- Beginners - Haskell.org

Haskell's Scrap Your Boilerplate (SYB) - applying transformation only once instead of everywhere
by Duc le anh 04 Jun '13

04 Jun '13

What's the best way to apply a transformation to a tree only once instead of everywhere using SYB? For instance, in the following simplified expression, there are several instances of Var "x", and I want to replace the first instance with Var "y" only. data Exp = Var String | Val Int | Plus Exp Exp |... myExp = Val 5 `Plus` Var "x" `Plus` Val 5 `Plus` Var "x" ... This can't be done using the everywhere combinator since it will try to transform all instances of Var "x" toVar "y". -- Duc A. Le PS: I also asked this on stackoverflow but figured that I might get a faster response here. Thank you!

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Re: [Haskell-beginners] FFI, export a ByteString
by Antoine Latter 02 Jun '13

02 Jun '13

The function 'castStablePtrToPtr' does not actually return a pointer you can usefully de-reference in C - it just gives you a value that will fit into a 'void*' in C-land. A StablePtr is, conceptually, just a pointer-sized index into an array maintained by the Haskell runtime, you can only de-reference it in Haskell. But you can pass it around in C because it fits into a pointer-sized variable or struct member. On Thu, Apr 11, 2013 at 10:59 AM, Kees Bleijenberg <k.bleijenberg(a)lijbrandt.nl> wrote: > My function in Haskell takes a CString and returns a ByteString (of Word8). > > I tried to put this function in a Haskell made Windows dll. I want to use > this function in a program written in C. > > > > The most simple program that compiled was: > > > > Import qualified B.ByteString as B > > import Foreign.StablePtr > > import Foreign.C.String > > import Foreign.Ptr > > > > foreign export stdcall parseRuitType :: CString -> IO (Ptr ()) > > parseRuitType s = do > > p <- (newStablePtr ( B.pack [10,11,12])) > > return $ castStablePtrToPtr p > > > > For testing I ignore s. > > > > In the C-program : > > hs_init(NULL,NULL) > > res = parseRuitType (“5” ) > > No errors, but the returned value res (or the dereferenced res) makes no > sense (not 10,11,12). > > > > My idea was: the result of the function(a ByteString of Word8) must be > locked. No garbage collection. But newStablePtr does not return a pointer. > So I have to convert the newStablePtr to a real pointer and then coerce this > in the C program to a pointer to a array of word8…? > > I’ve tried to return the ByteString in the Haskell function. But now the > compiler complains: Unacceptable result type in foreign declaration : > B.ByteString > > > > I’am missing the big picture. Any ideas? > > > > Greetings > > Kees > > > > > > > > > > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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data-files for test-suite
by harry 02 Jun '13

02 Jun '13

My cabal file has a test-suite section which requires an input file. I can get it with data-files and getDataFileName, but presumably this will be installed with the package. However, I only need it for running the exitcode-stdio-1.0. Is there another way of accessing test data?

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Re: [Haskell-beginners] Q 2 of 2: GUI and turnkey compiler?
by Gan Uesli Starling 02 Jun '13

02 Jun '13

I think I need to define what is meant by turn-key compiler. That is what they called it in JForth. In Perl this same feature goes by the term "packed archive" and in LabVIEW they call it a "built application". It is a feature in those languages whereby you can issue an standalone *.exe having within itself, complete unto itself, all that is needed to run the script, program, application. The output appears as a single file to the end user with the expected extension of *.exe but functions in fact more like those ZIP installers which also come with *.exe extensions. The size will be bloated, since packed archive has within it all from the parent language, modules included, that are needful to run the script, at least for Perl. For JForth it was much the same. For LabVIEW the output of a "build" requires to have the "runtime engine" installed, but which, once installed, is good for ALL applictions and will run them but not allow editing. So, it would be something to allow an author to issue programs which end-users would NOT have to know anything about Haskell itself and would have to, at most, perform a two-step, wholly automatic installation procedure. Short of this, anything I might aspire to give away free to the public en masse, could not conceivably be written in Haskell. In which case, I'll respectfully bow out from endeavoring to learn it myself, however useful it serves for many another purpose.

