Re: [Haskell-cafe] Using lenses
Lenses for nested sum types e.g. Either. On 03/10/2013 6:08 PM, Simon Peyton-Jones simo...@microsoft.com wrote: (I sent this to ‘libraries’ but Kim-Ee suggested adding Café, where so many smart people hang out.) ** ** Friends ** ** Some of you will know that I’ve promised to give a talk about Edward’s lens library http://hackage.haskell.org/package/lens at the Haskell Exchange http://skillsmatter.com/event/scala/haskell-exchange in London next Wednesday (9th). I did this to give everyone (including me) a break from GHC hackery, and also to force me to learn about this lens voodoo that everyone is twittering about. Edward generously gave me quite a bit of one-to-one attention last week (my hair is still standing on end), but this message is to ask your help too. *Specifically, I’d like to give some compelling use-cases*. If you are using the lens library yourself, could you spare a few minutes to tell me how you are using it? I expect to cover Lens and Traversal but not Prism. ** ** The use-case everyone starts with is nested records, but I’d like to go beyond that. The next levels seem to be: **· **Lenses as views of data that isn’t “really there” e.g. regarding a record with rectangular coordinates as having polar coordinates too. **· **Lenses and Traversals that focus on elements of finite maps (Control.Lens.At) ** ** What else? I’m sure you are using them in all sorts of cool ways that I would never think of, and I’d love to know. ** ** Please don’t tell me anything secret! To give everyone the benefit I may just concatenate all the replies and send to you all, so please say if you don’t want me to do that with yours. ** ** And don’t burn too many cycles on this...I don’t want to waste your time, and I can always get back to you if I can’t understand what you say. Sooner is better than later...Weds is coming. ** ** Simon “Edward’s prophet” PJ ** ** ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Traversals of monomorphic containers
These questions are exactly what Control.Lens answers. On 04/09/2013 12:50 PM, Mario Blažević blama...@acanac.net wrote: On 09/02/13 06:53, Nicolas Trangez wrote: # Redirected to haskell-cafe On Sun, 2013-09-01 at 14:58 +0400, Artyom Kazak wrote: Would this be an appropriate place to propose adding mapM_ (and then possibly mapM) to bytestring library? Was it suggested before? If yes, why was it rejected? This got me wondering: there are several type-classes useful for polymorphic container types, e.g. Functor, Foldable Traversable which all apply to some type of kind (* - *). Are there related things for monomorphic containers, like ByteString, Text or some newtype'd Vector with fixed element type, e.g. class MFunctor f a where mfmap :: (a - a) - f - f instance MFunctor ByteString Word8 where mfmap = ByteString.map I'm not aware of this particular class, but I have considered it. In the end I've chosen to generalize the class to FactorialMonoid instead: class Monoid m = FactorialMonoid m where ... foldMap :: Monoid n = (m → n) → m → n ByteString and Text are instances of the class, and so are lists, maps, and other containers, and Sum and Product as well. http://hackage.haskell.org/**packages/archive/monoid-** subclasses/0.3.2/doc/html/**Data-Monoid-Factorial.htmlhttp://hackage.haskell.org/packages/archive/monoid-subclasses/0.3.2/doc/html/Data-Monoid-Factorial.html or (maybe even better) class MFunctor f where type Elem mfmap :: (Elem - Elem) - f - f instance MFunctor ByteString where type Elem = Word8 mfmap = ByteString.map and similar for other classes. Nicolas __**_ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/**mailman/listinfo/haskell-cafehttp://www.haskell.org/mailman/listinfo/haskell-cafe __**_ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/**mailman/listinfo/haskell-cafehttp://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mapFst and mapSnd
class BinaryFunctor f where bimap :: (a - c) - (b - d) - f a b - f c d mapFst = (`bimap id`) mapSnd = bimap id On 31/05/2013 12:16 PM, Shachaf Ben-Kiki shac...@gmail.com wrote: On Thu, May 30, 2013 at 7:12 PM, Shachaf Ben-Kiki shac...@gmail.com wrote: One generalization of them is to lenses. For example `lens` has both, _1, _2, such that mapPair = over both, mapFst = over _1, etc., but you can also get fst = view _1, set _2 = \y' (x,_) - (x,y'), and so on. (Since both refers to two elements, you end up with view both = \(x,y) - mappend x y.) The types you end up with are simple generalizations of mapFoo, with just an extra Functor or Applicative (think mapMFoo): both :: Applicative f = (a - f b) - (a,a) - f (b,b) both f (x,y) = (,) $ f x * g y _2 :: Functor f = (a - f b) - (e,a) - f (e,b) _2 f (x,y) = (,) x $ f y With an appropriate choice of f you can get many useful functions. I spoke too quickly -- your mapPair is something different. Indeed bimap (or (***), if you prefer base) is the place to find it -- lenses don't really fit here. My both is for mapping one function over both elements. Shachaf ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] LambdaJam 2013
