- Haskell-Cafe - Haskell.org

Operator precedence and associativity on happy grammars
by j.romildo＠gmail.com 07 May '09

07 May '09

Hello. I am learning how to use Happy (a LALR(1) parser generator) and I have a question on a grammar based on an example from the manual. The input file to Happy is attached to this message. The grammar is: Exp -> let var = Exp in Exp Exp -> Exp + Exp Exp -> Exp - Exp Exp -> Exp * Exp Exp -> Exp / Exp Exp -> ( Exp ) This grammar is ambiguous, but the conflicts that will appear in the parser table can be resolved with precedence and associativity declarations for the operators that surround expressions: %right in %left + - %left * / giving right associativity to the in token, and left associativity to the arithmetic operators, and giving lower precedence to in, followed by + and -, and then by * and /. I see that if the token in is not given a precedence declaration, the grammar still is ambiguous, as can be seen in the example of parsing the expression: let v = e1 in e2 + e3 which could be parsed as (let v = e1 in e2) + e3 or let v = e1 in (e2 + e3) Giving the token in a lower precedence makes the parser choose the last option. My question is about the associativity of the token in? Does it make any difference giving it left associativity, right associativity, or making it non associative? Regards. José Romildo

1 0

traversing a tree using monad.cont
by Anatoly Yakovenko 07 May '09

07 May '09

So I am trying to traverse a tree in a specific order, but i have no idea where the things that i am looking for are located, and i want to avoid explicit backtracking. I was thinking i could do it with the continuation monad. Here is what i have module TestCont where import Control.Monad.Cont import Control.Monad.Identity import Control.Monad.State.Lazy --our stupid tree data Tree a = Tree [Tree a] | Leaf a --traverse all the branches search (Tree ts) next = do mapM_ (\ ti -> (callCC (search ti))) ts next $ () search tt@(Leaf a) next = do cur <- lift get case ((cur + 1) == a) of True -> do --the current leaf is what we want, update the state and return lift $ put a return $ () False -> do --the current leaf is not what we want, continue first, then try again next () search tt (\ _ -> error "fail") t1 = Leaf 1 t2 = Leaf 2 t3 = Tree [t1,t2] t4 = Leaf 3 t5::Tree Int = Tree [t4,t3] run = runIdentity (runStateT ((runContT $ callCC (search t5)) return) 0) it seems like next isn't quite doing what i want, because i don't think I ever try again after i call next $ () in the second clause. Any ideas? Thanks, Anatoly

4 7

IO help
by applebiz89 07 May '09

07 May '09

I havent done much IO at all in haskell, only within the function itself. However I want to get the input from the interface for the function and havent done this before. In my main function, I want to ask the user what they would like to do 'become fan' for example, then with their choice initiate the function and ask for the appropriate input for that function. This is the code below: main :: IO() do putStr "Hi there! what is your name: " fanName = getLine do putStr "1 = Insert film, 2 = Become a Fan, 3 = The number of fans of a film, 4 = Film released in a year: " input = getLine read input :: Int (if input == 1 then main x = insertFilm [] else if input == 2 then main x = becomeFan [] else if input == 3 then main x = numberOfFans []) Say they choose film in a given year function function: filmsInGivenYear :: Int -> [Film] -> [String] filmsInGivenYear filmYear films = [ title | (Film title director year fans) <- films, year == filmYear] I need to ask the user what filmYear they want to insert. But i need to do this from the main function...I chose to do an if statement to choose what function they want, but i dont know where to go from there? any help will be greatly appreciated. thanks apple -- View this message in context: http://www.nabble.com/IO-help-tp23423403p23423403.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

4 3

Interesting Thread on OO Usefulness (scala mailing list)
by Paolo Losi 07 May '09

07 May '09

Hi all, I'm following an interesting thread on the scala mailing list: http://www.nabble.com/-scala--usefulness-of-OOP-td23268250.html Martin Odersky advocates the OO features of the scala language proposing an interesting problem where the OO approach seams valuable. I would be very much interested in seeing an Haskell solution to that problem. Any haskell guru want to take a stub at it or give an opinion from a pure FP point of view? Thanks Paolo

9 11

calling a variable length parameter lambda expression
by Nico Rolle 07 May '09

07 May '09

Hi everyone. I have a problem. A function is recieving a lambda expression like this: (\ x y -> x > y) or like this (\ x y z a -> (x > y) && (z < a) my problem is now i know i have a list filled with the parameters for the lambda expression. but how can i call that expression? [parameters] is my list of parameters for the lambda expression. lambda_ex is my lambda expression is there a function wich can do smth like that? lambda _ex (unfold_parameters parameters) best regards

7 9

WolframAlpha
by Vasili I. Galchin 07 May '09

07 May '09

Hello, http://blog.wolframalpha.com/2009/05/01/the-secret-behind-the-computational… <<< this is obviously a Wolfram Inc. blog so maybe not totally objective ... but here is a snippet that speaks in Haskell's favor: As a result, the five million lines of *Mathematica* code that make up Wolfram|Alpha are equivalent to many tens of millions of lines of code in a lower-level language like C, Java, or Python. * I am some what familiar with Mathematica and it's multi-paradigm nature (like F#, OCaml, etc.). In any case, I would like the Haskell community to view WolframAlpha as a challenge. For what is it worth I presently cabalizing Swish .... Based on my reading of WolframAlpha it is a semantic web ... i.e. formal knowledge representation! all Google results => http://www.google.com/search?q=WolframAlpha&ie=utf-8&oe=utf-8&aq=t&rls=com.… Kind regards, Vasili *

3 2

cabal parse problems
by Vasili I. Galchin 07 May '09

07 May '09

Hello, I am trying to cabalize a package (swish .. semantic web) but am running into parse error: vigalchin@ubuntu:~/FTP/Haskell/Swish-0.2.1$ runhaskell Setup.hs configure Setup.hs: swish.cabal:24: Parse of field 'exposed-modules' failed. vigalchin@ubuntu:~/FTP/Haskell/Swish-0.2.1$ Below: is a fragment from my .cabal file. Line #24 starts with "Exposed-modules:" ..... ..... ..... Data-Files: README Exposed-modules: Swish.HaskellRDF.BuiltInDatatypes, Swish.HaskellRDF.BuiltInMap, Swish.HaskellRDF.BuiltInMapTest, Swish.HaskellRDF.BuiltInRules, Swish.HaskellRDF.ClassRestrictionRule, Swish.HaskellRDF.ClassRestrictionRuleTest, Swish.HaskellRDF.Datatype, Swish.HaskellRDF.GraphClass, Swish.HaskellRDF.GraphMatch, Any ideas why I am getting parse error? Pretty vague error message! Kind regards, Vasili

2 1

Vector-like data structure
by Krzysztof Skrzętnicki 07 May '09

07 May '09

Hi I'm looking for a data structure with following characteristics: 1. O(1) lookup 2. O(1) modification 3. amortized O(1) append 4. O(1) size query This roughly characterizes C++ vector<> class. I'm ready to implement it myself, but first I would like to ask if anyone knows package with similar data structure. If there are many, which one would you choose and why? Best regards Christopher Skrzętnicki

4 5

instance Monad (Except err)
by Martijn van Steenbergen 07 May '09

07 May '09

Hello, Mr. McBride and mr. Paterson define in their Applicative paper: > data Except e a = OK a | Failed e > instance Monoid e => Applicative (Except e) where ... Sometimes I'd still like to use >>= on Excepts but this "feels" wrong somehow, because it doesn't use monoids nicely like the Applicative instance does. Are there any good reasons such a Monad instance shouldn't be defined? Does it violate any laws, for example? Thanks, Martijn.

