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Problem over Context Free Grammar Transitive Closure Algorithm
by Victor Silva 24 Nov '05

24 Nov '05

Folks this is a parser for Context Free Grammars, for some reason when I go simplify the grammar trying to remove productions that substitute variables like in the case: S -> A, A -> B, B -> a, it works perfectly and returns S -> a, the problem is when the root symbol is on the listthat must be replaced as in: S -> aXa | bXb , X -> a | b | S | £(empty) instead of returning S -> aXa | bXb, X -> a | b | £ | aXa | bXb it ruturns X as X -> a | b | £ | S <- this S is wrong! it should return X -> a | b | £ | aXa | bXb! Im not sure about where is the mistake. If you can give me hints it would be grat. I will be checking the mail and also hanging over #haskell at freenode ifyou want to contact me directly. Thanks in advance for the help! Victor

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RE: [Haskell-cafe] Function application like a Unix pipe
by Scherrer, Chad 23 Nov '05

23 Nov '05

Henning Thielemann <lemming(a)henning-thielmann.de> writes: > Since > (a . b) x > a $ b x > a (b x) > > are equivalent, do you also want to reverse function and > argument in order to match argument order of . and $ ? > > That is > x (b . a) > x b $ a > (x b) a > ? I'm sorry, I'm not sure I understand your question. Are you asking whether one should be allowed to write x f instead of f x? I don't think anyone is advocating this, but is can be convenient to have an infix operator for this purpose. Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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RE: [Haskell-cafe] Function application like a Unix pipe
by Scherrer, Chad 23 Nov '05

23 Nov '05

Henning Thielemann <lemming(a)henning-thielmann.de> writes: > I want to say, that the order of symbols for ($), (.) and > function application is consistent. This is a fine thing. I > think that shall not be distroyed by giving ($) and (.) > reversed argument order. I see. I like the argument order also, since it so nicely reflects mathematical notation. But I do think there's a place for (flip ($)) and (flip (.)). The problem is that the assignment of fixities is much more subtle and requires a consideration of what should be considered "proper style". Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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RE: [Haskell-cafe] GHCi always optimizes?
by Simon Peyton-Jones 23 Nov '05

23 Nov '05

| Anyway, it's strange that you are experiencing crashes. IIRC, there | were problems if you mixed modules compiled with different levels of | optimisation in the same program, but I am not sure it still happens. If you can make a reproducible crash, please tar it up and send it to us. It shouldn't happen. Ever. Simon

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RE: [Haskell-cafe] Function application like a Unix pipe
by Scherrer, Chad 23 Nov '05

23 Nov '05

Udo Stenzel <u.stenzel(a)web.de> writes: > The unix pipe is actually function composition. Its argument > (standard > input) isn't explicitly mentioned Then it seems Unix must overload the "|" operator. I typically use it to do things like grep "." *.hs | wc So I think of the types as being grep "." *.hs :: String wc :: String -> Int -- ok, not really, but it shows the point better. So we'd have to have (|) :: a -> (a -> b) -> b And (flip ($)) is the only thing that makes sense. Is it the case that a Unix pipe is analagous to (flip ($)) or (flip (.)) depending on the context? Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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RE: [Haskell-cafe] Spurious program crashes
by Simon Marlow 23 Nov '05

23 Nov '05

On 23 November 2005 13:29, Joel Reymont wrote: > On Nov 23, 2005, at 1:18 PM, Simon Marlow wrote: > >> After subsequent dicsussion, do you still think something strange was >> going on here? > > Yes, but in a different thread. The "Postmortem" one. > >> so this basically loops until there are no messages in the channel, >> and then exits. Is that what you wanted, or did you want it to keep >> reading from the channel until told to die? > > I probably made a mistake someplace as I wanted to read until told to > die but ONLY if the channel was empty. I replaced that code with > Tomasz's elegant solution so now I read until Nothing is read from > the channel. > > logger :: Handle -> IO () > logger h = > do ss <- getChanContents parent > logger' ss > where logger' [] = return () > logger' (Nothing:_) = return () > logger' ((Just x):xs) = > do putStrLn x > hPutStrLn h x > logger' xs > yield The yield is unnecessary. Also, getChanContents is considered by some (me included) to be poor style, because it relies on lazy I/O. This should work just as well: logger h = do m <- readChan parent case m of Nothing -> return () Just x -> do hPutStrLn h x; logger h Cheers, Simon

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RE: [Haskell-cafe] Spurious program crashes
by Simon Marlow 23 Nov '05