Hey haskell guys, LambdaJam 2013 is a functional programming conference to be held in Brisbane in early May. Call for submissions ends this Friday 08 March. If you are planning to make a submission, please make sure you do soon! If you are stuck somehow, I'd love to be able to help you get to the goal. So feel free to send me an email or I am on IRC (dibblego), lurking around the #haskell channel or privmsg if you like. Personally, I would love see more haskell submissions :) LambdaJam2013 call for papers: http://www.yowconference.com.au/lambdajam/Call.html -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] CoArbitrary
On 09/02/13 15:08, Roman Cheplyaka wrote: I don't think the question was about generating functions... FWIW, both QuickCheck and SmallCheck generate functions. There was also an interesting paper at the last ICFP by Koen related to this. But I think Tony is looking for some kind of a pattern here... Roman * Stephen Tetley stephen.tet...@gmail.com [2013-02-09 10:50:45+] I think GAST - the Clean equivalent of Quickcheck - generates functions. There are certainly quite a few papers by members of the Clean team documenting how they generate them. On 9 February 2013 07:07, Tony Morris tonymor...@gmail.com wrote: [...] I hope I have phrased this in a way to make the point. I found it a bit difficult to articulate and I do wonder (hope!) that others encounter similar scenarios. Thanks for any tips! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Yeah I am looking for a possible pattern. I am struggling to explain the problem. Perhaps some code will help. This code compiles, however the function problem is the one I am looking for. There are two data structures here: 1) Op, which is a functor 2) FreeOp, which is a monad that arises from the Op functor (i.e. an instance of the free monad) There are some functions for demonstration: 1) productOp -- An example of zipping two FreeOp instances, just to show that you can (though, this is trivial by the monad) 2) booleanOp -- Produces a FreeOp Bool by using the IntOp constructor for Op 3) coproductOp -- An example of splitting out two FreeOp instances, to show this is possible too. * The question is, what about a function FreeOp b - FreeOp (a - b)? * Can I constrain the 'a' type variable somehow to come up with something similar to CoArbitrary (QuickCheck)? * Can I generalise this idea i.e. not just FreeOp? Or for CoArbitrary, not just for Gen? * Is there a pattern here that is currently not part of my mental tool kit? I am struggling to see it; maybe just it's not there. As always, thanks for any pointers! Begin code... data Op a = DoubleOp (Double - a) | IntOp (Int - a) data FreeOp a = Suspend (Op (FreeOp a)) | Point a examples productOp :: FreeOp a - FreeOp b - FreeOp (a, b) productOp a b = do a' - a b' - b return (a', b') boolOp :: FreeOp Bool boolOp = Suspend (fmap Point (IntOp even)) coproductOp :: FreeOp a - FreeOp b - FreeOp (Either a b) coproductOp a b = boolOp = \p - if p then fmap Left a else fmap Right b The Problem problem :: -- ? c = -- ? other arguments FreeOp b - FreeOp (a - b) problem = error what constraints on 'a' to allow an implementation of this function that uses the argument? -- fmap const -- type-checks, but ignores the argument, unlike e.g. QuickCheck which uses CoArbitrary to perturb that result with the argument. support libraries instance Functor Op where fmap f (DoubleOp g) = DoubleOp (f . g) fmap f (IntOp g) = IntOp (f . g) instance Functor FreeOp where fmap f = (=) (return . f) instance Monad FreeOp where return = Point Suspend o = f = Suspend (fmap (= f) o) Point a = f = f a -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] CoArbitrary
Hello, In the QuickCheck library, there is a type-class called CoArbitrary. It is defined like so: class CoArbitrary a where coarbitrary :: a - Gen b - Gen b -- Gen is a monad Its purpose is to allow generation of functions. In other words, for taking Gen x - Gen (a - x), which could be done rather degenerately (fmap const) but QuickCheck constrains with (CoArbitrary a) to perturb the resulting value instead of ignoring (const) it. It has always puzzled me in the general sense of thinking about the scenario: f x - f (a - x) and whether the CoArbitrary is a good/general solution, perhaps for the specific case of f=Gen or maybe even more generally. -- approximate (a - f x - f x) -- e.g. CoArbitrary to perturb the result - f x -- e.g. Gen x - f (a - x) -- e.g. Gen (a - x) So I often wonder about what might be a better (and perhaps more general) constraint to produce functions f x - f (a - x) for a given Monad f. I was wondering if there is an existing abstraction (or paper) that might point me in that direction. It is a problem that I encounter on occasion in general programming and I am using Arbitrary/CoArbitrary as an example to help make my point, but I am dissatisfied (for reasons that I am unsure about) with the solution provided by CoArbitrary. What about other monads? For example, what is a general constraint to be placed to permit a function Maybe x - Maybe (a - x) that does not simply const away the argument. I hope I have phrased this in a way to make the point. I found it a bit difficult to articulate and I do wonder (hope!) that others encounter similar scenarios. Thanks for any tips! -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Control.bimap?
