[Haskell-cafe] Re: StateT IO Action on `onKeyPress`
Ryan Ingram ryani.s...@gmail.com writes: Something like this: -- Replaces runStateT for callbacks that might affect the state invert :: IORef s - StateT s IO a - IO a invert r m = do s - readIORef r (a, s') - runStateT m s writeIORef r s' return a -- Replaces liftIO when the action called might use invert revert :: IORef s - IO a - StateT s IO a revert r m = do s - get writeIORef r s a - liftIO m s' - readIORef r put s' return a Then you use ... `onKeyPress` (\event - invert windowListRef (...)) I'm not sure where in your code revert is required; I don't know when WindowListT might call back into IO. If you want to be extra safe, make the IORef be a Maybe WindowList and make sure it's Nothing except between 'revert' and 'invert'. Result is ReaderT is cleaner and simpler. Thanks for your help! -- Andy -- ryan On Mon, May 4, 2009 at 1:02 AM, Andy Stewart lazycat.mana...@gmail.com wrote: Hi Ryan, Ryan Ingram ryani.s...@gmail.com writes: Hi Andy. The GTK bindings use IO for their callbacks, not any custom monad like your WindowListT. I suggest, instead of StateT s IO a, you use ReaderT (IORef s) IO a: putR :: s - ReaderT (IORef s) IO () putR s = do r - ask liftIO $ writeIORef r s getR :: ReaderT (IORef s) IO s getR = ask = liftIO . readIORef Otherwise, there are techniques to use an IORef to hold onto the state while calling into IO (which might make callbacks), and then read it back out and put it in the state while running your action. But it's simpler to just switch to ReaderT I'm curious another techniques that use IORef hold on state. Can you implement a simple example that make my code pass `onKeyPress`? Thanks! -- Andy -- ryan On Sun, May 3, 2009 at 8:27 AM, Andy Stewart lazycat.mana...@gmail.com wrote: 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 --
Re: [Haskell-cafe] applicative challenge
On Mon, May 4, 2009 at 11:49 PM, Conor McBride co...@strictlypositive.org wrote: Remember folks: Missiles need miffy! Quote of the week ! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] applicative challenge
On 4 May 2009, at 23:15, Thomas Hartman wrote:
{-# 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 ()
Conor's already give you a comprehensive explanation of why
Applicative can't be used to do this, but that doesn't mean you can't
use applicative style!
How about...
echo = unlines . takeWhile (not . blank) . lines
seemsToWork = interact echo
Bob
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Re: [Haskell-cafe] A brief note on n_k patterns and Hackage
On Tue, May 5, 2009 at 1:04 AM, Richard O'Keefe o...@cs.otago.ac.nz wrote: I never really understood why it was thought to be relevant, but I was challenged to show that n+k patterns occurred in Hackage. Why is it relevant? /M -- Magnus Therning(OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] setResourceLimit
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? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] How to understand the fmap here ?
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to understand the fmap here ?
On 5 May 2009, at 11:27, z_axis wrote: 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! As the type signature of fmap explains, it transforms a function. Specifically, it starts with a function (a - b), and it transforms it to accept an 'a' inside a functor instead of just an a, and return a 'b' inside the same functor instead of just a b. In other words, fmap applies functions inside containers. We can see from floatLocation that it returns a pair inside a container - specifically, an X container. Fmap takes snd, and transforms it to work on values inside the X. So, snd has type (a,b) - b, thus fmap snd has type f (a,b) - f b. In this case, the type it's being applied to is X (ScreenId, W.RationalRect), so f unifies with X, a with ScreenID and b with W.RationalRect. Making snd `fmap` floatLocation w hav the type X W.RationalRect. Finally, the bind into rr there takes it out of the X monad all together, getting you a W.RationalRect. You may want to read this article which explains some of Haskell's abstraciton mechanisms: http://noordering.wordpress.com/2009/03/31/how-you-shouldnt-use-monad/ Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to understand the fmap here ?
2009/5/5 z_axis z_a...@163.com: 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! X is a Functor (and a Monad). floatLocation w :: X (ScreenId, W.RationalRect) -- i.e. f is X and a is (ScreenId, W.RationalRect) snd :: (ScreenId, W.RationalRect) - W.RationalRect fmap :: (same as snd) - X (ScreenId, W.RationalRect) - X W.RationalRect Makes sense ? Cheers, Thu ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to understand the fmap here ?
