January 2008 - Haskell-Cafe

Re: [Haskell-cafe] Mission: To take args from an n-tuple ... generally
by Martin Hofmann 31 Jan '08

31 Jan '08

> > I wondered, why not take an n-tuple of arguments s.t. > > > > multApply' :: (a1->a2->...->an->o) -> (a1,(a2,(...(an,o)...))) -> > o > > > I'm not sure what you're trying to do here. Why is there an o in the > argument? Also, do you really mean the number of arguments expected to > match the number of arguments given? > You are right. It should rather be multApply' :: (a1->a2->...->an->o) -> (a1,(a2,(...(an-1,an)...))) -> o as e.g. in: multApply' take (2,['a','b','c']) > Also you might want to check out Olegon polyvariadic functions: > http://okmij.org/ftp/Haskell/vararg-fn.lhs Thanks, I will have a look at this. Martin

1 0

CFP - PLMMS 2008
by Jacques Carette 31 Jan '08

31 Jan '08

FIRST CALL FOR PAPERS Second Workshop on Programming Languages for Mechanized Mathematics (PLMMS 2008) http://events.cs.bham.ac.uk/cicm08/workshops/plmms/ As part of CICM / Calculemus 2008 Birmingham, UK, 28-29 July 2008 This workshop is focused on the intersection of programming languages (PL) and mechanized mathematics systems (MMS). The latter category subsumes present-day computer algebra systems (CAS), interactive proof assistants (PA), and automated theorem provers (ATP), all heading towards fully integrated mechanized mathematical assistants that are expected to emerge eventually (cf. the objective of Calculemus). The two subjects of PL and MMS meet in the following topics, which are of particular interest to this workshop: * Dedicated input languages for MMS: covers all aspects of languages intended for the user to deploy or extend the system, both algorithmic and declarative ones. Typical examples are tactic definition languages such as Ltac in Coq, mathematical proof languages as in Mizar or Isar, or specialized programming languages built into CA systems. Of particular interest are the semantics of those languages, especially when current ones are untyped. * Mathematical modeling languages used for programming: covers the relation of logical descriptions vs. algorithmic content. For instance the logic of ACL2 extends a version of Lisp, that of Coq is close to Haskell, and some portions of HOL are similar to ML and Haskell, while Maple tries to do both simultaneously. Such mathematical languages offer rich specification capabilities, which are rarely available in regular programming languages. How can programming benefit from mathematical concepts, without limiting mathematics to the computational worldview? * Programming languages with mathematical specifications: covers advanced "mathematical" concepts in programming languages that improve the expressive power of functional specifications, type systems, module systems etc. Programming languages with dependent types are of particular interest here, as is intentionality vs extensionality. * Language elements for program verification: covers specific means built into a language to facilitate correctness proofs using MMS. For example, logical annotations within programs may be turned into verification conditions to be solved in a proof assistant eventually. How need MMS and PL to be improved to make this work conveniently and in a mathematically appealing way? These issues have a very colorful history. Many PL innovations first appeared in either CA or proof systems first, before migrating into more mainstream programming languages. Some examples include type inference, dependent types, generics, term-rewriting, first-class types, first-class expressions, first-class modules, code extraction etc. However, such innovations were never aggressively pursued by builders of MMS, but often reconstructed by programming language researchers. This workshop is an opportunity to present the latest innovations in MMS design that may be relevant to future programming languages, or conversely novel PL principles that improve upon implementation and deployment of MMS. We also want to critically examine what has worked, and what has not. Why are all the languages of mainstream CA systems untyped? Why are the (strongly typed) proof assistants so much harder to use than a typical CAS? What forms of polymorphism exist in mathematics? What forms of dependent types may be used in mathematical modeling? How can MMS regain the upper hand on issues of "genericity" and "modularity"? What are the biggest barriers to using a more mainstream language as a host language for a CAS or PA/ATP? Submission ---------- Submission works through EasyChair http://www.easychair.org/conferences/?conf=plmms2008 Two kinds of papers will be considered: * Full research papers may be up to 12 pages long. Authors of accepted papers are expected to present their work on the workshop in a regular talk. * Position papers may be up to 4 pages long. The workshop presentation of accepted position papers consists of two parts: a stimulating statement of certain issues or challenges by the author, followed by a discussion in the plenum. Papers should use the usual ENTCS style http://www.entcs.org/prelim.html (11 point version), and will be reviewed by the program committee. Informal workshop proceedings will be circulated as a technical report. We also plan post-workshop proceedings of improved research papers, or position papers that have been completed into full papers, as a special issue in a journal; papers from both PLMMS 2007 and 2008 will be considered here (details to follow). Programme Committee ------------------- Jacques Carette (Co-Chair) (McMaster University, Canada) John Harrison (Intel Corporation, USA) Hugo Herbelin (INRIA, Ecole polytechnique, France) James McKinna (Radboud University Nijmegen, Netherlands) Ulf Norell (Chalmers University, Sweden) Bill Page Christophe Raffalli (Universite de Savoie, France) Josef Urban (Charles University, Czech Republic) Stephen Watt (ORCCA, University of Western Ontario, Canada) Makarius Wenzel (Co-Chair) (Technische Universitaet Muenchen, Germany) Freek Wiedijk (Radboud University Nijmegen, Netherlands) Important Dates --------------- * Submission deadline - 5 May 2008 * Notification of acceptance - 6 June 2008 * Final version - 7 July 2008 (approximately) * Workshop - 28-29 July 2008