Check out Control.Lens on hackage. On 13/12/12 07:54, Gregory Guthrie wrote: I found a nice idiom for a graph algorithm where the pairs of nodes representing links could be merged into node lists by something like: ns = nub $ map fst g--head nodes ne = nub $ map snd g -- tail nodes And found a nicer approach: (ns,ne) = (nub***nub) unzip g Or perhaps: (ns.ne) = bimap nub nub $ unzip g-- from Control.Bifunctor The SO reference I saw described bimap as a way to map a function over a pair, and it seemed like a great match, but I cannot find the bimap function, and cabal reports no package Control.Bifunctor. ?? --- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Segment Tree based Set
It is not a Set, but a Map. Of course, I could use it to implement the function I need with something like: type SSet a = STree [()] a, but then I'd have to unnecessarily go beyond Haskell98. Hoping there might be an interval tree or segment tree specifically for this task. On 29/10/12 18:36, Roman Cheplyaka wrote: If you searched hackage, you'd find http://hackage.haskell.org/package/SegmentTree Roman * Tony Morris tonymor...@gmail.com [2012-10-29 15:38:07+1000] Er, oops. ...can be implemented as: \a rs - let s = Set.fromList (rs = \(a, b) - [a..b]) in a `member` s Something like that! On Mon, Oct 29, 2012 at 2:48 PM, Tony Morris tonymor...@gmail.com wrote: Hi, I was wondering if anyone knows of a package implementing a fast lookup for an element in ranges. For example, this operation: Ord a = a - [(a, a)] - Bool ...can be implemented: \a rs - let s = Set.fromList rs in a `member` s This is not particularly efficient. A segment tree seems like a more appropriate data structure to store the ranges. Does such a library exist? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Segment Tree based Set
Yeah that looks useful indeed. I am surprised there isn't a DIET on hackage. On Tue, Oct 30, 2012 at 3:55 AM, Stephen Tetley stephen.tet...@gmail.comwrote: Are Martin Erwig's diets anything close? http://web.engr.oregonstate.edu/~erwig/diet/ On 29 October 2012 04:48, Tony Morris tonymor...@gmail.com wrote: Hi, I was wondering if anyone knows of a package implementing a fast lookup for an element in ranges. For example, this operation: Ord a = a - [(a, a)] - Bool ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Segment Tree based Set
Hi, I was wondering if anyone knows of a package implementing a fast lookup for an element in ranges. For example, this operation: Ord a = a - [(a, a)] - Bool ...can be implemented: \a rs - let s = Set.fromList rs in a `member` s This is not particularly efficient. A segment tree seems like a more appropriate data structure to store the ranges. Does such a library exist? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Segment Tree based Set
Er, oops. ...can be implemented as: \a rs - let s = Set.fromList (rs = \(a, b) - [a..b]) in a `member` s Something like that! On Mon, Oct 29, 2012 at 2:48 PM, Tony Morris tonymor...@gmail.com wrote: Hi, I was wondering if anyone knows of a package implementing a fast lookup for an element in ranges. For example, this operation: Ord a = a - [(a, a)] - Bool ...can be implemented: \a rs - let s = Set.fromList rs in a `member` s This is not particularly efficient. A segment tree seems like a more appropriate data structure to store the ranges. Does such a library exist? -- Tony Morris http://tmorris.net/ -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldr (.) id
It's the Endo monoid. ? :t ala Endo foldMap -- see newtype package ala Endo foldMap :: Foldable t = t (a - a) - a - a ? ala Endo foldMap [(+1), (*2)] 8 17 ? :i ala ala :: (Newtype n o, Newtype n' o') = (o - n) - ((o - n) - b - n') - b - o' -- Defined in Control.Newtype On 27/10/12 04:41, Greg Fitzgerald wrote: Hi Haskellers, 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. Thanks, Greg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why Kleisli composition is not in the Monad signature?
As a side note, I think a direct superclass of Functor for Monad is not a good idea, just sayin' class Functor f where fmap :: (a - b) - f a - f b class Functor f = Apply f where (*) :: f (a - b) - f a - f b class Apply f = Bind f where (=) :: (a - f b) - f a - f b class Apply f = Applicative f where unit :: a - f a class (Applicative f, Bind f) = Monad f where Same goes for Comonad (e.g. [] has (=) but not counit) ... and again for Monoid, Category, I could go on... On 17/10/12 04:14, David Thomas wrote: I think the version below (with a Functor or Applicative superclass) is clearly the right answer if we were putting the prelude together from a clean slate. You can implement whichever is easiest for the particular monad, use whichever is most appropriate to the context (and add optimized versions if you prove to need them). I see no advantage in any other specific proposal, except the enormous advantage held by the status quo that it is already written, already documented, already distributed, and already coded to. Regarding mathematical purity of the solutions, this is in every way isomorphic to a monad, but we aren't calling it a monad because we are describing it a little differently than the most common way to describe a monad in category theory strikes me as *less* mathematically grounded than we are calling this a monad because it is in every way isomorphic to a monad. On Tue, Oct 16, 2012 at 7:03 AM, AUGER Cédric sedri...@gmail.com wrote: So I think that an implicit question was why wouldn't we have: class Monad m where return :: a - m a kleisli :: (a - m b) - (b - m c) - (a - m c) bind = \ x f - ((const x) = f) () join = id=id :: (m (m a) - m a) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Flipping type constructors
