- Haskell-Cafe - Haskell.org

Kind-agnostic type classes
by Florian Weimer 03 Oct '08

03 Oct '08

I'm trying to encode a well-known, informally-specified type system in Haskell. What causes problems for me is that type classes force types to be of a specific kind. The system I'm targeting however assumes that its equivalent of type classes are kind-agnositic. For instance, I've got class Assignable a where assign :: a -> a -> IO () class Swappable a where swap :: a -> a -> IO () class CopyConstructible a where copy :: a -> IO a class (Assignable a, CopyConstructible a) => ContainerType a class (Swappable c, Assignable c, CopyConstructible c) => Container c where size :: (Num i, ContainerType t) => c t -> IO i I suppose I could address this with creating aliases for the relevant classes, but I wonder if there is a better workaround. I see that the ML module system has a similar restriction on type sharing. Is there a fundamental reasons behind this?

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Re: [Haskell-cafe] Haskell participating in big science like CERN Hadrian...
by Rafael Gustavo da Cunha Pereira Pinto 03 Oct '08

03 Oct '08

> > > > On Fri, Oct 3, 2008 at 07:47, Dougal Stanton <dougal(a)dougalstanton.net>wrote: > >> 2008/10/3 Galchin, Vasili <vigalchin(a)gmail.com>: >> > Hello, >> > >> > One of my interests based on my education is "grand challenge >> science". >> > Ok .. let's take the CERN Hadrian Accelerator. >> > >> > Where do you think Haskell can fit into the CERN Hadrian effort >> > currently? >> > >> > Where do you think think Haskell currently is lacking and will have >> to >> > be improved in order to participate in CERN Hadrian? >> >> Is that the experiment where Picts are accelerated to just short of >> the speed of light in order to smash through to the Roman Empire? ;-) >> > Man, that was almost as good as the Large Hardon Collider<http://largehardoncollider.com/> !!! > >> I don't know what the main computational challenges are to the LHC >> researchers. The stuff in the press has mostly been about >> infrastructure --- how to store the gigabytes of data per second that >> they end up keeping, out of the petabytes that are produced in the >> first place (or something). >> >> > There is a lot of data filtering, looking for the right trigger event... -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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Is Haskell-Cafe down?
by Benjamin L.Russell 03 Oct '08

03 Oct '08

About two hours ago, I sent two follow-ups to the "Announcing OneTuple-0.1.0" thread on the Haskell-Cafe mailing list, neither of which has appeared in either my newsreader or my Inbox, even though they have both appeared on the Web archive; viz.: [Haskell-cafe] Re: Announcing OneTuple-0.1.0 http://www.haskell.org/pipermail/haskell-cafe/2008-October/048633.html [Haskell-cafe] Re: Announcing OneTuple-0.1.0 http://www.haskell.org/pipermail/haskell-cafe/2008-October/048636.html Is Haskell-Cafe down? -- Benjamin L. Russell

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mysql (hsql-mysql-1.7) on Linux with GHC 8.3 - cabal issues
by Anton Tayanovskyy 03 Oct '08

03 Oct '08

Hello, I had some difficulty building hsql and hsql-mysql (native driver) packages under GHC 8.3 on Fedora 9. Namely, they do not install with cabal install, because of a few API changes. Is this considered all right as GHC 8.3 is still experimental, or should the packages be updated? If so, can somebody please do it. I love when things just work with cabal install! The problem looks very basic -- just some Cabal file incompatibilities. However I am not experienced with the Cabal build process, so I'll just write what I did to get it to work on my platform, maybe it will help someone. == hsql.cabal == name: hsql version: 1.7 license: BSD3 author: Krasimir Angelov <ka2_mail(a)yahoo.com> category: Database description: Simple library for database access from Haskell. exposed-modules: Database.HSQL, Database.HSQL.Types build-depends: base, old-time build-type: Simple ghc-options: -O2 extensions: ForeignFunctionInterface, TypeSynonymInstances, CPP, RankNTypes, DeriveDataTypeable == hsql-mysql.cabal == name: hsql-mysql version: 1.7 license: BSD3 author: Krasimir Angelov <kr.angelov(a)gmail.com> category: Database description: MySQL driver for HSQL. ghc-options: -O2 build-depends: base, hsql, Cabal, old-time extensions: ForeignFunctionInterface, CPP include-dirs: Database/HSQL, /usr/include/mysql build-type: Simple extra-source-files: Database/HSQL/HsMySQL.h extra-libraries: mysqlclient extra-lib-dirs: /usr/lib/mysql exposed-modules: Database.HSQL.MySQL == hsql-mysql*/Setup.lhs == #!/usr/bin/runghc \begin{code} import Distribution.Simple main = defaultMain \end{code} Of course this hsql-mysql.cabal should not mention Fedora-specific locations as /usr/lib/mysql and /usr/include/mysql but I don't know how to make that generic. The original version had a big custom Setup.lhs file which wouldn't compile with the new Cabal and which I couldn't tweak myself. Thanks, --A

