- Haskell-Cafe - Haskell.org

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

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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.

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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.

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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

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applicative challenge
by Thomas Hartman 05 May '09

05 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

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Re: Tests
by John Goerzen 05 May '09

05 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 > >

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Re: [Haskell-cafe] ANN: atom-0.0.3
by Tom Hawkins 05 May '09

05 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 > >

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Performance counters
by Andrew Coppin 05 May '09

05 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

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using interact with state, was Re: [Haskell-cafe] applicative challenge
by Thomas Hartman 05 May '09

05 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 >> >

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Re: FW: Re: [Haskell-cafe] calling a variable length parameter lambda expression
by Ryan Ingram 05 May '09

05 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 >>> > >>> >

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