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Help a Newbie
by Matthew Morvant 20 Feb '04

20 Feb '04

I am trying (and have been for quite some time) to repair a broken Haskell script. I keep getting stuck on Last generator in do {...} must be an expression. I am betting that this is not uncommon. Can someone please help me understand? code5 :: Parser code5 = do dd <- code3 do ddd <- code3 do dddd <- code3 return "ce" code4 :: Parser code4 = do dd <- code3 do dde <- code3 digit ddd <- many (do digit) q <- code5 return (q : "ss") Thanks in advance for helping Matthew

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RE: [Haskell-cafe] setPermissions bug?
by Simon Marlow 20 Feb '04

20 Feb '04

> On Wed, Feb 18, 2004 at 12:31:34PM -0000, Simon Marlow wrote: > > > I presume that this is a bug in setPermissions? > > > > Yes, it's a bug. However, since > getPermissions/setPermissions doesn't > > really give you enough control in Unix, I suggest you use > the functions > > available from System.Posix instead (fileStatus, fileMode, > setFileMode > > etc.). > > Well, in my case all I want to do is to copy permissions from > one file to > another, so get/setPermissions would have been fine. But yeah, I've > already switched to using setFileMode etc. Actually if you want to copy permissions from one file to another, then get/setPermissions is *not* what you want. It will only copy across the owner's permissions. The problem is that the Permissions type is fully defined and non-abstract; there's no place to store extra platform-specific permission bits. Cheers, Simon

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hOp
by sebc＠macs.hw.ac.uk 19 Feb '04

19 Feb '04

The RTS of GHC is truly an amazing piece of software. By removing the system-specific bits, and just adding a few lines of C and assembly code, we were able to turn it into a standalone micro-kernel that can be extended in our favorite language. A screenshot is available here: http://etudiants.insia.org/~jbobbio/hOp-20040219.png Everything from the base hierarchical library not under the System subdirectory is available. This includes threads and communication primitives and the foreign interface, which means experimenting with writing drivers in Haskell should be reasonably easy. We hope to make a source release soon. -- Sébastien Carlier Jérémy Bobbio

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[Q] What calls what
by Andre Betz 19 Feb '04

19 Feb '04

Hi, I have a question about the following function. I don't know how the definition after "where" are called. For example, in ( D x2 (-y1) , b ++ b2 ) there is x2 defined in the where-block. So does that mean that it calls the iteratin at line 7? But what is with b2, does it also calls the line 7? Who calls the definition in line 3 and who calls the def in line 7? Many thanks Andre 1 test :: ( D , [Int] ) -> ( D , [S] ) -> ( D , [Int] ) 2 test ( D x y , b ) ( D dx dy , st ) = ( D x2 (-y1) , b ++ b2 ) 3 where ( x1 , y1 ) = head $ dropWhile ( ( > 0 ) . snd ) $ iterate inc ( x , y ) 4 5 inc ( n , m ) = ( n+1 , m+1 ) 6 7 ( b2 , x2 ) = head $ do 8 S ( D x3 sty ) pl _ sb <- st 9 guard $ sty == -y1 10 return ( [1,2,3] ++ sb , x3 + sb) -- ********************************************************** ** * André Betz * * * ---------- * * * EMail: mail(a)andrebetz.de * WW * * Web: http://www.andrebetz.de * /__\ * * ICQ: 117759780 * | oo | _WWWWW_ * * Mobil: +49 (0)162 8847 391 * (|_()_|) / o o \ * * Office: +49 (0)9131 82611 27 * \__/ (| __O__ |) * * Home: +49 (0)911 6695 558 * /|\/|\ \ \___/ / * * Tel./Fax: +49 (0)89 1488 224189 * |||||||| /-------\ * * Snail: Hirschelgasse 34 * |||||||||=========| * * D-90475 Nürnberg * |||||||||=========| * ********************************************************** **

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Re: [Haskell] Parsec question: attempted 'notMatching' combinator
by Graham Klyne 18 Feb '04