1 0

Mission: To take args from an n-tuple ... generally
by Martin Hofmann 31 Jan '08

31 Jan '08

Dear Community. I have recently read Joel Koerwer's posting how to evaluate a function of type (a->a->...->a->a), taking the arguments from a list (http://haskell.org/pipermail/haskell-cafe/2006-October/018658.html) Therefore, he introduced a function multApply: multApply :: (a->a->...->a->a) -> [a] -> a I wondered, why not take an n-tuple of arguments s.t. multApply' :: (a1->a2->...->an->o) -> (a1,(a2,(...(an,o)...))) -> o I naively tried to modify Joel's code, but in vain. As far as I can deduce from the error messages, GHC fails to fix the accordant types. However, in my understanding the type solely depends on the type of the passed function. Or am I wrong? Maybe there is a reason nobody has tried it this way? Following the code. First Joel's original multApply and then my attempt to define multApply'. ------------------------ MultApply.hs ------------------------------ {-# OPTIONS -fglasgow-exts -fallow-overlapping-instances #-} module MultApply where class MultApply func arg | func -> arg where multApply :: func -> [arg] -> arg instance MultApply (a->a) a where multApply f (x:xs) = f x multApply f _ = error "MultApply: one too few args" instance MultApply cascade a => MultApply (a -> cascade) a where multApply f (x:xs) = multApply (f x) xs multApply f _ = error "MultApply: n too few args" -- some random examples oneArg = multApply sqrt [25..] twoArg = multApply (+) [1..] fiveArg = multApply (\a b c d e -> sqrt ((a+b)^2+(d-e)^2)-5*c) [13..] class MultApply' f arg out | f -> arg out where multApply' :: f -> arg -> out instance MultApply' (a->b) a b where multApply' f x = f x multApply' f _ = error "MultApply: one too few args" instance MultApply' cascade a2 b => MultApply' (a1 -> cascade) (a1,a2) b where multApply' f (x,xs) = multApply' (f x) xs multApply' f _ = error "MultApply: n too few args" -------------------End File ------------------------ Hope I didn't stumble in a stupid newbie question. Thanks a lot to everybody who could give me some explanation and hints. Cheers, Martin

2 1

Re: [Haskell] Why functional programming matters
by Don Stewart 31 Jan '08

31 Jan '08

stephan.friedrichs: > Simon Peyton-Jones wrote: > >[...] > > > >2. War stories from real life. eg "In company X in 2004 they rewrote > >their application in Haskell/Caml with result Y". Again, for my purpose > >I can't tell very long stories; but your message can give a bit more > >detail than one might actually give in a presentation. The more concrete > >and specific, the better. E.g. what, exactly, about using a functional > >language made it a win for you? > > > > We [1] implemented an ad-hoc chat system in Haskell in the SEP [2] at > the TU-Braunschweig. Wow! Looks like lots of great code in there, http://sep07.mroot.net/documentation/barracuda/ Text.ParserCombinators.Parsec.XML Codec.Encryption.PKCS1 Network.GnuTLS Data.Heap Will we see this packaged for hackage.haskell.org soon? Some of those modules look very reusable. -- Don

3 2

trying to use unix's openFD ..
by Galchin Vasili 30 Jan '08

30 Jan '08

Hello, The signature for openFD is openFd: FilePath -> OpenMode -> Maybe FileMode -> OpenFileFlags -> IO Fd I am currently reading http://www.haskell.org/ghc/docs/latest/html/libraries/unix/System-Posix-Fil… and diving down into links but can't find the definition of data type FileMode. ??? Basically I want to open in O_RDONLY filemode. Kind regards, Vasili

2 5

Improving 'project'
by Justin Bailey 30 Jan '08

30 Jan '08

All, project doesn't easily let me add a few columns to an existing query (or take a few columns away). Instead, each use of project requires me to build the entire list of columns I'd like to pass on by hand. Before I go further, if there is a way to do that, please let me know. An example of what I'd like to do is below. Imagine I want to add a "selected" boolean column to my query. select_customers = do cust <- table customers project ( (selectedField <<- False) # cust) So I get all the columns in my customers table, plus the "selected" column. Unfortunately the code above doesn't work. cust has type "Rel r", and project wants a "Record r", so I originally tried something like: select_customers = do let toRec :: Rel r -> Record r toRec _ = undefined ... project (unRel cust) But project needs to recover the "Expr" values associated with 'r' and this fails. I then came up with this solution, which lets me add columns at the head of a query: class RecoverExpr a b where toExpr :: a -> b instance RecoverExpr RecNil RecNil where toExpr rec = RecNil instance (FieldTag f, RecoverExpr r r) => RecoverExpr (RecCons f (Expr b) r) (RecCons f (Expr b) r) where toExpr ~rec@(RecCons _ r) = RecCons (attribute . fieldName $ recField $ rec) (toExpr r) recField :: RecCons f a b -> f recField _ = undefined rest :: (RecoverExpr r r) => Rel r -> r rest qry = let unRel :: Rel r -> r unRel _ = undefined in toExpr . unRel $ qry Then I can write: select_customers = do cust <- table customers_tbl ... project (selectField <<- False # (rest cust)) And it actually works. However, it seems over-engineered. My questions are: 1) Is there a way to make project behave like I want without these tricks? 2) If not 1, would it make more sense for the Query monad to pack around the original Expr values? 3) If not 2, how can the solution above be better? I'd like to extend it to adding/removing arbitrary columns and more control over column order. Thanks for any and all thoughts! Justin