I have a data-type that is similar to EitherT, however, I have ordered the type variables like so: data EitherT (f :: * - *) (a :: *) (b :: *) = ... This allows me to declare some desirable instances: instance Functor f = Bifunctor (EitherT f) instance Foldable f = Bifoldable (EitherT f) instance Traversable f = Bitraversable (EitherT f) However, I am unable to declare a MonadTrans instance: instance MonadTrans (EitherT a) -- kind error I looked at Control.Compose.Flip to resolve this, but it does not appear to be kind-polymorphic. http://hackage.haskell.org/packages/archive/TypeCompose/0.9.1/doc/html/src/Control-Compose.html#Flip I was wondering if there are any well-developed techniques to deal with this? Of course, I could just write my own Flip with the appropriate kinds and be done with it. Maybe there is a more suitable way? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Martin Odersky on What's wrong with Monads
Odersky is repeatedly wrong on this subject and specifically for the claim that you quote, the only response is simply not true. On 24/06/12 15:31, Jonathan Geddes wrote: Cafe, I was watching a panel on languages[0] recently and Martin Odersky (the creator of Scala) said something about Monads: What's wrong with Monads is that if you go into a Monad you have to change your whole syntax from scratch. Every single line of your program changes if you get it in or out of a Monad. They're not polymorphic so it's really the old days of Pascal. A monomorphic type system that says 'well that's all I do' ... there's no way to abstract over things. [0, 53:45] Thoughts? --J Arthur [0] - http://css.dzone.com/articles/you-can-write-large-programs ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Perth Functional Programmers meetup group launched
Hi Todd, I am over the other side. I help organise the Brisbane Functional Programming Group. http://bfpg.org/ Although we discuss FP in general, we have a pretty strong emphasis on Haskell. Let me know how it goes or if we can help out in any way! On 14/06/12 00:46, Todd Owen wrote: We are pleased to announce that a functional programming user group has recently been formed in Perth, Australia. Being a small community, we aim to keep the group as inclusive as possible, and welcome new members from all levels of experience and language backgrounds. (That said, Haskellers currently account for about half our membership, followed by F#, and then a heap of other languages). As this is a global mailing list, I would also like to invite our colleagues from around Australia, and indeed anywhere in the world, to get in touch if you are ever passing through Perth. We're always happy to share a beer and talk about programming! For more information, visit: http://www.meetup.com/PerthFP/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Unit and pair
On 09/05/12 03:49, MigMit wrote: On 8 May 2012, at 21:42, Felipe Almeida Lessa wrote: On Tue, May 8, 2012 at 2:36 PM, MigMit miguelim...@yandex.ru wrote: Hi café, a quick question. Is there a somewhat standard class like this: class Something c where unit :: c () () pair :: c x y - c u v - c (x, u) (y, v) ? I'm using it heavily in my current project, but I don't want to repeat somebody else's work, and it seems general enough to be defined somewhere; but my quick search on Hackage didn't reveal anything. I know about arrows; this, however, is something more general, and it's instances aren't always arrows. Are you aware of generalized arrows [1]? It's still a lot more than your Something, though. I've heard of them, but some instances of my Something class aren't categories either, which rules out GArrows too. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe data-lens has something similar (Tensor): http://hackage.haskell.org/packages/archive/data-lens/2.10.0/doc/html/Control-Category-Product.html -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Haskell source AST zipper with state
Is there a library to traverse a source AST keeping state? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Does somebody know about these functions?
On 01/03/12 14:40, wren ng thornton wrote: Of course, you can simplify the implementation by: inter f xs = zipWith f xs (tail xs) inter f = zipWith f * tail -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Not an isomorphism, but what to call it?
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 01/20/2012 07:24 AM, Sean Leather wrote: I have two types A and B, and I want to express that the composition of two functions f :: B - A and g :: A - B gives me the identity idA = f . g :: A - A. I don't need g . f :: B - B to be the identity on B, so I want a weaker statement than isomorphism. I understand that: (1) If I look at it from the perspective of f, then g is the right inverse or section (or split monomorphism). (2) If I look at from g, then f is the left inverse or retraction (or split epimorphism). But I just want two functions that give me an identity on one of the two types and I don't care which function's perspective I'm looking at it from. Is there a word for that? Regards, Sean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe It is not clear to me exactly what you are asking, so shot in the dark: injection or surjection? - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQEcBAEBAgAGBQJPGJNXAAoJEPxHMY3rBz0PHt0IAKP1lVcfDXZm00h4W1WQPDBT h6LB9nLlp0cgAh5CH06FsdQFqdtDVJNMkV7/9q3H/wTFOoscZHCTigr1G+vE/kA8 lh1/Gb3caQByt6rWkgD79998FL5ZCBdHN2HYh1o/RPBwA/BYxA041P92pE0EFTKB 1oylh5ldUfv8rEzvHhQVw0USrJ11uiZfn/T3+UrO2s2xLQZS1oTWNZhsKMccjB95 tYaqEw+20Q+8yBanVnDJFOqD3yPXIRBHkTSJTOFO+Y++oen4gXUzSJJ2lkpXLECE ojMNHD/9Yh43gCm1Jq3Wuz5B6mr+v+RTRuLkxiVMsK7wxW+lfmOgeMyxHyr8pxU= =aPtB -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functor0?