4 4

Current status of Annotations
by Tom Lokhorst 07 May '09

07 May '09

Hello, I've been reading about Annotations [1] as implemented during last years Summer of Code, and about some earlier ideas [3] [2]. It seems though that these annotations have never been merged into HEAD [4]. Could anyone elaborate on the current status of this proposal? Has there been any further work or discussion about this subject that I haven't been able to dig up? - Tom Lokhorst [1] http://hackage.haskell.org/trac/ghc/wiki/Annotations [2] http://www.haskell.org/pipermail/haskell-prime/2006-November/001893.html [3] http://hackage.haskell.org/trac/haskell-prime/ticket/88 [4] http://www.haskell.org/pipermail/haskell-cafe/2008-September/047293.html

3 2

Unfold fusion
by Adrian Neumann 07 May '09

07 May '09

Hello, I'm trying to prove the unfold fusion law, as given in the chapter "Origami Programming" in "The Fun of Programming". unfold is defined like this: unfold p f g b = if p b then [] else (f b):unfold p f g (g b) And the law states: unfold p f g . h = unfold p' f' g' with p' = p.h f' = f.h h.g' = g.h Foremost I don't really see why one would want to fuse h into the unfold. h is executed once, at the beginning and is never needed again. Can someone give me an example? So, this is what I got so far: unfold p f g.h = (\b -> if p b then [] else (f b): unfold p f g (g b).h = if p (h b) then [] else (f (h b)) : unfold p f g (g (h b)) = if p' b then [] else f' b: unfold p f g (h (g' b)) not very much. I kinda see why it works after I "unfold" some more, but I can't really prove it. I suspect I need some technique I haven't learned yet. I've heard about fixpoint induction, that looks promising, but Google knows very little about it. I hope somebody can give me some hints. Regards, Adrian

4 4

setResourceLimit
by brian＠lorf.org 07 May '09

07 May '09

I have a long-lived multithreaded server process that needs to execute programs and interact with them via Handles. The programs could misbehave, like loop or hang, so I need to limit the real and CPU time they can take. I guess System.Posix.Resource.setResourceLimit sets limits on the current process, so I can't use it in the server process. Is it a good approach to write some utility program for the server to call that does setResourceLimit and executes the programs that could misbehave?

2 5

Reply to
by Ashok Gautham 07 May '09

07 May '09

I feel that the haskell mailing lists must have the reply-to field set, so that a person can reply to the list by just clicking reply. I am unsure if I am the only one facing this problem. --- Ashok `ScriptDevil` Gautham

6 9

Haskell Arrows Applications
by Hannousse 06 May '09

06 May '09

Hello, I'm interested to the concept of arrows in Haskell, however, I couldn't find a real application or example using this new technology in a real world application. All what I found are just academic examples and other people developing new specific libraries using arrows.Could someone, please, give me a reference to one of the real applications that uses arrows. Thank you for all Hannousse. -- View this message in context: http://www.nabble.com/Haskell-Arrows-Applications-tp23388417p23388417.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

ST.Lazy vs ST.Strict
by Tobias Olausson 06 May '09

06 May '09

Hello! I have a program that is using ST.Strict, which works fine. However, the program needs to be extended, and to do that, lazy evaluation is needed. As a result of that, I have switched to ST.Lazy to be able to do stuff like foo y = do x <- something xs <- foo (y+1) return (x:xs) However, when running the program compiled with ST.Lazy, the following is outputted: [tobsi@wobsi]$ ./runnerLazy looper.hex runnerLazy: <<loop>> The very same program compiled with ST.Strict outputs: [tobsi@wobsi]$ ./runner looper.hex 83298556 The code that is actually computing stuff is this: loopSys :: Int -> CPU s Int loopSys cc = do instr <- fetch if instr == 0xEA --NOP then return cc else do c <- execute instr loopSys $! (cc+c) The CPU type looks as follows: type CPU s a = ReaderT (SysEnv s) (ST s) a The program is run like runReaderT (loopSys 0) which in turn is being runST'd and then printed Does anyone know why the program just outputs <<loop>> when compiled under ghc 6.10.2, and runs perfectly fine under ghc 6.8.2? The program is compiled with --make and -O2 Tobias Olausson tobsan(a)gmail.com

6 9

Writing a compiler in Hakell (continued 1)
by Rouan van Dalen 06 May '09

06 May '09

Hi everyone. Thanks for the speedy replies. Let me elaborate on my language and situation. The language I have in mind is a statically typed, hybrid (OOP + functional), strict language. It contains both functional and imperitive features (much the same as OCaml/F#) with ideas from haskell, scheme, smalltalk, mozart/oz, F#, C#. The syntax is kept as succint as possible, adding things that I think would be useful that are not defined in any of the inspirational languages. As for the target language, im not quite sure yet. I am doing a lot of work in .NET/C# at the moment, but I would eventually like to use my own programming language, instead of C#. I would also like to use my language for linux programming, so I will eventually support both Windows and Linux environments. My aim with this project is to further explore haskell and deepen my understanding of it, while at the same time creating something useful where I can explore my own ideas about programmnig languages and concepts. I don't really intend to write "the next programming language" used by billions of people. If others find it useful, that's awsome. I definitely intend to use my language for my own projects and to make it freely availble on the net. So it is not just for learning or fun, it is more focused on implementing a decent programming language that can be used for commercial applications (and fun). The compiler for my language should have a decent compile time, comparing to compilers written in C. The approach that I have in mind is to piggy back on existing languages/compilers until their implementation is no longer adequite or I have time to write my own code generator. So the first stage would be something like translate to C# and then use the C# compiler to obtain an executable. I want the first steps to go as quick as possible so that I can refine the language and test ideas quickly, and once the language has stabalized, concentrarte on my own full fledged compiler (if it is necessary). I think I will try Parsec as a starting point for creating a parser. Thanks again everyone. Regards Rouan

2 1

List comprehension
by applebiz89 06 May '09

06 May '09

Hi, I think I need to use a list comprehension for this function but Im not good with list comprehension. this is what I have so at the moment? filmsInGivenYear :: Int -> [Film] -> [String] filmsInGivenYear filmYear ?= [ title | year <- (Film title director year fans) , year == filmYear] (this code wont compile - error given '?Syntax error in expression (unexpected `;', possibly due to bad layout)') Is there an alternative solution instead of comprehension? I want the function to be given a release date and then return all films with that date. Thanks -- View this message in context: http://www.nabble.com/List-comprehension-tp23389686p23389686.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

7 6

A brief note on n_k patterns and Hackage
by Richard O'Keefe 06 May '09

06 May '09

I never really understood why it was thought to be relevant, but I was challenged to show that n+k patterns occurred in Hackage. Last week I downloaded it. I had to miss a couple of days looking after my daughter, but today I got back to it. Guess what? I found one. Having found one I stopped looking. I don't want to start another thread, but having promised that I'd look I thought I should follow that up.

