April 2003 - Haskell-Cafe

RE: [OT[ proving programs for novices
by David Bakin 16 Apr '03

16 Apr '03

How would this book relate - in subject, difficulty, audience - to: The Science of Programming by Gries? Algebra of Programming by Bird, De Moor, Moor? Thanks! - Dave -----Original Message----- From: Doaitse Swierstra [mailto:doaitse@cs.uu.nl] Sent: Tuesday, April 15, 2003 12:38 PM To: Bill Wood Cc: Doaitse Swierstra; haskell-cafe(a)haskell.org Subject: Re: [OT[ proving programs for novices One might be interested in the newly published book by Roland Backhouse: "Program Construction : Calculating Implementations from Specifications" http://www.wiley.com/remtitle.cgi?0470848820 I quote from the publishers web site: "The ever-increasing dependence of our lives and livelihoods on the correct functioning of computer software means that logic and program correctness are core elements of all good computer science degrees. This book presents both these topics in one self-contained text. The focus of the book is on "correct-by-construction" program design -- the discipline of calculating programs from their specifications. Modern, calculational logic is introduced in combination with key program construction principles, such as the assignment axiom, loop invariants and bound functions. This material is intertwined with motivational discussion, programming examples and challenging problem-solving exercises, bringing the book alive for its intended audience, undergraduates in computer science and mathematics, as well as professional programmers wishing to further develop their programming skills. The book covers the elements of logic and program correctness that form the foundations of further study --- the logical connectives and their algebraic properties, induction, quantifiers and program construction rules. Substantial examples of program construction are included. Many exercises are provided, all with detailed solutions. " Doaitse Swierstra _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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[OT[ proving programs for novices
by Chris Moline 15 Apr '03

15 Apr '03

hi. this isnt a haskell question but i am hoping you will forgive it. i was wondering where i could find books/websites/tutorials on how to construct proofs and how to prove programs. preferably books that are aimed at novices with some programming experience but little math/logic experience. i have paul hudaks hsoe book which has some material on proving properties of programs. sincerely chris moline

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wiki problems
by Isaac Jones 14 Apr '03

14 Apr '03

Greetings, Not sure who to send this to: I noticed on the wiki that the HaskellNewbies page was blank for some reason, and when I attempted to put something there, I got the below error. I tried this over the course of two days just to make sure there wasn't actually an editing conflict. "Editing Conflict Another author has modified this page since you checked it out. So that you won't interfere with the other author's work, you need to reload, re-edit, and submit again. Sorry!" peace, isaac

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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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