December 2007 - Haskell-Cafe

Do real programs need IO? (was IO is a bad example for Monads)
by Conal Elliott 10 Dec '07

10 Dec '07

On Dec 9, 2007 10:07 AM, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: > Interactive programmes without using IO? Cool :) And how! > I think you misunderstood Lennart. Thanks for checking. In this case, I think I understood Lennart fine and that he was saying what you're saying. > Would you deny that any useful programme has to do at least some of the following: > -accept programme arguments at invocation > -get input, be it from a keyboard, mouse, reading files, pipes... > -output a result or state info, to the monitor, a file, a pipe... === If by "programme", you mean the code I write, then I'm happy to deny that my programme has to do these things. Examples below. If you include a stateful RTS, then no I don't deny it. > I think Lennart was referring to that, you HAVE to know a little IO to write > programmes, at least getArgs, getLine, putStr(Ln), readFile, writeFile, > appendFile. And therefore some use of the IO monad has to be taught > relatively early. Explicit imperative programming is just one way to deal with input & output, not the only way. As proof, see FRP, Pan, or TV programs, which contain uses of none of these functions. (Nor could they, as these libraries are functional, having IO-free types and semantics.) Moreover, use of imperative programming sacrifices some of the semantic simplicity & composability that makes FP so appealing. That's why I'd like to see this belief in its necessity dispelled. That said, I don't think the existing functional (non-IO) approaches to interaction are quite there yet with the flexibility of imperative programming. It will take more work to get them there, and that work is mostly likely to be pursued by people who doubt the necessity of IO for writing "real programs". In that sense, Lennart's and your statements are self-fulfilling prophechies, as are mine. BTW, if you haven't seen it already, please check out http://haskell.org/haskellwiki/TV . The TV (tangible values) approach includes a simple algebra of interfaces (input/output) and keeps separable from the core computation. The separability allows the interface parts to be composed in parallel with the core part. For instance, when two function-valued TVs are composed, the interfaces are peeled off, so that the core functions can be composed directly. The output half of one interface and the matching input half of the other are discarded. The remaining input and output halves are recombined into a new interface, which is used as the interface of the composed TV. The core interface algebra can be used for text stream i/o, GUIs, and many other possible styles of information passing. I mention TV, because it's an example of combining the purity & composability I love about FP with the usability a "real" app. For more about this combination, please see my Google tech talk "Tangible Functional Programming: a modern marriage of usability and composability" ( http://conal-elliott.blogspot.com/2007/11/tangible-functional-programming-m…) That talk focus on end-user composability, but the essential points apply as well to explicit programming. As I mentioned before, TV (a) is currently less flexible than imperative/IO programming, and (b) has the composability, guaranteed safety, and amenability to reasoning of pure functional programming. Cheers, - Conal Am Sonntag, 9. Dezember 2007 18:31 schrieb Conal Elliott: > > > IO is important because you can't write any real program without using > > > it. > > > > Ouch! I get awfully discouraged when I read statements like this one. > The > > more people who believe it, the more true it becomes. If you want to do > > functional programming, instead of imperative programming in a > functional > > language, you can. For instance, write real, interactive programs in > FRP, > > phooey, or TV. And if you do, you'll get semantic simplicity, powerful > & > > simpler reasoning, safety and composability. > > > > - Conal > > > On Dec 8, 2007 1:26 AM, Lennart Augustsson <lennart(a)augustsson.net> wrote: > > [...] > > IO is important because you can't write any real program without using it. > > So why not teach enough of it to get people off the ground straight away? > > People who hang around long enough to do some more Haskell programming > > will run into the other monads sooner or later. But IO is an unavoidable step to > > writing Haskell programs.