Hello z_axis, Tuesday, May 5, 2009, 1:27:16 PM, you wrote: floatLocation :: Window - X (ScreenId, W.RationalRect) rr - snd `fmap` floatLocation w class Functor f where fmap :: (a - b) - f a - f b looks ok. X===f, fmap executes floatLocation w :: X (ScreenId, W.RationalRect) === f a and then applies snd :: (ScreenId, W.RationalRect) - W.RationalRect === a-b to result, so that entire (snd `fmap` floatLocation w) has type X W.RationalRect === f b -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is Haskell a Good Choice for Web Applications? (ANN: Vocabulink)
Chris Forno (jekor) wrote: The idea is that I spent years studying different languages, generally with a textbook. The textbooks tend to focus on teaching rules and grammar, with a little bit of vocabulary and dialog each chapter. I think the focus should be reversed. I think it largely depends on the learner. Some people find vocabulary easier, or more interesting, others not. I have a hard time learning a lot of isolated facts (e.g. vocabulary), but I find it easier and more enjoyable to learn a rule that I can apply many times. But I know people who are the exact opposite. I wouldn't want to make an absolute rule. I generally like rules that will save me a lot of memorization. I hate rules that force me to memorize a lot. I am not good at memorization. I consider myself to be a highly logically-oriented (audio-digital?) learning type, as I expect many programmers are. However, I still don't remember most grammar lessons. The only way I successfully became fluent in a language (Esperanto) was through immersion, and that wouldn't have been possible without a decent vocabulary to start with. I totally understand, and I agree. And with only a few exceptions, I would say that vocabulary is more useful than grammar (even if I find the former harder to learn). That said, I cause Esperanto as a good example of a language with rules that make learning easier. In Esperanto, the ending of a word tells you if the word is a noun, a verb, an adjective, a subject, an object, etc. Knowing these rules makes it much easier for you to learn Esperanto. When I learn a language, I like learning rules that will make language learning easier. That being said, Esperanto, and even Japanese sentence structure perhaps is not as different as an agglutinative language like German. I'll need to study it more to find out. In the specific case of German, word order is a lot more important than any other language I know. You can get everything else about grammar wrong, but as long as you put the words in the right place people will probably understand you. But if you get everything else right, and put the words in the wrong place, you won't be understood. Absolutely. I'm not trying to claim that you only need 1,000 words to become fluent, like some courses claim. Ok. I probably misunderstood something. The idea is that once you can begin to read with a dictionary by your side you'll begin learning much faster because you can focus on reading what *you* are interested in rather than some contrived dialog from a textbook. In my case, the things I'm interested in are too technical :-( I've had a hard time finding things that are interesting and are simple enough for me to read in German. But I'll get better. So far I've been focusing on Japanese. I only have 15 or so stories currently. They take a bit of time to create ;) For now, the navigation is basically to click the Latest Links link in the header bar or in the Latest Links box. Ok. Cheers, Daniel. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] applicative challenge
That's slick, but is there some way to use interact twice in the same program?
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) -}
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:
thart...@ubuntu:~/haskell-learning/lazy-n-strictrunghc sequencing.hs
a
first time: second time: a
b
b
If someone can explain the subtleties of using interact when you run
out of stdio here, it would be nice to incorporate this into
http://www.haskell.org/haskellwiki/Haskell_IO_for_Imperative_Programmers#IO
where it talks about how using interact is the easy way to approach
these types of problems. Not *that* easy though, as this scenario
suggests.
2009/5/5 Thomas Davie tom.da...@gmail.com:
On 4 May 2009, at 23:15, Thomas Hartman wrote:
{-# 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 ()
Conor's already give you a comprehensive explanation of why Applicative
can't be used to do this, but that doesn't mean you can't use applicative
style!
How about...
echo = unlines . takeWhile (not . blank) . lines
seemsToWork = interact echo
Bob
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Re: [Haskell-cafe] applicative challenge
Thomas Hartman tphya...@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: thart...@ubuntu:~/haskell-learning/lazy-n-strictrunghc 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] applicative challenge
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: thart...@ubuntu:~/haskell-learning/lazy-n-strictrunghc sequencing.hs a first time: second time: a b b If someone can explain the subtleties of using interact when you run out of stdio here, it would be nice to incorporate this into Essentially, what's happening here is that interact consumes *all* of standard input, and runs your function on it. This means that as you've realised here, your function must do *all* of the processing of input in one go – but this is good! This means our IO is restricted to one tiny little corner of the program, and we get to write pure Haskell everywhere else :) What's going on with your top function on the other hand is that (.) is not `after` in the sense you're thinking. If you want one grabby to consume some of the input, but not all of it you'd need to return a pair containing the output, and the unconsumed input. where it talks about how using interact is the easy way to approach these types of problems. Not *that* easy though, as this scenario suggests. The key here is that it's more composable than using IO – IO can change all kinds of wierd state, and result in two functions doing totally different things depending on when they're called. This means you can't reliably stick bits of IO code together. With pure functional code though, referential transparency guarentees that you can. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] applicative challenge
seems to be the same behavior whether in ghci or compiled with ghc. 2009/5/5 Ketil Malde ke...@malde.org: Thomas Hartman tphya...@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: thart...@ubuntu:~/haskell-learning/lazy-n-strictrunghc 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Parsec - Custom Fail
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
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Re: [Haskell-cafe] Parsec - Custom Fail
Hi,
When we needed to do something similar with Parsec, we chose to pack the
relevant source position into the error string (you can just use
Show/Read, plus a special sequence of characters to indicate where the
position ends and the real error starts). We then unpack it outside
runParser before issuing the error to the user. If there's no packed
position found, we just use the Parsec position.