8 10

Thread on scala ml: Usefulness of OOP
by Paolo Losi 06 May '09

06 May '09

Hi all, I'd like to mention an interesting thread on the scala mailing list about usefulness of OOP. http://www.nabble.com/-scala--usefulness-of-OOP-td23268250.html Martin Odersky, scala creator, advocates the use of OOP using an interesting toy problem. I would be very interested about the haskell point of view on the topic. Is there any haskell guru that wants to take a stub at the problem or give his opinion? Thanks Paolo

2 1

applicative challenge
by Thomas Hartman 06 May '09

06 May '09

{-# LANGUAGE NoMonomorphismRestriction #-} import Data.List import Control.Monad import Control.Applicative -- Can the function below be tweaked to quit on blank input, provisioned in the applicative style? -- which function(s) needs to be rewritten to make it so? -- Can you tell/guess which function(s) is the problem just by looking at the code below? -- If so, can you explain what the strategy for doing so is? notQuiteRight = takeWhile (not . blank) <$> ( sequence . repeat $ echo ) echo = do l <- getLine putStrLn l return l -- this seems to work... is there a way to make it work Applicatively, with lifted takeWhile? seemsToWork = sequenceWhile_ (not . blank) (repeat echo) sequenceWhile_ p [] = return () sequenceWhile_ p (mx:mxs) = do x <- mx if p x then do sequenceWhile_ p mxs else return () blank x = "" == x

8 13

Re: Tests
by John Goerzen 06 May '09

06 May '09

Guenther Schmidt wrote: > let me first of all thank you for providing the HDBC package. Haskell > would be a much, much less usefull language without a working database > interface. I could certainly not have written the app in Haskell > without it and in any other language I know writing this app would have > been much more difficult. Thanks! I'm glad you found it (and Real World Haskell) helpful. > The problem is what's in the database. > > You'd think there'd be a "Günni" in the database, right? > > Wrong! > > At least this is where your library and Sqlite disagree. Sqlite with any > GUI client doesn't show a "Günni", it shows a "G!$%§$§%nni". So do MS > Access and MySql btw. And now that is REALLY weird, because I can't duplicate it here. I wrote this little Haskell program: import Database.HDBC import Database.HDBC.Sqlite3 main = do dbh <- connectSqlite3 "foo.db" run dbh "CREATE TABLE foo (bar text)" [] run dbh "INSERT INTO foo VALUES (?)" [toSql "Günther"] run dbh "INSERT INTO foo VALUES (?)" [toSql "2-G\252nther"] commit dbh disconnect dbh And when I inspect foo.db with the sqlite3 command-line tool: /tmp$ sqlite3 foo.db SQLite version 3.5.9 Enter ".help" for instructions sqlite> select * from foo; Günther 2-Günther Exactly correct, as expected. I can read it back correctly from Haskell, too: import Database.HDBC import Database.HDBC.Sqlite3 import qualified System.IO.UTF8 as U main = do dbh <- connectSqlite3 "foo.db" results <- quickQuery' dbh "SELECT * from foo" [] mapM_ ((print :: String -> IO ()) . fromSql . head) results mapM_ (U.putStrLn . fromSql . head) results disconnect dbh and when I run this: /tmp$ ./foo3a "G\252nther" "2-G\252nther" Günther 2-Günther I wonder if there is something weird about your environment: non-unicode terminals, databases, editors, or something? For me, it Just Works as it should. > > For now I managed to rollback the UTF8 code in the HDBC-2.1 and got my > app to work as needed. > > I hope you find this info useful, thanks once more > > Günther > >

1 0

Re: [Haskell-cafe] ANN: atom-0.0.3
by Tom Hawkins 06 May '09

06 May '09

I pushed out a new version of atom that incorporates some of John's recommendations (thanks John). And it ships with a slightly better example. The release also includes means to extract code coverage to track which atom rules have fired during testing. -Tom http://hackage.haskell.org/cgi-bin/hackage-scripts/package/atom On Tue, May 5, 2009 at 4:06 AM, John Van Enk <vanenkj(a)gmail.com> wrote: > If any one has been looking at this and wants another example, I've finished > two posts on the topic of Atom. > > These posts depend on a patch: > http://code.sw17ch.com/blog/atom/atom_0.0.2_sw17ch_1.diff > > Part 1: http://blog.sw17ch.com/wordpress/?p=84 <- Brief discussion and > describes the patch > Part 2: http://blog.sw17ch.com/wordpress/?p=111 <- Writing a program to > blink an LED. > > Feed back would be appreciated! > > /jve > > On Sun, Apr 26, 2009 at 4:39 PM, Tom Hawkins <tomahawkins(a)gmail.com> wrote: >> >> Atom is a DSL in Haskell for designed hard realtime embedded programs. >> At Eaton, we are using it to control hydraulic hybrid refuse trucks >> and shuttle buses. After my talk at CUFP >> (http://cufp.galois.com/2008/schedule.html) a few people inquired >> about atom -- I finally had a chance to upload it to Hackage. >> >> http://hackage.haskell.org/cgi-bin/hackage-scripts/package/atom-0.0.2 >> >> Some new enhancements: >> - A simple rule scheduler for load balancing. >> - Signal probing for debug and data logging. >> - Functional coverage to monitor which rules have executed. >> - Started integration with the Yices SMT solver for bounded model >> checking. >> >> Experiences with our Eaton project: >> - 5K lines of Haskell/atom replaced 120K lines of matlab, simulink, >> and visual basic. >> - 2 months to port simulink design to atom. >> - 3K lines of atom generates 22K lines of embedded C. >> - Design composed of 450 atomic state transition rules. >> - Rules with execution periods from 1ms to 10s all scheduled at >> compile time to a 1 ms main loop. >> - 3 minute compilation time from atom source to ECU. >> - Atom design clears electronic/sw testing on first pass. >> - Currently in vehicle testing with no major issues. >> >> >> Tom >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe > >

1 0

Performance counters
by Andrew Coppin 06 May '09

06 May '09

OK, so I'm pretty sure I'm not the first person to run into this... I've got a fairly large, complex program. It seems to work fine. But now I want to add some counters to measure various things. Trouble is, to do that, I'd have to "infect" the entire program with boilerplate code to propogate these counters. At best I could wrap it up in a monad, but it's still annoying to have to make 90% of the code in the program monadic just to carry these performance counters around. Does anybody know of a better solution? I could use the dreaded unsafePerformIO to prod some IORefs, but now you have to start sprinkling NOINLINE pragmas around and hope you know what you're doing... Scary! o_O

