Re: [Haskell-cafe] Wanted: Haskell binding for libbdd (buddy)
On 20/08/2012, at 11:19 PM, Johannes Waldmann wrote: Are there any Haskell bindings for BDD libraries (reduced ordered binary decision diagrams)? E.g., it seems buddy is commonly used http://packages.debian.org/squeeze/libbdd-dev and it has an Ocaml binding. My hBDD bindings are on Hackage. I once had a binding to buddy but found CUDD to have superior performance for my application. cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why does (++ !) bla return bla! and not !bla?
Hilco, On 22/06/2012, at 2:54 PM, Hilco Wijbenga wrote: I'm going through the excellent http://learnyouahaskell.com tutorial. So far it's been pretty easy to follow but now I ran into something that (when I later started reading about maps) do not seem to fully grasp. I think I'm close to understanding why (++ !) bla returns bla! instead of !bla but I seem to be missing the last step. :-) I noticed that ((++) !) bla does indeed return !bla. So it seems to be related to the infix property of ++? The types of (++) !, ((++) !), and (++ !) are all the same so that doesn't tell me much. This stuff is in a beginner's tutorial? (!?) This is purely a syntactic issue. These things are called sections. It might be more obvious if we put in some lambda abstractions, which I hope your tutorial has already introduced: (++ !) = (\x. x ++ !) (! ++) = (\y. ! ++ y) Yes, it is related to the infix property of ++. You can get similar things going with arbitrary binary (two argument) functions like so: app = (++) -- or whatever (`app` !) = (\x. x `app` !) = (\x. app x !) (and the other way around) cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Theoretical question: are side effects necessary?
Christopher, On 16/03/2012, at 11:23 PM, Christopher Svanefalk wrote: there is a question I have been thinking about a bit. In short, we could simply formulate it like this: Are there any problems which cannot be solved a side effect-free language (such as Haskell)? In other words, are there problems that would explicitly demand semantics that can only be provided by a language allowing direct modification of external state variables, such as Java and C++? If not, are there problems which are simply infeasible to solve with a side effect-free language? Start here: http://www.cs.ox.ac.uk/people/geraint.jones/morehaste.html and dig through their references. I don't think a logarithmic factor is ever going to make the difference between feasible and infeasible. :-) cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functor in terms of Arrow
Tom, On 19/02/2012, at 3:21 AM, Tom Schouten wrote: Does AFunctor below have a standard name? It's a generalization of the Functor class in terms of Arrow instead of (-): fmap :: Functor f = (i - o) - f i - f o afmap :: Arrow a, AFunctor f = a i o- a (f i) (f o) It pops up in less general form (AFunctor = []) in iterated functions (difference equations / state space models), where the arrow is the update function parameterized by state type: data Iter s i o = Iter ((s,i) - (s,o)) instance Arrow (Iter s) I think you can work with Arrow transformers instead. See: http://hackage.haskell.org/packages/archive/arrows/latest/doc/html/Control-Arrow-Transformer-Stream.html#t:StreamArrow It may be that you can generalise to arbitrary functors, but satisfying the Arrow laws may require some care. cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Question: Lazy Incremental Evaluation and Haskell?
Ben, On 07/10/2011, at 8:55 AM, Benjamin Redelings I wrote: My question is, roughly, is there already an existing framework for incremental evaluation in Haskell? Margnus Carlsson did something monadic several years ago. http://dl.acm.org/citation.cfm?doid=581478.581482 Perhaps there is an implementation on Hackage or on his website. This stuff also goes by the moniker adaptive computation. See the references and citations of that paper for more on this. cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is Harper right that Haskell cannot model the natural numbers?
On 03/05/2011, at 1:25 PM, Richard O'Keefe wrote:
In one of his blog posts, Robert Harper claims that the natural numbers
are not definable in Haskell.
SML datatype nat = ZERO | SUCC of nat
Haskell data Nat = Zero | Succ Nat
differ in that the SML version has strict constructors, and so only
finite instances of nat can be constructed, whereas Haskell has
lazy constructors, so
inf = Succ inf
is constructible, but that's not a natural number, and it isn't
bottom either, so this is not a model of the natural numbers.
Fair enough, but what about
data Nat = Zero | Succ !Nat
where the constructors *are* strict? It's perfectly good Haskell 98
as far as I can see. Now Nat itself isn't _quite_ a model of the
naturals because it includes bottom, but then an SML function
returning nat can also fail, so arguably SML's nat could or should be
thought of as including bottom too.
What am I missing?
Do read the comments attached to the blog post (and also maybe at reddit).
I believe the traditional model for ML-like languages in denotational
semantics is to lift the type returned by a function. So ML's Nat type is
taken to be an inductively-defined set (with the discrete order) and your
divergent ML function really has type a - Nat_{lift} for whatever type a you
had in mind. I think Moggi's encoding into monads makes this clear - you use
the Maybe/Evaluation/Strict monad to handle divergent arguments.
cheers
peter
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Re: [Haskell-cafe] How to keep cabal and ghci package versions in sync?
