December 2006 - Haskell-Cafe

Cabal licence files
by Dougal Stanton 22 Dec '06

22 Dec '06

The cabal setup recognises a small set of licences which I don't think are well explained. I'm trying to put together a canonical list for setting up new projects. GPL: <http://www.gnu.org/licenses/gpl.txt> LGPL: <http://www.gnu.org/licenses/lgpl.txt> Thankfully the FSF are particularly anal^W well-organised about this kind of thing and they make it very easy to determine what each licence is and how to apply it BSD3/BSD4: There appears to be one explicitly-named BSD licence on the OSI site. There is also a closely related MIT licence. I don't know which would be which, if these are the two referred to in the licence data type. <http://www.opensource.org/licenses/bsd-license.php> <http://www.opensource.org/licenses/mit-license.html> PublicDomain: The Creative Commons people have a public domain licence, though I am uncertain whether it is intended for code. Maybe this doesn't matter when what you're saying is "I give up all rights"? <http://creativecommons.org/licenses/publicdomain/> AllRightsReserved: This is the very opposite of a PD licence and, I believe, the default licence when there is no other explicitly stated. However it would be nice to say as much rather than relying on people's legal knowledge. ;) OtherLicense: This one's easy enough... If anyone can fill in gaps and correct the errors in the above I think it would be useful. Obviously if this has been discussed elsewhere just point me off in the right direction! :) Cheers, D.

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Showing the 1 element tuple
by Neil Mitchell 22 Dec '06

22 Dec '06

Hi, A weird question, what does the 1 element tuple look like? () -- 0 element tuple (,) a b -- 2 element tuple (,,) a b c -- 3 element tuple () a - meaningless (a) - a in brackets GHC has the 1 element unboxed tuple, (# a #), and all the other sizes unboxed as well, but how would you visually represent the 1 element boxed tuple? As it happens, Yhc _does_ have the 1 element tuple, you just can't use it from normal programs, its only created by desugarings of class instances. I'd quite like to change this, and I also need to render the 1 element tuple in some way, so wondered if anyone had any good ideas (or if there is even some sort of convention) I currently use ()1, but don't like that as it doesn't follow Haskell rules - the () and 1 would be separate lexemes. My other thought is (?), where ? is something appropriate - but all the appropriate things I can think of would either not be a lexeme (i.e. 1), having an existing meaning (i.e. . | etc) or seem wrong. Thoughts? Neil

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ST and Transformers
by J. Garrett Morris 22 Dec '06

22 Dec '06

Hello everyone, I recently found myself attempting to use ST at the base of a stack of transformers, ala: type Test t r = ErrorT String (ST t) r I imagined, given the type of ST, that I would need a run function along the lines of: runTest :: (forall t. Test t r) -> Either String r (which was, in fact, exactly what I wanted). Unfortunately, even with that type signature, I can't find a way to convince the compiler that the state thread is properly universal in the call to runST. Is it possible to write this function, or is ST (and other monads that use its style of threading) not transformable? Thanks, /g -- It is myself I have never met, whose face is pasted on the underside of my mind.

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"automonadization" of code?
by Adam Megacz 22 Dec '06

22 Dec '06

Is there any work on automatic translation of code in some tiny imperative language into Haskell code that uses the ST and/or IO monads (or perhaps even pure functional code)? For example, the user writes something vaguely like array = newArray (1,100) 1 for x=2 to 99 array[x] := array[x-1]+array[x+1] And it is transformed into something like foldl (>>=) (newArray (1,100) 1) $ map (\n arr -> do a <- readArray arr (n-1) b <- readArray arr (n+1) writeArray arr n (a+b) return arr) [2..99] Obviously the "small imperative language" would have to be highly restricted and carefully chosen in order for the translation to always work and be predictable. I'm interested in any existing work on choosing such a sublanguage. Thanks! - a -- PGP/GPG: 5C9F F366 C9CF 2145 E770 B1B8 EFB1 462D A146 C380

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Re: Shrinking the Prelude: The "categorical" approach
by Imam Tashdid ul Alam 22 Dec '06

22 Dec '06

so the main issues are these: Bulat Ziganshin raised: Num is intrinsically bound to the compiler. sad. so let's leave Num out. this basically means we will avoid "abstract algebra" in general. that is, forget "groups", "rings", for now, the principle focus will be "functor", "monad"... Henning Thielemann provided links to CAL: hi Henning, would you mind if we "steal" some of the good looking names in your Prelude? Brian Hulley raised one serious issue: even if we were to define a "newbie" friendly Prelude, which should be preferred, alphanumeric names or operators? I have a thing for operators, they look good! but Brian's issue is solid, the precedence isn't documented by Haddock. any Haddock developer reading this? so mplus stays. I would still suggest plus/zero over mplus/zero but Haskell's scoping is rather liberal. I suggest the convention that Control.Monad is always imported as M. this would mean we use M.plus, M.zero, M.lift and so on. Diego Navarro said: this will confuse beginners. I disagree. I think type classes and monads are the basics of Haskell (before that, it's just commonsense ;)... ) so if the error message successfully conveys the message that there's a problem, that should be enough to inspire the beginner to start on the basics. besides, people will ultimately use the standard Prelude. what I am trying to do is to prove that with a good choice of names programs written in Haskell can look more "comprehensible" to beginners. like Gosling said about Java, the language syntax has one very good property: it's easy to read it. Garrett Morris seems to agree with me on this. Greg Fitzgerald raised the further issue of monads being overlooked by the beginners. I think "do notation considered harmful" blog was at least partially right: it's better to expose the concept rather than hiding it. so with enough people interested (I assume) it's time to move this discussion to the wiki. the wiki code is not to be taken seriously, it's a "proof-of-concept", a "guideline". cheers! Imam __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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API for reading a big binary file
by Ranjan Bagchi 21 Dec '06