On 01/16/2012 03:26 PM, Evan Laforge wrote: Often when I define some type that wraps something else, I find myself writing a function like the following: newtype Thing = Thing X liftThing f (Thing x) = Thing (f x) It's like a Functor, but I can't make it an instance because Functor requires that the type be parametric. So I've been using type families to make a kind of 0 argument functor: class Functor0 a where type Elem a :: * fmap0 :: (Elem a - Elem a) - a - a instance Functor0 Thing where type Elem Thing = X fmap0 = liftThing Is there a name for this? A better way to do it? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ? http://hackage.haskell.org/packages/archive/newtype/0.2/doc/html/Control-Newtype.html ? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] strict, lazy, non-strict, eager
On 24/12/11 17:54, Yves Parès wrote: See that's typically the speech that scares people away from Haskell... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Wait what? I find it intriguing, helpful, provocative and potentially helpful toward the common goal of helping others. I am interested in further commentary. I'm not scared and you shouldn't be either. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] strict, lazy, non-strict, eager
On 24/12/11 18:41, Gregory Crosswhite wrote: On Dec 24, 2011, at 6:22 PM, Tony Morris wrote: Wait what? I find it intriguing, helpful, provocative and potentially helpful toward the common goal of helping others. I am interested in further commentary. I'm not scared and you shouldn't be either. Asking honest questions is imminently reasonable; accusing others of being incompetent hypocrites --- especially when you go to great length to make it clear that this is *exactly* what you are doing --- is not. The former is helpful, but the latter is poisonous. Yves's point is that doing the latter risks scaring people away. Cheers, Greg If I am an incompetent hypocrite, I want to know more. Toughen up. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Lens with merge (Semigroup)
A regular Lens can be represented as follows: data CoState a = CoState (a - b) a newtype Lens a b = Lens (a - CoState b a) I once read about a lens representation that permits a general merge operation. I forget where I read it -- I think it was #haskell IRC. However, as I recall, perhaps incorrectly, the merge operation involves a Semigroup[1] and helps to overcome the fact that Lens is not an Arrow and so does not have a () operation. I am interested to know what this Lens operation is and the associated merge operation. [1] -- Approximate data SemigroupT f a = SemigroupT (a - a - f a) -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Smarter do notation
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Tasty. On 04/09/11 12:34, Daniel Peebles wrote: Hi all, I was wondering what people thought of a smarter do notation. Currently, there's an almost trivial desugaring of do notation into (=), (), and fail (grr!) which seem to naturally imply Monads (although oddly enough, return is never used in the desugaring). The simplicity of the desugaring is nice, but in many cases people write monadic code that could easily have been Applicative. For example, if I write in a do block: x - action1 y - action2 z - action3 return (f x y z) that doesn't require any of the context-sensitivty that Monads give you, and could be processed a lot more efficiently by a clever Applicative instance (a parser, for instance). Furthermore, if return values are ignored, we could use the ($), (*), or (*) operators which could make the whole thing even more efficient in some instances. Of course, the fact that the return method is explicitly mentioned in my example suggests that unless we do some real voodoo, Applicative would have to be a superclass of Monad for this to make sense. But with the new default superclass instances people are talking about in GHC, that doesn't seem too unlikely in the near future. On the implementation side, it seems fairly straightforward to determine whether Applicative is enough for a given do block. Does anyone have any opinions on whether this would be a worthwhile change? The downsides seem to be a more complex desugaring pass (although still something most people could perform in their heads), and some instability with making small changes to the code in a do block. If you make a small change to use a variable before the return, you instantly jump from Applicative to Monad and might break types in your program. I'm not convinced that's necessary a bad thing, though. Any thoughts? Thanks, Dan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQEcBAEBAgAGBQJOYuR6AAoJEPxHMY3rBz0PRE8IAMK8sQTzxtgRYeWcyP6JmWso Yl3eDUjny2uMSzIkifJix/t7tYuYG092H6SvA5VhgVBPQUd8LnZH/91X3PDGANBu ufjmCJLuN5+bgeNxvyzBHwz5iYM3GOkPhGvpJ3hJzYFIBlDVnVmMNoCDkki46/nq xJ/gsAIwfgpe4+Ll1LWu9DjVaQHb9nWmdBpTvpbXb7W+WEX7MHIsVsP/KysVFZkc XwPESJntb7oTHCcS3q1GEVTYdMFNKHlWOFcrdkGGQlegvwfjdt221oVDNToZi4z1 wJ268MdvXLSVIcU+JHLYxElQj6zrf2D51oQbHWLS/wlHRnpZHU5gtmrMTKvPvf8= =d1uz -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pointed, but not Applicative
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 28/08/11 01:41, Sönke Hahn wrote: Hi! I was reading through the Typeclassopedia ([1]) and I was wondering which type could be an instance of Pointed, but not of Applicative. But I can't think of one. Any ideas? Sönke [1] http://www.haskell.org/haskellwiki/Typeclassopedia ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Pointed f = Pointed (StateT s f) but not Applicative f = Applicative (StateT s f) - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQEcBAEBAgAGBQJOWhtaAAoJEPxHMY3rBz0Pa3oIAMrqoyv4DW39VjIXwzV3/4Ir W5s0+fdoPj7h1j6eyCB81VcDHNtGQmWhZ3+g2AhHo1jLAzmH8G5ACdD1c1FeF2dn a0iO7uvH5sM0xovpsqUwZC8BkomdeAnRuYF5Ohzar5M/Ip2BD0k7QpIWJt3RdLZm uCpwDnsQ2foHJCJYlGmmGkpzDAnkwePOfER93KrKXmzHqQxhS0oACQy6LKfXODTM +d2VVzzb4tWuzijXE4NflpdtW/4jSs3gVFmkZ7BmXSg8XxZO3naO/y4gtrU4YVjw 7TKo4IOIygQVMsFbdV2WZHprMHU/VaM6MTByiNECyB0q/yhJhsXtGsd9eeR2jng= =X4nM -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Simon PJ in Australia
