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Robert Harper: Summer of Programming Languages

Planet Haskell - Mon, 07/21/2014 - 11:22pm

Having just returned from the annual Oregon Programming Languages Summer School, at which I teach every year, I am once again very impressed with the impressive growth in the technical sophistication of the field and with its ability to attract brilliant young students whose enthusiasm and idealism are inspiring.  Eugene was, as ever, an ideal setting for the summer school, providing a gorgeous setting for work and relaxation.  I was particularly glad for the numerous chances to talk with students outside of the classroom, usually over beer, and I enjoyed, as usual, the superb cycling conditions in Eugene and the surrounding countryside.  Many students commented to me that the atmosphere at the summer school is wonderful, filled with people who are passionate about programming languages research, and suffused with a spirit of cooperation and sharing of ideas.

Started by Zena Ariola a dozen years ago, this year’s instance was organized by Greg Morrisett and Amal Ahmed in consultation with Zena.  As usual, the success of the school depended critically on the dedication of Jim Allen, who has been the de facto chief operating officer since it’s inception.  Without Jim, OPLSS could not exist.  His attention to detail, and his engagement with the students are legendary.   Support from the National Science Foundation CISE Division, ACM SIGPLANMicrosoft Research, Jane Street Capital, and BAE Systems was essential for providing an excellent venue,  for supporting a roster of first-rate lecturers, and for supporting the participation of students who might otherwise not have been able to attend.  And, of course, an outstanding roster of lecturers donated their time to come to Eugene for a week to share their ideas with the students and their fellow lecturers.

The schedule of lectures is posted on the web site, all of which were taped, and are made available on the web.  In addition many speakers provided course notes, software, and other backing materials that are also available online.  So even if you were not able to attend, you can still benefit from the summer school, and perhaps feel more motivated to come next summer.  Greg and I will be organizing, in consultation with Zena.  Applying the principle “don’t fix what isn’t broken”, we do not anticipate major changes, but there is always room for improvement and the need to freshen up the content every year.  For me the central idea of the summer school is the applicability of deep theory to everyday practice.  Long a dream held by researchers such as me, these connections become more “real” every year as the theoretical abstractions of yesterday become the concrete practices of today.  It’s breathtaking to see how far we’ve come from the days when I was a student just beginning to grasp the opportunities afforded by ideas from proof theory, type theory, and category theory (the Holy Trinity) to building beautiful software systems.  No longer the abstruse fantasies of mad (computer) scientists, these ideas are the very air we breathe in PL research.  Gone are the days of ad hoc language designs done in innocence of the foundations on which they rest.  Nowadays serious industrial-strength languages are emerging that are grounded in theory and informed by practice.

Two examples have arisen just this summer, Rust (from Mozila) and Swift (from Apple), that exemplify the trend.  Although I have not had time to study them carefully, much less write serious code using them, it is evident from even a brief review of their web sites that these are serious languages that take account of the academic developments of the last couple of decades in formulating new language designs to address new classes of problems that have arisen in programming practice.  These languages are type safe, a basic criterion of sensibility, and feature sophisticated type systems that include ideas such as sum types, which have long been missing from commercial languages, or provided only in comically obtuse ways (such as objects).  The infamous null pointer mistakes have been eradicated, and the importance of pattern matching (in the sense of the ML family of languages) is finally being appreciated as the cure for Boolean blindness.  For once I can look at new industrial languages without an overwhelming sense of disappointment, but instead with optimism and enthusiasm that important ideas are finally, at long last, being recognized and adopted.  As has often been observed, it takes 25 years for an academic language idea to make it into industrial practice.  With Java it was simply the 1970’s idea of automatic storage management; with languages such as Rust and Swift we are seeing ideas from the 80’s and 90’s make their way into industrial practice.  It’s cause for celebration, and encouragement for those entering the field: the right ideas do win out in the end, one just has to have the courage to be irrelevant.