On 27/04/2011, at 3:04 AM, Rogan Creswick wrote: At the moment, cabal-dev ghci just uses the -package-conf and -no-user-package-conf flags to restrict ghci to the sandboxed and global package dbs. It's difficult to do more without parsing the content of the project's cabal file, and that parser isn't exposed through a library api (it's in cabal-install), which makes it a little difficult to use. Can't you use cabal as a library to do this? I had a crack at figuring out a ghci command line in TBC (testing by convention): https://github.com/peteg/TBC while waiting for this cabal feature to be implemented: http://hackage.haskell.org/trac/hackage/ticket/382 Apparently there's a GSoC project that will nail this. You can fire up TBC with the verbose flag and it will give you a very long ghc invocation line that is strongly similar to what cabal uses to compile your project. It doubtlessly is incomplete, and probably bit rotten. I will try to make it work with the latest Haskell Platform release in the coming weeks. I welcome patches, but you're better off fixing cabal itself. cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Asynchronous Arrows need Type Specialization - Help!
David, On 21/03/2011, at 4:18 PM, David Barbour wrote: I was giving Control.Arrow a try for a reactive programming system. The arrows are agents that communicate by sending and returning time-varying state. Different agents may live in different 'vats' (event-driven threads) to roughly model distributed computing. For the most part, the state varies asynchronously - i.e. a file updates at a different rate than the mouse position. Anyhow, I ran into a problem: The (***) and () operations, as specified in Control.Arrow, are inherently synchronization points. Indeed. Take a look here: http://www.haskell.org/haskellwiki/Research_papers/Monads_and_arrows#Arrows In particular, ProdArrows -- Arrows for Fudgets by Magnus Carlsson. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] lambda calculus and equational logic
Ben, comments from the peanut gallery: On 13/07/2010, at 11:51 PM, Ben Lippmeier wrote: What kind of equality do you use for getChar :: IO Char ? Surely this is easy: getChar denotes a particular IO action, which is always the same thing (i.e. self-identical and distinct from all other IO actions). By massively complicated you mean harder than the simplest case. Haskell's do-notation makes the state of the world implicit, and performing the desugaring makes it explicit again -- but then that state isn't really the state of the word... ... which tells you that the passing-the-world semantics is not a faithful model of I/O. In the archives of this list there are many comments on exactly this topic. A couple of quick pointers: - Wouter Swierstra wrote a paper on this. http://www.cse.chalmers.se/~wouter/repos/IOSpec/index.html - Resumptions are a better model: http://www.haskell.org/pipermail/haskell-cafe/2003-August/004892.html Intuitively the state monad used by GHC works because it only needs to track dependencies between I/O actions - there is no need for the compiler to impose a total order on everything going on in the world (taken to be all of the context that could influence the execution of the program). For interesting programs it cannot, anyway. cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: FRP for game programming / artifical life simulation
Ben,
On 29/04/2010, at 6:16 AM, Ben wrote:
[...]
newtype STAuto s a b = STAuto { unSTAuto : (a, s) - (b, s) }
As Felipe observes in detail, this can be made to work. He uses Read and Show
for serialisation, but clearly you can use whatever you like instead.
I just wanted to add that one can go to town with the approach: after you
understand Felipe's stuff, take a look at Caspi and Pouzet's coalgebraic
streams stuff. (I'd recommend looking at both the tech report and the published
paper, and there is some Haskell code too.)
BTW I was referring (off-list) to the original Arrows paper by John Hughes.
cheers
peter
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Re: [Haskell-cafe] Bulk Synchronous Parallel
On Thu, Apr 22, 2010 at 4:07 AM, Aaron D. Ball aarondball+hask...@gmail.com wrote: I don't need a tool that automatically figures out how to distribute any workload in an intelligent way and handles all the communication for me. If I have the basic building block, which is the ability to serialize a Haskell expression with its dependencies and read them into another Haskell instance where I can evaluate them, I can handle the other pieces, which are - passing strings back and forth in whatever way is convenient - deciding how to divide up my workload. Alice/ML is the place to look for this technology. http://www.ps.uni-saarland.de/alice/ The project may be dead (I don't know), but they did have the most sophisticated take on pickling that I've seen. It's an ML variant, with futures, running on top of the same platform used by Mozart/Oz of CTM fame. Why do you want to pass strings around? cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Beginning of a meta-Haskell [was: An issue with the ``finally tagless'' tradition]
Thanks Oleg! Brad: On 24/09/2009, at 3:54 PM, o...@okmij.org wrote: and interpret it several times, as an integer -- testpw :: Int testpw = (unR power) (unR 2) ((unR 7)::Int) -- 128 My type function allows one to remove the '::Int' annotation, which is especially useful in situations where you cannot give an annotation due to 'show . read'-style ambiguity. Conversely one gives up some useful polymorphism (in my case, sometimes it would be nice to have multiple boolean types). Another nit with Oleg's code is that you can only interpret Bool with Bool, in the interpreter: class QBool repr where true, false :: repr Bool if_ :: repr Bool - repr w - repr w - repr w If 'repr' is merely a Haskell98-style type constructor, it cannot analyse its argument. Hence there are two choices: use the argument (Bool) or don't (see the pretty printer). I doubt you could implement a very pleasant interpreter using the latter option, but see http://web.cecs.pdx.edu/~brianh/talks/talk20081010.pdf if you want to try. Again, using a type function here allows you to choose an arbitrary type to represent Bool (and so forth). Trying to do this with fundeps is possible but syntactically heavy. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] An issue with EDSLs in the ``finally tagless'' tradition