18 Feb '04

[To Haskell-cafe...] At 16:57 17/02/04 -0500, Andrew Pimlott wrote: >On Tue, Feb 17, 2004 at 07:48:52PM +0000, Graham Klyne wrote: > > Thanks! That got me going, though not with quite what you suggested. > > > > I ended up with this: > > [[ > > notMatching :: Show a => GenParser tok st a -> GenParser tok st () > > notMatching p = do { a <- try p ; unexpected (show a) } <|> return () > > ]] > > which does the required job for me. > >Oops, that does look better. I knew the try had to go somewhere. :-) > >The only remaining problem is when p succeeds but does not consume any >input, eg eof. In this case, the <|> return () cannot distinguish it >from p failing. > >I didn't realize at first what a dirty trick this function uses: It >distinguishes success or failure of p by whether any input from the >first part (before <|> return ()) was consumed. I don't think it is >possible (or desirable!) to get this approach 100% right: By "erasing" >the success of p, you lose the information you need. > >What about a more prosaic implementation: > > notFollowedBy' :: Show a => GenParser tok st a -> GenParser tok st () > notFollowedBy' p = do res <- do a <- try p; return $ Just a > <|> > return Nothing > case res of Just a -> unexpected (show a) > Nothing -> return () I don't see why that would work where the above case does not; i.e. when p consumes no input. Or indeed, your later, preferred version: [[ notFollowedBy' :: Show a => GenParser tok st a -> GenParser tok st () notFollowedBy' p = join $ do a <- try p; return (unexpected (show a)) <|> return (return ()) ]] >This works for the tests I've tried, but there's one little quirk with >error reporting: I never really cared so much about that bit! #g -- > aNoBC = do char 'a' > notFollowedBy' $ do char 'b'; char 'c' > > *Main> parseTest (aNoBC >> char 'e') "abe" > parse error at (line 1, column 2): > unexpected "e" > expecting "c" or "e" > >It seems that parsec both misreports which token is unexpected (should >be "b", and thinks that the failure to match "c" is a problem, even >though notFollowedBy' succeeded. > > > Using your version caused the notMatching parser to be equivalent to: > > return () > > presumably, because the failure was protected by the try combinator? > >Or perhaps more accurately, the success (of p) was protected by the try! >Ie, the unexpected and the try together undid the tell-tale token >consumption of p. > >Andrew ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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RE: [Haskell-cafe] setPermissions bug?
by Simon Marlow 18 Feb '04

18 Feb '04

> I've run into a problem using Directory.setPermissions (or > equivalently System.Directory.setPermissions). > > Basically the problem is that > > getPermissions f >>= setPermissions f > > is not a noop. Instead it is equivalent to > > chmod o-r f > chmod g-r f > etc > > I presume that this is a bug in setPermissions? Yes, it's a bug. However, since getPermissions/setPermissions doesn't really give you enough control in Unix, I suggest you use the functions available from System.Posix instead (fileStatus, fileMode, setFileMode etc.). Cheers, Simon

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setPermissions bug?
by David Roundy 18 Feb '04

18 Feb '04

I've run into a problem using Directory.setPermissions (or equivalently System.Directory.setPermissions). Basically the problem is that getPermissions f >>= setPermissions f is not a noop. Instead it is equivalent to chmod o-r f chmod g-r f etc I presume that this is a bug in setPermissions? The documentation claims that Note that to change some, but not all permissions, a construct on the following lines must be used. makeReadable f = do p <- getPermissions f setPermissions f (p {readable = True}) Which certainly implies that it is possible to change only some permissions, contrary to what happens in the actual implementation. It also seems that the Permissions data type could include in it additional (hidden, since it isn't portable perhaps) information in order to make the above code work. Then the only major question would be whether setPermissions f (p {writeable = True}) should make the file world-writeable or just user-writeable. I don't know about that. But at a minimum, it should preserve world-readability. -- David Roundy http://www.abridgegame.org

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Re: [Haskell] updating graphs non-destructively
by Graham Klyne 18 Feb '04

18 Feb '04

[to Haskell-cafe] At 19:29 17/02/04 -0500, S. Alexander Jacobson wrote: >Thank you for the link to FGL. I also looked at >the boilerplate stuff. If *feels* like there >should be a way to embed the graph stuff in the >boilerplate stuff to allow non-destructive update >of arbitrary object graphs without handcoding the >mapping of the datastructure to an object graph? > >Is this possible? Has anyone does this? Any >reason to believe its a bad idea? > >Alternatively, I could decide up front to >represent my data as RDF triples (amenable to a >graph system), but I'd rather take advantage of >Haskell's type system. Hmmm (speaking as an RDF advocate) ... that seems like an odd reason for using RDF. Much of the value of RDF is, I would say, as means to exchange information *between* programs rather than for internal structuring of data. An RDF-graph-based interface to data provides for some uniformity of access and processing that can be useful. In my own work, I have at least two different internal representations for RDF data (neither of which have been designed for efficiency). #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Wiki manager change
by Keith Wansbrough 16 Feb '04

16 Feb '04

Hi all... the Haskell Wiki http://haskell.org/hawiki/ is under new management. From now on, please contact Shae Erisson <shae(a)ScannedInAvian.com> with your requests, comments, or feedback on the Wiki site - or, of course, improve and extend the site yourself! Thanks all for your contributions - please keep them coming, and make this resource even more useful. Cheers, --KW 8-) Former Haskell Wiki manager -- Keith Wansbrough <kw217(a)cl.cam.ac.uk> http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.

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[Q] Using Class
by Andre Betz 13 Feb '04

13 Feb '04

Hi, I'm a really beginner to haskell and I have a question about using classes. I don't now really what this function does. So I will try to use it to see what it will put out position :: ( Enum a , Eq b ) => b -> [b] -> [a] position x = position_snd . zip ( enumFrom $ toEnum 1 ) where position_snd [] = [] position_snd ((p,y):ys) | x == y = p : position_snd ys | otherwise = position_snd ys My roblem is now that I don't know how to use it. I tried "head position 2 [1,2,3,4]" but I always get an error. Thanks for help André

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