- Haskell-Cafe - Haskell.org

Computing sums
by Andrew Coppin 20 Feb '10

20 Feb '10

The other day, I found myself writing the following code: data Property x y = forall s. Property s (x -> s -> s) (s -> y) step :: x -> Property x y -> Property x y step x (Property s f g) = Property (f x s) f g read :: Property x y -> y read (Property s _ g) = g s pure :: (x -> y) -> Property x y pure f = Property undefined const f fold :: (x -> y -> y) -> y -> Property x y fold f y = Property y f id fold1 :: (x -> x -> x) -> Property x x fold1 f = Property Nothing (\x mx -> maybe (Just x) (Just . f) mx) fromJust (>==>) :: Property x y -> Property y z -> Property x z p0 >==> p1 = Property (p0,p1) (\x (q0,q1) -> let w0 = step x q0 in (w0, step (read w0) q1)) (\(_,q1) -> (read q1)) (>==<) :: Property x y -> Property x y' -> Property x (y,y') p0 >==< p1 = Property (p0,p1) (\x (q0,q1) -> (step x q0, step x q1)) (\(q0,q1) -> (read q0, read q1)) sum = fold (+) 0 count = pure (const 1) >==> sum minimum = fold1 min maximum = fold1 max Have I just invented arrows? (And if so, why is the above code nowhere to be seen in the libraries?)

4 6

What happened in Ohloh?
by Paul Johnson 20 Feb '10

20 Feb '10

If you go to http://www.ohloh.net/languages/compare?l0=haskell&measure=projects and look at the number (not percentage) of Haskell projects you see it rise exponentially until the start of 2008 and then suddenly drop away. Does anyone know what happened? Assuming this is just an artefact because they aren't scanning Haskell project hosts, can we get them to fix it? I'd like to use this kind of graph at work as evidence that Haskell is on a growth trajectory. Paul.

3 4

Haskell - Heterogeneous Data Structures - Nested Pairs and functional references
by Günther Schmidt 20 Feb '10

20 Feb '10

Hallo Heinrich, entschuldige bitte wenn ich Dich ausserhalb der Cafe-Liste kontaktiere, aber so langsam wird's mir peinlich.... Auf meinen Post "Heterogeneous Data Structures - Nested Pairs and functional references" vom 16.02.2010 in der Cafe-Liste habe ich eine interessante Antwort erhalten von Alexander Solla, die ich nicht verstanden haben. Als ich nachgehakt habe, hat er freundlicherweise sofort eine Antwort geliefert die ich noch weniger verstanden habe ... Jetzt trau' ich mich nicht mehr nochmals zu fragen, denn ich glaube ich würd's wieder nicht verstehen. Kannst Du mir vielleicht erklären was er mir da eigentlich geschickt hat? Grüße Günther

1 0

fast, strongly typed heterogeneous collections
by Alberto G. Corona 19 Feb '10

19 Feb '10

Hi What is the speed of hProjectByLabel/s in Data.HList.Record? . The documentation in hackage <http://hackage.haskell.org/packages/archive/HList/latest/doc/html/Data-HLis…>does not mention it. Perhaps this is because Hlist is designed for supposedly short records and this does not matter too much. looking at the code I guess that everything in HList is O(n) because it uses type-level list structure, but I think that nothing prevents from using a type level balanced tree, Data.Map style, instead. This would permit to implement large heterogeneous Maps with fast access and strongly typed. AFAIK, the alternative is to use a fast data container with data elements encapsulated within Data.Dynamic, that is weakly typed, produces type errors at runtime, which is unsatisfactory and dangerous. Any comments?

4 4

Cabal broken on Mac OS X?
by Oleg Lobachev 19 Feb '10

19 Feb '10

Hello all, I suspect that it's me and not the cabal, but everything looks fine and still fails. Would someone mind to tell me, what am I doing wrong? $ cabal -V cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library $ ghc -V The Glorious Glasgow Haskell Compilation System, version 6.12.1 $ which ghc /usr/bin/ghc $ which cabal /usr/local/bin/cabal $ cabal install cabal-install cabal: failed to parse output of 'ghc-pkg dump' That's the problem! As it is Mac OS X 10.6.2, here is some information on the architecture and the executables used: $ uname -v Darwin Kernel Version 10.2.0: Tue Nov 3 10:37:10 PST 2009; root:xnu-1486.2.11~1/RELEASE_I386 $ file /Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.1/ghc /Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.1/ghc: Mach-O executable i386 $ file `which cabal` /usr/local/bin/cabal: Mach-O executable i386 $ file `which ls` /bin/ls: Mach-O universal binary with 2 architectures /bin/ls (for architecture x86_64): Mach-O 64-bit executable x86_64 /bin/ls (for architecture i386): Mach-O executable i386 $ ghc --make hello.hs [1 of 1] Compiling Main ( hello.hs, hello.o ) Linking hello ... $ ./hello Hello world! $ file ./hello ./hello: Mach-O executable i386 The output of ghc-pkg dump is here: http://www.mathematik.uni-marburg.de/~lobachev/tmp/ghc-pkg.txt I have installed the Haskell Platform and then the new GHC via official pkg files. Greetings, Oleg

