- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Class Quantification
by Sean Leather 30 Sep '08

30 Sep '08

> Now I would like to generalise 'foo' and 'bar' to 'bla' so that I can > write: > > testBla1 = bla fromInteger 1 :: (Int, Float) > testBla2 = bla read "1" :: (Int, Float) > > My question is how to define 'bla'. > > I can write: > > > bla :: (forall b. a -> b) -> a -> (c, d) > > bla f x = (f x, f x) > > But then 'testBla1' gives the following expected error: > > Could not deduce (Num b) from the context () > arising from a use of `fromInteger' > > [...] > > So, somehow I need to quantify over the type class. > > bla :: forall cls. (cls c, cls d) => (forall b. cls b => a -> b) -> a -> > (c, d) > > But this isn't legal. > > Is there another way of defining 'bla'? > The simple, from-the-gut answer involves the enumeration of all the classes that you need in the context. > bla :: (Read c, Read d, Num c, Num d) => (forall b. (Read b, Num b) => a -> b) -> a -> (c, d) > bla f x = (f x, f x) But perhaps you're looking for potentially unknown classes? Sean

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Distributing Haskell binaries as OSX App Bundles
by Stephen 30 Sep '08

30 Sep '08

I wrote a command-line program recently for a friend in haskell. However, he's far away and not particularly computer literate. I sent him the raw binaries, but they came up with errors about not being able to find libgmp stuff. So then I thought I should probably be able to somehow distribute all the stuff together in a .app bundle. However, the only experience I have of doing this sort of stuff before is via xcode. What's the best way to go about this? It just occurred to me now that maybe cmake is the way to go? Any advice that might save me some time would be really appreciated. Cheers, Stephen

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Haskell versus F#, OCaml, et. al. ...
by Galchin, Vasili 30 Sep '08

30 Sep '08

Hello, Frank mode on ... ;^) In terms of functionality, where is Haskell superior vs inferior to ML, Caml, OCaml, F#, Erlang, etc.? E.g. in terms of library functionality? Regards, Vasili

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GHC, names of inferred type variables
by Shiqi Cao 30 Sep '08

30 Sep '08

Hi, Error messages from GHC contain inferred type variables, is there anyway to find out which term an inferred type variable is for(if the term exists)? Thanks, Shiqi

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Class Quantification
by Bas van Dijk 30 Sep '08

30 Sep '08

Hello, I was writing some Haskell when I stumbled on the following problem: With the following language extension... > {-# LANGUAGE RankNTypes #-} ... it's possible to define 'foo' and 'bar' like so: > foo :: (Num c, Num d) => (forall b. Num b => a -> b) -> a -> (c, d) > foo f x = (f x, f x) > bar :: (Read c, Read d) => (forall b. Read b => a -> b) -> a -> (c, d) > bar f x = (f x, f x) Which allows us to write: > testFoo = foo fromInteger 1 :: (Int, Float) > testBar = bar read "1" :: (Int, Float) Now I would like to generalise 'foo' and 'bar' to 'bla' so that I can write: testBla1 = bla fromInteger 1 :: (Int, Float) testBla2 = bla read "1" :: (Int, Float) My question is how to define 'bla'. I can write: > bla :: (forall b. a -> b) -> a -> (c, d) > bla f x = (f x, f x) But then 'testBla1' gives the following expected error: Could not deduce (Num b) from the context () arising from a use of `fromInteger' Possible fix: add (Num b) to the context of the polymorphic type `forall b. a -> b' In the first argument of `bla', namely `fromInteger' In the expression: bla fromInteger 1 :: (Int, Float) In the definition of `testBla1': testBla1 = bla fromInteger 1 :: (Int, Float) And 'testBla2' gives a similar error complaining that it can't deduce (Read b) from the context. So, somehow I need to quantify over the type class. bla :: forall cls. (cls c, cls d) => (forall b. cls b => a -> b) -> a -> (c, d) But this isn't legal. Is there another way of defining 'bla'? Thanks, Bas

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Re: [Haskell-cafe] not able to compile Happy GLR examples
by brianalexander1＠cableone.net 30 Sep '08

30 Sep '08

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Red-Blue Stack
by Matthew Eastman 29 Sep '08

29 Sep '08

Hey guys, This is probably more of a question about functional programming than it is about Haskell, but hopefully you can help me out. I'm new to thinking about things in a functional way so I'm not sure what the best way to do some things are. I'm in a data structures course right now, and the assignments for the course are done in Java. I figured it'd be fun to try and implement them in Haskell as well. The part of the assignment I'm working on is to implement a RedBlueStack, a stack where you can push items in as either Red or Blue. You can pop Red and Blue items individually, but the stack has to keep track of the overall order of items. i.e. popping Blue in [Red, Red, Blue, Red, Blue] would give [Red, Red, Blue] All push and pop operations on the stack have to be done in O(1) time. It was easy to do in Java since you can just keep track of everything with a few pointers, but it took a while to get the Haskell version working. Maybe I'm just not used to the functional way of doing things. I originally had this: data RBSItem a = Red a | Blue a data RedBlueStack a = RBS { red :: [RBSItem a], blue :: [RBSItem a], overall :: [RBSItem a] } But there was no way to keep popping in O(1) time because I would have to walk through the overall list when removing items. I eventually came up with: data ShowColour = PlusRed | MinusRed | PlusBlue | MinusBlue data RedBlueStack a = RBS { red :: [a], blue :: [a], order :: [ShowColour] } popRed :: RedBlueStack a -> RedBlueStack a popRed (RBS (r:rs) b o) = RBS rs b (MinusRed : o) popRed rbs@(RBS [] _ _) = rbs -- As an aside here, would it be better to put "popRed = id" for the catch-all in popRed instead of what I have? -- I don't know proper Haskell coding style yet. remUseless :: [ShowColour] -> [ShowColour] remUseless order@(x:xs) | x == MinusRed = remShowColour PlusRed xs | x == MinusBlue = remShowColour PlusBlue xs | otherwise = order where remShowColour r (c:cs) | c == r = cs | otherwise = c : remShowColour r cs remShowColour _ [] = error "Incorrect Stack Order" So instead of walking through the overall list, I just have to add a MinusRed or MinusBlue to the order list. This makes the pop and push functions operate in O(1) time, but it seems a bit excessive, because whenever the overall order of the stack is needed (e.g. printing the stack) I need to clean up the order list. I was just wondering whether this is the best way to implement something like this, keeping track of changes made instead of making the changes. If anyone has any ideas for other ways of implementing this I'd love to see them. I didn't take into account that Haskell is lazy. Will that have any effect on the running time? Probably not for a simple program like this, but for larger ones and more complex data structures and algorithms, I'm guessing it would? Thanks, Matt