Thanks,
Neil.
mwin...@brocku.ca wrote:
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
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Re: [Haskell-cafe] Parsec - Custom Fail
mwin...@brocku.ca wrote: 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. If you already know what position you want to report the error at, then why bother calling setPosition to let parsec know? Just do: fail (show pos ++ : ++ msg) Parsec will then result in a ParseError with its own ideas of location, but you can ignore that. HTH, Martijn. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] List comprehension
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. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List comprehension
Hello applebiz89, Tuesday, May 5, 2009, 7:20:35 PM, you wrote: 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)') you forget films list variable: filmsInGivenYear filmYear films = [ title | (Film title director year fans) - films, year == filmYear] -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List comprehension
2009/5/5 applebiz89 applebi...@hotmail.com: 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. Hi, I suggest you start by some simple examples, then move to your goal. For instance you can try to write a function which takes in input a list of Int and outputs a list of those Int that are odd. f [1,2,3,4,5] = [1,3,5] You can generalise this function to accept a predicate. g even [1,2,3,4,5] = [2,4] Returning back to your problem, can you write a function that takes a Film and returns its release date ? Then you can write a function which test if that date match a given one. Then, finally, you can write a function which get a list of Film and outputs only those satisfying your desire. Cheers, Thu ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List comprehension
Hi, applebiz89 wrote: Hi, I think I need to use a list comprehension There is no need to use list comprehensions, there is always a way to express the same thing without them. In fact, list comprehensions are defined as syntactic shorthands for this other way. filmsInGivenYear :: Int - [Film] - [String] filmsInGivenYear filmYear ?= [ title | year - (Film title director year fans) , year == filmYear] Let's look at the type of your function. You have two arguments of types Int and [Film], so your definition should have two argument names: filmsInGivenYear filmYear films = ... Now, we want to look at each of the films filmsInGivenYear filmYear films = [ ... | film - films ... ] The part film - films means: Look at each element of films in turn, and name the current one film. But we actually want to look inside the film (we want to check the year). filmsInGivenYear filmYear films = [ ... | Film title director year fans - films ...] This means to look at each of the films in turn, and match the current film against the pattern (Film title director year fans). So if the current film has been created by a call like (Film ...), we will get the various data fields in the variables title, directory, year and fans. Now we want to check that the year is correct. filmsInGivenYear filmYear films = [ ... | Film title director year fans - films, filmYear == year] The part (filmYear == year) means that we are only interested in such films where year is filmYear. Finally, we want to return the name of the remaining films. filmsInGivenYear filmYear films = [title | Film title director year fans - films, filmYear == year] Hope that helps, Tillmann PS. I'm not a native speaker, but shouldn't it be movies and not films? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec - Custom Fail
Thanks, but I want a nice solution not another, even more complicated, workaround. On 5 May 2009 at 17:10, Martijn van Steenbergen wrote: mwin...@brocku.ca wrote: 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. If you already know what position you want to report the error at, then why bother calling setPosition to let parsec know? Just do: fail (show pos ++ : ++ msg) Parsec will then result in a ParseError with its own ideas of location, but you can ignore that. HTH, Martijn. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List comprehension
On Tue, May 05, 2009 at 05:36:12PM +0200, Tillmann Rendel wrote: PS. I'm not a native speaker, but shouldn't it be movies and not films? Both are correct. =) -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A brief note on n_k patterns and Hackage