On 08/19/2011 05:56 PM, Simon Peyton-Jones wrote: This is a message for *Australian FPers* -- sorry to spam the rest of you. I'm crossing the planet to Australia for YOW (Melbourne Dec 1-2, Brisbane Dec 5-6). http://www.yowconference.com.au/YOW2011/ I'm committed for the conference dates of course, but I could spend a few days before or afterwards, hanging out with some of you guys. Hack on GHC, give a talk, eat pizza, university departments, Haskell users groups, surfing (on water, that is) Do drop me (not the list) a line if any of that sounds fun. Simon PS: John Hughes is coming to Yow too, so you could try luring him into your lair too. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ...and I will be running a Haskell/FP workshop. I'm as keen as Simon to meet others from further afield with similar interests. http://www.yowconference.com.au/YOW2011/general/workshopDetails.html?eventId=3552 PS: we have a significant FP community here in Brisbane with nearly 250 members http://bfpg.org/ -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Data.Time
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 I recently set out to write a library that required a decent time library. Having only had a flirt with Data.Time previously, I assumed it would be robust like many other haskell libraries. I don't know about consensus, but I have been massively let down. I won't go in to the details, since this is not the point -- I don't wish to complain - -- I wish to get on with it. So, assuming the consensus is in agreement, is there a reasonable alternative to Data.Time (I looked on hackage and nothing seemed to have come close)? Am I wrong in assuming Data.Time is pretty useless? If I am right, and there is no alternative, I see no option but to take an excursion into writing my own. Ultimately, I am just trying to avoid this. Tips welcome. - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk4Ggx4ACgkQmnpgrYe6r61BRQCfbn+1jqNSjR+lxM+4h3gpvAMM VskAoKxqDCETyVAaOdoYDmFJGz1fOGd/ =IC7O -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Iteratee stack-consumption
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 I have written iteratees on the JVM, which does not include tail-call in its instruction set. They incur a stack frame per iteratee input, which can be expensive and unable to scale beyond a few thousand. Is there some trick to permitting this? Some sort of chunking the enumeratee or something? I was unable to find any good general hints in any of the hackage libraries. Thanks for any pointers. - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk3jNHEACgkQmnpgrYe6r63QcACfSeZvvy+iYVisZ3e/dhWQd3D6 790AnRZCjJT6eCY21XlsVqcpTuamfuXY =ADo5 -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] *GROUP HUG*
On 24/05/11 22:41, Johannes Waldmann wrote: Then tell me, why does calculating the length of a (Haskell) list has O(n) complexity. Infiniticity aside, tail would become O(n) if you store a length with each cons cell. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] *GROUP HUG*
On 25/05/11 16:46, Eugene Kirpichov wrote: data FList a = FNil | FCons Int a (FList a) empty = FNil len FNil = 0 len (FCons n _) = n cons x xs = FCons (1 + len xs) x xs tail (FCons _ _ xs) = xs My mistake, sorry. Currently looking for original reason to have accidentally come to believe that. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] *GROUP HUG*
Just laugh mate. It's the best possible outcomes sometimes. On 24/05/11 15:10, Gregory Crosswhite wrote: Hey everyone, Okay, this will sound silly, but I ventured into the Scala mailing list recently and asked an ignorant question on it, and I was shocked when people reacted not by enlightening me but by jumping on me and reacting with hostility. I bring this up not to badmouth the Scala community (they are apparently going through growing pains and will hopefully mature with time!) but just because it made me appreciate just how awesome you guys are, so I just feel the need to publicly express my admiration and thank to everyone on this list for having fostered such an incredibly professional, fanatically nonhostile, and generally pleasant place to talk about Haskell!!! *GROUP HUG* Okay, I'm done now. :-) Cheers, Greg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Robert Harper on monads and laziness
On 02/05/11 17:54, Ketil Malde wrote: I'm following Harper's blog, Existential Type¹, which I find to be an enjoyable and entertainingly written tirade about the advantages of teaching functional programming - specifically ML - to students. Of course, he tends to be critical of Haskell, but it's nice to get some thought provoking opinion from somebody who knows a bit about the business. Recently, he had a piece on monads, and how to do them in ML, and one statement puzzled me: There is a particular reason why monads had to arise in Haskell, though, which is to defeat the scourge of laziness. My own view is/was that monads were so successful in Haskell since it allowed writing flexible programs with imperative features, without sacrificing referential transparency. Although people are quick (and rightly so) to point out that this flexibility goes way beyond IO, I think IO was in many ways the killer application for monads. Before IO, we had very limited functionality (like 'interact' taking a 'String - String' function and converting it into an exectuable program) to build real programs from. Laziness does require referential transparency (or at least, it is easier to get away with the lack of RT in a strict language), so I can see that he is indirectly correct, but RT is a goal in itself. Thus, I wonder if there are any other rationale for a statement like that? -k ¹ http://existentialtype.wordpress.com/ Interesting how I have been authoring and subsequently using monads in scala for several years and it is strictness that gets in the way more than anything. http://github.com/scalaz/scalaz/ I have been teaching functional programming for quite a while, both in universities and outside of academia, and I am of the opinion that Haskell's suitability in first place has no close second place. I wonder why I am wrong, but this post (and previous) is hardly persuasive. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Python is lazier than Haskell
On 27/04/11 20:02, Thomas Davie wrote: This completely misses what laziness gives Haskell – it gives a way of completing a smaller number of computations than it otherwise would have to at run time. The hope being that this speeds up the calculation of the result after the overhead of laziness is taken into account. This is not what laziness gives us. Rather, it gives us terminating programs that would otherwise not terminate. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How large is the Haskell community ?