5 6

using interact with state, was Re: [Haskell-cafe] applicative challenge
by Thomas Hartman 06 May '09

06 May '09

Aha! There is in fact a way to fit this specification into the applicative paradigm. I'm a bit muzzy as to what it all means, but I must say, aesthetically I'm rather pleased with the result: module Main where import Control.Monad.State import Control.Applicative import Control.Applicative.State -- applicative-extras on hackage -- works t18 = interact $ evalState f18 where f18 = return paint `ap` grabTillBlank `ap` grabTillBlank paint first second = "first\n" ++ first ++ "second\n" ++ second grabTillBlank = State $ \s -> let (beg,end) = break null . lines $ s in (unlines beg, (unlines . drop 2 $ end)) -- And, with applicative extras: t19 = interact $ evalState f19 where f19 = paint <$> grabTillBlank <*> grabTillBlank paint first second = "first\n" ++ first ++ "second\n" ++ second 2009/5/5 Thomas Hartman <tphyahoo(a)gmail.com>: >> interact (\s -> let (first,second) = span (not . null) (lines s) > in unlines ("first":first++"second":takeWhile (not.null) second)) > > So, that didn't quite do the right thing, and it seemed like using > span/break wouldn't scale well for more than two iterations. Here's > another attempt, which is a little closer I think, except that it > seems to be using some sort of half-assed state without being explicit > about it: > > module Main where > > t17 = interact f17 > f17 s = let (first,rest) = grabby s > (second,_) = grabby rest > in "first\n" ++ first ++ "second\n" ++ second > > grabby :: String -> (String,String) > grabby s = > let (beg,end) = break null . lines $ s > in (unlines beg, (unlines . drop 2 $ end)) > > > 2009/5/5 Ketil Malde <ketil(a)malde.org>: >> Thomas Hartman <tphyahoo(a)gmail.com> writes: >> >>> That's slick, but is there some way to use interact twice in the same program? >> >> No :-) >> >>> t10 = >>> let f = unlines . takeWhile (not . blank) . lines >>> in do putStrLn "first time" >>> interact f >>> putStrLn "second time" >>> interact f >>> >>> this results in *** Exception: <stdin>: hGetContents: illegal >>> operation (handle is closed) -} >> >> Yes. Interacting uses hGetContents, and hGetContents semi-closes (or >> fully-closes) the handle. If you do it from GHCi, you only get to run >> your program once. >> >>> I also tried >>> >>> t15 = >>> let grabby = unlines . takeWhile (not . blank) . lines >>> top = ("first time: " ++) . grabby . ("second time: " ++) . grabby >>> in interact top >> >>> but that didn't work either: >>> thartman@ubuntu:~/haskell-learning/lazy-n-strict>runghc sequencing.hs >>> a >>> first time: second time: a >>> b >>> b >> >> Well - the input to the leftmost grabby is "second time" prepended to >> the input from the first, and then you prepend "first time" - so this >> makes sense. >> >> Something like this, perhaps: >> >> interact (\s -> let (first,second) = span (not . null) (lines s) >> in unlines ("first":first++"second":takeWhile (not.null) second)) >> >>> If someone can explain the subtleties of using interact when you run >>> out of stdio here, it would be nice to incorporate this into >> >> hGetContents - there can only be one. >> >> -k >> -- >> If I haven't seen further, it is by standing in the footprints of giants >> >

1 0

Re: FW: Re: [Haskell-cafe] calling a variable length parameter lambda expression
by Ryan Ingram 06 May '09

06 May '09

On Tue, May 5, 2009 at 10:05 AM, Nico Rolle <nrolle(a)web.de> wrote: > I dont't understand why u ask me that but the lambda expression will > only get values not functions as a parameter. > a = 1 > b = 2 > c = 3 > >>> What if I call >>> a (+)? > > this won't happen in my use case. > regards nico Here's an example for a single fixed argument and return types: class LambdaApply lam where lamApply :: lam -> [Integer] -> Bool instance LambdaApply Bool where lamApply b [] = b lamApply _ _ = error "Too many arguments in argument list" instance LambdaApply r => LambdaApply (Integer -> r) where lamApply _ [] = error "Not enough arguments in argument list" lamApply f (x:xs) = lamApply (f x) xs The problem is generalizing this to any type, because of the base case: instance LambdaApply r where lamApply r [] = r lamApply _ _ = error "Too many arguments in argument list" overlaps with function types such as (Integer -> a). Since you say this is only for value types, if you're willing to encode each value type you care about, you can make this work in a somewhat more general way. But unless you are just using it for syntactic sugar (which seems unlikely for this use case), it's usually a signal that you are doing something wrong. -- ryan >>> Von: Ryan Ingram <ryani.spam(a)gmail.com> >>> Gesendet: 05.05.09 18:58:32 >>> An: Nico Rolle <nrolle(a)web.de> >>> CC: haskell-cafe(a)haskell.org >>> Betreff: Re: [Haskell-cafe] calling a variable length parameter lambda expression >> >>> This is a Hard Problem in Haskell. >>> >>> Let me ask you, how many parameters does this function take? >>> a = (\x -> x) >>> >>> How many parameters does this function take? >>> b = (\f x -> f x) >>> >>> How many parameters does this function take? >>> c = (\f x y -> f x y) >>> >>> What if I call >>> a (+)? >>> >>> -- ryan >>> >>> On Tue, May 5, 2009 at 9:49 AM, Nico Rolle <nrolle(a)web.de> wrote: >>> > Hi everyone. >>> > >>> > I have a problem. >>> > A function is recieving a lambda expression like this: >>> > (\ x y -> x > y) >>> > or like this >>> > (\ x y z a -> (x > y) && (z < a) >>> > >>> > my problem is now i know i have a list filled with the parameters for >>> > the lambda expression. >>> > but how can i call that expression? >>> > [parameters] is my list of parameters for the lambda expression. >>> > lambda_ex is my lambda expression >>> > >>> > is there a function wich can do smth like that? >>> > >>> > lambda _ex (unfold_parameters parameters) >>> > >>> > best regards >>> > _______________________________________________ >>> > Haskell-Cafe mailing list >>> > Haskell-Cafe(a)haskell.org >>> > http://www.haskell.org/mailman/listinfo/haskell-cafe >>> > >>> >