23 Nov '05

On 21 November 2005 16:43, Joel Reymont wrote: > I'm being quite careful with resources these days. The outstanding > issues are > > 1) Crashes on Mac OSX that are not reproduced on Linux, Windows, etc. > > 2) Some kind of a problem with Chan. getChanContents retrieves things > smoothly, readChan only does it for the first few lines. Simon? > Anyone? After subsequent dicsussion, do you still think something strange was going on here? The code does look strange: logger :: MVar () -> IO () logger die = do empty <- isEmptyChan parent unless empty $ do x <- readChan parent putStrLn x alive <- isEmptyMVar die when (alive || not empty) $ logger die so this basically loops until there are no messages in the channel, and then exits. Is that what you wanted, or did you want it to keep reading from the channel until told to die? STM is a better solution, as already suggested. Without STM, the best way to do this is to multiplex everything into a single channel (i.e. send the die message down the channel). > 3) Different performance of the logger thread on Mac OSX and Windows. > > I'm having thousands of threads write their trace messages to a Chan. > The logger On Windows I only see the first few lines of output when > using isEmptyChan/readChan to retrieve values in a loop. On Mac OSX I > do see smooth output. Context switch behaviour might be different between MacOS X and Windows. With the above code, it might be that the logger thread found an empty channel at sp,e point and exited. Does that make sense? > On Windows I run out of memory because all the output sent to the > chan builds up and is never processed. I can process it by replacing > isEmptyChan/readChan with getChanContents but then my logger thread > hangs forever (right semantics) and hangs everything else that waits > for the logger thread to check an MVar and exit. yes, because the logger thread has exited. Cheers, SImon

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Re: [Haskell-cafe] Function application like a Unix pipe
by Scherrer, Chad 23 Nov '05

23 Nov '05

"Albert Lai" <trebla(a)vex.net> writes: > I offer a simpler, more direct, and pre-existing correspondence between a functional > programming construct and unix pipes: Maybe my point wasn't clear. Of course this idea of comparing lazy evaluation to Unix pipes is very old (long before July 2004, I'm sure). The point I'm making is that there is an old idea that may be underused. We use ($) all over the place, but if there are a lot of them (and especially if they are spread over several lines) it becomes awkward to read the whole thing backward to trace through the function from beginning to end. In these cases, it's much simpler to use (\|) = flip ($) -- (#) seems to me too pretty for other purposes to use it here. infixl 0 \| -- Again, why can't this be negative or Fractional?? What I'm asking is really a question of pedagogy and style. This style seems reasonable to me. OTOH, there are some reasons not to do things in this way. Maybe any function big enough to benefit from writing it this way should be broken up anyway. Or maybe getting used to this style where the laziness is right in your face could make it more difficult for people to learn to reason through less obvious laziness. I'm really trying to figure out whether this approach is worth pursuing, rather than imply that this is a completely original idea. Chad Scherrer Computational Mathematics Group Pacific Northwest National Laboratory "Time flies like an arrow; fruit flies like a banana." -- Groucho Marx

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Records (was Re: [Haskell] Improvements to GHC)
by Joel Reymont 23 Nov '05

23 Nov '05

I second this motion! I rather like Simon's proposal. On Nov 17, 2005, at 5:00 PM, Fraser Wilson wrote: > Yeah, I thought you might have tried that at some point :-) > > I like http://research.microsoft.com/~simonpj/Haskell/records.html > > cheers, > Fraser. > > On 11/17/05, Joel Reymont <joelr1(a)gmail.com > wrote: Don't get me > started, please :-). I tried making each field a > separate class but then needed to compose records of difference field > instances which led to HList which led to GHC eating up all my memory > and crashing, etc. > > I can see where you are going but if I have 250 records with shared > fields then that's a whole lot of extra boiler plate code to marshall > between the functions with prefixes to the class method > implementations. The road to hell is paved with good intentions ;-). > > Thanks for the tip, though. > > On Nov 17, 2005, at 2:12 PM, Fraser Wilson wrote: > > > To solve this problem I just made them all instances of a class > > with a gameId function. Still, not ideal. -- http://wagerlabs.com/

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RE: Re[2]: [Haskell-cafe] using of data types as kinds
by Simon Peyton-Jones 23 Nov '05

23 Nov '05

Sorry should be fixed now (or shortly when the mirror does its stuff) Simon | -----Original Message----- | From: Bulat Ziganshin [mailto:bulatz@HotPOP.com] | Sent: 22 November 2005 19:28 | To: Simon Peyton-Jones | Cc: haskell-cafe(a)haskell.org | Subject: Re[2]: [Haskell-cafe] using of data types as kinds | | Hello Simon, | | Tuesday, November 22, 2005, 8:17:38 PM, you wrote: | | SPJ> I have not written anything about what I plan to do in GHC, but | SPJ> basically it amounts to allowing you to use a data type as a | SPJ> kind. Busy doing GADTs and impredicativity at the moment though | | Simon, i can't download file | | http://research.microsoft.com/~simonpj/papers/boxy/boxy-pldi.ps.gz | | reffered at http://research.microsoft.com/~simonpj/papers/boxy/ | | | -- | Best regards, | Bulat mailto:bulatz@HotPOP.com | |

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