- Haskell-Cafe - Haskell.org

linesPS is not analagous to lines
by David Roundy 14 Apr '03

14 Apr '03

I've just switched my code to use PackedStrings rather than Strings for file contents, and ran into what appears to be a bug in the Data.PackedString module in ghc 5.04.3. unlines is the inverse of lines, since lines converts empty lines into empty strings. linesPS, however, ignores empty lines. :( It seems like it would be a good idea to either fix this (which might not be a good idea, if existing code depends on its current behavior), or add a note in the documentation that it isn't analagous to lines, so that other people won't make the same mistake based on its similar name. My example program that demonstrates this: import Data.PackedString main = do temp <- return $ packString "hello\nworld\n\nI\njust\nskipped\n\n a line\n" putStr $ "The unlines of linesPSed version:\n\n" putStr $ unlines $ map unpackPS $ linesPS temp putStr $ "The unlines of linesed version:\n\n" putStr $ unlines $ lines $ unpackPS temp -- David Roundy http://www.abridgegame.org

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(no subject)
by Eileen Head 13 Apr '03

13 Apr '03

I have two version of factorial as follows: > fac = 1.0: zipWith (*) [1.0 ..] fac > fact = 1.0: [ f | f <- zipWith (*) [1.. ] fact ] Could some explain why does facBad run out of stack memory while fact work fine? i.e. ... > take 5 fact [1.0,1.0,2.0,6.0,24.0] ...> take 5 facBad [1.0 ERROR - Control stack overflow Thanks

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Re: Combining Monads
by Dominic Steinitz 12 Apr '03