On Tue, May 5, 2009 at 1:30 AM, Magnus Therning mag...@therning.org wrote: On Tue, May 5, 2009 at 1:04 AM, Richard O'Keefe o...@cs.otago.ac.nz wrote: I never really understood why it was thought to be relevant, but I was challenged to show that n+k patterns occurred in Hackage. Why is it relevant? Showing that they occur was not relevant. Showing the number of occurrences relative to the number of packages, function definitions, and other features was relevant (in my opinion): http://www.mail-archive.com/haskell-cafe@haskell.org/msg58320.html But, I phrased the challenge poorly and this is what we get :) I was just curious if it's a feature worth holding on it (eg., it gets a lot of good use). Sadly, just me commenting here about that goal will probably rekindle the thread we would all like to avoid... My sincerest apologizes if that happens. Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A brief note on n_k patterns and Hackage
On Mon, May 4, 2009 at 5:23 PM, John Van Enk vane...@gmail.com wrote: Which package? In 1251 packages, I found 20 source files out of 15144 source files that use (n+k) patterns: queuelike-1.0.9/Data/Queue/Stack.hs queuelike-1.0.9/Data/Queue/TrieQueue.hs hinze-streams-1.0/Data/Stream/Hinze/Stream.hs compression-0.1/Codec/Compression/Utils.hs bytestring-0.9.1.4/tests/lazybuildcons.hs bytestring-0.9.1.4/tests/lazybuild.hs liboleg-0.1.1/Language/TypeLC.hs pqueue-mtl-1.0.7/Data/Queue/Stack.hs pqueue-mtl-1.0.7/Data/Queue/Queue.hs IOSpec-0.2/examples/Echo.hs pxsl-tools-1.0.1/src/PxsltFormat.hs pdf2line-0.0.1/pdf2line.hs stream-fusion-0.1.2.1/tests/Test/SmallCheck/Partial.hs queue-0.1.1.3/src/Data/Queue/Instances/STM.hs frag-1.1.2/src/AFRP.hs th-fold-0.0.0.1/src/Language/Haskell/TH/Fold.hs tetris-0.27178/Util.hs random-fu-0.0.0.2/src/Data/Random/Distribution/Binomial.hs random-fu-0.0.0.2/src/Data/Random/Source/PureMT.hs The method that I used to extract them may have missed some files (e.g. anything that is Haskell source but needs to be preprocessed), but I just wanted to save people the trouble of finding them themselves ;) j3h -- For the curious, those who wish to search other sources, or those who wish to improve on my results, I found these by I downloaded the full Hackage tarball, unpacked it and ran: find ./ -type f \( -name \*.hs -o \*.lhs \) | xargs npk where npk is the following program: import System.Environment ( getArgs ) import Control.Monad ( when ) import Data.Generics ( cast, GenericQ, gcount ) import Language.Haskell.Exts ( Module, ParseResult(..), Pat(..), parseFile ) isNPlusK :: GenericQ Bool isNPlusK x = case cast x of Just (PNPlusK _ _) - True _ - False hasNPlusK :: Module - Bool hasNPlusK = ( 0) . gcount isNPlusK printIfNPlusK :: FilePath - IO () printIfNPlusK fn = do res - parseFile fn case res of ParseOk mod - when (hasNPlusK mod) $ putStrLn fn _ - return () main :: IO () main = mapM_ printIfNPlusK = getArgs ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Interesting Thread on OO Usefulness (scala mailing list)
Am Montag, 4. Mai 2009 13:35 schrieb Bulat Ziganshin: Hello Paolo, Monday, May 4, 2009, 2:05:44 PM, you wrote: Martin Odersky advocates the OO features of the scala language proposing an interesting problem where the OO approach seams valuable. i know two problems in Haskell/GHC that require OO-loke features - extensible exceptions and GUI widget types hierarchy Note that you don’t need different types for different kinds of GUI widgets if you use Functional Reactive Programming (FRP). You need different types if you use OO because you have to explicitely modify widgets after you have created them, and what modifications you are allowed to do, depends on the kind of widget. With FRP, you specify the behavior over all time when you create the widget, so no need for later method calls. Best wishes, Wolfgang ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Interesting Thread on OO Usefulness (scala mailing list)
Hello Wolfgang, Tuesday, May 5, 2009, 8:27:17 PM, you wrote: i know two problems in Haskell/GHC that require OO-loke features - extensible exceptions and GUI widget types hierarchy Note that you don’t need different types for different kinds of GUI widgets if you use Functional Reactive Programming (FRP). You need different types if you use OO because you have to explicitely modify widgets after you have created them, and what modifications you are allowed to do, depends on the kind of widget. With FRP, you specify the behavior over all time when you create the widget, so no need for later method calls. (i don't know anything about FRP) i think it doesn't change anything. the main reason why i need common operations is because i write generic procedures. another erason os what i can't remember 100 individual 'setSize' operations for 100 types of widgets. so we need to have some generic names, short of they are used due initialization or later -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANN: BUG FIX release of regex-tdfa-1.1.2