1 0

Parsec - Custom Fail
by mwinter＠brocku.ca 06 May '09

06 May '09

Hi, I am using parsec to parse a small programming language. The language is typed and I need to do some type checking, too. I have decided to do the parsing and type checking simultaneously in the my parsec parser. This approach avoids to keep source code positions in the data type in order to produce suitable error messages during type checking. Anyhow, because type errors are usually detected after parsing some code I need produce error messages with an earlier source position. Unfortunately, there is no function that produces an error taking a position as parameter. I tried the following: myFail :: SourcePos -> String -> GenParser tok st a myFail pos msg = setPosition pos >> fail msg This is already a workaround because I am modifying the position in the parser just to produce an error message. But even worse, it does not work. If I use this function as in: test :: Either ParseError () test = runParser (char '(' >> myFail (newPos "" 100 100) "Test") () "" "(" the position of the error is still the original position (line 1, column 2). As far as I can tell setPosition does not take effect until another symbol is read. This could be achieved by simply using anyToken if we are not at the end of the input (as in the example above). I came up with the following: myFail :: SourcePos -> String -> GenParser Char st a myFail pos msg = do { State toks _ st <- getParserState; setParserState $ State ('d':toks) pos st; anyToken; fail msg } This code works but it is not nice at all. The function myFail is not longer polymorphic in the type of tokens since we need an element of this data type in order to add it temporarily to the input ('d' above). My guess is that one has to enforce strictness at some point in order to work with the first approach, but I was not successful. Any ideas? Thanks, Michael

4 4

ANN: BUG FIX release of regex-tdfa-1.1.2
by ChrisK 06 May '09

06 May '09

Hello, While occasionally and slowly updating the future version of regex-tdfa I found a bug that exists in the released 1.1.1 version. It was just a matter of passing the wrong value into a function, so was easy to fix when I figured it out. The test case triggered an impossible "error" call and so I know this can kill a program that uses a buggy version of this library. Please upgrade regex-tdfa to version 1.1.2 which is available on hackage at: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-tdfa Cheers, Chris

1 0

How to understand the fmap here ?
by z_axis 05 May '09

05 May '09

The following code snippets is from xmonad: -- Given a window, find the screen it is located on, and compute -- the geometry of that window wrt. that screen. floatLocation :: Window -> X (ScreenId, W.RationalRect) --....... rr <- snd `fmap` floatLocation w Prelude> :i fmap class Functor f where fmap :: (a -> b) -> f a -> f b It seems it is different from the definition of fmap ? sincerely! 2009-05-05 z_axis

4 3

StateT IO Action on `onKeyPress`
by Andy Stewart 05 May '09

05 May '09

Hi all, I have a function named `keymapTest` need moand state WindowListT, and WindowListT is `type WindowListT = StateT WindowList IO`. when i add "(\event -> keymapTest winList event >> return False)" after `onKeyPress` for handle key press event, i got GHC error: Manatee.hs:57:58: Couldn't match expected type `IO a' against inferred type `WindowListT Bool' In the first argument of `(>>)', namely `keymapTest winList event' In the expression: keymapTest winList event >> return False In the second argument of `onKeyPress', namely `(\ event -> keymapTest winList event >> return False)' So function `onKeyPress` just accept *one* IO-action? Have a way to fix above problem? Any help? Thanks! -- Andy Below is source code of Manatee.hs file. ------------------------------> Manatee.hs start <------------------------------ module Main where import Text.Printf import Data.Monoid import Data.List import Data.Maybe import Control.Monad import Control.Monad.State import Control.Applicative import Data.IORef import Graphics.UI.Gtk hiding (Window, windowNew, get) import Graphics.UI.Gtk.SourceView import Graphics.UI.Gtk.Abstract.Widget import Manatee.Event import Manatee.Buffer import Manatee.WindowList import Manatee.Pane import Manatee.Statusbar import Manatee.Utils import Manatee.Window import qualified Data.Set as Set import qualified Graphics.UI.Gtk.Windows.Window as W import qualified Graphics.UI.Gtk.Gdk.Events as E main :: IO () main = do -- Init. initGUI -- Root frame. rootFrame <- W.windowNew rootFrame `onDestroy` mainQuit -- quit main loop when root frame close -- Root frame status. windowFullscreen rootFrame -- fullscreen -- Windows list. let windowsList = WindowList 0 Set.empty evalStateT (do -- Window 1 window1 <- windowNewWithBuffer DTop "test" liftIO $ containerAdd rootFrame $ windowPaned window1 (window2, window3) <- windowSplitVertically window1 (window4, window5) <- windowSplitHorizontally window3 winList <- windowListGetList liftIO $ rootFrame `onKeyPress` (\event -> keymapTest winList event >> return False) -- Handle window content synchronous. windowHandleSynchronous ) windowsList -- Loop widgetShowAll rootFrame -- display all widget mainGUI keymapTest :: [Window] -> E.Event -> WindowListT Bool keymapTest winList event = do window <- liftIO $ windowFindFocus winList case window of Just x -> handleKeyPress x event Nothing -> return False handleKeyPress :: Window -> E.Event -> WindowListT Bool handleKeyPress window ev = do liftIO $ case eventTransform ev of Nothing -> return False Just e -> do let display = statusbarOutputSubitemSetText $ paneStatusbar $ windowPane $ window eventName = eventGetName e case eventName of -- Window commands. "M-t" -> display "windowSplitVertically" -- "M-t" -> windowSplitVertically window >> return False _ -> display $ printf "%s undefined." eventName ------------------------------> Manatee.hs end <------------------------------

2 5

Re: Using Takusen is a real intellectual challenge
by oleg＠okmij.org 05 May '09

05 May '09

Perhaps you might find the following code, relying on low-level Takusen's functions, useful http://darcs.haskell.org/takusen/Database/PostgreSQL/Test/pgaccess.hs The code is a stand-alone program that does the following: docstrings = [ "A helper to access a PostgreSQL from a `scripting' language.", "This function attempts to closely emulate `dbaccess' of Informix.", "The goal is to have my Scheme-to-Informix interface work with", "PostgreSQL with minimal changes.", "", "Synopsys:", " dbaccess <conn-parms>", "where <conn-parms> is the string of database connection parameters.", "", "The program connects to the database and listens on its standard", "input for a SQL command. A command is an ASCII sequence terminated by", "NL-NULL-NL.", "If the command is other than SELECT or COPY, we submit it to the database.", "On success, no reply is generated, and we wait for a new command.", "If the command starts with a /*simple*/ SELECT, we expect few rows.", "We submit the query to the database and write the result to stdout.", "If the command was SELECT, we make a query and write the result,", "in groups, to the stdout. Each group of results has the form:", " 0 NL", " for the last group of results", " <nrows> <space> <ncols> NL", " followed by the matrix of <nrows> * <ncols> field values in the", " row-major order. Each field value has the form", " <length>NL<value>", " where <length> is the length of the value proper (no NL).", "", "If the command was COPY, we assume ", " COPY /*<len> fname*/ table ... FROM STDIN...", " where <len> is the length of the file name. We open the file and", " send its contents to the server", "", "Any error causes the termination of this program.", "We log all the results to stderr." ] The above was my attempt at comments shared between compile-time and run-time. HERE documents would have certainly helped; I hope Haskell will get them one day. As the comments show, pgaccess is a very simple program to endow any scripting language (Scheme in my case) to communicate with a PostgreSQL database. The scripting language communicates with pgaccess via a pipe; thus the language itself needs to know nothing about databases and need not depend on any database libraries. The code has been designed keeping incremental processing firmly in mind. It was written about three years ago and in constant use. The program runs for days and indeed moves quite large amount of data to and from the database (the amount of data in a transaction can be larger than the amount of available memory).