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I'm translating <All about Monads> to Chinese
by Albert Lee 10 Dec '07

10 Dec '07

I have been confussed by monad for a long time. and I can't stand for it any more. so I start to translate the tutorial <All About Monads> to my mother language Chinese. My English is not good enough, so this work is only for my own study~ I know there are some other Chinese fellow in this list, wish it would be helpful. I will work for one chapter everyday. the address is: http://www.kamang.net

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Re: [Haskell] IVars
by Conal Elliott 10 Dec '07

10 Dec '07

(moving to haskell-cafe) > readIVar' :: IVar a -> a > readIVar' = unsafePerformIO . readIVar > so, we do not need readIVar'. it could be a nice addition to the libraries, maybe as "unsafeReadIVar" or "unsafeReadMVar". The same argument applies any to pure function, doesn't it? For instance, a non-IO version of succ is unnecessary. My question is why make readIVar a blocking IO action rather than a blocking pure value, considering that it always returns the same value? - Conal On Dec 8, 2007 11:12 AM, Marc A. Ziegert <coeus(a)gmx.de> wrote: > many many answers, many guesses... > let's compare these semantics: > > readIVar :: IVar a -> IO a > readIVar' :: IVar a -> a > readIVar' = unsafePerformIO . readIVar > > so, we do not need readIVar'. it could be a nice addition to the > libraries, maybe as "unsafeReadIVar" or "unsafeReadMVar". > but the other way: > > readIVar v = return $ readIVar' v > > does not work. with this definition, readIVar itself does not block > anymore. it's like hGetContents. > and... > > readIVar v = return $! readIVar' v > > evaluates too much: > it wont work if the stored value evaluates to 1) undefined or 2) _|_. > it may even cause a 3) deadlock: > > do > writeIVar v (readIVar' w) > x<-readIVar v > writeIVar w "cat" > return x :: IO String > > readIVar should only return the 'reference'(internal pointer) to the read > object without evaluating it. in other words: > readIVar should wait to receive but not look into the received "box"; it > may contain a nasty undead werecat of some type. (Schrödinger's Law.) > > - marc > > > > > > Am Freitag, 7. Dezember 2007 schrieb Paul Johnson: > > Conal Elliott wrote: > > > Oh. Simple enough. Thanks. > > > > > > Another question: why the IO in readIVar :: IVar a -> IO a, instead > > > of just readIVar :: IVar a -> a? After all, won't readIVar iv yield > > > the same result (eventually) every time it's called? > > Because it won't necessarily yield the same result the next time you run > > it. This is the same reason the stuff in System.Environment returns > > values in IO. > > > > Paul. > > _______________________________________________ > > Haskell mailing list > > Haskell(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell > > > > >

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ANNOUNCE: xmonad 0.5: a tiling window manager
by Don Stewart 09 Dec '07