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] calling a variable length parameter lambda expression
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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] calling a variable length parameter lambda expression
Short answer: that's impossible. Well, with some oleging it should be possible, but the very fact that you're trying to do something like this indicates that you're doing something wrong. Where did this list of parameters came from? May be, you can apply your function to them one at a time, as they arrive? On 5 May 2009, at 20:49, Nico Rolle 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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] 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 nro...@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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec - Custom Fail
Am Dienstag 05 Mai 2009 17:38:35 schrieb mwin...@brocku.ca: Thanks, but I want a nice solution not another, even more complicated, workaround. I'm afraid you're out of luck there. Parsec carries a ParseError around even for successful parses (where it's a SourcePos and and empty list of messages). When binding two parsers, if the second doesn't consume any input, for the overall result it calls mergeErrorReply, which calls mergeError from Text.ParserCombinators.Parsec.Error: mergeError :: ParseError - ParseError - ParseError mergeError (ParseError pos msgs1) (ParseError _ msgs2) = ParseError pos (msgs1 ++ msgs2) So that doesn't look at the position of the second error :( You could change the sources of parsec, the least intrusive would probably be to modify mergeError: mergeError :: ParseError - ParseError - ParseError mergeError (ParseError _ []) pe2 = pe2 mergeError (ParseError pos msgs1) (ParseError _ msgs2) = ParseError pos (msgs1 ++ msgs2) or you could employ an ugly workaround like setPosAndFail :: tok - SourcePos - String - GenParser tok st a setPosAndFail dummy pos msg = do setPosition pos inp - getInput setInput (dummy:inp) tokenPrim (const ) (\p _ _ - p) Just fail msg myFail :: SourcePos - String - GenParser Char st a myFail = setPosAndFail 'a' to pretend you actually consumed some input. It works: *TestWorkAround test Left (line 100, column 100): Test but isn't pretty. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] applicative challenge
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 ke...@malde.org: Thomas Hartman tphya...@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: thart...@ubuntu:~/haskell-learning/lazy-n-strictrunghc 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: FW: Re: [Haskell-cafe] calling a variable length parameter lambda expression
On Tue, May 5, 2009 at 10:05 AM, Nico Rolle nro...@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.s...@gmail.com Gesendet: 05.05.09 18:58:32 An: Nico Rolle nro...@web.de CC: haskell-cafe@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 nro...@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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] calling a variable length parameter lambda expression
Nico Rolle nro...@web.de writes: A function is recieving a lambda expression like this: (\ x y - x y) or like this (\ x y z a - (x y) (z a) And the type of that function is..? 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 Would this work? data LambdaExpr a = L0 a | L1 (a - a) | L2 (a - a - a) | ... apply :: LambdaExpr a - [a] - a apply (L0 x) _ = x apply (L1 f) (x:_) = f x apply (L2 f) (x1:x2:_) = f x1 s2 : -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
using interact with state, was Re: [Haskell-cafe] applicative challenge
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 tphya...@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 ke...@malde.org: Thomas Hartman tphya...@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: thart...@ubuntu:~/haskell-learning/lazy-n-strictrunghc 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ST.Lazy vs ST.Strict
This simple implementation of CPU does not behave as expected in the
latest version of ghc using ST.Lazy since it updates the `pc` in the
wrong order.
When we use ghc-6.8 the code works as expected both with lazy and strict ST.
How is that? How do we fix this so we can use ghc-6.10.