1 0

ANN: atom-0.0.2
by Tom Hawkins 05 May '09

05 May '09

Atom is a DSL in Haskell for designed hard realtime embedded programs. At Eaton, we are using it to control hydraulic hybrid refuse trucks and shuttle buses. After my talk at CUFP (http://cufp.galois.com/2008/schedule.html) a few people inquired about atom -- I finally had a chance to upload it to Hackage. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/atom-0.0.2 Some new enhancements: - A simple rule scheduler for load balancing. - Signal probing for debug and data logging. - Functional coverage to monitor which rules have executed. - Started integration with the Yices SMT solver for bounded model checking. Experiences with our Eaton project: - 5K lines of Haskell/atom replaced 120K lines of matlab, simulink, and visual basic. - 2 months to port simulink design to atom. - 3K lines of atom generates 22K lines of embedded C. - Design composed of 450 atomic state transition rules. - Rules with execution periods from 1ms to 10s all scheduled at compile time to a 1 ms main loop. - 3 minute compilation time from atom source to ECU. - Atom design clears electronic/sw testing on first pass. - Currently in vehicle testing with no major issues. Tom

3 3

Using Takusen is a real intellectual challenge
by Gü?nther Schmidt 05 May '09

05 May '09

Hi, well I'm *trying* to use Takusen to export from one database into the other. I'm using Takusen for this so that I can guarantee that during the export not a single huge result is assembled (possibly in memory) and then insert in one lump into the second database. Trying to figure out how to do that in Takusen has left me totally exhausted, my brain is pudding and I need to have a lay down now. I couldn't even figure out whether that's at all possible. help Günther

2 1

Odd error message from ghc --make
by Jeff Heard 05 May '09

05 May '09

I was just running a test that has compiled before just fine, and now I'm getting this rather odd message: EventBusTest.hs:20:9: Couldn't match expected type `Bus a' against inferred type `buster-1.1:App.EventBus.Bus [buster-1.1:App.EventBus.EData a1]' Expected type: MVar (Bus a) Inferred type: MVar (buster-1.1:App.EventBus.Bus [buster-1.1:App.EventBus.EData a1]) In the expression: mk In the first argument of `bus', namely `[mk]' Any ideas? I mean, it looks like it type checks to me.

1 0

Re: [Haskell-cafe] HDBC 2.1, UTF8 and Umlauts
by John Goerzen 05 May '09

05 May '09

Guenther Schmidt wrote: > Hi John, > > thanks for taking the time. It actually is \252 that turned into > something else because of my email client, damn the thing. OK, perhaps we have some confusion here. Are you saying that you entered the Unicode characters directly into your Haskell source as literals? In other words, you did not type: backslash two five two but instead just typed the umlaut on the keyboard? If so, that won't work directly -- I think. Maybe somebody can correct me on this, but my hunch is that would save the umlaut as UTF-8 when you save the .hs file. Then you will get a String which is supposed to have decoded Unicode data, instead having encoded UTF-8 data. You could wrap it with Codec.Binary.UTF8.String.decodeString from utf8-string and see if that helps. If it does, that'll be your problem. It's a complicated topic, I know. And the scary thing is that Unicode makes this all *easier*. > > I'll do some further investigating and give you some more details when I > have them, thanks in advance. > > Günther > > > John Goerzen schrieb: >> On Mon, May 04, 2009 at 04:44:04PM +0200, Guenther Schmidt wrote: >> >>> Hi John, >>> >>> I'm trying stuff like: >>> >>> dbc <- connectSqlite3 "somedatabase" >>> run dbc "insert into someTable values (?)" [toSql "Günni"]. >>> >> SO what do you get back after adding: >> >> commit >> r <- quickQuery' dbc "select * from someTable" >> print r >> >> Just knowing it's garbled doesn't help. Need to know *how* it's >> garbled. >> >> But the problem is that \374 isn't Unicode at all. It's ISO-8859-1. >> You're not actually giving it Unicode data to start with. I believe >> the proper sequence is \252. >> >> For all I know, \374 may not even be a valid Unicode encoding (haven't >> tested it). >> >> Try \252. >> >> >> >>> I also tried: >>> >>> dbc <- connectSqlite3 "somedatabase" >>> run dbc "insert into someTable values ('Günni')" []. >>> >>> So since this is Haskell code I presume it's in UTF-8, my emacs stores >>> all my *.hs files as UTF-8 >>> >>> In either case the "ü" becomes garbled. >>> >>> With the previous version of HDBC, 1.1.6, this worked just fine. >>> >>> >>> It also garbles any Umlauts coming *out*, the source is an UTF-8 sqlite3 >>> db file. >>> >>> Günther >>> >>> >>> >>> John Goerzen schrieb: >>> >>>> GüŸnther Schmidt wrote: >>>> >>>> >>>>> Hi guys, >>>>> >>>>> for some reason, any way I try, all the Umlauts get garbled with HDBC 2.1. >>>>> HDBC 1.16 worked fine with any backend (ODBC, Sqlite3, ... what have you). >>>>> >>>>> Anybody else had similar problems and knows how to solve this? >>>>> >>>>> >>>> You need to be more specific, but it is likely you are trying to send >>>> something to HDBC that isn't encoded in UTF-8. HDBC 2.x has a global >>>> preference for UTF-8 now, actually partly to resolve complaints like this. >>>> >>>> If you are feeding it ISO-8859-1 data or somesuch, try giving it UTF-8 >>>> instead. >>>> >>>> -- John >>>> >>>> >>> > > >

3 2

Darcs and binary files
by Gü?nther Schmidt 05 May '09

05 May '09

Hi, I've gotten used to darcs now, and use it for my project. There is one quirk though, I also added some rather large binary files, an sqlite database and an MS Access database to the repository. Now whenever I do a push from a branch into the main trunk the system runs out of memory. Guess I should have left the binary files out of there? Günther

