September 2008 - Haskell-Cafe

Re: [Haskell-cafe] Re: Python's big challenges, Haskell's big advantages?
by Richard A. O'Keefe 18 Sep '08

18 Sep '08

It may be of interest that although Erlang has been doing lightweight concurrency for >20 years, - you can choose whether you want to use an SMP version that has as many schedulers as there are cores (plus internal locking as needed) or a non-SMP version with one scheduler (and no internal locking); both versions are standard and it's only a performance issue, not a semantics issue - performance sometimes goes one way, sometimes the other - there was a "one UNIX process per Erlang process" implementation; I have a copy. The community interest in it was, shall we say, massively underwhelming. It might also be interesting to note that the experimental operating system K42 from IBM does _all_ user-visible threading in user-land. This includes thread switching and even I/O blocking and unblocking; all done in user-land. I don't think we've begun to explore all the options yet.

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about openTempFile
by Manlio Perillo 17 Sep '08

17 Sep '08

Hi. After reading the chapter about IO in the "Real Word Haskell" book, I have noted that there is no support for opening a temporary file that will be automatically removed at program termination. The Python tempfile module, as an example, implements a wrapper around mkstemp function that does exactly this, and the code is portable; on Windows it uses O_TEMPORARY_FILE flag, on POSIX systems the file is unlink-ed as soon as it is created (but note that the code is not signal safe - well, many functions in the Python standard library are not signal safe). There are reasons why GHC library does not implement this? The Python version also set the FD_CLOEXEC, O_NOINHERIT and O_NOFOLLOW flags (where available). The GHC version, instead, set the O_NOCTTY flag. Manlio Perillo

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Re: [Haskell-cafe] Re: Python's big challenges, Haskell's big advantages?
by Donn Cave 17 Sep '08

17 Sep '08

Quoth Jonathan Cast <jonathanccast(a)fastmail.fm>: ... | Say what? This discussion is entirely about performance --- does | CPython actually have the ability to scale concurrent programs to | multiple processors? Well, ostensibly the discussion also has something to do with Haskell. On that premise, may I say I hope we're not inspired by Python's example to treat concurrency as a religious matter. The C Python community is forced to take the position that interpreter- level threading is not worth the price, because they don't have it and can't conveniently get it. I doubt we'll ever know for sure whether they're right or wrong, since Python comes with other issues that could also take blame or credit for its success at large scale applications. The Haskell community seems to be convinced as a whole that user-level threads are the way to go, but at most that's just a majority, not a consensus. Who cares!? Given the ability to take either path, Haskell programmers will solve our religious argument - we'll know who gets to heaven with working, high performance, reliable applications. That is, assuming that programmers of each persuasion can find Haskell suited to their model. As far as I know, though, that isn't a really sure thing. I've read things like "thread RTS can't/shouldn't support fork" and "would like to be able to get rid of the non-threaded RTS", with respect to GHC. At least until there is more empirical evidence that it's a waste of time, Haskell implementations should leave this to the programmer - support processes, OS threads, or user threads without undue work-around burdens. Otherwise we'll be stuck like the Python community is, defending this approach as a religious argument. We have enough of those already. Donn Cave, donn(a)avvanta.com

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How to check if two Haskell files are the same?
by Mauricio 17 Sep '08

17 Sep '08

Hi, I would like to write a Haskell pretty-printer, using standard libraries for that. How can I check if the original and the pretty-printed versions are the same? For instance, is there a file generated by GHC at the compilation pipe that is always guaranteed to have the same MD5 hash when it comes from equivalent source? Thanks, Maurício

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Float instance of 'read'
by Mauricio 17 Sep '08

17 Sep '08

Hi, A small annoyance some users outside english speaking countries usually experiment when learning programming languages is that real numbers use a '.' instead of ','. Of course, that is not such a problem except for the inconsistence between computer and free hand notation. Do you think 'read' (actually, 'readsPrec'?) could be made to also read the international convention (ie., read "1,5" would also work besides read "1.5")? I'm happy to finaly use a language where I can use words of my language to name variables, so I wonder if we could also make that step. Thanks, Maurício

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Predicativity?
by Wei Hu 17 Sep '08

17 Sep '08

Hello, I only have a vague understanding of predicativity/impredicativity, but cannot map this concept to practice. We know the type of id is forall a. a -> a. I thought id could not be applied to itself under predicative polymorphism. But Haksell and OCaml both type check (id id) with no problem. Is my understanding wrong? Can you show an example that doesn't type check under predicative polymorphism, but would type check under impredicative polymorphism? Thanks!

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Semantic Domain, Function, and denotational model.....
by Greg Meredith 17 Sep '08