-- --
module Main where
import Control.Monad.Reader
import Control.Monad.ST.Lazy
import Data.STRef.Lazy
import Data.Array.ST
import Int
data Refs s = Refs
{ memory:: STArray s Int8 Int8
, pc:: STRef s Int8
}
type CPU s a = ReaderT (Refs s) (ST s) a
type Address = Int8
type OPCode = Int8
alterVar v f = asks v = lift . flip modifySTRef f
getVar v = asks v = lift . readSTRef
setVar v a = asks v = lift . flip writeSTRef a
readMem :: Int8 - CPU s Int8
readMem addr = asks memory = lift . flip readArray addr
writeMem :: Address - Int8 - CPU s ()
writeMem addr v = asks memory = \r - lift $ writeArray r addr v
fetch :: CPU s OPCode
fetch = getVar pc = \v - alterVar pc (+1) readMem v
execute :: OPCode - CPU s ()
execute op = case op of
0x4 - alterVar pc (+100) -- should run this
_ - error should never match this
initCPU :: ST s (Refs s)
initCPU = do
m - newArray_ (0,30)
p - newSTRef 0
return (Refs m p)
main :: IO ()
main = do
print $ runST (initCPU = runReaderT m)
where
m = do
writeMem 0 0x4
writeMem 1 0x10
op - fetch
execute op
getVar pc
___
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Re: [Haskell-cafe] Performance counters
Magnus Therning wrote: Andrew Coppin wrote: Stuff like how many times does this function get called? How what's the maximum depth it recurses to? That kind of thing. It won't help you, but wouldn't it be the kind of thing that'd fit in the GHC runtime? Do you also require that the counters are available to the program itself? (This is starting to sound like something Don mentioned in his talk in London...) [I'm getting *really* tired of my ISP thinking that 30% of the traffic from Haskell-cafe is spam. If only there was a way to whitelist it or something... repeatedly clicking this is not spam doesn't seem to get the message across.] Um... hmm, that's a good question. Basically I've written a program that does adaptive sampling, and I want to see how much it's recursing. There are two parameters you can adjust: the minimum step size, and the maximum error tollerance. I'd like to know which of the two limits the program is reaching. (In other words, does the error eventually fall below the threshold, or does the step size become too small first?) I guess *ideally* I'd like the end user to be able to find this out from the running program... but I guess to do that there really is no other way than to sprinkle monads or unsafe I/O around the place. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ST.Lazy vs ST.Strict
Am Dienstag 05 Mai 2009 21:42:00 schrieb Tobias Olausson: This simple implementation of CPU does not behave as expected in the latest version of ghc using ST.Lazy since it updates the `pc` in the wrong order. When we use ghc-6.8 the code works as expected both with lazy and strict ST. How is that? How do we fix this so we can use ghc-6.10. Fix 1: compile with optimisations. The sample code worked here with that. Fix 2: change fetch: fetch :: CPU s OPCode fetch = getVar pc = \v - alterVar pc (+1) readMem v The lazy ST doesn't actually read the STRef before readMem v is called, so it reads the altered STRef and consequently the wrong memory address. Make sure that v is determined before the STRef is altered, e.g. by fetch = getVar pc = \v - v `seq` (alterVar pc (+1) readMem v) or fetch = do v - getVar pc w - readMem v alterVar pc succ return w ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Performance counters
2009/5/5 Andrew Coppin andrewcop...@btinternet.com: Magnus Therning wrote: Andrew Coppin wrote: Stuff like how many times does this function get called? How what's the maximum depth it recurses to? That kind of thing. It won't help you, but wouldn't it be the kind of thing that'd fit in the GHC runtime? Do you also require that the counters are available to the program itself? (This is starting to sound like something Don mentioned in his talk in London...) [I'm getting *really* tired of my ISP thinking that 30% of the traffic from Haskell-cafe is spam. If only there was a way to whitelist it or something... repeatedly clicking this is not spam doesn't seem to get the message across.] Um... hmm, that's a good question. Basically I've written a program that does adaptive sampling, and I want to see how much it's recursing. There are two parameters you can adjust: the minimum step size, and the maximum error tollerance. I'd like to know which of the two limits the program is reaching. (In other words, does the error eventually fall below the threshold, or does the step size become too small first?) I guess *ideally* I'd like the end user to be able to find this out from the running program... but I guess to do that there really is no other way than to sprinkle monads or unsafe I/O around the place. Hi, Since your recursive function has to test if the recursion should be ended or continued, when it ends, you know the reason. So why not just wrap your result inside something like: data Result a = ThresholdReached a | StepSizeTooSmall a ? Also, passing an accumulator to the function to record the number of steps should be straightforward. Cheers, Thu ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ST.Lazy vs ST.Strict
On Sun, May 3, 2009 at 11:27 AM, Tobias Olausson tob...@gmail.com wrote: 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) As Ryan points out, this will not do what you want. But that is incidental, not essential: foo y = do x - something fmap (x:) $ foo (y+1) Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ST.Lazy vs ST.Strict
On Tue, May 5, 2009 at 3:27 PM, Luke Palmer lrpal...@gmail.com wrote: On Sun, May 3, 2009 at 11:27 AM, Tobias Olausson tob...@gmail.com wrote: 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) As Ryan points out, this will not do what you want. But that is incidental, not essential: foo y = do x - something fmap (x:) $ foo (y+1) Questioning my own reasoning, I must apologize. I was wrong, these two programs are identical and both do what you want. Any references to the state *after* the infinite chain of foos will result in _|_, but as long as something is the only place that state calls occur, you will be fine. I also suspect that ST.Lazy should be no less defined than ST.Strict in all cases, modulo unsafe operations of course (you aren't doing those, are you?), so you have encountered a bug. Minimize the test case and submit a bug report :-) Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: atom-0.0.3