2 1

Re: [Haskell-cafe] HDBC 2.1, UTF8 and Umlauts
by John Goerzen 05 May '09

05 May '09

Günther Schmidt wrote: > Am 04.05.2009 um 21:07 schrieb John Goerzen: > >> Günther Schmidt wrote: >>> Hi John, >>> >>> I'm afraid so. >>> >>> If it came back as an SqlString "G\252nni" then it propably wouldn't >>> be a problem. >> Can you boil this down to a few-line self-contained test program so I >> can try it myself? I do have test cases for Unicode stuff and they >> are >> all passing here. I would like to be able to eliminate your >> environment >> as a culprit. >> > I'll try, it will take me some time since I've never written a test in > haskell. You don't really have to make a special unit test. Just a simple client program that creates a table, inserts some data, reads it back, and demonstrates the corruption. > That'll be harder to do since I'm on XP here, but I'll try that too, No problem. It's just runghc setup configure -f buildtests runghc setup build and just run runtests >>> >>> >>> >>> Am 04.05.2009 um 20:47 schrieb John Goerzen: >>> >>>> Günther Schmidt wrote: >>>>> Hi John, >>>>> >>>>> what I just noticed is that *all* strings come back as >>>>> SqlByteStrings. >>>> That's normal, and pretty much irrelevant, since fromSql takes care >>>> of it. >>>> >>>> It's documented, even: the SqlByteString is assumed to be in UTF-8, >>>> and >>>> is decoded when converted to a String. >>>> >>>> It is not correct to have \252 in the SqlByteString. The proper >>>> sequence there is \xc3\xbc. When converted to String, *then* it >>>> should >>>> be \252. >>>> >>>> Are you positive you're seeing \252 in the SqlByteString? That >>>> doesn't >>>> make any sense to me. It's not a valid UTF-8 encoding. >>>> >>> How have Umlauts been behaving on your end? Were they as mean to you >>> as they were to me? >>> >>> >>> >>>>> ie. I get my "Günni", (G\252nni), back as an SqlByteString "G >>>>> \252nni" >>>>> instead of an SqlString "G\252nni". >>>>> >>>>> So when I cast, ie. fromSql x :: String, I get an "G\65533nni", >>>>> which >>>>> is where the garbling occurs. >>>>> >>>>> >>>>> BTW this is the 3rd time now that I'm writing the same bloody >>>>> email, >>>>> my email client "clipped" the previous 2. >>>>> >>>>> Günther >>>>> >>>>> >>> > >

1 0

Re: [Haskell-cafe] HDBC 2.1, UTF8 and Umlauts
by John Goerzen 05 May '09

05 May '09

Günther Schmidt wrote: > Hi John, > > I'm afraid so. > > If it came back as an SqlString "G\252nni" then it propably wouldn't > be a problem. Can you boil this down to a few-line self-contained test program so I can try it myself? I do have test cases for Unicode stuff and they are all passing here. I would like to be able to eliminate your environment as a culprit. Incidentally, could you run make test in the HDBC-sqlite3 source directory? > > > > > Am 04.05.2009 um 20:47 schrieb John Goerzen: > >> Günther Schmidt wrote: >>> Hi John, >>> >>> what I just noticed is that *all* strings come back as >>> SqlByteStrings. >> That's normal, and pretty much irrelevant, since fromSql takes care >> of it. >> >> It's documented, even: the SqlByteString is assumed to be in UTF-8, >> and >> is decoded when converted to a String. >> >> It is not correct to have \252 in the SqlByteString. The proper >> sequence there is \xc3\xbc. When converted to String, *then* it >> should >> be \252. >> >> Are you positive you're seeing \252 in the SqlByteString? That >> doesn't >> make any sense to me. It's not a valid UTF-8 encoding. >> > > How have Umlauts been behaving on your end? Were they as mean to you > as they were to me? > > > >>> ie. I get my "Günni", (G\252nni), back as an SqlByteString "G\252nni" >>> instead of an SqlString "G\252nni". >>> >>> So when I cast, ie. fromSql x :: String, I get an "G\65533nni", which >>> is where the garbling occurs. >>> >>> >>> BTW this is the 3rd time now that I'm writing the same bloody email, >>> my email client "clipped" the previous 2. >>> >>> Günther >>> >>> > >

1 0

Re: [Haskell-cafe] HDBC 2.1, UTF8 and Umlauts
by John Goerzen 05 May '09

05 May '09

Günther Schmidt wrote: > Hi John, > > what I just noticed is that *all* strings come back as SqlByteStrings. That's normal, and pretty much irrelevant, since fromSql takes care of it. It's documented, even: the SqlByteString is assumed to be in UTF-8, and is decoded when converted to a String. It is not correct to have \252 in the SqlByteString. The proper sequence there is \xc3\xbc. When converted to String, *then* it should be \252. Are you positive you're seeing \252 in the SqlByteString? That doesn't make any sense to me. It's not a valid UTF-8 encoding. > > ie. I get my "Günni", (G\252nni), back as an SqlByteString "G\252nni" > instead of an SqlString "G\252nni". > > So when I cast, ie. fromSql x :: String, I get an "G\65533nni", which > is where the garbling occurs. > > > BTW this is the 3rd time now that I'm writing the same bloody email, > my email client "clipped" the previous 2. > > Günther > >

1 0

When is it OK to create a new mailing list?
by Mads Lindstrøm 05 May '09

05 May '09

Hi I wanted a mailing list for my project WxGeneric and I am wondering when it is OK to do so? How big must the potential audience be? Is there any kind of etiquette or guidelines? Here http://haskell.org/mailman/admin it says that I must have "the proper authority" to create a mailing list. What is meant by "proper authority"? Can I just try to create one and see if I am successful? Or must I request someone to do it? Regards, Mads Lindstrøm

6 5

HDBC 2.1, UTF8 and Umlauts
by Günther Schmidt 04 May '09

04 May '09

Hi guys, for some reason, any way I try, all the Umlauts get garbled with HDBC 2.1. HDBC 1.16 worked fine with any backend (ODBC, Sqlite3, ... what have you). Anybody else had similar problems and knows how to solve this? Günther

2 1

Estimating the time to garbage collect
by Neil Davies 04 May '09

04 May '09

Hi With the discussion on threads and priority, and given that (in Stats.c) there are lots of useful pieces of information that the run time system is collecting, some of which is already visible (like the total amount of memory mutated) and it is easy to make other measures available - it has raised this question in my mind: Given that you have access to that information (the stuff that comes out at the end of a run if you use +RTS -S) is it possible to estimate the time a GC will take before asking for one? Ignoring, at least for the moment, all the issues of paging, processor cache occupancy etc, what are the complexity drivers for the time to GC? I realise that it is going to depend on things like, volume of data mutated, count of objects mutated, what fraction of them are live etc - and even if it turns out that these things are very program specific then I have a follow-on question - what properties do you need from your program to be able to construct a viable estimate of GC time from a past history of such garbage collections? Why am I interested? There are all manners of 'real time' in systems, there is a vast class where a statistical bound (ie some sort of 'time to complete' CDF) is more than adequate for production use. If this is possible then it opens up areas where all the lovely properties of haskell can be exploited if only you had confidence in the timing behaviour. Cheers Neil