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ANN: Haskell-Embedded System Design: ForSyDe 3.0 and Tutorial
by Alfonso Acosta 03 Oct '08

03 Oct '08

Hi everyone, I am glad to announce the 3.0 release of ForSyDe's implementation, now available from HackageDB. The ForSyDe (Formal System Design) methodology has been developed with the objective to move system design (e.g. System on Chip, Hardware and Software systems) to a higher level of abstraction. ForSyDe is implemented as a Haskell-embedded behavioral DSL (Domain Specific Language). >From this release, ForSyDe includes a new deep-embedded DSL and embedded compiler. We have also published tutorial which should be much more user-friendly than the Haddock documentation and the ForSyDe research papers. ForSyDe includes two DSL flavours which offer different features: 1) Deep-embedded DSL (ForSyDe.Signal) Deep-embedded signals, based on the same concepts as Lava, are aware of the system structure. Based on that structural information ForSyDe's embedded compiler, can perform different analysis and transformations. o Thanks to Template Haskell, computations are expressed in Haskell, not needing to specifically design a DSL for that purpose. o Embedded compiler backends: + Simulation + VHDL (with support for Modelsim and Quartus II) + GraphML (with yFiles graphical markup support.) o Synchronous model of computation. o Support for components. o Support for fixed-sized vectors à la VHDL. 2) Shallow-embedded DSL (ForSyDe.Shallow.Signal) Shallow-embedded signals are modeled as streams of data isomorphic to lists. Systems built with them are unfortunately restricted to simulation, however, shallow-embedded signals provide a rapid-prototyping framework with which to experiment with Models of Computation (MoCs). o Synchronous MoC. o Untimed MoC. o Continuous Time MoC. o Domain Interfaces allow connecting various subsystems with different timing (domains) regardless of their MoC. o Deep-embedded models can be integrated through simulation. Links ==== ForSyDe website: http://www.ict.kth.se/org/ict/ecs/sam/projects/forsyde/www/ ForSyDe tutorial: http://www.ict.kth.se/org/ict/ecs/sam/projects/forsyde/www/files/tutorial/t… HackageDB package page: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ForSyDe Darcs repository: darcs get http://www.ict.kth.se/org/ict/ecs/sam/projects/forsyde/www/darcs/ForSyDe/

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Re: [Haskell-cafe] A question about constraints
by Luke Palmer 03 Oct '08

03 Oct '08

On Thu, Oct 2, 2008 at 7:51 AM, jean-christophe mincke <jeanchristophe.mincke(a)gmail.com> wrote: > Hello, > > Thank you for your comments. > > Would not it be feasible to add constraints at type definition, something > like, in a somewhat free style syntax > > data String2 = String2 (s::String) with length s <= 5 > > and with a polymorphic type > > data List5 a= List5 (l::[a]) with length l <= 5 Well, yeah, it is possible that to a language. However, it's a question of how far you take it. What do you want that to do? Is it a runtime check on the constructor? Is it a compile-time guarantee? If it's runtime, how lazy is it -- i.e. when does it check? If it's compile-time, how do you enforce it? Basically it dumps the contents of pandora's box all over the design space, so it's easier to leave it out and just let module abstraction take care of the hard questions. There are definitely ways to simulate it: You can simulate the runtime checks as follows: subtype :: (a -> Bool) -> (a -> b) -> a -> b subtype p f x = if p x then f x else error "Subtype constraint failed" string2 :: String -> String2 string2 = subtype (\s -> length s < 5) String2 -- And don't expose String2 from the module, so string2 is the only way to make them But to give an example of why this is not a straightforward thing to answer, here's a different way which might also be correct, depending on what you want: string2 :: String -> String2 string2 s = partial 5 s where partial 0 _ = error "Subtype constraint failed" partial n [] = [] partial n (x:xs) = x:partial (n-1) xs Which lazily checks the constraint; i.e. only errors if a value is demanded beyond the index 5 and exists. Implementing Compile-time checks is typically much harder, and demands a bit more cleverness, since the way you do it is different for each type of constraint you have. For this example, you could create an algebra of lengthed lists: import Prelude hiding (++) -- Type level numbers; Z = 0, S n = n + 1. data Z data S n -- Lists of length exactly n data Listn n a where Nil :: Listn Z a Cons :: a -> Listn n a -> Listn (S n) a -- Type-level addition of numbers type family Plus m n :: * type instance Plus Z n = n type instance Plus (S m) n = S (Plus m n) -- Typed append; appends the lists, adds the lengths. -- The compiler verifies that the implementation actually does this! (++) :: Listn m a -> Listn n a -> Listn (Plus m n) a Nil ++ ys = ys Cons x xs ++ ys = Cons x (xs ++ ys) -- Type-level less than or equal; represents the type of *proofs* that m <= n. data m :<= n where LeN :: n :<= n LeS :: m :<= n -> m :<= S n -- A List5 a is a function which takes a proof than n <= 5, and returns a Listn n a type List5 a = forall n. n :<= S (S (S (S (S Z)))) -> Listn n a So that was a bit of work, wasn't it? But this solution is quite expressive; the compiler will not even compile your code if you try to construct a list with more than 5 elements. Furthermore, you can actually write most things you'd expect to because of Haskell's great GADT typechecking features :-) The downside is that you have to redefine all the standard list operations, because, well, they have different types now. The above is approaching what dependently typed languages do. My favorite is Coq (a lot of haskell folks like Agda 2), and in it you can express directly the constraint you want without all this type boilerplate: Definition List5 a := { l : List a | length l <= 5 }. But internally it is doing something very similar to the above Haskell program. This also has the property that it is impossible to write a well-typed program that constructs a List5 of length > 5. In addition, you can use all the standard list functions, however you have to prove what they do to the constraint: Theorem append_length : forall m n a (xs ys : List a), (length xs <= m) -> (length ys <= n) -> (length (xs ++ ys) <= m+n). (* prove prove prove.... *) Qed. And then you can use that theorem to prove that the parts of your program that use List5 are well-typed. Whew, so now that we're done with that, in summary, it depends on your situation how you want to do it, and there's really no "easy answer". I hope you got something out of seeing these techniques. Adding notation like that you suggest is a challenge for the language, since a runtime constraint changes the strictness properties of the object, and a compile-time constraint written that concisely actually does require full dependent types. Best to leave it to the users. Luke