Brad: On 24/09/2009, at 11:59 AM, excerpts of what Brad Larsen wrote: We then try to define an evaluator: -- instance PolyArithExpr E where -- constant = E -- addP e1 e2 = E (eval e1 + eval e2) -- bzzt! The instance definition for `addP' is not type correct: Could not deduce (Num a) from the context () arising from a use of `+' at /home/blarsen/mail.lhs:42:20-36 One way to remedy this is to change the class definition of PolyArithExpr so that `addP' has a Num constraint on `a': class PolyArithExprFixed exp where pae_constant :: a - exp a pae_add :: Num a = exp a - exp a - exp a which allows us to define an evaluator: instance PolyArithExprFixed E where pae_constant = E pae_add e1 e2 = E (eval e1 + eval e2) I find this ``fix'' lacking, however: to define a new interpretation of the EDSL, we may be forced to change the DSL definition. This is non-modular, and seems like an instance of the expression problem. (There may be a multiparameter type class solution for this.) A better fix is to shift the type constraint to the class head, e.g.: class PolyArithExprFixed exp a where pae_constant :: a - exp a pae_add :: exp a - exp a - exp a then: instance PolyArithExprFixed E Integer where ... or if you want to be more general: instance Num n = PolyArithExprFixed E n where ... but note that this instance precludes any other ones for PolyArithExprFixed without allowing overlapping instances and hence some Olegs / Olegging. See, e.g.: http://www.soi.city.ac.uk/~ross/papers/fop.html (the slides link) Ambiguity is IMHO best handled with a judicious application of type (or data) families, but you can get surprisingly far by simply requiring that every class member mention all type variables in the class head. YMMV of course. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] An issue with EDSLs in the ``finally tagless'' tradition
Brad, On 24/09/2009, at 2:45 PM, Brad Larsen wrote: On Thu, Sep 24, 2009 at 12:22 AM, Peter Gammie pete...@gmail.com wrote: [...] Ambiguity is IMHO best handled with a judicious application of type (or data) families, but you can get surprisingly far by simply requiring that every class member mention all type variables in the class head. YMMV of course. Can you say more about the use of type/data families? Multi-parameter type classes generally lead to ambiguity - see the classic paper(s) by Simon PJ et al for some good examples. In the context of Arrows, I used type/data families to make the representation type (e.g. Integer) a function of the arrow (e.g. a direct semantics), which makes life a bit easier for using this stuff but is not completely faithful to what I wanted. (Imagine you want to mix Integer and Double in the same expression.) Hope this helps! cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell] Re: 20 years ago
On 28/07/2009, at 11:35 AM, Richard O'Keefe wrote: It's true that the abstract speaks of a more biological scheme of protected universal cells interacting only through messages that could mimic any desired behavior, but that's basically _it_ for biology, if we are to believe Kay, and even then, its semantics are a bit like having thousands of and thousands of comptuers all hooked together by a very fast network and Philosophically, Smalltalk's objects have much in common with the monads of Leibnitz (bringing us neatly back to Haskell (:-)). But Richard (or am I arguing with Kay?) - monads don't interact. http://en.wikipedia.org/wiki/Monadology I'd take that to be their defining characteristic - Leibniz is trying to overcome Cartesian mind/body dualism here. Perhaps Smalltalk objects are regulated by pre-established harmonies... and have no need to talk to each other. Or perhaps that is the part that lies outside Kay's identification, in which case one might say the ontologies of monads and Smalltalk have superficial similarities but the deeper structure diverges significantly. (I think my point is well-defended by the above wikipedia article - the OO insights are thin on the ground, and are at least quite tangled.) How are you going to relate Leibniz's monads and Haskell's? I can't find my way, neatly or otherwise. :-P Ah, I see, Haskell has an interface to the best of all possible worlds... :-) cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell] Re: 20 years ago
On 28/07/2009, at 12:59 PM, Richard O'Keefe wrote: On Jul 28, 2009, at 2:25 PM, Peter Gammie wrote: But Richard (or am I arguing with Kay?) - monads don't interact. You're arguing with Alan Kay here: the reference to Leibniz was his. The key link here is (Wikipedia): Leibniz allows just one type of element in the build of the universe (sic.). In precisely the same way, Alan Kay allowed just one kind of 'thing' in his computational universe: object. Just as in the lambda calculus, everything is a function and in set theory everything is a set, so in Smalltalk _everything_ (including classes and the number 42 and anonymous functions) is an object. Yea gods, that's the thinnest use of monads ever. The concept that lead to idealism, away from mind-body dualism is reduced to ... monism. Awesome. http://en.wikipedia.org/wiki/Monism He could've cited just about any of the major philosophies for that - and I'm not going to talk about religions. How are you going to relate Leibniz's monads and Haskell's? I can't find my way, neatly or otherwise. :-P Verbally. Sure, but I was hoping you'd explain why Wadler uses the pineal gland allusions in his COMPREHENDING MONADS (capitals denoting paper title). That structure was *exactly* what Leibniz was doing his best to avoid in his monadology. I think Wadler was making a joke. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] TBC: Testing By Convention
Hello, Mark and I would like to announce our test harness, which has features complementary to existing harnesses. TBC provides two main features: - It attempts to compile and run all tests, even if some do not compile or run. - Aspiring to the write-it-once principle, tests following conventions require a lot less boilerplate. It is at an embryonic stage of development, and would greatly benefit from feedback and/or patches. :-) Hackage: http://hackage.haskell.org/package/TBC git: http://github.com/peteg/TBC/ Enjoy! cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] TBC: Testing By Convention