2 2

Why is Haskell used so little in the industry?
by Gabi 19 Feb '10

19 Feb '10

I posted a question about it in StackOverflow (http://stackoverflow.com/questions/2284875/why-is-haskell-used-so-little-in…) Haskell gets some beating there. Please Hasellers, join the discussion and give your opinion there. -- Regards, Gabi http://bugspy.net

1 0

ANNOUNCE: vty-ui 0.3
by Jonathan Daugherty 19 Feb '10

19 Feb '10

I'm happy to announce the release of vty-ui 0.3. Get it from Hackage: http://hackage.haskell.org/package/vty-ui Or get the source with darcs: http://repos.codevine.org/vty-ui Project homepage: http://codevine.org/vty-ui/ This version of vty-ui features a richer rendering engine, generalized text transformations, and a more functional style. If you've written any applications to use vty-ui, your type signatures and usage of a few of the built-in widget types will need to change but the API is largely the same. In particular, the largest change was a refactoring inspired by Luke Palmer's blog post, "Haskell Antipattern: Existential Typeclass," which targeted vty-ui (among other libraries) in its treatment of the topic: http://lukepalmer.wordpress.com/2010/01/24/haskell-antipattern-existential-… Here's a summary of changes: General * Replace Widget type class with a concrete Widget type * Move text widget into Graphics.Vty.Widgets.Text * Add support for generalized text "formatters" * Replace WrappedText widget type with a text-wrapping formatter * Provide a regex-based text-highlighting formatter (see the demo for an example) * Refactor the rendering engine to provide "address" information so you can get coordinates and size for rendered widgets * Add a few QuickCheck tests (and related fixes) Enjoy! -- Jonathan Daugherty

1 0

Implementing "unionAll"
by Leon Smith 19 Feb '10

19 Feb '10

With the urging and assistance of Omar Antolín Camarena, I will be adding two functions to data-ordlist: mergeAll and unionAll, which merge (or union) a potentially infinite list of potentially infinite ordered lists, under the assumption that the heads of the non-empty lists appear in a non-decreasing sequence. Union takes two sorted lists and produces a new sorted list; an element occurs in the result as many times as the maximum number of occurrences in either list. The unionAll function generalizes this behavior to an infinite number of lists. A reasonable implementation of mergeAll is: > import Data.List.Ordered(merge, union) > mergeAll :: Ord a => [[a]] -> [a] > mergeAll = foldr (\(x:xs) ys -> x : merge xs ys) [] However, for many inputs, we can do better; the library implementation of mergeAll is based on H. Apfelmus's article "Implicit Heaps", which presents a simplification of Dave Bayer's "venturi" algorithm. The difference is that the foldr version uses a line of comparisons, whereas "venturi" uses a tree of comparisons. http://apfelmus.nfshost.com/articles/implicit-heaps.html http://www.mail-archive.com/haskell-cafe@haskell.org/msg27612.html However, as Omar pointed out to me, the following implementation of unionAll has a flaw: > unionAll :: Ord a => [[a]] -> [a] > unionAll = foldr (\(x:xs) ys -> x : union xs ys) [] Namely unionAll [[1,2],[1,2]] should return [1,2], whereas it actually returns [1,1,2]. After some work, I believe I have generalized H. Apfelmus's algorithm to handle this; however it seems a bit complicated. I would love feedback, especially with regard to simplifications, bugs, testing strategies, and optimizations: > unionAll' :: Ord a => [[a]] -> [a] > unionAll' = unionAllBy compare > data People a = VIP a (People a) | Crowd [a] > unionAllBy :: (a -> a -> Ordering) -> [[a]] -> [a] > unionAllBy cmp xss = loop [ (VIP x (Crowd xs)) | (x:xs) <- xss ] > where > loop [] = [] > loop ( VIP x xs : VIP y ys : xss ) > = case cmp x y of > LT -> x : loop ( xs : VIP y ys : xss ) > EQ -> loop ( VIP x (union' xs ys) : unionPairs xss ) > GT -> error "Data.List.Ordered.unionAll: assumption violated!" > loop ( VIP x xs : xss ) > = x : loop (xs:xss) > loop [Crowd xs] = xs > loop (xs:xss) = loop (unionPairs (xs:xss)) > > unionPairs [] = [] > unionPairs [x] = [x] > unionPairs (x:y:zs) = union' x y : unionPairs zs > > union' (VIP x xs) (VIP y ys) > = case cmp x y of > LT -> VIP x (union' xs (VIP y ys)) > EQ -> VIP x (union' xs ys) > GT -> error "Data.List.Ordered.unionAll: assumption violated!" > union' (VIP x xs) (Crowd ys) = VIP x (union' xs (Crowd ys)) > union' (Crowd []) ys = ys > union' (Crowd xs) (Crowd ys) = Crowd (unionBy cmp xs ys) > union' xs@(Crowd (x:xt)) ys@(VIP y yt) > = case cmp x y of > LT -> VIP x (union' (Crowd xt) ys) > EQ -> VIP x (union' (Crowd xt) yt) > GT -> VIP y (union' xs yt) -- Leon