21 Dec '06

Hi -- I'm still learning to use ghc effectively, and I'm trying to use it for more-or-less mundane programming tasks, hopefully in a way which is a lot more elegant than I'd do in Java or Perl. I've got a big [around a gigabyte] binary file, filled with identical binary structures (imagine a C process writing structs). I'd like to process/analyze them efficiently. In C or even Java, i'd memory map the file and extract the data I need. Is there a fast way to do this using ghc? I can extract fields by using a ByteString, but I may not be using it fast enough: I've had to write my own routines to extract ints, longs and doubles. The other option is to write the loading code largely in C, but I'm not clear how to write a data type which maps well to a C struct. Any help / examples would really be appreciated. Thanks, Ranjan

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cabal running multiple preprocessors on one file - suggestion
by Marc Weber 21 Dec '06

21 Dec '06

Am I right that the way to do this now is create your own preprocessor run on files havirg your extionsion? What about naming the source files eg Module.hs.cpp.di.gc.chs ? This would mean first run c2hs, then greencard thin DrIft then cpp and put the results in Module.hs.cpp.di.gc.chs 1) Module.hs.cpp.di.gc 2)` Module.hs.cpp Module.hs pro : you see immediately which preprocessors are to be invoked you don't need write your own preprocessor handler.. con : There are to much possibilities to use doesFileExist? so the way to get the source file is using getDirectoryContents and filter by regex \.hs.* \.lhs.* matches and take longest filename (So that 1) will be perefered over 2) You need a customizable editor to set filetype by regular expression rather by (.gc .hs .lhs .chs ..) What do you think, would this be an enhancement? I'd like to implement this and provide a darcs patch to cabal 1.1.7 Any ideas/ comments on this? Marc

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Haskell Side Effect
by Ashley Yakeley 21 Dec '06

21 Dec '06

Since learning Haskell, I can now count in Spanish! See: "one" in Spanish, "two" in Spanish, "three" in Spanish, "four" in Spanish.. -- Ashley Yakeley

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Timer with Haskell GUI application
by asilovy 21 Dec '06

21 Dec '06

Hi all, I'm still trying to figure out why, in the Graphics.HGL library, is the timer not treated as the other events (mouse clics, key pressing... ? For instance see : http://haskell.org/ghc/docs/latest/html/libraries/HGL/Graphics-HGL-Window.h… As I understand, the timer in a HGL GUI application must be treated by waiting for a tick to occur (rather than doing useful things and being warned with maybeGetWindowEvent if one occurs). This sounds stupid to me and gives rise to non responding situations. Im a misunderstanding something ? Is it a weakness of HGL and does it force me to change all my approach and turn to another library ? Any help would be appreciated. Thanks Alain -- View this message in context: http://www.nabble.com/Timer-with-Haskell-GUI-application-tf2864634.html#a80… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Shrinking the Prelude: The "categorical" approach
by Imam Tashdid ul Alam 20 Dec '06

20 Dec '06

hi guys, I was just wondering if anyone is interested is a "quasi"-project of rewriting the Prelude (only shrinking it as measured by the total number of names imported, read along...) the idea is (just to be substantially different in approach) to develop an "alternate history" of Haskell. take map for example, and fmap, I don't think they should be named different (fmap is ugly, not suggestive, and conceptually the same). mplus could be renamed (++) (they are conceptually the same, mzero looks odd, name it empty, or simply zero) although I think concat should be replaced by msum (or the other way around preferably :) ? msum is a bad name) I am somewhat confused by join. concat seems to match join's type but I don't think the ideas coincide. and so on. in particular, we would want: * no backwards compatibility. the Prelude as is it, is good enough. we are defining a new module. * clean categorical hierarchy of type classes (category theorists wanted!) promoting uniformity. * cleaner names. foldl1 means nothing. absolutely nothing. what's the 1 for? * our Prelude only contains typeclasses, datatypes, and functions of utmost conceptual (and semantic, such as error, undefined and seq) importance, everything else goes to other modules. our Prelude is not going to be a place for "convenient" declarations. * the suffix method of naming (liftM2) is considered messy. we would prefer a seperate module (promoting qualified import). this is a fun project. we will not rewrite the Prelude, we'll merely rename it. I would suggest the name TheOtherPrelude to make our intentions clear. the conveniences (like concatMap) goes to TheOtherPrelude.Extension. I suggest we do it on the wiki. anyone interested just open a page with your name of choice. I am not doing it only because there's no point doing it if no one's interested. cheers, Imam __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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