On 27/07/2009, at 2:26 PM, Alexander Dunlap wrote: Can it return an exit status based on whether or not all tests passed? If not, that would be a very useful feature that I have not seen in any other testing frameworks. It could, and that's probably true of the other harnesses out there. The git repo for TBC now has this feature; it's a bit of a hack as some of the functions in Cabal that TBC uses have overly restrictive types. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [ANNOUNCE] Bindings for libguestfs
Richard, I remember having similar problems to you when trying to use the FFI. The following comments are suggestions as to what helps in practice, and not a claim that the situation can't be improved. On 13/05/2009, at 8:40 PM, Richard W.M. Jones wrote: Specific things would be: Recommendation to use 'throwDyn' to throw exceptions, yet this function doesn't exist in my version of GHC. In fact, none of the code here: http://www.haskell.org/haskellwiki/FFICookBook#Raising_and_handling_exceptions works in GHC at all, as far as I could tell. The shifting exceptions story is a PITA. Compatibility is the price of progress. No good examples available on how to use ForeignPtr. Yet I'd argue that almost any non-trivial modern C library bindings will have some concept of an opaque object, so this is the most vital example. (The examples in RWH ch 17 weren't enough for me). I'd suggest you download a mature library binding close to what you want to do and ape that. For example: 1. got lots of C structures? look at a GTK binding. 2. got a fairly imperative API? look at a database binding, or HOpenGL. 3. got a fairly pure API? look at a BDD or numerical package binding. and so forth. The FFI itself is intended to be minimal, just enough to do the job. The idea was that extra tools (most prominently hsc2hs and c2hs, historically greencard) would support large bindings. It's funny you say opaque object. ForeignPtr only handles pointers. In one library I relied on, the opaque objects were represented with ints. It'd be nice to track arbitrary values the way that ForeignPtr tracks C pointers. I'd really like to know what parameters and return types are permissible for 'foreign import ccall' statements. By trial and error I found out. Is the FFI spec unclear on this point? How do I convert to/from booleans? Can you use the methods of the Enum class? How do I really use C structures? Real examples, please. I'd suggest looking into c2hs. Even if you don't use it, the code it generates is quite illuminating, especially how it pushes values between C and Haskell. How can I free a C string that is returned from a 'caller frees' C function? Does Foreign.Marshall.Alloc.free work for this? This is tricky. Rather than give you a definite answer, I'd suggest you check with the FFI spec. IIRC in principle the FFI's malloc/free could be distinct from those in C land, but in practice (GHC) they coincide. Again, reflecting my own inexperience with Haskell, I found 'withCString', 'maybeWith', 'withMany', 'withArray0' etc to be both undocumented and extremely confusing to use. They're documented in the FFI spec. Granted using them is not necessarily intuitive, which is where looking at existing code might help. How do I specify a 64 bit int? Using 'Int64#' just causes syntax errors. Did you look into the Data.Word library? What's a good emacs editing mode for Haskell code? My emacs thinks Haskell code is LISP ... There's been a Haskell mode for at least a decade now. It's packaged with every emacs I've used in the past few years. http://www.haskell.org/haskell-mode/ To editorialise a bit, I think the original FFI spec is very clearly written, and the contradictions you allude to lie in fact in the more recent wiki writeups; the Haskell specs are lucid and intended to be usable, and not just by the compiler writers. One last gotcha: be very careful about strictness if you're trying to graft a pure API onto the library. Hope this helps! cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems with Validate from HaXML
Hello, I have the same problems. I believe I sent Malcolm some patches. If not, look here: http://peteg.org/haskell/HaXml/ I am away from my laptop right now so I cannot readily tell you whether a darcs get will do the job. In any case, there is a bug in the DTD parser that is easily fixed. (Try downloading the Parser.hs file if the darcs get fails.) Hope that helps. cheers peter On 6/12/08, Samuel Silva [EMAIL PROTECTED] wrote: Hello I'm working with large DTDs from http://www.recordare.com/dtds/, particulary partwise.dtd and timewise.dtd. I check these DTDs with some tools, like Altova XML Spy and Microsoft Visual Studio, and DTDs are valid. However Validate tool from HaXML doesn't validate same DTDs. Result from Validate is: $ Validate partwise.dtd Validate: In a sequence: in content spec of ELEMENT decl: note When looking for a non-empty sequence with separators: Expected % but found | in file note.mod at line 51 col 45 used by file partwise.dtd at line 120 col 1 when looking for a content particle when looking for a content particle Problematic definition is in note.mod file, presented below: !ELEMENT note (((grace, %full-note;, (tie, tie?)?) | (cue, %full-note;, duration) | (%full-note;, duration, (tie, tie?)?)), instrument?, %editorial-voice;, type?, dot*, accidental?, time-modification?, stem?, notehead?, staff?, beam*, notations*, lyric*) I'm trying using DtdToHaskell tool from HaXML to build parser from MusicXML to Haskell types. There are other ways to build XML parser using HaXML? I'm already using HaXML, with HaXML combinators, to write MusicXML. Thanks. -- Don't hug that, Hugs ME! Samuel Silva ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: What is the maturity of Haskell Web Frameworks
