June 2012 - Haskell-Cafe

Program for displaying graph state in lambda interpreter?
by Benjamin Redelings 02 Jun '12

02 Jun '12

Hi, I have written an interpreter that operates on the lambda calculus augmented with letrec, constructors, case, primitive objects, and builtin operations. I'd like to display the internal state of the intepreter at various points so that I can, um, debug the "programs" that I've written. Currently I've got about 3500 nodes, and I'm dumping a graph that I format using the 'dot' program in graphviz. I might have "let x = 1+2 in x*x" represented as. x/1: <2>+<3> 2: 1.0 3: 2.0 4: <1>*<1> where <1>, <2>, <3>, represent various memory locations. I can display this in graphviz, where each node is a rectangle containing text like "<1>*<1>" and there are arrows from the rectangle to <1>. This works as long as the number of nodes isn't very large. However, it would be nice to know of any programs that are better suited for this. For example, if I could write <>*<> and draw arrows from the placeholders "<>" to the memory location being referenced, that would be easier to read. This is done here http://en.wikibooks.org/wiki/Haskell/Graph_reduction to illustrate that 'square (1+2)' doesn't evaluate 1+2 twice. Any ideas? thanks, -BenRI P.S. If this is too off-topic, please let me know.

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SafeSemapore
by Chris Kuklewicz 02 Jun '12

02 Jun '12

This message is in three parts: a problem, a solution, and a request for help. The problem is that Control.Concurrent.QSem, QSemN, and SampleVar are all still broken. GHC ticket #3160 is still open [1]. These three synchronization mechanisms can be irretrievable broken by a killThread on a blocked waiter. The solution is that I am please to announce that SafeSemaphore has been updated to 0.9.0 on hackage [2] and github. These provides safe solutions to replace QSem and QSemN, actually several such solutions. See the github [2] page (scroll down for README) for a summary of all the modules. The request for help is that I would like to unbreak the Haskell Platform by replacing the guts of QSem, QSemN, and SampleVar with SafeSemaphore. Do you think my replacement is correct or buggy? Can we get #3160 closed? Replacing these will preserve their API but may tinker with corner case undocumented behavior. Should waiters block in FIFO order? Should QSemN starve big or small requests to be fairer? [1] http://hackage.haskell.org/trac/ghc/ticket/3160 (three years old!) [2] http://hackage.haskell.org/package/SafeSemaphore/ [3] https://github.com/ChrisKuklewicz/SafeSemaphore Sincerely, Chris Kuklewicz

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Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
by wren ng thornton 01 Jun '12

01 Jun '12

On 6/1/12 6:11 AM, Gábor Lehel wrote: > On Fri, Jun 1, 2012 at 6:29 AM, wren ng thornton<wren(a)freegeek.org> wrote: >> >> TypeFamilies (aka TFs) >> These are really nifty and they're all the rage these days. In >> a formal sense they're equivalent to fundeps, but in practice >> they're weaker than fundeps. > > Is that still true? The reason used to be that we didn't have > superclass equalities, but we do have them now since 7.2. The only > drawbacks I know of relative to FDs are that it's sometimes more > typing, not supported by GeneralizedNewtypeDeriving, and doesn't allow > OverlappingInstances (ick). The superclass equalities was a big thing, but the disparity remains even still. The main problem is that type inference with fundeps remains more powerful than type inference for TFs. There are still patterns of use which infer/check easily with fundeps but which don't have an equivalent TF implementation. I haven't messed with TFs enough to know the details here, but there are other folks on the list who do. In addition there's the issue of overlapping instances, which is currently being discussed elsewhere. -- Live well, ~wren

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Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
by wren ng thornton 01 Jun '12

01 Jun '12

On 6/1/12 12:45 AM, Jonathan Geddes wrote: > Thanks, Wren, I really appreciate the detailed response! Though I am > surprised that Template Haskell isn't on your list. From the little I know > of TH it seems like all of the interesting generic/generative stuff is done > with TH. Do the other extensions subsume the need for TH, or is it just not > terribly interesting? TH is plenty interesting, but it's a very different sort of direction to head. The extensions I mentioned are the ones I think everyone assumes a seasoned Haskeller will know. With the exception of TFs and GADTs, those extensions have all been around since the days of GHC 6.6 and Hugs. Thus, most people consider them normal parts of "Haskell" even if they're not in in the Report. Consequently, understanding them is necessary to understand most of the non-H98 code on Hackage. TH, on the other hand, is very much GHC-only and that's unlikely to change. Also, there's a lot of unnecessary(?) grunge there. The basic theory of staged computation which led to TH was laid out in the MetaML/MetaOCaml papers. The version in MetaML is a lot nicer in terms of the theory, so I'd suggest people start there before diving into TH. One of the nice things in MetaML is that there's no limit on the levels of meta-ness. This was removed from TH because it would require having the compiler present in every executable; a sensible limitation, though it complicates the theory. Another major difference is that MetaML only did staging at the term level, whereas TH also allows splices in type signatures, splices which generate type class instances, etc. These extensions make TH *much* more powerful than MetaML, but also make it *much* harder to understand and reason about. IMO the formal theory of these non-term splices hasn't been worked out very well, which is part of the reason why TH is so grungy to work with. I'm a big fan of staged computation, and I'd definitely suggest any sophisticated functional programmer should read the MetaML papers (and the TH papers if so inclined, and the Flask papers to get a different perspective on the theme). But very few packages require understanding TH in order to understand them, and even fewer require understanding TH in order to use them. -- Live well, ~wren