I hope to find the time to comment more meaningfully on the recent developments in practical programming languages, including Rust and Swift, but also languages such as Go and OCaml that are also making inroads into programming practice.  (I’ve had quite enough to say about Haskell for the time being, so I’ll give that one a rest, but with a tip of the hat to its enormous popularity and influence, despite my criticisms.)  But for now, let me say that the golden age of programming language research is here and now, and promises to continue indefinitely as we develop a grand unified theory of programming and mathematics.

Filed under: Programming, Research, Teaching Tagged: OPLSS14, programming languages, Rust, Swift
Categories: Offsite Blogs

OverlappingInstances-extension-like for type families

haskell-cafe - Mon, 07/21/2014 - 10:29pm
Hello haskell-café, I just started working with type families, and come with some difficulties. Now I have a class: class (...) ⇒ Complexable s where type ComplexSignalType s ... Is there a possibility to have this two instances (in any way)? instance (RealFloat e) ⇒ Complexable [e] where type ComplexSignalType [e] = [ℂ e] instance (RealFloat e) ⇒ Complexable [ℂ e] where type ComplexSignalType [ℂ e] = [ℂ e] Thanks. -- Leza Morais Lutonda, Lemol-C 50 Aniversario de la Cujae. Inaugurada por Fidel el 2 de diciembre de 1964 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe< at >
Categories: Offsite Discussion

Proposal: end lazy IO results with errors/exceptions

libraries list - Mon, 07/21/2014 - 9:16pm
Currently, withFile "foo" hGetContents >>= putStrLn prints out an empty line, the better to confuse newbies. I propose modifying the lazyRead function in GHC.IO.Handle.Text that currently reads lazyRead :: Handle -> IO String lazyRead handle = unsafeInterleaveIO $ withHandle "hGetContents" handle $ \ handle_ -> do case haType handle_ of ClosedHandle -> return (handle_, "") SemiClosedHandle -> lazyReadBuffered handle handle_ _ -> ioException (IOError (Just handle) IllegalOperation "hGetContents" "illegal handle type" Nothing Nothing) to something like lazyRead :: Handle -> IO String lazyRead handle = unsafeInterleaveIO $ withHandle "hGetContents" handle $ \ handle_ -> do case haType handle_ of ClosedHandle -> return (handle_, error "Forcing the result of a lazy read led to an attempt to read from a closed handle.") SemiClosedHandle -> lazyReadBuffered handle handle_
Categories: Offsite Discussion


General haskell list - Mon, 07/21/2014 - 8:45pm
****************************************************************** CALL FOR PARTICIPATION TASE 2014: 8th Theoretical Aspects of Software Engineering Symposium 1-3 September 2014, Changsha, China ****************************************************************** ---------------- OVERVIEW ---------------- Modern society is increasingly dependent on software systems that are becoming larger and more complex. This poses new challenges to the various aspects of software engineering, for instance, software dependability in trusted computing, interaction with physical components in cyber physical systems, distribution in cloud computing applications, etc. Hence, new concepts and methodologies are required to enhance the development of software engineering from theoretical aspects. TASE 2014 aims to provide a forum for people from academia and industry to communicate their latest results on theoretical advances in software engineering. TASE 2014 is the 8th in the TA
Categories: Incoming News

GHCI on ARM (Nvidia Jetson TK1)

haskell-cafe - Mon, 07/21/2014 - 8:01pm
Hi There, I would like to ask you what is the situation with ghci on arm. I bought Jetson TK1 and I am really sad that ghci does not works there. I have found mentioned somewhere that this should be fixed in 7.4.2 but still it is not. Can somebody clarify what is the situation at the moment and what are further plans? Best regards, T.
Categories: Offsite Discussion

Type family stopped compiling on upgrade from GHC 7.6.3 to 7.8.3

glasgow-user - Mon, 07/21/2014 - 6:51pm
Hi, I was experimenting a bit with type families recently and ran into a bit of an issue. Given that I don't know type families that well yet, I was wondering if I made an error somewhere. One thing is that I can't find any relevant changes in the GHC release notes for 7.8.1, .2 or .3. Maybe this code contains an error which 7.6.3 simply wasn't able to find? Thanks. --------
Categories: Offsite Discussion