On 05/06/2008, at 5:31 PM, apfelmus wrote: WASH/CGI has something in that direction. I don't know a short introduction, but have a look at sections 4 and 8 of the implementation notes http://www.informatik.uni-freiburg.de/~thiemann/WASH/draft.pdf I think there is also some work done about using Arrows for this problem, cf John Hughes' original paper. (I hear the use of Arrows was inessential, see the followup by Thiemann.) IMH-and-probably-offtopic-O, the continuation-based approach is semantically nice but I don't think anyone has reconciled it with the general desire for clean and tidy URIs. Going to: http://example.com/continuations/45345345 is not anywhere near as aesthetic / phishing proof / ... as going to e.g. http://example.com/comments/edit/16 (Though I grant that hpaste URIs are tantalisingly mysterious.) The general problem is providing a way for the programmer to (schematically) specify the URIs and handlers without getting bogged down in state machine encodings. If someone's got some good ideas here, I'd love to hear about it (on the web-devel mailing list, perhaps). cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Ghc / cgi static linking
On 02/06/2008, at 5:26 AM, Don Stewart wrote: pieter: Yes, it is entirely possible to statically link entire CGI apps. You might want to watch out for a bug in GHC 6.8.2 that means GHC's - static flag doesn't work. (At least for me, at least on Debian: the - lpthread flag is passed before the -lrt one, and symbols are left unresolved as a result.) Apparently the near-to-release 6.8.3 will fix this issue. Presumably dons is using a more recent GHC than 6.8.2, or other (BSD?) platforms are not affected. Note also you may have to tweak sundry .cabal files to add extra- libraries fields. As an example, I added this to HSQL's PostgreSQL backend to get it to statically link: extra-libraries: pq, crypt, pthread (If anyone cares you need to build PostgreSQL without kerberos as that doesn't seem to statically link any more.) cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: the Network.URI parser
On 28/05/2008, at 12:28 PM, Miguel Mitrofanov wrote: I am taking comments on a web forum from arbitrary people. The interpretation of the HTML occurs at the user's browser. A lot of people will be using outdated browsers (IE 5.5 / 6), ergo security (at the source) becomes my problem. I cannot force them to upgrade their browsers. I think this is very wrong for two reasons. First of all, the more web sites care of old browsers, the later people will upgrade them, therefore preventing the progress in Web (though IE 5.5 is not THAT old and bad, so this argument is not so strong). In Russia we some times say that a user with an outdated browser is an EPTH (Evil Pinocchio To Himself, don't ask me about source of this term). I am not encouraging people to stick with IE 5.5, I am trying to prevent them from being exploited when visiting my site. It is a good- faith-best-effort, not something I will formally prove. Secondly, I don't think that filtering HTML coming from an arbitrary user is a good idea. HTML is not very human-readable and too complex to achieve real safety without lots of work. My suggestion is to use some home-grown wiki-like syntax - it's easier to enter (*bold* instead of bbold/b), easier to read (and your users would sometimes read their comments before posting - to check correctness), and easier to process, since it can't have security holes you're not aware of. Did you read my post? I validate the XHTML against a restricted variant of the XHTML 1.0 Strict DTD. I want to ensure that if it validates, it is safe, as I explained before. I think the style attribute is unsafe, so I can remove it from the DTD. (We can simulate the effect of style by providing pre-made CSS classes and vetting the class attribute.) I am sure you can generalise from here. As for some other kind of markup: if my users were sophisticated enough to use something else, then I would use it. The target audience is not very literate, let alone computer literate. But you're right, we are off topic. Sorry to reply to your post then, I couldn't resist. :-/ cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: static linking
Hello, Further on my static linking woes: debian ~$ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 -debian ~$ cat T.hs main :: IO () main = putStrLn Hello world -debian ~$ ghc -optl-static -static --make T.hs Linking T ... /usr/lib/gcc/i486-linux-gnu/4.2.3/../../../../lib/ librt.a(timer_create.o): In function `timer_create': (.text+0x107): undefined reference to `pthread_once' ... references to missing pthread symbols Does anyone have static linking working for GHC 6.8.2 and can inform me of the magic incantation? Get the latest development version from darcs, perhaps? BTW when I said move to the end, I meant move to the end of -l flags. cheers peter On 27/05/2008, at 10:37 AM, Peter Gammie wrote: Hello, I am having a bit of trouble static linking my program using GHC 6.8.2. In brief: rt uses pthread, but -lrt is the final thing on the command line passed to the linker, and so the pthread symbols are not resolved. Manually moving -lpthread to the end of the collect2 command line does the trick. The question is how to do this using ghc. This is on Debian, using the 6.8.2-5 from unstable. I can provide the command lines if that is helpful. Note the same setup worked fine under GHC 6.6.1. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: the Network.URI parser
On 27/05/2008, at 6:08 PM, Neil Mitchell wrote: It most certainly is a security flaw. In the src of an img, yes, probably. In the href of a link, its a completely valid thing to do - and one that I've done loads of times. The URI is fine, its just the particular location that is dodgy. Sure, but for other reasons (potential inaccessibility) I am quite happy to ban JavaScript from URIs. (Not all URIs, just the ones coming from untrusted users.) whole pile of dodgy URIs. Most get culled (in my case) by the HaXml parser and/or XHTML 1.0 Strict validation, and now I hope to eliminate the rest by carefully handling the URIs. I don't think that's possible. A URI can validly have javascript, and can validly be a lot of things which are unsafe. Sure, I now realise my notion of allowable URI goes beyond (is an additional restriction of) the RFC. One way to show the URI is valid is to fetch what it is pointing to, and ensure it is an image or whatever. On that topic, does anyone have any good advice for handling these things? My advice is that you are targeting security at the wrong level. You shouldn't be cleaning the HTML to get a secure page, you should be having the level that interprets