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 vane...@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 tomahawk...@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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Tests
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] applicative challenge
half-assed state for a real state solution, there's follow up here: http://groups.google.com/group/haskell-cafe/browse_thread/thread/d6143504c0e80075 2009/5/5 Thomas Hartman tphya...@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 ke...@malde.org: Thomas Hartman tphya...@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: thart...@ubuntu:~/haskell-learning/lazy-n-strictrunghc 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Thread on scala ml: Usefulness of OOP
On May 4, 2009, at 06:19 , Paolo Losi wrote: 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? Did you read the HaskellWiki link given in the comments? http://haskell.org/haskellwiki/OOP_vs_type_classes The top of the page suggests it's incomplete but there is a lot of stuff in there including example code. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH PGP.sig Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A brief note on n_k patterns and Hackage
On 5 May 2009, at 8:30 pm, Magnus Therning wrote: On Tue, May 5, 2009 at 1:04 AM, Richard O'Keefe o...@cs.otago.ac.nz wrote: I never really understood why it was thought to be relevant, but I was challenged to show that n+k patterns occurred in Hackage. Why is it relevant? Some people think that the popularity of a feature in an openly accessible collection of Haskell sources is relevant to whether it should continue to be supported, in a way that, say, appearance in textbooks, utility to beginners, and contribution to readability are not. Apparently. I don't understand this. n+k patterns are the ONLY Haskell98 feature scheduled for removal from Haskell'. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A brief note on n_k patterns and Hackage
ok: On 5 May 2009, at 8:30 pm, Magnus Therning wrote: On Tue, May 5, 2009 at 1:04 AM, Richard O'Keefe o...@cs.otago.ac.nz wrote: I never really understood why it was thought to be relevant, but I was challenged to show that n+k patterns occurred in Hackage. Why is it relevant? Some people think that the popularity of a feature in an openly accessible collection of Haskell sources is relevant to whether it should continue to be supported, in a way that, say, appearance in textbooks, utility to beginners, and contribution to readability are not. Apparently. This was the main reason it was kept in Haskell 98, 11 years ago. New textbooks don't use it (RWH doesn't talk or recommend the use of n+k) I don't understand this. n+k patterns are the ONLY Haskell98 feature scheduled for removal from Haskell'. For multiple reasons, summarised here: http://hackage.haskell.org/trac/haskell-prime/wiki/RemoveNPlusK -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] List comprehension
2009/5/6 Bulat Ziganshin bulat.zigans...@gmail.com:
Hello applebiz89,
Tuesday, May 5, 2009, 7:20:35 PM, you wrote:
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)')
you forget films list variable:
filmsInGivenYear filmYear films =
[ title | (Film title director year fans) - films, year == filmYear]
And this is nicer if you define Film with records:
-- just guessing as to types
data Film = Film { title :: String, director :: String, year :: Int, fans ::
[Fan] }
filmsInGivenYear filmYear films = [title | film - films, year film == filmYear]
-- and then it's probably easier written as:
filmsInGivenYear filmYear films = filter (\film - year film == filmYear) films
-- or slightly simpler:
filmsInGivenYear filmYear = filter (\film - year film == filmYear)
-- or with Data.Function.Predicate (shameless plug)
http://hackage.haskell.org/packages/archive/predicates/0.1/doc/html/Data-Function-Predicate.html:
filmsInGivenYear filmYear = filter (year `equals` filmYear)
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Re: [Haskell-cafe] calling a variable length parameter lambda expression
On 6 May 2009, at 4:49 am, Nico Rolle 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) Your first function has type Ord a = a - a - Bool so your list of parameters must have type Ord a = [a] and length 2. Your second function -- and why do you have the excess parentheses? they make it harder to read -- has type Ord a = a - a - a - Bool so your list of parameters must have type Ord a = [a] and length 3. 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) but in both cases lambda_ex wants a single Ord, so unfold_parameters parameters would have to return just that Ord. You _could_ fake something up for this case using type classes, but the big question is WHY do you want to do this? It makes sense for Lisp or Scheme, but not very much sense for a typed language. Why are you building a list [x,y] or [x,y,z] rather than a function (\f - f x y) or (\f - f x y z) so that you can do parameters lambda_ex? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is Haskell a Good Choice for Web Applications? (ANN: Vocabulink)