Another Cabal question

haskell-cafe - Mon, 07/21/2014 - 2:16pm
Dear all, I have another issue: I have a Cabal package (call it A) that runtime depends on another (say B), where B provides (only) an executable that is compiled with the Make build-type. Now, I would like to list B under the build-depends of A (since there is no way to specify having just a run-time dependency), but it seems that when I cabal install B the package is not registered (although the executable does in fact become available to use within the standard directory.) So when I then compile A with Cabal it will complain the package is not registered. Does anyone know whether the behaviour for B is normal, or whether this is a bug of some kind? If the former, what is an easy and clean way of dealing with this situation. best, Jur
Categories: Offsite Discussion

Robert Harper: Parallelism and Concurrency, Revisited

Planet Haskell - Mon, 07/21/2014 - 10:36am

To my delight, I still get compliments on and criticisms of my post from three years ago (can it possibly be that long?) on parallelism and concurrency.  In that post I offered a “top down” argument to the effect that these are different abstractions with different goals: parallelism is about exploiting computational resources to maximize efficiency, concurrency is about non-deterministic composition of components in a system.  Parallelism never introduces bugs (the semantics is identical to the sequential execution), but concurrency could be said to be the mother lode of all bugs (the semantics of a component changes drastically, without careful provision, when composed concurrently with other components).  The two concepts just aren’t comparable, yet somehow the confusion between them persists.  (Not everyone agrees with me on this distinction, but neither have I seen a rigorous analysis that shows them to be the same concept.  Most complaints seem to be about my use of the words “parallelism” and “concurrency” , which is an unavoidable problem, or about my temerity in trying to define two somewhat ill-defined concepts, a criticism that I’ll just have to accept.)

I’ve recently gotten an inkling of why it might be that many people equate the two concepts (or see no point in distinguishing them).  This post is an attempt to clear up what I perceive to be a common misunderstanding that seems to explain it.  It’s hard for me to say whether it really is all that common of a misunderstanding, but it’s the impression I’ve gotten, so forgive me if I’m over-stressing an obvious point.  In any case I’m going to try for a “bottom up” explanation that might make more sense to some people.

The issue is scheduling.

The naive view of parallelism is that it’s just talk for concurrency, because all you do when you’re programming in parallel is fork off some threads, and then do something with their results when they’re done.  I’ve previously argued that this is the wrong way to think about parallelism (it’s really about cost), but let’s just let that pass.  It’s unarguably true that a parallel computation does consist of a bunch of, well, parallel computations.  So, the argument goes, it’s nothing but concurrency.  I’ve previously argued that that’s not a good way to think about concurrency either, but we’ll let that pass too.  So, the story goes, concurrency and parallelism are synonymous, and bullshitters like me are just trying to confuse people and make trouble.

Being the troublemaker that I am, my response is, predictably, nojust no.  Sure, it’s kinda sorta right, as I’ve already acknowledged, but not really, and here’s why: scheduling as you learned about it in OS class (for example) is an altogether different thing than scheduling for parallelism.  And this is the heart of the matter, from a “bottom-up” perspective.

There are two aspects of OS-like scheduling that I think are relevant here.  First, it is non-deterministic, and second, it is competitive.  Non-deterministic, because you have little or no control over what runs when or for how long.  A beast like the Linux scheduler is controlled by a zillion “voodoo parameters” (a turn of phrase borrowed from my queueing theory colleague, Mor Harchol-Balter), and who the hell knows what is going to happen to your poor threads once they’re in its clutches.  Second, and more importantly, an OS-like scheduler is allocating resources competitively.  You’ve got your threads, I’ve got my threads, and we both want ours to get run as soon as possible.  We’ll even pay for the privilege (priorities) if necessary.  The scheduler, and the queueing theory behind it (he says optimistically) is designed to optimize resource usage on a competitive basis, taking account of quality of service guarantees purchased by the participants.  It does not matter whether there is one processor or one thousand processors, the schedule is unpredictable.  That’s what makes concurrent programming hard: you have to program against all possible schedules.  And that’s why you can’t prove much about the time or space complexity of your program when it’s implemented concurrently.