09 Dec '07

The xmonad dev team is pleased to announce the 0.5 release of xmonad! http://xmonad.org xmonad is a tiling window manager for X. Windows are arranged automatically to tile the screen without gaps or overlap, maximising screen use. Window manager features are accessible from the keyboard: a mouse is optional. xmonad is extensible in Haskell, allowing for powerful customisation. Custom layout algorithms, key bindings and other extensions may be written by the user in config files. Layouts are applied dynamically, and different layouts may be used on each workspace. Xinerama is fully supported, allowing windows to be tiled on several physical screens. Features: * Very stable, fast, small and simple. * Automatic window tiling and management * First class keyboard support: a mouse is unnecessary * Full support for tiling windows on multi-head displays * Full support for floating windows * XRandR support to rotate, add or remove monitors * Per-workspace layout algorithms * Per-screens custom status bars * Easy, powerful customisation and reconfiguration in Haskell * Large extension library * Extensive documentation and support for hacking Get it! More information, screenshots, documentation, tutorials and community resources are available from the xmonad home page: http://xmonad.org The 0.5 release, and its dependencies, are available from hackage.haskell.org, here: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad **The headlines** xmonad 0.5 is a major new release of xmonad, allowing, for the first time, dynamic extension in Haskell without requiring recompilation. Extension and configuration of xmonad is now faster, simpler, and more flexible. All configuration is done via the ~/.xmonad/xmonad.hs file. Example config files and screenshots are available on the wiki: http://haskell.org/haskellwiki/Xmonad/Config_archive This marks the final break from its dwm origins. As a result, xmonad is now also much easier to package, distribute and extend, as recompilation of xmonad is not required to extend it. New features: * xmonad is now a library, as is the collection of xmonad extensions * xmonad is configured by editing ~/.xmonad/xmonad.hs * simpler layout hooks type and interface * small EDSL for writing window management rules * support for ghc 6.8.x * xmonad now requires Cabal 1.2 or newer * small bug fixes Extensions: xmonad comes with a huge library of extensions (now more than 5 times the size of xmonad), contributed by viewers like you. Extensions enable pretty much arbitrary window manager behaviour to be implemented by users, in Haskell, in the config files. Examples include: * ion-like window tabs * status bar support * X property and hints support * altnerative layout algorithms For more information on using and writing extensions see the webpage. The library of extensions is available from hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-contrib Brought to you by the xmonad team: Spencer Janssen Don Stewart Jason Creighton Andrea Rossato David Roundy Brent Yorgey with code contributions from: Brandon Allbery Chris Mears Shachaf Ben-Kiki Eric Mertens Alec Berryman Neil Mitchell Gwern Branwen Devin Mullins Joachim Breitner Daniel Neri Alexandre Buisse Stefan O'Rear Nick Burlett Simon Peyton Jones Peter De Wachter Hans Philipp Annen Aaron Denney Karsten Schoelzel Nelson Elhage Michael Sloan Shae Erisson Ivan Tarasov Joachim Fasting Alex Tarkovsky Michael Fellinger Christian Thiemann David Glasser Joe Thornber Kai Grossjohann Matsuyama Tomohiro Dave Harrison Daniel Wagner Juraj Hercek Ferenc Wagner Sam Hughes Jamie Webb Miikka Koskinen Klaus Weidner David Lazar nornagon Lucas Mai timthelion Robert Marlow Valery V. Vorotyntsev Dougal Stanton Joel Suovaniemi Mats Jansborg Dmitry Kurochkin Clemens Fruhwirth As well as the support of many others on the #xmonad and #haskell IRC channels, and the wider Haskell and window manager communities. Thanks to everyone for their support!

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Parsing integers [was: Re: [Haskell-cafe] help]
by Tim Chevalier 09 Dec '07

09 Dec '07

On 12/9/07, Ryan Bloor <ryanbloor(a)hotmail.com> wrote: > > hi > > I have a function parseInt... which needs an error guard for when the input > is not an Int. > > parseInt :: Parser > parseInt [] = [] > parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) > in [(EInt (read digits), removeSpace rest)] > > Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" > gives...[(Int 5, Int 2, "if")] > so, op is '+' or "&&". I am unsure of how to begin... > > parseBinaryOp :: String -> String -> [(Expr, Expr, String)] > parseBinaryOp op str > Check out: http://www.haskell.org/haskellwiki/Homework_help If this isn't homework, try posting again and phrasing your question in the form of a question; saying "I am unsure of how to begin" doesn't help us help you think about the problem. Cheers, Tim -- Tim Chevalier * catamorphism.org * Often in error, never in doubt "[Teaching children to read] will make America what we want it to be, a literate country and a hopefuller country." -- George W. Bush

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Point and link
by Andrew Coppin 09 Dec '07

09 Dec '07

Hi guys. Here's a fairly basic question. I have several ideas for programs that I'd like to write. They all involve placing "units" of some kind, and then drawing "connections" between those units. How feasible is it to program such a thing in Haskell? Where would you start? (Gtk2hs is the only graphics API I'm familiar with using at present.)