the HTML be secure regardless of the input. I am taking comments on a web forum from arbitrary people. The interpretation of the HTML occurs at the user's browser. A lot of people will be using outdated browsers (IE 5.5 / 6), ergo security (at the source) becomes my problem. I cannot force them to upgrade their browsers. If anyone knows of the state-of-the-art in this area, I'd appreciate a pointer. http://htmlpurifier.org/live/smoketests/printDefinition.php doesn't seem to think the style attribute is unsafe. Have they not been following the MySpace fiascos? Safety is a property of the HTML viewer, not of the HTML or CSS. Well, yes and no. I am heavily restricting the XHTML I accept (e.g. no scripts, no style attribute, ...), in an attempt to keep things visually accessible and avoid phishing attacks. I was alluding to the use of absolute positioning in CSS. If I had a CSS parser I might allow the style attribute. Safety for me involves making sure that what is displayed is trustworthy and easily identifiable as such. This is not something the HTML viewer can always help with. I think we're off-topic enough for me to stop here. Thanks for your comments. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] library for drawing charts
Hello, Has anyone got some code for drawing charts? I don't mean graphs of functions, ala http://dockerz.net/twd/HaskellCharts and I don't mean calls to the Google Charts API ala http://community.livejournal.com/evan_tech/241080.html I would like something that can generate PNGs in memory, i.e. not directly to a file. This is close to what I have in mind: http://www.fred.net/brv/chart/ I'd like 2D pie charts, bar charts and something like a google-o-meter. Any pointers? cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: HaXml and the XHTML 1.0 Strict DTD
On 30/04/2008, at 5:32 PM, Malcolm Wallace wrote: Peter Gammie [EMAIL PROTECTED] wrote: The most-recent darcs version relies on a newer ByteString than I have, so it is not easy for me to test it. I believe there was a patch to fix this. Apparently only one version of the bytestring package (0.9.0.1) ever exported the 'join' function, and a different version with the same number (but not exporting 'join') was uploaded to Hackage! 'Join' has since been replaced by 'intercalate', which is available in all versions 0.9.x. Thanks. I don't doubt it works with a newer bytestring, I just can't readily use such a thing. A recent (this year) darcs version failed to parse the DTD, yielding this error: I didn't try the full XHTML DTD, but the fragment you included in your message was parsed just fine by the darcs version of HaXml/ DtdToHaskell. Can you please try the full XHTML 1.0 Strict DTD? At the same time, can you verify that it handles this part of it properly (circa line 854): !ELEMENT table (caption?, (col*|colgroup*), thead?, tfoot?, (tbody+|tr+)) Using a slightly hacked HaXml v1.13.3, I get this from DtdToHaskell: data Table = Table Table_Attrs (Maybe Caption) (OneOf2 [Col] [Colgroup]) (Maybe Thead) (Maybe Tfoot) (OneOf2 (List1 Tbody) (List1 Tr)) deriving (Eq,Show) My expectation is that we can have a table without a col or colgroup child. The W3 validator seems to agree with that interpretation. When I use the HaXml validator with this DTD I get this (e.g.): Element table should contain (caption?,(col*| colgroup*),thead?,tfoot?,(tbody+|tr+)) but does not. Element table should contain (col*|colgroup*) but does not. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: HaXml and the XHTML 1.0 Strict DTD
On 21/05/2008, at 5:44 PM, Malcolm Wallace wrote: Peter Gammie [EMAIL PROTECTED] wrote: !ELEMENT table (caption?, (col*|colgroup*), thead?, tfoot?, (tbody+|tr+)) Using a slightly hacked HaXml v1.13.3, I get this from DtdToHaskell: data Table = Table Table_Attrs (Maybe Caption) (OneOf2 [Col] [Colgroup]) (Maybe Thead) (Maybe Tfoot) (OneOf2 (List1 Tbody) (List1 Tr)) deriving (Eq,Show) This looks entirely correct to me. I realised that as soon as I sent it. :-) My expectation is that we can have a table without a col or colgroup child. Ah, yes I can see why that is permitted, but I guess HaXml's validator is not yet smart enough to be able to choose whether it has seen an empty list of col or an empty list of colgroup. :-) Here is a suggested fix. Let me know if it works for you. In src/Text/XML/HaXml/Validate.hs, around line 220, use the following diff over the local defn of 'choice': choice elem ns cps = -- return only those parses that don't give any errors [ rem | ([],rem) - map (\cp- checkCP elem (definite cp) ns) cps ] + ++ [ ns | all possEmpty cps ] where definite (TagName n Query) = TagName n None definite (Choice cps Query) = Choice cps None definite (Seq cps Query)= Seq cps None definite (TagName n Star) = TagName n Plus definite (Choice cps Star) = Choice cps Plus definite (Seq cps Star) = Seq cps Plus definite x = x + possEmpty (TagName _ mod) = mod `elem` [Query,Star] + possEmpty (Choice cps None) = all possEmpty cps + possEmpty (Choice _ mod)= mod `elem` [Query,Star] + possEmpty (Seq cps None)= all possEmpty cps + possEmpty (Seq _ mod) = mod `elem` [Query,Star] Fantastic, thanks, that seems to work fine. A couple of nits: your use of `elem` refers to Prelude.elem, so I added the Prelude as a qualified import as P and changed those shadowed references to `P.elem`. I will try to send you a patch against 1.13.3 with all these little bits and pieces, when my project is finished. Can you lay out some kind of plan for HaXml? (is 1.13.x now dead, is 1.19.x stable, ...?) This would help for new-ish projects like mine. Are there other places, apart from the validator, where a similar problem arises? I do not know, I am merely using the DTD and HTML parsers, the CFilter combinators, the pretty printer and the validator. They all seem fine on a cursory check. (In general HaXml has been working quite well. Thanks for producing such a long-lived and well-thought-out library.) cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] another Newbie performance question
You want to lazily read a CSV file? I had a crack at writing a module for that: http://peteg.org/blog/AYAD/Project/2008-04-11-LazyCSVParser.autumn It is not quite RFC compliant (I believe that is clearly commented upon). As I say there, anything based on (older versions of) Parsec is likely to be too strict in the general case. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] FFI Query: How to free a CString allocated in Haskell