Parallel scheduling is a whole ‘nother ball of wax.  It is (usually, but not necessarily) deterministic, so that you can prove bounds on its efficiency (Brent-type theorems, as I discussed in my previous post and in PFPL).  And, more importantly, it is cooperative in the sense that all threads are working together for the same computation towards the same ends.  The threads are scheduled so as to get the job (there’s only one) done as quickly and as efficiently as possible.  Deterministic schedulers for parallelism are the most common, because they are the easiest to analyze with respect to their time and space bounds.  Greedy schedulers, which guarantee to maximize use of available processors, never leaving any idle when there is work to be done, form an important class for which the simple form of Brent’s Theorem is obvious.

Many deterministic greedy scheduling algorithms are known, of which I will mention p-DFS and p-BFS, which do p-at-a-time depth- and breadth-first search of the dependency graph, and various forms of work-stealing schedulers, pioneered by Charles Leiserson at MIT.  (Incidentally, if you don’t already know what p-DFS or p-BFS are, I’ll warn you that they are a little trickier than they sound.  In particular p-DFS uses a data structure that is sort of like a stack but is not a stack.)  These differ significantly in their time bounds (for example, work stealing usually involves expectation over a random variable, whereas the depth- and breadth-first traversals do not), and differ dramatically in their space complexity.  For example, p-BFS is absolutely dreadful in its space complexity.  For a full discussion of these issues in parallel scheduling, I recommend Dan Spoonhower’s PhD Dissertation.  (His semantic profiling diagrams are amazingly beautiful and informative!)

So here’s the thing: when you’re programming in parallel, you don’t just throw some threads at some non-deterministic competitive scheduler.  Rather, you generate an implicit dependency graph that a cooperative scheduler uses to maximize efficiency, end-to-end.  At the high level you do an asymptotic cost analysis without considering platform parameters such as the number of processors or the nature of the interconnect.  At the low level the implementation has to validate that cost analysis by using clever techniques to ensure that, once the platform parameters are known, maximum use is made of the computational resources to get your job done for you as fast as possible.  Not only are there no bugs introduced by the mere fact of being scheduled in parallel, but even better, you can prove a theorem that tells you how fast your program is going to run on a real platform.  Now how cool is that?

[Update: word-smithing.]

Filed under: Programming, Research Tagged: concurrency, parallelism
Categories: Offsite Blogs

Is "avoid popularity at all costs" still a valid advice?

Haskell on Reddit - Mon, 07/21/2014 - 8:07am

If one has an idea or initiative that could help growing Haskell's popularity, should he think twice about doing it, or is that acceptable now (that it seems inevitable, anyway)?

submitted by SrPeixinho
[link] [7 comments]
Categories: Incoming News

Philip Wadler: Meditations on Using Haskell

Planet Haskell - Mon, 07/21/2014 - 3:27am
Bitemyapp - Meditations on Using Haskell explains why and how those in the trenches use Haskell, by quoting from conversations on an IRC channel.

ESo when i found haskell i slingshotted off through dependent and substructural types. Assuming that if a little was good a lot was better. Made it half way through TaPL and found pure type systems, coq, etc.I think the power to weight ratio isn’t there. I find that Haskell gives amazingly expressive types that have amazingpower for the amount of code you tie up in them and that are very resistant to refactoring.If i write agda and refactor I scrap and rewrite everything. If i write haskell, and get my tricky logic bits right?I can refactor it, split things up into classes, play all the squishy software engineering games to get a nice API I want. And in the end if it still compiles I can trust I didn’t screw up the refactoring with a very high degree of assurance.
CAdmittedly I’m not playing at the level E is, but this was my experience. I can make sweeping changes to my API, get all the bugs caught by the type system, and still have minimal code impact.
BThat is what I was getting at with the tweet about not using dynamically typed langs because I need to be able to prototype quickly and get rapid feedback.I think a lot of my friends thought i was just being trollish. Even just being able to see what would have to change if you changed your design slightly and being able to back it out quickly…
Categories: Offsite Blogs