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SCGI module
by Anton Tayanovskyy 02 Oct '08

02 Oct '08

Hello, SCGI module from Hackage is not working for me with Apache mod_scgi, I think because it uses LazyBytestring to hGetContents on a socket handle and the Apache side does not close the socket. Also, looking at the source I didn't see any fork* calls, so I assume it is not doing concurrency. So I ended up writing a replacement and would like to invite comments. It does forkIO per request and, if asked to, timeouts those threads. So far it works for me on simple examples from http://www.haskell.org/haskellwiki/Practical_web_programming_in_Haskell giving roughly 800 requests a second versus 250 on plain CGI (both through Apache). http://hpaste.org/10842 Thanks! Also, in general, is there any interest in using SCGI, or FastCGI (plain CGI, HappS, you name it) is vastly superior? --Anton

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Alex and Flex
by Manlio Perillo 02 Oct '08

02 Oct '08

Hi. I'm starting to write a CSS parser with Alex and Happy. The grammar is defined here: http://www.w3.org/TR/CSS21/grammar.html However I have noted that there are some difference in the syntax between Alex and Flex? What is the rationale? One more thing. Here: http://www.haskell.org/alex/doc/html/alex-files.html it seems that there is an error with macrodef := @smac '=' set | @rmac '=' regexp it should be macrodef := $smac '=' set | @rmac '=' regexp Thanks Manlio Perillo

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Climbing up the shootout...
by Don Stewart 02 Oct '08

02 Oct '08

Thanks to those guys who've submitted parallel programs to the language benchmarks game, we're climbing up the rankings, now in 3rd, and ahead of C :) http://shootout.alioth.debian.org/u64q/benchmark.php?test=all&lang=all Just one or two more parallel programs required... Submit them here, and we can test on quad core, http://haskell.org/haskellwiki/Shootout/Parallel -- Don

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System.Process
by Andrew Coppin 02 Oct '08

02 Oct '08

Much hair-pulling resulted today when I attempted to perform a small task. The System.Process module provides the runCommand function. This takes a complete command line and returns a ProcessHandle. No problem there. The module also provides the runProcess function, which enables you to set environment variables and so forth. However... this doesn't take a complete command line. It takes a path to an executable and a list of parsed arguments. Herein lies the problem: I have a program that accepts complete commands from a file and executes them. It works perfectly. And now I'd just like to set an environment variable while each command runs... But alas no, the only way to do that is with runProcess, which expects me to parse the command line first. (Before anybody remarks that "words" will do this, consider the "echo" command, which treats whitespace meaningfully.) In the end I resorted to reading the source code for the module. After about 30 minutes of hacking, I got something which typechecks but throws a runtime exception. It appears that runCommand looks for binaries in the system search path, but the code I've concocted doesn't. (Despite being cut and pasted from the definition of runCommand.) The only thing I can think of is that the source code I'm looking at (GHC 6.8.3) is different from the version of the library I have (GHC 6.8.1). Anyway, the point I'm trying to get to is... would it be hard for however maintains this library to add a little more flexibility in what it can do please?

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