6 16

Re: [Haskell-cafe] Category Theory woes
by Michael Matsko 18 Feb '10

18 Feb '10

Nick, Actually, clopen is a set that is both closed and open. Not one that is neither. Except in the case of half-open intervals, I can't remember talking much in topology about sets with a partial boundary. Category theory-wise. No one seems to have mentioned MacLane's "Categories for the Working Mathematician." Although, I don't seem to recall instant enlightenment when I picked it up. Mike ----- Original Message ----- From: "Nick Rudnick" <joerg.rudnick(a)t-online.de> To: "Michael Matsko" <msmatsko(a)comcast.net> Cc: haskell-cafe(a)haskell.org Sent: Thursday, February 18, 2010 4:54:03 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Hi Mike, of course... But in the same spirit, one could introduce a straightforward extension, «partially bordered», which would be as least as good as «clopen»... ;-) I must admit we've come a little off the topic -- how to introduce to category theory. The intent was to present some examples that mathematical terminology culture is not that exemplary as one should expect, but to motivate an open discussion about how one might «rename refactor» category theory (of 2:48 PM). I would be very interested in other people's proposals... :-) Michael Matsko wrote: Nick, That is correct. An open set contains no point on its boundary. A closed set contains its boundary, i.e. for a closed set c, Closure(c) = c. Note that for a general set, which is neither closed or open (say the half closed interval (0,1]), may contain points on its boundary. Every set contains its interior, which is the part of the set without its boundary and is contained in its closure - for a given set x, Interior(x) is a subset of x is a subset of Closure(x). Mike ----- Original Message ----- From: "Nick Rudnick" <joerg.rudnick(a)t-online.de> To: "Michael Matsko" <msmatsko(a)comcast.net> Cc: haskell-cafe(a)haskell.org Sent: Thursday, February 18, 2010 3:15:49 PM GMT -05:00 US/Canada Eastern Subject: Re: Fwd: [Haskell-cafe] Category Theory woes Hi Mike, so an open set does not contain elements constituting a border/boundary of it, does it? But a closed set does, doesn't it? Cheers, Nick Michael Matsko wrote: ----- Forwarded Message ----- From: "Michael Matsko" <msmatsko(a)comcast.net> To: "Nick Rudnick" <joerg.rudnick(a)t-online.de> Sent: Thursday, February 18, 2010 2:16:18 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Gregg, Topologically speaking, the border of an open set is called the boundary of the set. The boundary is defined as the closure of the set minus the set itself. As an example consider the open interval (0,1) on the real line. The closure of the set is [0,1], the closed interval on 0, 1. The boundary would be the points 0 and 1. Mike Matsko ----- Original Message ----- From: "Nick Rudnick" <joerg.rudnick(a)t-online.de> To: "Gregg Reynolds" <dev(a)mobileink.com> Cc: "Haskell Café List" <haskell-cafe(a)haskell.org> Sent: Thursday, February 18, 2010 1:55:31 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Gregg Reynolds wrote: On Thu, Feb 18, 2010 at 7:48 AM, Nick Rudnick < joerg.rudnick(a)t-online.de > wrote: IM(H??)O, a really introductive book on category theory still is to be written -- if category theory is really that fundamental (what I believe, due to its lifting of restrictions usually implicit at 'orthodox maths'), than it should find a reflection in our every day's common sense, shouldn't it? Goldblatt works for me. Accidentially, I have Goldblatt here, although I didn't read it before -- you agree with me it's far away from every day's common sense, even for a hobby coder?? I mean, this is not «Head first categories», is it? ;-)) With «every day's common sense» I did not mean «a mathematician's every day's common sense», but that of, e.g., a housewife or a child... But I have became curious now for Goldblatt... * the definition of open/closed sets in topology with the boundary elements of a closed set to considerable extent regardable as facing to an «outside» (so that reversing these terms could even appear more intuitive, or «bordered» instead of closed and «unbordered» instead of open), Both have a border, just in different places. Which elements form the border of an open set?? As an example, let's play a little: Arrows: Arrows are more fundamental than objects, in fact, categories may be defined with arrows only. Although I like the term arrow (more than 'morphism'), I intuitively would find the term «reference» less contradictive with the actual intention, as this term Arrows don't refer. A *referrer* (object) refers to a *referee* (object) by a *reference* (arrow). Categories: In every day's language, a category is a completely different thing, without the least Not necesssarily (for Kantians, Aristoteleans?) Are you sure...?? See http://en.wikipedia.org/wiki/Categories_(Aristotle) ... If memory serves, MacLane says somewhere that he and Eilenberg picked the term "category" as an explicit play on the same term in philosophy. In general I find mathematical terminology well-chosen and revealing, if one takes the trouble to do a little digging. If you want to know what terminological chaos really looks like try linguistics. ;-) For linguistics, granted... In regard of «a little digging», don't you think terminology work takes a great share, especially at interdisciplinary efforts? Wouldn't it be great to be able to drop, say 20% or even more, of such efforts and be able to progress more fluidly ? -g _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Fwd: [Haskell-cafe] Category Theory woes
by Michael Matsko 18 Feb '10