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Abstracting Data.List
by Giacomo Tesio 01 Jun '13

01 Jun '13

I'm looking for a class abstracting Data.List. I mean something that enable the use of say head, map, take and so on into different kind of types. Is there already something like that? Giacomo

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help with IO and guards
by Miro Karpis 01 Jun '13

01 Jun '13

Please, I'm trying to do some user user input validation, but it looks like my approach is not correct. Here is my code: textInput :: IO () textInput | dataIn == "bye" = return () | otherwise = textInput where dataIn <- getLine the idea is that when user inputs "bye" the program should end/return. Problem is that I'm getting 'parse error on input `<-'' error. How can I achieve it? Another question I have is: how can I have more actions in guard, when the condition is valid. Something like (write on IO + return if dataIn == "bye): textInput :: IO () textInput *| dataIn == "bye" = (puStrLn "quitting", return () ) -- * | otherwise = textInput where dataIn <- getLine Millions of thanks for any kind of input.. Cheers, Miro

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Re: [Haskell-beginners] Q 2 of 2: GUI and turnkey compiler?
by Hans Georg Schaathun 01 Jun '13

01 Jun '13

On Fri, May 31, 2013 at 11:37:47AM -0700, Tim Perry wrote: > GHC is a compiler. > (...) > That said, it is *nice* when compilers will statically link your executable. Isn't there a linker for that part of the job? :-) -- :-- Hans Georg

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Sankey Diagram with monads
by Adrian May 01 Jun '13

