May 2013 - Beginners - Haskell.org

14 Aug '13

Every OO language which supports generics allows a declaration such as List<Show> alist, where Show is an interface. Any type implementing Show can be put in alist, and any Show operation can be performed on the alist's members. No casts, wrappers, or other special types and plumbing are needed. Why isn't it possible to do this directly in Haskell?

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can this be made shorter?
by Emmanuel Touzery 21 Jun '13

21 Jun '13

hi, i want to know if a string contains only "." and "/" characters. i do something like that: null . dropWhile (\c -> c == '.' || c == '/') now it's a shame, because if I wanted only to check for "." then I would have: null . dropWhile (=='.') afaik in scala you can say: null . dropWhile (_ == '.' || _ == '/') which is a bit more compact than the haskell... I was thinking to use "and" but I'm not sure it would end up being readable... Any idea? Or I am trying too hard to make it compact? Emmanuel

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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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Re: [Haskell-beginners] Q 2 of 2: GUI and turnkey compiler?
by Hans Georg Schaathun 31 May '13

31 May '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 31 May '13

31 May '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 31 May '13

31 May '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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Why Functional Programming Matters <-- this will make you smarter (Part 2)
by Costello, Roger L. 31 May '13

31 May '13

Hi Folks, I completed the second half of my summary of John Hughes' outstanding paper titled, Why Functional Programming Matters. This second half describes the importance of lazy evaluation and function composition. I converted all of Hughes' examples into Haskell. It contains a lengthy example showing how to implement the game of tic-tac-toe, and how lazy evaluation enables the implementation of tic-tac-toe to have a level of modularity that would otherwise not be attainable. (Modularity is important. Hughes writes, "Modularity is the key to successful programming") Here is my summary: http://www.xfront.com/Haskell/Why-Functional-Programming-Matters-Part-2.pdf /Roger

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Re: [Haskell-beginners] Q 1 of 2: How stable is Haskell?
by Hans Georg Schaathun 31 May '13

31 May '13

> On 30/05/13 23:50, Gan Uesli Starling wrote: > > As compared against Python and Perl, how stable is Haskell? I must say, I find the comparison odd. The transition from imperative to declarative programming must be much more mind-bogging than figuring out how to deal with instability. Surely, there are both more and less stable options within each paradigm. On Fri, May 31, 2013 at 12:23:00AM +0100, Mateusz Kowalczyk wrote: > I think you're actually asking ``how much does Haskell change between > revisions'' and not how stable it is. If you're looking for a language > where your program will work with a compiler that's 5 years old or 5 > years from now, this probably isn't it. It's not like programs break > daily because of the changes but I believe there is no hesitation to > change stuff because some program from 5 years ago might no longer > compile. It may be useful to distinguish between stability of the language and stability of the libraries. Talking about libraries, you are probably right. In fact, I have tried to install a number of libraries from hackage recently, and only about half of them succeeded. The typical problem with the others seem to be changes in their dependencies. Typical age is about 3-6 years, I think. Python however, is not even stable in terms of core language. Although library evolution can also break your program, such breakage is generally easier to repair. The problem for haskell, of course, is that the community is too small to sustain a standard library of the extent seen for python or java. Hence one relies much more on beta versions and stale libraries. That does not make haskell any less stable; it is a very different kind of problem. -- :-- Hans Georg

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Re: [Haskell-beginners] Q 2 of 2: GUI and turnkey compiler?
by Hans Georg Schaathun 31 May '13

31 May '13

On Thu, May 30, 2013 at 06:54:54PM -0400, Gan Uesli Starling wrote: > 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. Do you mean a GUI library? Or a GUI interface to the interpreter? There are plenty of GUI libraries. _Real World Haskell_ from O'Reilly presents one approach, and is otherwise a good read at http://book.realworldhaskell.org > Also to make > give-away apps, there needs to be a turn-key compiler, The glasgow haskell compiler (ghc) is a compiler. > something > into which I'd feed code and out would come an *.exe (albeit quite > bloated in size) well, you can always rename the file to *.exe or whatever else you want. I cannot see how that's relevant. -- :-- Hans Georg

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