18 Feb '10

----- Forwarded Message ----- From: "Michael Matsko" <msmatsko(a)comcast.net> To: "Nick Rudnick" <joerg.rudnick(a)t-online.de> Sent: Thursday, February 18, 2010 2:16:18 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Gregg, Topologically speaking, the border of an open set is called the boundary of the set. The boundary is defined as the closure of the set minus the set itself. As an example consider the open interval (0,1) on the real line. The closure of the set is [0,1], the closed interval on 0, 1. The boundary would be the points 0 and 1. Mike Matsko ----- Original Message ----- From: "Nick Rudnick" <joerg.rudnick(a)t-online.de> To: "Gregg Reynolds" <dev(a)mobileink.com> Cc: "Haskell Café List" <haskell-cafe(a)haskell.org> Sent: Thursday, February 18, 2010 1:55:31 PM GMT -05:00 US/Canada Eastern Subject: Re: [Haskell-cafe] Category Theory woes Gregg Reynolds wrote: On Thu, Feb 18, 2010 at 7:48 AM, Nick Rudnick < joerg.rudnick(a)t-online.de > wrote: IM(H??)O, a really introductive book on category theory still is to be written -- if category theory is really that fundamental (what I believe, due to its lifting of restrictions usually implicit at 'orthodox maths'), than it should find a reflection in our every day's common sense, shouldn't it? Goldblatt works for me. Accidentially, I have Goldblatt here, although I didn't read it before -- you agree with me it's far away from every day's common sense, even for a hobby coder?? I mean, this is not «Head first categories», is it? ;-)) With «every day's common sense» I did not mean «a mathematician's every day's common sense», but that of, e.g., a housewife or a child... But I have became curious now for Goldblatt... * the definition of open/closed sets in topology with the boundary elements of a closed set to considerable extent regardable as facing to an «outside» (so that reversing these terms could even appear more intuitive, or «bordered» instead of closed and «unbordered» instead of open), Both have a border, just in different places. Which elements form the border of an open set?? As an example, let's play a little: Arrows: Arrows are more fundamental than objects, in fact, categories may be defined with arrows only. Although I like the term arrow (more than 'morphism'), I intuitively would find the term «reference» less contradictive with the actual intention, as this term Arrows don't refer. A *referrer* (object) refers to a *referee* (object) by a *reference* (arrow). Categories: In every day's language, a category is a completely different thing, without the least Not necesssarily (for Kantians, Aristoteleans?) Are you sure...?? See http://en.wikipedia.org/wiki/Categories_(Aristotle) ... If memory serves, MacLane says somewhere that he and Eilenberg picked the term "category" as an explicit play on the same term in philosophy. In general I find mathematical terminology well-chosen and revealing, if one takes the trouble to do a little digging. If you want to know what terminological chaos really looks like try linguistics. ;-) For linguistics, granted... In regard of «a little digging», don't you think terminology work takes a great share, especially at interdisciplinary efforts? Wouldn't it be great to be able to drop, say 20% or even more, of such efforts and be able to progress more fluidly ? -g _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

2 3