Chris Forno (jekor) wrote: The idea is that I spent years studying different languages, generally with a textbook. The textbooks tend to focus on teaching rules and grammar, with a little bit of vocabulary and dialog each chapter. I think the focus should be reversed. This varies wildly by textbook, with some bias for the language being taught. Personally I've found too many vocabulary textbooks and far too few grammar textbooks (that is, actual *grammar* textbooks not sentence-sized-vocabulary textbooks). Obviously grammar is very important. But is reading about it effective for everyone? In my experience learning and teaching languages, this too varies wildly by learner. Some people do better with an examples first or vocabulary based style where they must come to an intuition of the grammar rules; other people (such as myself) do better with a rules first or grammar based style where they must come to learn vocabulary on their own. Neither variety of person is superior nor, as far as I can tell, more common at large; so any good teacher or textbook should balance these bottom up and top down approaches. IMO vocabulary is easy to learn, it just takes time, whereas grammar is harder to figure out on one's own, and so is the better thing for a teacher to focus on. However, this says little about reference material (as opposed to learning material), and study guides walk a line between reference and teaching. JGram http://jgram.org/pages/viewList.php is an interesting study guide that takes a middle path, treating syntactic patterns the same as it does lexemes. This is particularly appropriate for a language like Japanese where it's not always immediately apparent whether something belongs to the grammar vs the lexicon. The only way I successfully became fluent in a language (Esperanto) was through immersion, This is, hands down, the best way to learn any language. For it to work, as you say, some vocabulary is necessary; however, I think the amount of vocabulary needed at first is not so large as some think. Daily small-talk for getting/giving directions, ordering food, and the like comprise a large portion of beginner's language and requires remarkably little breadth of vocabulary (a couple hundred words or so). Small-talk also includes some of the most obscure and difficult-to-master grammatical patterns like greetings, getting the right tone of politeness/familiarity, and knowing what sorts of sentence fragments and other ungrammatical patterns are perfectly acceptable. And of course it has very forgiving sentence and a rather simple grammar, but I'm finding the experience to be very similar with Japanese so far. That being said, Esperanto, and even Japanese sentence structure perhaps is not as different as an agglutinative language like German. I'll need to study it more to find out. Actually, Japanese is agglutinative too (moreso than German is). The basic structures of Japanese are quite simple, however the details needed for fluency are quite intricate. Phrase order is rather free, though it is not entirely free and it is easy to reorder things so that they no longer make sense to native speakers. Aside from a few of the common mistakes beginners make, if you mess up the cases/particles you'll end up with gibberish. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] setResourceLimit
On May 5, 2009, at 04:52 , br...@lorf.org wrote: 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? An alternative is to forkProcess and have the child setResourceLimit and then executeFile. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH PGP.sig Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] runhaskell CLI parameters
Hello, I have forgotten the runhaskell CLI parameters ... sigh. In particular I want to a local build of a set of of package: runhaskell Setup.hs configure --user??? I just did a runhaskell -? which didn't tell me a lot! Kind regards, Vasili ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] runhaskell CLI parameters
On May 5, 2009, at 23:39 , Vasili I. Galchin wrote: I have forgotten the runhaskell CLI parameters ... sigh. In particular I want to a local build of a set of of package: runhaskell Setup.hs configure --user??? That's not an option to runhaskell, but an option to Setup.hs. runhaskell Setup.hs configure --help to see what options you can use with the configure command. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon universityKF8NH PGP.sig Description: This is a digitally signed message part ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: runhaskell CLI parameters
Vasili I. Galchin vigalc...@gmail.com writes: Hello, I have forgotten the runhaskell CLI parameters ... sigh. In particular I want to a local build of a set of of package: runhaskell Setup.hs configure --user??? I just did a runhaskell -? which didn't tell me a lot! Try runhaskell Setup.hs --help. runhaskell's help isn't very useful here, because the program you're interested in is actually Setup.hs. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANNOUNCE: The Haskell Platform
We're pleased to announce the first release of the Haskell Platform: a single, standard Haskell distribution for every system. Download the Haskell Platform 2009.2.0 installers and specification: http://hackage.haskell.org/platform/ The Haskell Platform is a blessed library and tool suite for Haskell culled from Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. What you get: http://hackage.haskell.org/platform/contents.html With regular time-based releases, we expect the platform will grow into a rich, indispensable development environment for all Haskell projects. Distro maintainers that support the Haskell Platform can be confident they're fully supporting Haskell as the developers intend it. Developers targetting the platform can be confident they have a trusted base of code to work with. *Please note that this is a beta release*. We do not expect all the installers to work perfectly, nor every developer need met, and we would appreciate feedback. You can help out by packaging the platform for your distro, or reporting bugs and feature requests, or installing Haskell onto your friends' machines. The process for adding new tools and libraries will be outlined in coming weeks. The Haskell Platform would not have been possible without the hard work of the Cabal development team, the Hackage developers and maintainers, the individual compiler, tool and library authors who contributed to the suite, and the distro maintainers who build and distribute the Haskell Platform. Thanks! -- The Platform Infrastructure Team ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