01 Jun '13

Hi all, Take a look at this disaster area, or just scroll down to where I come to the point... ======================= type SankeyBrain = (P2,CircleFrac,Double) -- like a turtle plus width data SankeyWorld tb = SankeyWorld ((Trail R2,Trail R2),tb) --outgoing and returning trails, plus brain emptySankey :: SankeyWorld SankeyBrain emptySankey = SankeyWorld ((mempty,mempty),(origin,0,0)) sankeyFrom:: CircleFrac -> Double -> SankeyWorld SankeyBrain sankeyFrom a w = SankeyWorld ((mempty,mempty),(p2 (0,0),a,w)) -- kick off with an angle and width instance Monad SankeyWorld where return a = SankeyWorld ((mempty,mempty), a) --never use this (SankeyWorld l) >>= f = let (SankeyWorld r) = f (snd l) in -- out = left then right, return = right then left SankeyWorld ( (((fst.fst) l <> (fst.fst) r),((snd.fst) r <> (snd.fst) l)),(snd r) ) sankeyVia :: Double -> SankeyBrain -> SankeyWorld SankeyBrain sankeyVia d (p,a,w) = let -- draw parallel lines and move them into place l1 = hrule 1 # scaleX d # translateX (d/2) # translateY (w/2) # rotate a # translate (origin .-. p) l2 = hrule 1 # scaleX (-d) # translateX (d/2) # translateY (-w/2) # rotate a # translate (origin .-. p) in SankeyWorld ( ( l1 , l2 ) , ( p .+^ (unitX # scale d # rotate a), a, w) ) sankeyTo :: SankeyBrain -> SankeyWorld SankeyBrain sankeyTo (p,a,w) = SankeyWorld ( --arrow at the end of the flow ( hrule w # translateX (w/2) # translateY (w/2) # rotate (-1/8::CircleFrac) # scale (0.7071) # rotate a # translate (origin .-. p) , hrule (-w) # translateX (w/2) # translateY (-w/2) # rotate (1/8::CircleFrac) # scale (0.7071) # rotate a # translate (origin .-. p) ), (p,a,w)) sankeyTurn r a' (p,a,w) = let (outr, inr, qu) = if a'>=0 then (r, -w-r, -0.25::CircleFrac) else (-w-r, r, 0.25::CircleFrac) in SankeyWorld ( -- turn a corner with nice round edges ( arc' outr (a+qu) (a+a'+qu) # translate (unitY # rotate (a+a' )# scale w) , arc' inr (a+a'+qu) (a+qu) # translate (unitY # rotate (a+a' )# scale w) ),(p,a+a',w)) -- bump... sankeySplit :: [(Double, SankeyBrain -> SankeyWorld SankeyBrain)] -> SankeyBrain -> SankeyWorld SankeyBrain sankeySplit fs (p,a,w) = let (placed,_) = ( foldl ( \(l,t) -> \(i,c) -> ( l++[( ( p .+^ (unitY # rotate a # scale (((t+i/2)-0.5)*w)), a, w*i) ,c )],t+i) ) ([],0) fs ) in foldl (\(SankeyWorld ((lo,lr),lb)) -> \(SankeyWorld ((ro,rr),rb)) -> SankeyWorld ( ( lo <> ro , rr <> lr ), rb ) ) emptySankey $ map (\(b,f)-> f b) placed SankeyWorld ((turtb,turta),_) = {- This is the bit that fails: sankeyFrom 0 5 >>= sankeyVia 5 >>= sankeySplit [ (0.3, sankeyVia 10 ) , (0.7, sankeyVia 15 ) ] -} sankeyFrom 0 5 >>= sankeyVia 5 >>= sankeyTurn 1 (-0.125) >>= sankeyVia 10 >>= sankeyTurn 1 (0.25) >>= sankeyTo -- >>= turn 0.25 >>= forward 10 >>= turn 0.25 >>= forward 20 >>= turn 0.25 >>= forward 10 pic3 = (strokeT (close ( turtb<>turta) )) # fc red ====================== The idea is that SankeyWorld is a monad containing two trails (outbound and inbound) and a turtle-like state. I bind it onto functions like SankeyBrain -> SankeyWorld, whereby >>= passes the state across. >>= draws the left hand outward trail, then the right hand outward trail, then the right hand inward trail, then the left hand inward trail, so it all makes a nice polygon and I can colour it in. sankeyFrom angle width is already a monad, sankeyVia length is such a function and I could have sankeyTo contain () in place of the brain (i.e. state) cos you're not supposed to continue from it. The tricky bit is splitting the flow. I want a function that takes the brain, splits the width according to named shares and shoves each share into a function SankeyBrain -> SankeyWorld that might have lots more stages and splits downwind. It was all going fine until I discovered that if I can say m >>= f, then I can't say f >>= f. So I don't know how to write the bits after the split. Silly me. But what should I do instead to model Sankey diagrams splitting? Is MonadPlus the trick? If so, am I gonna have to make [SankeyWorld] a monad as well? TIA, Adrian. PS: I rarely have any use for the polymorphism of the parameter to Monad. In this case, it's a SankeyBrain, end of story. Is there a simpler kind of monad that doesn't throw this complication at me?

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Q 2 of 2: GUI and turnkey compiler?
by Gan Uesli Starling 01 Jun '13

01 Jun '13

Perl and Python both have GUI interfaces, which are handy for publishing little give-away apps, as I used to do quite a lot in Perl/Tk and also in ancient of days on JForth. Also to make give-away apps, there needs to be a turn-key compiler, something into which I'd feed code and out would come an *.exe (albeit quite bloated in size) that ran on any PC which did not have Haskell already installed. So, are there such things also for Haskell? A GUI and a means to issue stand-alone apps?

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Q 1 of 2: How stable is Haskell?
by Gan Uesli Starling 31 May '13

31 May '13

As compared against Python and Perl, how stable is Haskell? That is to say, between Python's 3 and 2 being so different, you pretty much need to know both. And with Perl 6 being "on the way" for so very long, and it too being a turnabout in many ways from Perl 5, it is discouraging to be keeping on with Perl 5 the way I've been doing. All that said, I know that they model the forthcoming Perl 6 in Haskell, which is how I heard of it. And ever since then, I've been thinking if I were to take up a new language, Haskell had maybe ought be the one.

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