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Fwd: Parsing integers [was: Re: [Haskell-cafe] help]
by Tim Chevalier 09 Dec '07

09 Dec '07

[reply forwarded to list] ---------- Forwarded message ---------- From: Ryan Bloor <ryanbloor(a)hotmail.com> hi sorry for vagueness. I am writing a basic Parser from scratch. So far I have functions; # removeSpaces # match - which checks if a string is a substring of another # orParser which combines two parser's abilities # Basic pasrers like... parseInt, parseTrue, parseFalse, parseBool using the orParser on True and False. What I want to do now is have a parseBinaryOp that recognises: parseBinaryOp "+" "(5 + 2) if" >>>gives>> [(EInt 5, EInt 2, "if")] So I think that I have to split the initial string into four parts. "+" becomes op '(' becomes tokenF ')' becomes tokenB "5" becomes e1 "2" becomes e2 parseBinaryOp :: String -> String -> [(Expr, Expr, String)] parseBinaryOp op str = let (tokenF,e1,op,e2,tokenB) = I am not sure how to go about separating the string for how I need it using my other functiuons. Ryan ________________________________ > Date: Sun, 9 Dec 2007 13:07:23 -0800 > From: catamorphism(a)gmail.com > To: ryanbloor(a)hotmail.com > Subject: Parsing integers [was: Re: [Haskell-cafe] help] > CC: haskell-cafe(a)haskell.org > > On 12/9/07, Ryan Bloor <ryanbloor(a)hotmail.com> wrote: > > > > hi > > > > I have a function parseInt... which needs an error guard for when the input > > is not an Int. > > > > parseInt :: Parser > > parseInt [] = [] > > parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) > > in [(EInt (read digits), removeSpace rest)] > > > > Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" > > gives...[(Int 5, Int 2, "if")] > > so, op is '+' or "&&". I am unsure of how to begin... > > > > parseBinaryOp :: String -> String -> [(Expr, Expr, String)] > > parseBinaryOp op str > > > > Check out: > http://www.haskell.org/haskellwiki/Homework_help > > If this isn't homework, try posting again and phrasing your question > in the form of a question; saying "I am unsure of how to begin" > doesn't help us help you think about the problem. > > Cheers, > Tim > > -- > Tim Chevalier * catamorphism.org * Often in error, never in doubt > "[Teaching children to read] will make America what we want it to be, > a literate country and a hopefuller country." -- George W. Bush ________________________________ Are you the Quizmaster? Play BrainBattle with a friend now!

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help
by Ryan Bloor 09 Dec '07

09 Dec '07

hi I have a function parseInt... which needs an error guard for when the input is not an Int. parseInt :: ParserparseInt [] = []parseInt xs = let (digits, rest) = span isDigit (removeSpace xs) in [(EInt (read digits), removeSpace rest)] Also... I have a function that does this... parseBinaryOp "+" "(5 + 2) if" gives...[(Int 5, Int 2, "if")]so, op is '+' or "&&". I am unsure of how to begin... parseBinaryOp :: String -> String -> [(Expr, Expr, String)]parseBinaryOp op str Thankyou Ryan _________________________________________________________________ Get free emoticon packs and customisation from Windows Live. http://www.pimpmylive.co.uk

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ARM back end?
by Greg Fitzgerald 09 Dec '07

09 Dec '07

Anybody know of an ARM back end for any of the Haskell compilers? Thanks, Greg

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ST Monad - what's wrong?
by Nicu Ionita 09 Dec '07

09 Dec '07

Hi, I'm trying to use the ST monad in order to turn an almost pure function into a pure one: reading a precalculated list of primes into a prime set. But the following code brings an error: > primes :: Set Integer > primes = runST $ getPrimes "primes10h7.txt" > getPrimes :: String -> (forall s. ST s (Set Integer)) > getPrimes file = > do cont <- unsafeIOToST (readFile file) > let set = fromList $ map read $ lines cont > return set And here is the error: Couldn't match expected type `forall s. ST s a' against inferred type `ST s (Set Integer)' In the second argument of `($)', namely `getPrimes "primes10h7.txt"' In the expression: runST $ (getPrimes "primes10h7.txt") In the definition of `primes': primes = runST $ (getPrimes "primes10h7.txt") Compiled with GHC 6.6.1 with extensions activated. Nicu

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