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Requesting Feedback: I Love Haskell, but can't find a place to use it
by Jonathan Geddes 01 Jun '12

01 Jun '12

I love Haskell. It is my absolute favorite language. But I have a very hard time finding places where I can actually use it! I had hoped that compiling Haskell to C with -fvia-C (or would it be just -C?) would allow Haskell to run in new, uncharted territory such as Android (with NDK), IOS, Google's NaCl, etc. But today I learned that GHC's C backend has been deprecated! Is it more difficult than I am imagining to get Haskell to work in these environments? Is it simply a matter of low interest in this kind of work? Or something more fundamental? Am I missing something? I'm hoping that the Haskell->JavaScript efforts will mature enough to make Haskell viable for client-side web apps. (I think the first sign of this will be a self-hosting Haskell->JavaScript compiler.) I use Haskell for Server-Side code with various web frameworks, but over the years more and more of the app logic is moved into client-side JavaScript, leaving the server-side code as little more than a simple validation and security layer over the database and other services. Haskell doesn't have any trouble with this, of course, but it's not exactly a role where it can shine. (Of course this is not true of ALL server-side code, just the kind of apps I have been writing.) So anyway I'd like to request feedback: where can I use Haskell besides simple CLI utilities, dull server code, or project Euler problems? Even if it's just to contribute to getting Haskell in the environments mentioned above, any feedback is welcome! Thanks for reading, --J Arthur

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Re: [Haskell-cafe] Requesting Feedback: I Love Haskell, but can't find a place to use it
by Doug McIlroy 01 Jun '12

01 Jun '12

> > I love Haskell. It is my absolute favorite language. > > But I have a very hard time finding places where I can actually use it! > > have you considered "your head" as such a place that should be easy to find. Excellent advice. Haskell shines unusually brightly on applications that have an algebraic structure. And laziness relieves a plethora of sequencing concerns. I particularly treasure one experience: I asked a guru about the complexity of converting regular expressions to finite-state automata without epsilon transitions (state transitions that don't produce output). The best he knew was O(n^3), which is the cost of removing epsilon transitions from arbitrary automata. Then I wrote about a dozen lines of Haskell to do the job--and running time turned out to be O(n^2). Once I'd written the code, it became clear how to do it in other languages, but I never would have found the theorem without the help of Haskell. (This without even bringing in the heavy artillery of higher-order functions and monads.) Doug McIlroy

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Re: [Haskell-cafe] Requesting Feedback: I Love Haskell, but can't find a place to use it
by Doug McIlroy 01 Jun '12

01 Jun '12

> > I love Haskell. It is my absolute favorite language. > > But I have a very hard time finding places where I can actually use it! > > have you considered "your head" as such a place that should be easy to find. An excellent reason. Haskell shines unusually brightly on applications that have an algebraic structure. Laziness relieves a plethora of sequencing concerns. I particularly treasure one experience: I asked a guru about the complexity of converting regular expressions to finite-state automata without epsilon transitions (state transitions that don't produce output). The best he knew was O(n^3), which is the cost of removing epsilon transitions from arbitrary automata. Then I wrote about a dozen lines of Haskell to do the job--and running time turned out to be O(n^2). Once I'd written the code, it became clear how to do it in other languages, but I never would have found the theorem without the help of Haskell. (This without even bringing in the heavy artillery of higher-order functions and monads.) Doug McIlroy

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Tests by properties: origin?
by Yves Parès 01 Jun '12

01 Jun '12

Out of curiosity, does someone know if QuickCheck was the first test framework working through test by properties associated with random generation or if it drew the idea from something else? Because the idea has be retaken by a lot of frameworks in several languages (see http://en.wikipedia.org/wiki/Quickcheck) but I can't find what was QuickCheck inspiration. (This question pop up when arguing with a Java user about what Haskell brings to the field of development, he pretends it would be astonishing that QuickCheck is the first one using this testing paradigm).

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Confused by ghci output
by Clark Gaebel 01 Jun '12

01 Jun '12

*X> 3^40 `mod` 3 == modexp2 3 40 3 False *X> modexp2 3 40 3 0 *X> 3^40 `mod` 3 0 I'm confused. Last I checked, 0 == 0. Using GHC 7.4.1, and the file x.hs (which has been loaded in ghci) can be found here: http://hpaste.org/69342 I noticed this after prop_sanemodexp was failing. Any help would be appreciated, - Clark

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