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 13/02/11 08:37, Erik de Castro Lopo wrote: Aaron Gray wrote: I am wondering if mailing list statistics would be the best guide ? I am the organiser of FP-Syd, the Sydney (Australia) functiona prgramming group. Of the people who are regaular attendees to FP-Syed meetings, who say they are haskell users, I have seen less than 50% of these people post to this or other haskell mailing lists. Mailing list statistics may not be a good guide. Erik I am the co-organiser of Brisbane Functional Programming Group with nearly 200 members. I have not seen anywhere near 50% post here. - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk1XDbEACgkQmnpgrYe6r63g7ACghYtq7+lyba3S/UscZ34+DEvx hxEAoJ0uDPTwNhM2LnvUekpmTG7C5SNV =O/WP -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generalizing catMaybes
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Thanks guys for all the solutions. A slight correction below. On 09/01/11 03:54, David Menendez wrote: Naturally, if you also have pure and fmap, you also have a monad. You have a pointed functor but not necessarily a monad. There are many pointed functors that are not monads. The paper, Applicative Programming with Effects (McBride, Paterson) lists a couple. - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk0o4AgACgkQmnpgrYe6r62z4wCgk4A1njS5lLH3RHtxfnIkVGTL t3sAoKNm7HjVQyk/Gb1AL5LxahRHPmKN =5D4j -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Generalizing catMaybes
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 I am wondering if it possible to generalise catMaybes: (Something f, SomethingElse t) = t (f a) - t a I have being doing some gymnastics with Traversable and Foldable and a couple of other things from category-extras to no avail. Perhaps someone else's brain is molded into an appropriate shape to reveal an answer! - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk0n0lwACgkQmnpgrYe6r6155gCeLjYizQ/5w1r2qkecbEqiQqq5 ihIAn1bmmK/qNFxM2sSusqjJu/g2/lH7 =+SdM -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] UTF-8 BOM
I am reading files with System.IO.readFile. Some of these files start with a UTF-8 Byte Order Marker (0xef 0xbb 0xbf). For some functions that process this String, this causes choking so I drop the BOM as shown below. This feels particularly hacky, but I am not in control of many of these functions (that perhaps could use ByteString with a better solution). I'm wondering if there is a better way of achieving this goal. Thanks for any tips. dropBOM :: String - String dropBOM [] = [] dropBOM s@(x:xs) = let unicodeMarker = '\65279' -- UTF-8 BOM in if x == unicodeMarker then xs else s readBOMFile :: FilePath - IO String readBOMFile p = dropBOM `fmap` readFile p -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (Co/Contra)Functor and Comonad
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 ...regardless of the utility of a contravariant functor type-class, I strongly advocate for calling it Contrafunctor and not Cofunctor. I have seen numerous examples of confusion over this, particularly in other languages. On 24/12/10 12:16, Mario Blažević wrote: On Thu, Dec 23, 2010 at 5:25 PM, Stephen Tetley stephen.tet...@gmail.com mailto:stephen.tet...@gmail.com wrote: On 23 December 2010 21:43, Mario Blažević mblaze...@stilo.com mailto:mblaze...@stilo.com wrote: Why are Cofunctor and Comonad classes not a part of the base library? [SNIP] Later on I found that this question has been raised before by Conal Elliott, nearly four years ago. http://www.haskell.org/pipermail/libraries/2007-January/006740.html - From a somewhat philistine persepective, that Conal's question went unanswered says something: Does anyone have useful functionality to go into a Cofunctor module (beyond the class declaration)? Successful post-H98 additions to Base (Applicative, Arrows, ...) brought a compelling programming style with them. For Comonads, Category-extras does define some extra combinators but otherwise they have perhaps seemed uncompelling. There are plenty of potential Cofunctor instances on Hackage, as I've pointed out. The other side of the proof of the utility of the class would be to find existing libraries that could be parameterized by an arbitrary functor: in other words, some examples in Hackage of class Cofunctor c = ... instance Cofunctor c = ... f :: Cofunctor c = ... This would be rather difficult to prove - such signatures cannot be declared today, and deciding if existing declarations could be generalized in this way would require a pretty deep analysis. The only thing I can say is build it and they will come. To turn the proof obligation around, what could possibly be the downside of adding a puny Cofunctor class to the base library? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk0UIJ0ACgkQmnpgrYe6r62kWgCeNwZnYLetOFevK6bpCBE/joKO 2QQAniaX4IGzAmdjEC8kdDV27upUTsBw =NP27 -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] dot-ghci files
I teach haskell quite a lot. I recommend using .ghci files in projects. Today I received complaints about the fact that ghci will reject .ghci if it is group-writeable. I didn't offer an opinion on the matter. I am wondering if these complaints have legitimate grounds i.e. maybe you want to have group write on that file for some reason. I'd appreciate some comments on this issue. Thanks. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Load testing
Does there exist a package for convenient load-testing against a website? e.g. making lots of HTTP requests against a server, including timing, and collecting the results? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] category-extras clash with transformers