On 29/04/2008, at 12:58 PM, Brandon S. Allbery KF8NH wrote: On Apr 29, 2008, at 1:45 , Verma Anurag-VNF673 wrote: A naïve question I have now after reading your mail. How do I call MarshallAlloc.free from my C code because that's where I need to free it? Provide a Haskell wrapper function to deallocate / free it, as a parallel to the function that allocates and returns it to the C code. I would also point out the existence of withCString and friends. Tread carefully if you care about Unicode! cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] HaXml and the XHTML 1.0 Strict DTD
Is anyone using HaXml to validate XHTML Strict? The old 1.13.2 version has some bugs in how it handles attributes that stop me from using it. It handled the DTD parsing fine. The most-recent darcs version relies on a newer ByteString than I have, so it is not easy for me to test it. A recent (this year) darcs version failed to parse the DTD, yielding this error: validate: In a sequence: in content spec of ELEMENT decl: head When looking for a non-empty sequence with separators: In a sequence: Expected % but found | in file xhtml1 at line 252 col 50 when looking for a content particle when looking for a content particle This is the context: !-- Document Head ===-- !ENTITY % head.misc (script|style|meta|link|object)* !-- content model is %head.misc; combined with a single title and an optional base element in any order -- !ELEMENT head (%head.misc;, ((title, %head.misc;, (base, %head.misc;)?) | (base, %head.misc;, (title, %head.misc; I appreciate any advice on this. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] FFI Query: How to free a CString allocated in Haskell
On 28/04/2008, at 7:23 PM, Bulat Ziganshin wrote: Hello Verma, Monday, April 28, 2008, 4:11:51 PM, you wrote: newCString str Now once I call this function from C code, I am freeing the allocated memory using free function. I want to confirm that this is the right thing to do. yes, i've traced this function down to mallocArray - mallocBytes - malloc() calls Depending on how you feel about it, Bulat's response either entirely answers your question or is neither here nor there. If you want to write portable code, you should check with the FFI spec. If you just want to write code that works with GHC as it presently is, you can strace it or whatever. As Bulat has already given you the pragmatic answer, allow me to grind my axe a little. In the FFI spec on p33, we find: newCString :: String - IO CString newCStringLen :: String - IO CStringLen Allocate a memory area for a Haskell string and marshal the string into its C representation. There are two variants of the routine, one for each supported string representation. The memory area allocated by these routines may be deallocated using MarshalAlloc.free. OK, so back on p19 we find: free :: Ptr a - IO () Free a block of memory that was allocated with malloc, mallocBytes, realloc, reallocBytes, or any of the allocation functions from MarshalArray (see Section 5.9). and we can draw the inference (reading between the lines) that this function need not have a relation to C's malloc. (Roughly, the intention is to allow a Haskell implementation to use whatever memory management it likes.) So no, using C's free is not the right thing to do. It will probably work in the real world, but you should try to use Haskell's free if possible. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: HDBC, character encoding
On 03/04/2008, at 9:10 PM, John Goerzen wrote: On 2008-04-01, Peter Gammie [EMAIL PROTECTED] wrote: I added some stuff to HSQL (not HDBC) and HaskellDB so that UTF8 can be used to talk to the database. It's not very pretty though, so I haven't tried to get it merged. Do you have a diff somewhere? I'd be interested in your approach, to see if the same thing would make sense in HDBC. My approach is simple: when creating a connection to the database, ask for two functions (a codec pair) and stash them with the rest of the DB state. The types involved are [Word8] and String. I started out trying to use ByteString instead of [Word8] but it didn't bring much to the table in my case. I had generally been of the opinion that HDBC should just pass data through, however it is obtained, and let the programmer set the encoding as desired. If this approach isn't working for people, I'd like to fix it, but want to make sure it's done right. Assuming you're talking about the FFI's mandated behaviour, I'm not very comfortable with the encoding depending on the C locale, for two main reasons: - the program I'm hacking runs on some web host's server that is an unknown quantity. - GHC has a few quirks in how it implements the FFI, better to avoid the part that interprets characters. If you want to be general, I guess you could abandon String (at the HDBC level) and use [Word8]/ByteString everywhere, then provide some convenience wrappers. Then you can uniformly handle binary and textual data. I am not familiar with your project though. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: HDBC, character encoding
On 04/04/2008, at 9:27 PM, John Goerzen wrote: I can see this being a performance and ease-of-use win in some situations. I don't think it's an actual feature difference, though. If you can represent it as a [Word8], you can represent it as a [Char], and it will be converted to the same underlying data for the trip through FFI. Sure. I prefer to keep things separate: Char is for Unicode characters, [Word8]/ByteString is for concrete representations. It's more about type safety than efficiency. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HDBC, character encoding
On 31/03/2008, at 11:42 PM, Bjorn Bringert wrote: 2008/3/26 Adrian Neumann [EMAIL PROTECTED]: Hi, I wrote a CGI program to access a Postgres database using HDBC. [...] I think that Peter Gammie (copied) has some code to deal with this. I added some stuff to HSQL (not HDBC) and HaskellDB so that UTF8 can be used to talk to the database. It's not very pretty though, so I haven't tried to get it merged. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] modeling ANSI C structs in Haskell?
