.

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Re: [Haskell-cafe] Very large data structures in the heap
by Kim-Ee Yeoh 20 Feb '15

20 Feb '15

On Tue, Feb 17, 2015 at 8:26 PM, Dr. Olaf Klinke < o.klinke(a)dkfz-heidelberg.de> wrote: > On Tue, 17 Feb 2015, Kim-Ee Yeoh wrote: > > >> On Tue, Feb 17, 2015 at 5:05 PM, Dr. Olaf Klinke < >> o.klinke(a)dkfz-heidelberg.de> wrote: >> I'd like to separate serialization (IO) from traversal of the >> stucture (pure), but how can one avoid the entire heap object from being >> built? Can lazyness be exploited? >> >> >> Something I didn't quite understand from your email is whether >> >> 1. the data structure is being constructed and you'd like it >> simultaneously serialized to disk >> >> or >> >> 2. it's already serialized and you'd like to perform pure traversals on >> it. >> >> Problem 1 is one of compute then write, problem 2 is one of read then >> compute. >> >> Lazy I/O, as in unsafeInterleaveIO, excels at 2. >> >> -- Kim-Ee >> >> >> > Both. > > The data structure is a search tree, which is intended to be used multiple > times, so 1. applies. I guess that the only way to circumvent the memory > bottleneck would be to serialize the part of the structure already > constructed and use the already serialized part for constructing the rest, > somehow without de-serializing it again. > To an extent, the virtual memory of a modern operating system solves the memory bottleneck. I'm curious what you were hoping for when thinking of exploiting laziness. If the search tree is truly huge, you'd probably resort to fancy (or fuzzy or both) algorithms to break up the work. > > 2. also applies. Queries to the search tree traverse different paths of > it, but one can not rule out that multiple queries will eventually have > traversed the entire tree. Thus lazily de-serializing it may not be enough > unless the garbage collector disposes of parts of the tree after each > query. Which it won't, if we say something like this: > > search :: FilePath -> IO (Query -> Result) > search treeOnDisk = do > searchtree <- decodeFile treeOnDisk > return (\query -> searchFor query searchtree) > No, gc _will_ reclaim memory if decodeFile uses unsafeInterleaveIO. Take a look at the lazy bytestrings package. p.s. Your search function looks odd. Surely you mean something with the type signature: FilePath -> Query -> IO Result ? -- Kim-Ee

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Pattern match checking for GADTs
by Simon Peyton Jones 19 Feb '15

19 Feb '15

Friends George Karachalas, Tom Schrijvers, Dimitrios Vytiniotis, and I are working on finally cracking the problem of accurately reporting pattern-match overlap and redundancy warnings, in the presence of GADTs. You know the problem; consider vzip :: Vect n a -> Vect n b -> Vect n (a,b) vzip VN VN = VN vzip (VC x xs) (VC y ys) = VC (x,y) (vzip xs ys) data Vect :: Nat -> * -> * where VN :: Vect Zero a VC :: a -> Vect n a -> Vect (Succ n) a Are there any missing equations in vzip? No! But GHC complains anyway. We have lots of Trac tickets about this; e.g. https://ghc.haskell.org/trac/ghc/ticket/3927. We now have a draft paper (wait a week) and a prototype implementation, that fixes the problem. But we need your help. We'd like to try our prototype on real code, not just toy examples. So, could you send George a pointer to any packages you have, or know of, that * use GADTS (or other fancy type features) and * would benefit from accurate pattern-match overlap/redundancy warnings? Specifically: * Where you have had to add a catch-all f _ _ = error "impossible" to silence GHC from saying "missing patterns" * Or where you have added {-# OPTIONS_GHC -fno-warn-missing-patterns #-} to silence the warnings. * Or something else like that. George's email is in cc. Time is short - the ICFP deadline is on 27 Feb. So sooner is better than later for us. But later is better than never. Thank you! Simon

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build troubles with ghc binary
by briand＠aracnet.com 19 Feb '15

19 Feb '15

(i hope this is not a repost - sent it to haskell-cafe-bounces the first time) new thread because i have the details i should have provided in the first place. development system is centos 6.6 but i suspect something's broken because... i built and installed gmp-4.3.2 only to find that it's being build as a 32-bit library and will only build as 32-bits. which is really weird, because 64-bit system: 2.6.32-504.3.3.el6.x86_64 #1 SMP Wed Dec 17 01:55:02 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux however- no problem- i'll simply download the 32-bit version of the ghc binary, which was built on centos no less, and install that instead of the 64-bit version. so i do that, point configure at my gmp thusly ./configure --prefix=/home/denheyer/local --with-gmp-includes=/home/denheyer/local/include --with-gmp-libraries=/home/denheyer/local/lib and get this checking for path to top of build tree... utils/ghc-pwd/dist-install/build/tmp/ghc-pwd: error while loading shared libraries: libgmp.so.3: cannot open shared object file: No such file or directory configure: error: cannot determine current directory ok- i haven't seen it do that before, but it's probably because i'm pointing it away from it's internal libgmp. that's fine, i'll set LD_LIBRARY_PATH. I do that and make install starts to work. yay! and then i get THIS ghc-cabal: Bad interface file: dist-install/build/GHC/CString.hi magic number mismatch: old/corrupt interface file? (wanted 33214052, got 129742) somebody really hates me. so originally i had downloaded the xz file, and even though i figure it's pretty much impossible that the archive was actually corrupted, i decide to download the bzip version just in case. yeah- not surprisingly, the build still breaks. any ideas ?? Thanks, Brian

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FLTK bindings near completion. Need community assistance ...
by aditya siram 19 Feb '15

19 Feb '15

Hi all, I've been working on bindings [1] to the FLTK GUI toolkit [2] and I've come to the point where it's mostly complete but I could use some help from the community before I put it up on Hackage. My hope is to eventually fill the need in the Haskell ecosystem for a native GUI library that: - runs on all platforms - is easily installable - is small and fast - has a minimum of dependencies. Currently this library uses only base, stm, bytestring and c2hs. - is conservative with extensions. Currently the most recent extension required is GADTs. If any of this sounds good to you, I could use some help with: - testing the build on Linux and OSX. This should be as simple as installing FLTK, cloning the repo [1] and running `cabal build`. Better instructions are in included README. - getting it to build on Windows. I unfortunately don't have access to a Windows machine. - getting it to build on older GHC's. I used 7.8.3 because that is what I have, but it should work back to the first version of GHC that introduced GADTs. Unfortunately the included `Setup.hs` is non-trivial and I been having trouble getting it work with versions of Cabal older than 1.20 due to breaking API changes across versions. - building a single library. Currently the C/C++ wrappers and Haskell bindings are two separate statically linked libraries. They are copied to the same location so applications built with this package don't have to know that, but it would be ideal if it was all-in-one. - building a shared library on Mac OSX. Any other feedback you have is totally welcome. Thanks! -deech [1] https://github.com/deech/fltkhs [2] http://www.fltk.org/index.php

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