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Overload function application? (was: foldr (.) id)
by Greg Fitzgerald 27 Oct '12

27 Oct '12

Should library functions target Control.Category? We write for Num, IsString, and ListLike. Haskell overloads number literals and string literals, and awkwardly not List literals. Should Haskell overload function application too? How cool would it be to be able to leverage all of Haskell's libraries, and effectively none of the runtime, by implementing instances for literals and function application? Say, for example, to generate a call graph, or make the language strict, or target LLVM more directly. Anyone tried this? Thanks, Greg On Sat, Oct 27, 2012 at 11:39 AM, Greg Fitzgerald <garious(a)gmail.com> wrote: > > Alternatively: flip (foldr id) > > Very cool, but... > > Prelude> import qualified Data.Foldable as F > Prelude F> :t F.foldr id > F.foldr id :: F.Foldable t => b -> t (b -> b) -> b > > > {- Generalizing -} > > Prelude F> import qualified Control.Category as C > Prelude F C> :t F.foldr (C..) C.id > F.foldr (C..) C.id :: (F.Foldable t, C.Category cat) => t (cat b b) -> cat > b b > > > {- Sneaky type-specialization -} > > Prelude F C> :t F.foldr C.id > F.foldr C.id :: F.Foldable t => b -> t (b -> b) -> b > > > On Sat, Oct 27, 2012 at 3:09 AM, Ross Paterson <ross(a)soi.city.ac.uk>wrote: > >> On Fri, Oct 26, 2012 at 07:41:18PM +0100, Greg Fitzgerald wrote: >> > I've recently found myself using the expression: "foldr (.) id" to >> > compose a list (or Foldable) of functions. It's especially useful >> > when I need to map a function over the list before composing. Does >> > this function, or the more general "foldr fmap id", defined in a >> > library anywhere? I googled and hoogled, but no luck so far. >> >> Alternatively: flip (foldr id) >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Sparse records/ADTs
by Jon Fairbairn 27 Oct '12

27 Oct '12

Is there a convenient way of handling a data structure with lots of fields of different types that may or may not be filled in? Something equivalent to data D = D {a::Maybe A, b::Maybe B, c::Maybe C, …} but with better space efficiency and a more convenient empty object. An easy alternative is data E = Ea A | Eb B | Ec C | … type R = [E] which has a straightforward empty object, but one then must define getA e = listToMaybe [a | Ea a <- e] for each field, which is tedious (and O(n)). Obviously Templates would help, but is there an alternative I’ve missed? -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk

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Type-directed functions with data kinds
by Paul Visschers 27 Oct '12

27 Oct '12

Hello everyone, I've been playing around with the data kinds extension to implement vectors that have a known length at compile time. Some simple code to illustrate: > {-# LANGUAGE DataKinds, GADTs, KindSignatures #-} > > import Prelude hiding (repeat) > > data Nat = Zero | Succ Nat > data Vector (n :: Nat) a where > Nil :: Vector Zero a > Cons :: a -> Vector n a -> Vector (Succ n) a > > class VectorRepeat (n :: Nat) where > repeat :: a -> Vector n a > > instance VectorRepeat Zero where > repeat _ = Nil > > instance VectorRepeat n => VectorRepeat (Succ n) where > repeat x = Cons x (repeat x) In this code I have defined a repeat function that works in a similar way to the one in the prelude, except that the length of the resulting vector is determined by the type of the result. I would have hoped that its type would become 'repeat :: a -> Vector n a', yet it is 'repeat :: VectorRepeat n => a -> Vector n a'. As far as I can tell, this class constraint should no longer be necessary, as all possible values for 'n' are an instance of this class. I actually really just want to define a closed type-directed function and would rather not (ab)use the type class system at all. Is there a way to write the repeat function so that it has the type 'repeat :: a -> Vector n a' that I've missed? If not, is this just because it isn't implemented or are there conceptual caveats? Paul Visschers

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Enemy of the Mutable State
by Mark Lentczner 27 Oct '12

27 Oct '12

A Haskell poster I made that you all might enjoy: http://www.ozonehouse.com/mark/enemy/ - mzero