Call for opinions on whether to use Yesod or Django

Haskell on Reddit - Mon, 07/21/2014 - 2:56am


I'm a self taught Django user since forever, and I've been dabbling in Haskell for the past year or so, having gotten to the point where I can try to get into a bigger project in Haskell. I've read into Yesod, but I've done no actual work except for the occasional tutorial project.

I've no doubt in my mind that I would chose Haskell over Python for almost any kind of project, except for a web project.

However, what I find lacking in the Yesod environment are plugins. There's a huge Django ecosystem, with open-source plugins that do mostly anything from facebook integration to tagging to payment engines to you-name-it.

What are the Haskell community's thoughts about this state of affairs? Are there any Yesod resources I might have overlooked?

submitted by pashadia
[link] [14 comments]
Categories: Incoming News

Is there any facility in Haskell for automagically deciding which computations should be eager and which lazy?

Haskell on Reddit - Mon, 07/21/2014 - 1:27am

I'm not exactly sure how it would work under the hood, but I'm imagining a library that keeps statistics on how often an expression is actually evaluated and eagerly (and in parallel) evaluates the ones with a high enough chance of future use. Kind of like branch prediction in CPUs, except at a higher level. Does any such library or language feature exist? Does the concept even make sense?

submitted by amoralnihilist
[link] [17 comments]
Categories: Incoming News

applicative question

haskell-cafe - Sun, 07/20/2014 - 11:43pm
Hi guys, I am trying to simplify some code. A user provides a combination of boolean expressions and for each expression an async task is run until that expression becomes true. The boolean expressions have type (Event e => e -> Bool) eg (PC.availableFlag .&&. "asmith" .== PC.userId) Unfortunately the way I have coded this, the user needs to include the \r -> r ? for each expression, with a unique number indicating the position in the sequence (so I can keep track of the asyncs when restarting after a failure). The 'r' function just wraps the expression in a Concurrently constructor. r :: (forall e.Event e => (e -> Bool) -> Concurrently e) -> Concurrently a Here is a sample expression that someone might provide: expr1 = runit (\r -> (,) <$> r 1 (PC.availableFlag .&&. "asmith" .== PC.userId) <*> r 2 (22 .== SC.signalNumber)) So, is there a way to rewrite or simplify this to avoid having to pass the r + unique Int to each subexpression? Or, is there a different format that I
Categories: Offsite Discussion

An idea to help Haskell take off

Haskell on Reddit - Sun, 07/20/2014 - 9:08pm

What if we were to make a game in Haskell?

Specifically, a game that has very strong modding support, and a pretty fun concept. Minecraft and the Source engine games have had a lot of success with modding, and a ton of people have had to learn the modding language in order to work on it, and have had the incentive to. If we make this game modded with haskell, and if its successful, that would be an insane boost to the haskell community. It could even have a branching effect, where modders who learnt haskell for this game go and use it to make their own games, for that reason making the game open source would be a very good idea (a guideline for these modders to help conceptualize the structure of a bigger project than their mods.)

What do you think? Its undoubtedly a large project, and coming up with an idea is very tough, but if enough community members got on board i think itd be very doable and very fun to work on.

I also have an idea for a game that could be made. It might be scummy of us and im not sure what the legality of this is but the game Cube World is seriously loved and seriously poipular and only failed because its underdeveloped, and hard to mod. If we went the other way with that i doubt wed have a hard time finding success.

This might also be unrealistic... i know games are massive projects and I dont think the open source community has ever banded together to work on something with almost no purpose besides entertainment.

submitted by fruitbooploops
[link] [55 comments]
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