4 6

gcd
by Steve 04 May '09

04 May '09

[Question moved over from Haskell-Beginners] I had a look at the gcd definition in GHC 6.10.1 ghc-6.10.1/libraries/base/GHC/Real.lhs -- | @'gcd' x y@ is the greatest (positive) integer that divides both @x@ -- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) (-6)@ = @3@, -- @'gcd' 0 4@ = @4@. @'gcd' 0 0@ raises a runtime error. gcd :: (Integral a) => a -> a -> a gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined" gcd x y = gcd' (abs x) (abs y) where gcd' a 0 = a gcd' a b = gcd' b (a `rem` b) Why is gcd 0 0 undefined? http://en.wikipedia.org/wiki/Greatest_common_divisor says: "It is useful to define gcd(0, 0) = 0 and lcm(0, 0) = 0 because then the natural numbers become a complete distributive lattice with gcd as meet and lcm as join operation. This extension of the definition is also compatible with the generalization for commutative rings given below." An added advantage, for haskell, of defining gcd 0 0 = 0 is that gcd would change from being a partial function to a total function. Regards, Steve

8 11

Research in functional programming
by Louis Wasserman 04 May '09

04 May '09

Where might I find or submit a paper on functional data structures? Examples I've found so far include ICFP <http://www.icfpconference.org/> and the JFP <http://journals.cambridge.org/action/displayJournal?jid=JFP>, but Google hasn't found me anything else. Louis Wasserman wasserman.louis(a)gmail.com

4 4

*system*-time timeout?
by brian＠lorf.org 04 May '09

04 May '09

System.Timeout.timeout is OK for wall clock time, but can I limit the amount of system time an IO action can take?

1 0

converting IOException to Either in ErrorT
by brian＠lorf.org 04 May '09

04 May '09

I wrote this to make it a little nicer to catch IO exceptions and convert them to ErrorT failure: onExceptionThrowError :: (Error ce) => IO a -> (String -> ce) -> ErrorT ce IO a onExceptionThrowError a ce = liftIO (try a) >>= either (\(e :: IOException) -> throwError (ce (show e))) return So now I can do, e.g., writeFile fp s `onExceptionThrowError` SpecificErrorConstructor ... It works, but seems bad. Please let me know if you see ways to improve it. Thanks.

2 1

ANN: Silkworm game
by Duane Johnson 04 May '09

04 May '09

Reprinted from my blog post [1]: === The semester is over, my final project was a success (at least in that I passed the class) and it’s time now to release the game I made for Graphics 455: Silkworm! This is my first full application in Haskell. The process has been an enlarging experience–I’ve come to really enjoy the mental work that goes into thinking about a program in a functional way. I highly recommend the challenge to other software engineers. Silkworm combines the Hipmunk binding to Chipmunk 2D Game Dynamics with OpenGL, and GLFW (an alternative to GLUT). It’s built to work on Mac OS X, but it uses cross-platform libraries so it should be fairly easy to port to other platforms. The source code is here [2] and below are some screenshots [1] -- Duane Johnson (canadaduane) === [1] http://blog.inquirylabs.com/2009/05/02/silkworm-game-written-in-haskell/ [2] http://inquirylabs.com/downloads/Silkworm.tgz

6 11

Combining computations
by michael rice 04 May '09

04 May '09

I posted something similar about an hour ago but it seems to have gotten lost. Very strange. I've read that Monads can combine computations. Can a Maybe monad be combined with a List monad such that Nothing `mplus` [] ==> [] Just 1 `mplus` [] ==> [1] If not, can someone supply a simple example of combining computations? Michael

7 7

Haskell vs Clean (speed comparison)
by Daniel Carrera 04 May '09

04 May '09

Hi, I think the mail server may have been acting up earlier. I sent this to Haskell-beginners, but it more properly belongs here. I found something interesting. "General wisdom" is that Clean (or OCaml) is faster than Haskell. The claim is often followed by a link to the Debian shootout. But on closer inspection, I question this conclusion. The Debian shoot out actually has four sets of benchmarks: 1) Intel 32-bit one core. 2) Intel 32-bit quad-core. 3) Intel 64-bit one core. 4) Intel 64-bit quad-core. It turns out that Clean is only faster for (1). For the others, Haskell is faster. Here I compare Haskell, Clean, OCaml, Lisp SBCL, C# Mono and Fortran because they are all in the same ball mark: 32-bit sing core [1]: Lisp, Fortran, Clean, Haskell, C# Mono. 32-bit quad-core [2]: Haskell, C# Mono, Lisp, Clean, Fortran. 64-bit sing core [3]: Fortran, OCaml, Haskell, Clean, C# Mono, Lisp. 64-bit quad-core [4]: Haskell, OCaml, Lisp, C# Mono, Fortran, Clean. Notes: * The order is "fast language first". * There are no results for OCaml for 32-bit. * "Lisp" is "List SCBL" whatever that is. Tentative conclusions: 1) Haskell makes very good use of multiple cores. It smokes Clean. 2) For single core, they are neck and neck. Whether Clean is faster depends non the architecture. What do you think? Daniel. P.S. [1]http://shootout.alioth.debian.org/u32/benchmark.php?test=all&lang=all&d=d… [2]http://shootout.alioth.debian.org/u32q/benchmark.php?test=all&lang=all&d=… [3]http://shootout.alioth.debian.org/u64/benchmark.php?test=all&lang=all&d=d… [4]http://shootout.alioth.debian.org/u64q/benchmark.php?test=all&lang=all&d=…

4 7

using haskell for a project
by Nicolas Martyanoff 04 May '09

04 May '09

Hi, I don't think I already presented myself; I'm Nicolas, a 23y french student, trying to learn and use haskell. I've been using C for years, for all sort of tasks, and am quite comfortable with it. I'm also using it 40h a week in my internship for network systems, so I kind of know how to use it. I discovered Haskell some monthes ago, bought `Real World Haskell', quickly read, and enjoyed it. So now I'd want to use it for a small project of mine, a simple multiplayer roguelike based on telnet. I wrote a minimal server in C, and it took me a few hours. Now I'm thinking about doing the same in Haskell, and I'm in trouble. I don't really know how to map my ideas in haskell code. For example, a character can cast spells, so I'd have something like this in C: struct hashtable spells; struct character { int n_spells; struct spell **spells; }; I thought I could do something like this in haskell: spells = Data.Map.Map Int Spell data Character = Character { charSpells :: [Int] } But now I don't know how to dynamically add new spells (new spells can be created in my gameplay). Since I can't assign a new value to the `spells' variable (Data.Map.insert returns a new map), I just don't know where to go. I have the same problem for a bout every problem. I love writing pure functions in haskell, but as soon as I try to write some code involving states or side effects, I can't write a line. I just wanted a 2d array to store a zone, for example, dead simple in C, but this kind of link http://greenokapi.net/blog/2009/03/10/rough-grids-in-haskell make me shiver. Point is, I'd like to use haskell, but I don't know how, it seems totally alien. How did you manage to change the way you map ideas to code, from imperative to pure functional ? Thank you. Regards, -- Nicolas Martyanoff http://codemore.org khaelin(a)gmail.com

8 7

Array Binary IO & molecular simulation
by Grigory Sarnitskiy 03 May '09

03 May '09

4 8