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ANNOUNCE: pretty-tree
by Ivan Lazar Miljenovic 26 Oct '12

26 Oct '12

For a project I'm working on, I got sick of writing out trees by hand. Data.Tree has a drawTree function, but whilst it's better than nothing, I prefer a more top-down "traditional" approach to drawing trees. So I wrote a package to do just that: http://hackage.haskell.org/package/pretty-tree (I didn't think to check whether diagrams had capabilities to do so until after I finished, though in this case I prefer a textual approach anyway.) I'm more than happy to consider adding other forms of drawing trees (though off-hand I can't think of any), or of course you can send me patches since I'm taking advantage of the nice new shiny hub.darcs.net (thanks Darcs team!). I was going to put an example here but it only really comes out nicely if you're using a monospaced font, so have a look at the Haddock docs for the module once Hackage builds it (in hindsight, I should probably have come up with better labels before releasing the package... oh well :) -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Munich Haskell Meeting
by Heinrich Hördegen 26 Oct '12

26 Oct '12

Dear all, next Monday, 29th of October, our monthly Haskell Meeting will take place at 19h30 at Cafe Puck. Everybody interested in functional programming is wellcome. Please go to http://www.haskell-munich.de/dates for more information and push the button, if you intend to join. With hope to see you on Monday, Heinrich

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Problems translating Conor McBride's talk into Haskell + DataKind + KindPoly
by Ahn, Ki Yung 25 Oct '12

25 Oct '12

Most part of Conor's talk at ICFP, until just before the last stage where he heavily uses true value dependency for compiler correctness all the code seemed to be able to translate into Haskell with the new hot DataKinds and PolyKinds extension. I tried it in GHC 7.4.1 and it was possible to do it, but I got stuck and I had to make the generic list structure mono-kinded with kind signatures rather not use the PolyKinds extension. I wonder if this is just a but of GHC 7.4.1's implementation of PolyKinds, or a limitation of the DataKind design. I attached a literate Haskell script with this message that illustrates this problem. Thanks in advance for any comments including whether it runs in later versions or still has problems, and discussions about the DataKinds and PolyKinds extension. Ki Yung

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pragma to request tail recursion optimization (and perhaps [co]inductive recursion)
by Petr P 25 Oct '12

25 Oct '12

Hi, Haskell compilers optimize tail recursive functions as cycles, which improves both memory and CPU complexity. However, it's easy to make a mistake and break the conditions under which a function can be tail recursive and thus optimized. Is there a way to tell a Haskell compiler that a function must be tail recursive and if it cannot be done, fail the compilation? Like Scala's @tailrec? I'd say no, and believe it would be quite handy. I suppose this could be realized as a pragma. There could be also additional pragmas to ensure that a recursive function is inductive (the recursive call is on a strict sub-structure of one of the arguments) or co-inductive (the recursive call is guarded by a constructor). Best regards, Petr Pudlak

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Online haskell course
by graham＠fatlazycat.com 25 Oct '12

25 Oct '12

Anyone know of a similar course to the coursera scala one for haskell ? https://www.coursera.org/course/progfun Quite interesting online course that has graded assignments. Anything similar or anyone considering doing something similar for haskell ? Thanks Graham

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createProcess fails to find executable in Windows
by José Pedro Magalhães 25 Oct '12

25 Oct '12

Hi all, Consider the following program: module Test where > > import System.Process (readProcess) > > main :: IO () > main = readProcess "git" ["describe", "--tags"] "" >>= putStr > In Windows I get the following behaviour: > git --version > git version 1.7.10.msysgit.1 > > > ghc --version > The Glorious Glasgow Haskell Compilation System, version 7.6.1 > > > runghc Test > Test: git: createProcess: does not exist (No such file or directory) > The same happens with GHC 7.4.2. In Linux, however, it works as expected: $ git --version > git version 1.7.9.5 > $ ghc --version > The Glorious Glasgow Haskell Compilation System, version 7.4.1 > $ runghc Test.hs > Package-2.0-68-gacaf77a > Can anyone reproduce this result in Windows? Is this a bug or am I doing something wrong? Thanks, Pedro

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