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'par' - why has it the type a -> b -> b ?
by Henning Thielemann 29 Sep '08

29 Sep '08

What is the reason for implementing parallelism with 'par :: a -> b -> b'? Analogy to 'seq'? I thought parallelism would be introduced most naturally by a function which does two computations in parallel and puts together their results after completion. Say > par2 :: (a -> b -> c) -> (a -> b -> c) to be used like > par2 (+) expensiveComputationA expensiveComputationB I assume that par2 can be implemented this way: > par2 f x y = > f x (par x y) ?

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announcing darcs 2.1.0pre2
by Eric Kow 29 Sep '08

29 Sep '08

Hi everybody, The second pre-release of darcs 2.1 (formerly known as 2.0.3) is now available at http://darcs.net/darcs-2.1.0pre2.tar.gz We have increased the version number since the last pre-release because we are adopting an important new behaviour: The darcs initialize command now creates darcs-2 format repositories by default This will help new repositories to benefit from the improved handling of duplicate patches and to avoid the exponential time conflicts problem. Upcoming release schedule ------------------------- 2008-10-03: darcs 2.1.0pre3 2008-10-10: darcs 2.1.0 Work remaining -------------- * http://bugs.darcs.net/issue971 (Petr Ročkai) When using hashed or repositories, allow darcs check to deal with filenames that differ only in case when run on a case-insensitive filesystem. Petr has done 90% of the work for this -- he has implemented this for darcs repair -- so this shall largely be a question of refactoring. * http://bugs.darcs.net/issue1003 Allow darcs-transfer mode to deal with a specific set of missing files (namely _darcs/format) so that users can benefit from ssh connection sharing even if the remote end uses an old-fashioned repository. In the meantime, a simple workaround is to touch the _darcs/format file in the remote repoistory * http://bugs.darcs.net/issue1078 Fix a corner case in paths handling that makes darcs overly cautious if the repository is a symbolic link * http://bugs.darcs.net/issue1026 Improve the error messages for 'bug in get_extra' errors, which can happen if darcs is fooled into thinking that two patches are the same. Changes since the last pre-release ---------------------------------- * User Experience: Do not allow users to add files to a darcs repository if their filenames would be considered invalid under Windows. This can be overridden with the --reserved-ok flag (issue53, Eric Kow) * Bug Fix: Do not leave behind a half-gotten directory if darcs get fails (issue1041, Vlad Dogaru, David Roundy) * User Experience: notice when you are trying to pull from a seemingly unrelated repository, that is one with a sufficiently different history. This can be overridden with the --allow-unrelated-repos flag (Dmitry Kurochkin, David Roundy) * Bug Fix: Fix hang after a user input error (for example, EOF) (Judah Jacobson) * Quality Assurance: Improvements to documentation and online help (Simon Michael) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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Re: Haddock + Hoogle == Javadoc on steroids
by Simon Michael 29 Sep '08

29 Sep '08

Taking this to haskell-cafe.. http://joyful.com/repos/darcs-sm/api-doc is a mashup of haddock, hoogle and hscolour (and darcsweb, darcs-graph - see http://joyful.com/repos) It's rough but quite useful - a few minutes here gave me a much better understanding of the big picture of darcs code. By alternating shift & enter in the contents pane I could browse quickly through all modules. Improvement: one could do a lot of useful magic with javascript. But it would be more powerful to improve the tools, eg I'd like if haddock had frames/no-frames built in and hoogle could be made to work in either case. I haven't had time to work on this, currently I hard-code the target in hoogle and munge the haddock output slightly (see recent patch in darcs-unstable). As you say it would be great to keep improving this area and baking it into our tools and infrastructure. Highly accessible and efficient docs and code browsing tools help a lot! On Sep 27, 2008, at 1:49 PM, Jason Dagit wrote: > Simon, > > I'm wondering if you could find a way to make it "trivial" for people using cabal to combine haddock and hoogle the way you have for darcs? > > Some ideas: > 1) Depend entirely on cabal for the auto setup of things > 2) Provide the framed interface on your webpage also has a layout in emacs (so people can use either web or emacs) > 3) Package it so that people just 'cabal install hoodock && hoddock ./src' and then they are done for 90% of cases > 4) Provide demos for darcs and something else large like GHC > > What do you think? If you need help with it, I'm sure there are tons of people on Haskell-Cafe that would really dig this and help you with it. > > Thanks, I love it! > Jason Thanks! -Simon

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