1 1

Re: [Haskell-cafe] Simple network client
by Gary Bickford 30 Jan '08

30 Jan '08

One rather funky but effective solution might be to use the tftp protocol. No security, but simple, flexible and efficient. I think there are C libraries that implement it. This would take care of handshaking binary data. I have no idea if anyone has ever used it in Haskell. GB On Wed, 2008-01-30 at 12:50 -0500, haskell-cafe-request(a)haskell.org wrote: > robably not all of them, but some of them, definitely. > > If you want to transmit an arbitrary bytestring then I'm pretty sure > that transmitting a length word first is the way to go. An arbitrary > bytestring can have any value in it, so there are no values left to > act > as delimiters :) You'd have to have some kind of escaping mechanism, > like show, which is expensive. -- "Isn't it funny how the Global Village includes everybody but the villagers?" (http://wiki.laptop.org/go/OLPC_Publications)

1 0

ffi and pass by value structs
by Federico Squartini 30 Jan '08

30 Jan '08

Is there a way in the foreign function interface to pass to a function a struct by value? I explain it better. There are two ways in c to pass structs, as in the following example: /* begin c code */ struct couple{ int a; int b; }; //by value int print_couple (struct couple cpl) { printf("%d\n", cpl.a); printf("%d\n", cpl.b); return(0); } //by address int print_couple_ptr (struct couple *cpl) { printf("%d\n", cpl->a); printf("%d\n", cpl->b); return(0); } /* end c code */ Apparently in the ffi it is possible to import the second version, as: foreign import ccall "print_couple_ptr" printCouple::Ptr Couple->IO Int But not the first, as a struct it's not an atomic type. So how does one solve this issue? Cheers, Federico

2 1

NDP
by Stephan Friedrichs 30 Jan '08

30 Jan '08

Hi all, is someone familiar with compiling ndp (nested data parallel Haskell), "Speed with less convenience"-version? I followed the guide at http://www.haskell.org/haskellwiki/Data_Parallel_Haskell/PackageNDP but executing "make" in the examples directory issues the following error: (.text+0x8d5): undefined reference to `__stginit_parallelzm1zi0zi0zi0_ControlziParallelziStrategies_' manually compiling the dotp example on the link mentioned above doesn't work either: [...] (.data+0x5c): undefined reference to `ndp_DataziArrayziParallelziUnliftedziFlatziUArr_zdf14_closure' DotP.o: In function `sz3_srt': (.data+0x88): undefined reference to `ndp_DataziArrayziParallelziUnliftedziFlatziBasics_toU_closure' DotP.o: In function `sz3_srt': (.data+0x8c): undefined reference to `ndp_DataziArrayziParallelziUnliftedziFlatziUArr_zdf14_closure' /home/sfriedrichs/ghc/rts/libHSrts.a(Main.o): In function `real_main': Main.c:(.text+0x12): undefined reference to `__stginit_ZCMain' Main.c:(.text+0x2c): undefined reference to `ZCMain_main_closure' Does someone know, what's wrong? I'm using ghc-6.9.20080124 as source base (which compiled and works just fine). //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

4 7

Stacking monads - beginner design question
by Adam Smyczek 30 Jan '08

30 Jan '08

Hi, My application has to manage a data set. I assume the state monad is designed for this. The state changes in functions that: a. perform IO actions and b. return execution status and execution trace (right now I'm using WriteT for this). Is the best solution: 1. to build a monad stack (for example State -> Writer -> IO) or 2. to use IORef for the data set or 3. something else? Are monad stacks with 3 and more monads common? How could an example implementation look like? What I have for now is: -- Status data Status = OK | FAILED deriving (Show, Read, Enum) -- Example data set manages by state type Config = [String] -- WriterT transformer type OutputWriter = WriterT [String] IO Status -- example execute function execute :: [String] -> OutputWriter execute fs = do rs <- liftIO loadData fs tell $ map show rs return OK -- run it inside e.g. main (s, os) <- runWriterT $ execute files How do I bring a state into this, for example for: execute fs = do ?? conf <- get ?? -- get Config from state rs <- liftIO loadData conf fs ?? set conf ?? -- change Config and set to state tell "new state:" tell $ show conf return OK Do I have to use and how do I use StateT in this context: data DataState = StateT Config OutputWriter ?? and how do I run it runStateT . runWriterT? Thanks for help, Adam

4 4