12 Apr '03

Your program doesn't seem to have survived the transmission but try http://www.cse.ogi.edu/~mpj/pubs/springschool.html or http://www.cse.ogi.edu/~mpj/pubs/composing.html. For the theory, look at Triples, Toposes and Theories by Barr & Wells. I think it's chapter 7 but I haven't got it in front of me. Dominic Steinitz ----- Original Message ----- From: <haskell-cafe-request(a)haskell.org> To: <haskell-cafe(a)haskell.org> Sent: Saturday, April 12, 2003 5:01 PM Subject: Haskell-Cafe digest, Vol 1 #660 - 3 msgs > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-admin(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. Combining monads (Mark T.B. Carroll) > 2. Re: Combining monads (Scott Turner) > 3. Re: Combining monads (Glynn Clements) > > --__--__-- > > Message: 1 > Date: Sat, 12 Apr 2003 09:52:58 -0400 (EDT) > From: "Mark T.B. Carroll" <Mark.Carroll(a)Aetion.com> > To: haskell-cafe(a)haskell.org > Subject: Combining monads > > This message is in MIME format. The first part should be readable text, > while the remaining parts are likely unreadable without MIME-aware tools. > Send mail to mime(a)docserver.cac.washington.edu for more info. > > --481034957-302199926-1050155578=:555 > Content-Type: TEXT/PLAIN; charset=US-ASCII > > Hello! I am trying and failing to use one monad in another. A tiny example > is enclosed, where I try to use IO to help along a State-ful computation. > Is the real mistake in do_next? Is there a minimal rewrite that will get > this working that I can use as a template for other things? Or, is there a > good tutorial somewhere on this type of thing? Any pointers are much > appreciated! > > -- Mark > > --481034957-302199926-1050155578=:555 > Content-Type: TEXT/PLAIN; charset=US-ASCII; name="Test.hs" > Content-Transfer-Encoding: BASE64 > Content-ID: <Pine.LNX.4.44.0304120952580.555(a)cicero.aetion.internal> > Content-Description: Example of incompetence > Content-Disposition: attachment; filename="Test.hs" > > aW1wb3J0IENvbnRyb2wuTW9uYWQuU3RhdGUNCmltcG9ydCBEYXRhLkxpc3QN > Cg0KZ2V0X2xlbmd0aCA6OiBTdHJpbmcgLT4gSU8gSW50DQoNCmdldF9sZW5n > dGggbmFtZSA9IA0KICAgIGRvIGNvbnRlbnRzIDwtIHJlYWRGaWxlIG5hbWUN > CiAgICAgICByZXR1cm4gKGxlbmd0aCAobGluZXMgY29udGVudHMpKQ0KDQp1 > cGRhdGVfYmVzdF93aXRoIDo6IChTdHJpbmcsIEludCkgLT4gU3RhdGVUIChT > dHJpbmcsIEludCkgSU8gKCkNCg0KdXBkYXRlX2Jlc3Rfd2l0aCAoY3VycmVu > dF9uYW1lLCBjdXJyZW50X2NvdW50KSA9DQogICAgZG8gKGJlc3RfbmFtZSwg > YmVzdF9jb3VudCkgPC0gZ2V0DQogICAgICAgaWYgY3VycmVudF9jb3VudCA+ > IGJlc3RfY291bnQgDQoJICB0aGVuIHB1dCAoY3VycmVudF9uYW1lLCBjdXJy > ZW50X2NvdW50KQ0KCSAgZWxzZSByZXR1cm4gKCkNCg0KZG9fbmV4dCA6OiBT > dHJpbmcgLT4gU3RhdGVUIChTdHJpbmcsIEludCkgSU8gKCkNCg0KZG9fbmV4 > dCBuYW1lID0gDQogICAgZG8gY291bnQgPC0gZ2V0X2xlbmd0aCBuYW1lDQog > ICAgICAgdXBkYXRlX2Jlc3Rfd2l0aCAobmFtZSwgY291bnQpDQoNCmxvbmdl > c3RfZmlsZSA6OiBbU3RyaW5nXSAtPiBJTyAoU3RyaW5nLCBJbnQpDQoNCmxv > bmdlc3RfZmlsZSBuYW1lcyA9DQogICAgZXhlY1N0YXRlVCAoc2VxdWVuY2Vf > IChtYXAgZG9fbmV4dCBuYW1lcykpICgibm9uZSIsIDApDQoNCm1haW4gPSBk > byAobmFtZSwgY291bnQpIDwtIGxvbmdlc3RfZmlsZSBbIi9ldGMvaG9zdG5h > bWUiLCAiL2V0Yy9udHAuY29uZiIsICIvZXRjL3RpbWV6b25lIl0NCgkgIHB1 > dFN0ckxuICgiTG9uZ2VzdCBmaWxlIGlzICIgKysgbmFtZSArKyAiIHdoaWNo > IGlzICIgKysgc2hvdyBjb3VudCArKyAiIGxpbmVzIGxvbmciKQ0K > --481034957-302199926-1050155578=:555-- > > --__--__-- > > Message: 2 > From: Scott Turner <p.turner(a)computer.org> > To: "Mark T.B. Carroll" <Mark.Carroll(a)Aetion.com>, haskell-cafe(a)haskell.org > Subject: Re: Combining monads > Date: Sat, 12 Apr 2003 09:14:45 -0500 > > On Saturday 12 April 2003 09:52, Mark T.B. Carroll wrote: > > Hello! I am trying and failing to use one monad in another. > > Is there a minimal rewrite that will get this working ... ? > Yes. Just add lift. > do count <- lift $ get_length name > > > --__--__-- > > Message: 3 > From: Glynn Clements <glynn.clements(a)virgin.net> > Date: Sat, 12 Apr 2003 15:26:03 +0100 > To: "Mark T.B. Carroll" <Mark.Carroll(a)Aetion.com> > Cc: haskell-cafe(a)haskell.org > Subject: Re: Combining monads > > > Mark T.B. Carroll wrote: > > > Hello! I am trying and failing to use one monad in another. A tiny example > > is enclosed, where I try to use IO to help along a State-ful computation. > > Is the real mistake in do_next? Is there a minimal rewrite that will get > > this working that I can use as a template for other things? Or, is there a > > good tutorial somewhere on this type of thing? Any pointers are much > > appreciated! > > Combining monads isn't straightforward. There is a paper on the topic, > at: > > http://cm.bell-labs.com/cm/cs/who/wadler/topics/monads.html#combining-monads > > However, you can't really combine the IO monad in the way that the > above paper describes. > > The most direct way to fix your program is to use an IORef instead of > a state monad, e.g. > > > import IORef > > > update_best_with :: IORef (String, Int) -> (String, Int) -> IO () > > update_best_with ref (current_name, current_count) = do > > (best_name, best_count) <- readIORef ref > > when (current_count > best_count) $ do > > writeIORef ref (current_name, current_count) > > > do_next :: IORef (String, Int) -> String -> IO () > > do_next ref name = do > > count <- get_length name > > update_best_with ref (name, count) > > > longest_file :: [String] -> IO (String, Int) > > longest_file names = do > > ref <- newIORef ("none", 0) > > mapM_ (do_next ref) names > > readIORef ref > > A more elegant solution would be to generate a list of filename/length > pairs (e.g. using mapM), then just use maximumBy to select the > longest, e.g. > > > get_length' :: String -> IO (String, Int) > > get_length' name = do > > length <- get_length name > > return (name, length) > > > longest_file :: [String] -> IO (String, Int) > > longest_file names = do > > pairs <- mapM get_length' names > > return $ maximumBy cmp pairs > > where cmp (_, l1) (_, l2) = compare l1 l2 > > -- > Glynn Clements <glynn.clements(a)virgin.net> > > > --__--__-- > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > End of Haskell-Cafe Digest

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Combining monads
by Mark T.B. Carroll 12 Apr '03

12 Apr '03

Hello! I am trying and failing to use one monad in another. A tiny example is enclosed, where I try to use IO to help along a State-ful computation. Is the real mistake in do_next? Is there a minimal rewrite that will get this working that I can use as a template for other things? Or, is there a good tutorial somewhere on this type of thing? Any pointers are much appreciated! -- Mark

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writing compiler in haskell
by Debanjan Chatterjee 11 Apr '03