17 Sep '08

Daryoush, Hopefully, the other replies about proving the monad laws already answered your previous question: yes! As for notions of semantic domain and denotational model, these ideas go back quite a ways; but, were given solid footing by Dana Scott<http://en.wikipedia.org/wiki/Dana_Scott>. In a nutshell, we have essentially three views of a computation - Operational <http://en.wikipedia.org/wiki/Operational_semantics> -- computation is captured in a program and rules for executing it - Logical <http://en.wikipedia.org/wiki/Proof-theoretic_semantics> -- computation is captured in a proof and rules for normalizing it - Denotational <http://en.wikipedia.org/wiki/Denotational_semantics> -- computation is captured as a (completely unfolded) mathematical structure In the latter view we think of computations/programs as denoting some (usually infinitary) mathematical object. Our aim<http://en.wikipedia.org/wiki/Domain_theory>is to completely define the meaning of programs in terms of maps between their syntactic representation and the mathematical objects their syntax is intended to denote. (Notationally, one often writes such maps as [| - |] : Program -> Denotation.) For example, we model terms in the lambda calculus as elements in a D-infinity domain or Bohm trees or ... . Or, in more modern parlance, we model terms in the lambda calculus as morphisms in some Cartesian closed category, and the types of those terms as objects in said category. The gold standard of quality of such a model is full abstraction<http://en.wikipedia.org/wiki/Denotational_semantics#Full_abstraction>-- when the denotational notion of equivalence exactly coincides with an intended operational notion of equivalence. In symbols, - P ~ Q <=> [| P |] = [| Q |], where ~ and = are the operational and denotational notions of equivalence, respectively The techniques of denotational semantics have been very fruitful, but greatly improved by having to rub elbows with operational characterizations. The original proposals for denotational models of the lambda calculus were much too arms length from the intensional structure implicit in the notion of computation and thus failed to achieve full abstraction even for toy models of lambda enriched with naturals and booleans (cf the so-called full abstraction for PCF problem<http://en.wikipedia.org/wiki/Programming_language_for_Computable_Functions>). On the flip side, providing a better syntactic exposure of the use of resources -- ala linear logic -- aided the denotational programme. More generally, operational models fit neatly into two ready-made notions of equivalence - contextual equivalence<http://encyclopedia2.thefreedictionary.com/Contextual+equivalence>-- behaves the same in all contexts - bisimulation <http://en.wikipedia.org/wiki/Bisimulation> -- behaves the same under all observations Moreover, these can be seen to coincide with ready-made notions of equivalence under the logical view of programs. Except for syntactically derived initial/final denotational models -- the current theory usually finds a more "home-grown" denotational notion of equivalence. In fact, i submit that it is this very distance from the syntactic presentation that has weakened the more popular understanding of "denotational" models to just this -- whenever you have some compositionally defined map from one representation to another that respects some form of equivalence, the target is viewed as the denotation. Best wishes, --greg Date: Mon, 15 Sep 2008 10:13:53 -0700 From: "Daryoush Mehrtash" <dmehrtash(a)gmail.com> Subject: Re: [Haskell-cafe] Semantic Domain, Function, and denotational model..... To: "Ryan Ingram" <ryani.spam(a)gmail.com> Cc: Haskell Cafe <haskell-cafe(a)haskell.org> Message-ID: <e5b8e9790809151013k53fdc105i88f2f47f1f9b16bd(a)mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Interestingly, I was trying to read his paper when I realized that I needed to figure out the meaning of denotational model, semantic domain, semantic functions. Other Haskell books didn't talk about design in those terms, but obviously for him this is how he is driving his design. I am looking for a simpler tutorial, text book like reference on the topic. Daryoush On Mon, Sep 15, 2008 at 1:33 AM, Ryan Ingram <ryani.spam(a)gmail.com> wrote: > I recommend reading Conal Elliott's "Efficient Functional Reactivity" > paper for an in-depth real-world example. > > http://www.conal.net/papers/simply-reactive > > -- ryan > > On Sun, Sep 14, 2008 at 11:31 AM, Daryoush Mehrtash <dmehrtash(a)gmail.com> wrote: >> I have been told that for a Haskell/Functional programmer the process >> of design starts with defining Semantic Domain, Function, and >> denotational model of the problem. I have done some googling on the >> topic but haven't found a good reference on it. I would appreciate >> any good references on the topic. >> >> thanks, >> >> daryoush >> >> ps. I have found referneces like >> http://en.wikibooks.org/wiki/Haskell/Denotational_semantics which >> talks about semantic domain for "the Haskell programs 10, 9+1, 2*5" >> which doesn't do any good for me. I need something with a more real >> examples. >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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Mac OS X dylib woes
by John MacFarlane 17 Sep '08

17 Sep '08

I'm hoping some Haskell developers who use Macs can help me with this one. I can install pcre-light just fine using cabal install. But when I try to use it, I get this error: GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help :Loading package base ... linking ... done. Prelude> :m Text.Regex.PCRE.Light.Char8 Prelude Text.Regex.PCRE.Light.Char8> compile "h." [] Loading package array-0.1.0.0 ... linking ... done. Loading package bytestring-0.9.0.1 ... linking ... done. Loading package pcre-light-0.3.1 ... can't load .so/.DLL for: pcre (dlopen(libpcre.dylib, 10): image not found) OK, so it can't find the pcre library (which is in /opt/local/lib). I can fix that: export DYLD_LIBRARY_PATH=/opt/local/lib Now it works. But other things are broken! For example, I can't run vim, which looks for a library called libJPEG.dylib and now finds libjpeg.dylib in /opt/local/lib (case-insensitive file system!). The apple website recommends against setting DYLD_LIBRARY_PATH. Instead, they say, the paths to the libraries should be hard-coded into the binary: http://discussions.apple.com/thread.jspa?threadID=1670523&tstart=0 I'm sure others have had the same problem. What's the solution? John

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FRP (was: Semantic Domain, Function, and denotational model)
by John Lato 17 Sep '08

17 Sep '08

I just noticed that the "Simply Efficient Functional Reactivity" paper has been updated since I last looked; I'll have to read it again now. Is the library/code mentioned in the paper released or available anywhere at this time? Conal has left tantalizing hints scattered in various places... Thanks, John Lato

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GHC trouble on Leopard
by Miguel Vilaca 17 Sep '08

17 Sep '08

Hi, I tried to compile some code on Mac Os X (Intel) Leopard. I have GHC 6.8.3 installed - the installer from GHC webpage (GHC-6.8.3- i386.pkg). But when I run make I get this error ghc-6.8.3: could not execute: /Library/Frameworks/GHC.framework/ Versions/608/usr/lib/ghc-6.8.3/ghc-asm The work-around of removing the option -fvia-C pointed somewhere on the web is not an option here due to library dependecies. Any hints on how to solve this? Thanks in advance Miguel Vilaça

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