Vasili, On 04/02/2008, at 10:04 PM, Galchin Vasili wrote: I am reading through the FFI doc. Any suggestions on enabling Haskell programmers to model ANSI C structs that will be passed down to C run-time? The FFI spec is a wonderful document, but is of limited use in learning to use the FFI for practical tasks. I suggest you look into c2hs or some other tool that tries to help you with marshalling data structures (as compared to providing mechanisms sufficient for doing marshalling). I suggest you do this even if you don't end up using a tool, as the generated Haskell has some carefully designed idioms that will definitely help. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On 28/12/2007, at 4:21 PM, Miguel Mitrofanov wrote: How can I test that partial order in Haskell ? You can't. It's kinda internal. More specifically, if you try to compute a value, which is not maximal in this order, you'll get an error or loop forever. But you still can use such values in your programs if you don't try to compute them. The point is, maximal values are the only interesting ones, and all others are just approximations. Right, so when I say to GHCi: Prelude take 5 [1..] and it says: [1,2,3,4,5] then it really has computed the entirety of the infinite sequence [1..], and not some approximation? Perhaps it is better to say that in a lazy language both approximations and maximal values (least upper bounds of a chain) are interesting. In a strict language we only get our hands on the latter. The take lemma nicely captures this kind of thinking. (Ask google about the generalised take lemma, it's a nice paper.) Also note that the notion of equality that's being thrown around is observational equality, i.e. x=y implies that for all contexts (some expression with a suitably-typed hole), if you plug 'x' into the hole, and 'y' into hole, then those two expressions denote the same value. ((Roughly) equivalently, you can use the idea of Leibniz equality: there is no function that distinguishes them. These notions may diverge in Haskell - does anyone know?) As Lennart observed, any (semi-)computable equality predicate is not going to be that strong. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On 28/12/2007, at 5:50 PM, Miguel Mitrofanov wrote: Right, so when I say to GHCi: Prelude take 5 [1..] and it says: [1,2,3,4,5] then it really has computed the entirety of the infinite sequence [1..], and not some approximation? Of course not! In fact, it doesn't even compute the value 1. It just juggles with the bytes somehow, so that it seems that it computes the whole sequence and then applies take 5. But does it really matter if it computes 5 integers, or 10, or the whole sequence, at least if we are not concerned about performance? I'll bite. So what do you expect: take 5 [1,2,3,4,5,undefined] to do? How about: take 5 (1:2:3:4:undefined) ? So it does seem to matter how much of the list it evaluates... Perhaps it is better to say that in a lazy language both approximations and maximal values (least upper bounds of a chain) are interesting. Well, that depends of your notion of interesting. I defined it - I'm interested in things that are observationally equivalent. Please read the generalised-take-lemma paper. It will clear up some of this confusion. cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Wikipedia on first-class object
On 28/12/2007, at 7:19 PM, Miguel Mitrofanov wrote: So what do you expect: take 5 [1,2,3,4,5,undefined] to do? Nothing! It's a value, not an instruction! Dang, I knew I'd choose at least one wrong word in all of that. :-P What is it's value, then? ... and what is the value of the other example I gave? (What would GHCi print? What is another, perhaps even simpler, expression it is equal to? ... a substantive answer please! Then please square that with your comment that we don't care how much of the list gets evaluated.) (Jules, thanks for your response. My questions are Socratic, not rhetorical. I'm benefiting by having to choose my words very, very carefully. :-) cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Sending email from a Haskell program
Hello, Does anyone have a library for sending email from a Haskell program? I'd like something portable and cabalised. I note there is code in darcs to send email on Windows and UNIX-y systems, and also something in WASH (I know not what). Apparently there is an SMTP server of some kind (?) in HAppS, but I want to re- use the system's MTA. BTW is anyone else looking at the darcs codebase and thinking that there are many useful libraries lurking there? cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Does laziness make big difference?
Nick, Roughly, I'd say you can fudge laziness in data structures in a strict language without too much bother. (I don't have much experience with this, but the existence of a streams library for OCaml is the sort of thing I mean. There are plenty of papers on co- iterative streams and suchlike that show the general pattern.) If you wish to add control structures you would need to use the lazy keyword a lot, e.g.: if cond then *lazy* S1 else *lazy* S2 and for more complicated structures it's not going to be always clear what needs to be suspended. Laziness is a conservative default here. (If you want to write an EDSL in a non-lazy language, you'll need to use some kind of preprocessor / macros / ... - in other words, a two- level language - or do thunking by hand, as above, or live with doing too much evaluation.) One way to gauge how useful laziness really is might be to look through big ML projects and see how often they introduce thunks manually. A thunk there is usually something like fn () = ... IIRC. Also IIRC, Concurrent ML is full of them. Dare I say the tradeoff is between a relatively simple operational model (so you can judge space and time usage easily) and semantic simplicity (e.g. the beta rule is unrestricted, easing program transformation). cheers peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