11 Apr '03

Hi I am doing my Masters in Computing. The master's course is a conversion course for non-computing graduates. And, the only language taught in the course is Java. My undergraduate degree is in accounting. I would like to ask whether with my background I can learn Haskell and as well write a compiler for C-- in Haskell in three months for my dissertation. Is this a feasible project within the time frame? I would also like to mention that I have never written a compiler before. I would grateful if anyone can advise me on this matter. Sincerely Yours Debanjan

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Re: Threading monads
by Jon Cast 11 Apr '03

11 Apr '03

Jan-Willem Maessen <jmaessen(a)MIT.EDU> wrote: > Actually, though the operations of most monads constrain evaluation > order, an interesting property of the identity monad is that it should > not do so. This observation has led me to think long and hard about > the distinction between monadic and non-monadic computations in > Haskell (admittedly to no real conclusion). Why would you do this? I don't see what conclusion you could come to that would involving discarding the ID monad, so you'd still need to distinguish between different monads. OTOH, the do-notation or other monadic notations don't really add much to the ID monad. So, a non-monadic syntax would still be desirable for this very common special case. So, what could you possibly think would change? Jon Cast

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Re: Threading monads
by Jon Cast 10 Apr '03

10 Apr '03

Mark Carroll <mark(a)chaos.x-philes.com> wrote: > I could give an example, actually, which might make things > clearer. Take one of my unfoldM's: > unfoldM f base = > do fb <- f base > case fb of > Nothing -> return [] > Just (a, b) -> do rest <- unfoldM f b > return (a : rest) > Now, using the Identity monad I can write a wrapper for non-monadic > f's: > myUnfoldr f b = > runIdentity (unfoldM (return . f) b) > which works the same as a regular unfoldr. With a monadic function > and a wrapper that uses the Identity monad, I can offer both monadic > and non-monadic interfaces to the unfolding functionality that is only > implemented once. > Is this a good design idea? Are there better ones? Is myUnfoldr > necessarily less efficient than a "normal" unfoldr that doesn't have > monads underneath constraining evaluation order? Monads don't necessarily constrain the order of evaluation. The ID monad, in particular, is perfectly lazy. So, it doesn't place any more constraints on the order of evaluation than ordinary Haskell does. Furthermore, (although I can't check this atm) I suspect if you ask a Haskell compiler to specialize unfoldM for ID, you'll get a version that acts internally almost exactly like unfoldr---and that produces a myUnfoldr implementation precisely equal to unfoldr. Jon Cast

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Threading monads
by Mark T.B. Carroll 10 Apr '03

10 Apr '03

Occasionally I have a non-monadic function (call it "x") that takes a function (call it "y") as an argument and uses it for stuff. Then, one day I inevitably want to pass it a y that needs to do something monadically: e.g. when I wanted unfoldM instead of unfoldr. So, I have to rewrite x accordingly. If I want to write an x that is open to taking a monadic y but doesn't require it to be monadic, should I maybe write it monadically and provide a wrapper that puts non-monadic y's in the Identity monad? Might this wrapping approach cause less efficient code than writing an alternative non-monadic x for non-monadic y's? Am I making any sense at all? (-: -- Mark

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unfoldM
by Mark T.B. Carroll 09 Apr '03

09 Apr '03

I wanted a monadic unfoldr and couldn't see one so I thought I'd share two possibilities I'd been playing with: unfoldM_a :: Monad m => (b -> Maybe (m (a, b))) -> b -> m [a] unfoldM_a f base = case f base of Nothing -> return [] Just result -> do (a, b) <- result rest <- unfoldM_a f b return (a : rest) unfoldM_b :: Monad m => (b -> m (Maybe (a, b))) -> b -> m [a] unfoldM_b f base = do fb <- f base case fb of Nothing -> return [] Just (a, b) -> do rest <- unfoldM_b f b return (a : rest) test_case_a "" = Nothing test_case_a word@(_:xs) = Just (do a <- print (length word) return (a, xs)) test_case_b "" = return Nothing test_case_b word@(_:xs) = do a <- print (length word) return (Just (a, xs)) main = unfoldM_a test_case_a "foo" putStr "----\n" unfoldM_b test_case_b "foo" -- Mark

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announcing darcs
by David Roundy 09 Apr '03

09 Apr '03

(My apologies for a possibly off-topic post, but I don't know where else I might contact interested haskellers.) For anyone who might be interested, I've written a revision control system which is in haskell (called darcs), which is licensed under the GPL. It contains a few bits of code that you might find useful, such as an interface to libcurl (for lazily reading files over http or ftp), an LCS function and code for opening a pipe to sendmail as a Handle. Of course, I'd welcome anyone who wants to contribute to darcs itself. the darcs webpage is http://www.abridgegame.org/darcs/ darcs is currently unix-only (counting MacOS X as unix). A port to windows would be a fair amount of work, but probably would mostly be straightforward (mostly dealing with slashes versus backslashes and getting libcurl to work under windows). -- David Roundy http://www.abridgegame.org/darcs/

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