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 I have installed mtl-1.1.1.0 so that xmonad-contrib-0.9.1 would compile with GHC 6.12.1. http://permalink.gmane.org/gmane.comp.lang.haskell.xmonad/10603 Then I tried to installed category-extras-0.53.5, which clashed with transformers-0.2.2.0 for the Applicative/Monad instances for Either. Is there any way out of this problem? Thanks for any pointers. $ ghc-pkg list | grep mtl mtl-1.1.1.0 mtl-2.0.1.0 $ ghc-pkg list | grep transformers transformers-0.2.2.0 $ cabal install category-extras ... [39 of 99] Compiling Control.Monad.Either ( src/Control/Monad/Either.hs, dist/build/Control/Monad/Either.o ) src/Control/Monad/Either.hs:44:9: Duplicate instance declarations: instance Monad (Either e) -- Defined at src/Control/Monad/Either.hs:44:9-24 instance Monad (Either e) -- Defined in transformers-0.2.2.0:Control.Monad.Trans.Error src/Control/Monad/Either.hs:49:9: Duplicate instance declarations: instance Applicative (Either e) -- Defined at src/Control/Monad/Either.hs:49:9-30 instance Applicative (Either e) -- Defined in transformers-0.2.2.0:Control.Monad.Trans.Error src/Control/Monad/Either.hs:53:9: Duplicate instance declarations: instance MonadFix (Either e) -- Defined at src/Control/Monad/Either.hs:53:9-27 instance MonadFix (Either e) -- Defined in transformers-0.2.2.0:Control.Monad.Trans.Error cabal: Error: some packages failed to install: category-extras-0.53.5 failed during the building phase. The exception was: ExitFailure 1 - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkznxgQACgkQmnpgrYe6r60dkACfZQkYKbMOQuGfaVpFb2MfhJWD asAAn1/hoX+m/YpUOch3r4NsR99y2htz =IsZc -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] category-extras clash with transformers
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 21/11/10 04:43, Ross Paterson wrote: On Sat, Nov 20, 2010 at 10:58:44PM +1000, Tony Morris wrote: I have installed mtl-1.1.1.0 so that xmonad-contrib-0.9.1 would compile with GHC 6.12.1. http://permalink.gmane.org/gmane.comp.lang.haskell.xmonad/10603 Then I tried to installed category-extras-0.53.5, which clashed with transformers-0.2.2.0 for the Applicative/Monad instances for Either. Is there any way out of this problem? Thanks for any pointers. The instance in question is in the base package from GHC 7, which should avoid this problem of clashing orphans in the future. Unfortunately that doesn't help you -- I think both xmonad-contrib and category-extras need to be updated. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Thanks Ross, I just wish to confirm -- I'm totally screwed, right? - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkzoSnsACgkQmnpgrYe6r62K3QCgnxkRqnQbv0FlKBy1sfrxcoKC 1zcAoM295VGFZBo/OQR1Qq4jW1zI3C+D =FRrh -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] category-extras clash with transformers
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 21/11/10 08:41, Edward Z. Yang wrote: No, you're not totally screwed; you just need to bug xmonad-contrib and category-extras to fix their code. Edward I am wondering if upgrading from 6.12.1 to 6.12.3 will allow me to compile xmonad-contrib* and therefore, be ride of the old version of mtl and therefore, can install category extras. * $ cabal install xmonad-contrib Resolving dependencies... Configuring xmonad-contrib-0.9.1... Preprocessing library xmonad-contrib-0.9.1... Building xmonad-contrib-0.9.1... [ 1 of 180] Compiling XMonad.Util.Replace ( XMonad/Util/Replace.hs, dist/build/XMonad/Util/Replace.o ) XMonad/Util/Replace.hs:1:0: Warning: Module `Prelude' is deprecated: You are using the old package `base' version 3.x. Future GHC versions will not support base version 3.x. You should update your code to use the new base version 4.x. [ 2 of 180] Compiling XMonad.Util.CustomKeys ( XMonad/Util/CustomKeys.hs, dist/build/XMonad/Util/CustomKeys.o ) XMonad/Util/CustomKeys.hs:80:23: Not in scope: data constructor `Reader' cabal: Error: some packages failed to install: xmonad-contrib-0.9.1 failed during the building phase. The exception was: ExitFailure 1 - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkzoUrYACgkQmnpgrYe6r60E4QCfXjGuWsu9xLcEVn142+lJkzRQ +0QAoJpbYpioJaIeuygxZXKLHKb7fRBk =dBSD -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Non-hackage cabal source
I am trying to set up an apache server as an additional source to hackage for haskell packages. I have added to my ~/.cabal/config file: remote-repo: myhackage:http://myhackage/packages I am able to successfully make the HTTP requests: http://myhackage/packages/00-index.tar.gz http://myhackage/packages/MyPackage/0.0.1/MyPackage-0.0.1-tar.gz I can execute: $ tar -tvf 00-index.tar.gz -rw-r--r-- root/root 1491 2010-11-03 15:16 ./MyPackage/0.0.1/MyPackage.cabal However, when I try to unpack my package with cabal: $ cabal unpack MyPackage Downloading MyPackage-0.0.1... cabal: Failed to download http://myhackage/packages/package/MyPackage-0.0.1.tar.gz : ErrorMisc Unsucessful HTTP code: 404 Why is cabal even making this request? Why is it not making the request to http://myhackage/packages/MyPackage/0.0.1/MyPackage-0.0.1.tar.gz Thanks for any tips. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Re: A rant against the blurb on the Haskell front page
On 17/10/10 12:03, Ben Franksen wrote:
wren ng thornton wrote:
On 10/16/10 10:48 AM, Ben Franksen wrote:
Don Stewart wrote:
It is open source, and was born open source. It is the product of
research.
How can a language be open source, or rather, how can it *not* be open
source? The point of a (programming) language is that it has a published
('open') definition. Nothing prevents anyone from creating a proprietary
compiler or interpreter for Haskell, AFAIK.
Miranda[TM] is/was a proprietary language, quite definitively so. If
nothing else, this should be apparent by the fact that every reference
to it in research papers of the era (a) included the TM sigil, and (b)
had footnotes indicating who the IP holders are. That was before my
time, but I was under the impression that Haskell was open from the
beginning ---by express intention--- in order to enable work on lazy
functional languages without being encumbered by Miranda[TM]'s closed
nature.
For that matter, until rather recently Java was very much a closed
language defined by the runtime system provided by Sun Microsystems and
not defined by the sequence of characters accepted by that system, nor
by the behavior of the system when it accepts them. Sun even went
through some trouble to try to shut out competitive development of
runtime systems such as SoyLatte, IcedTea, and the like.
Even the venerable C language has a long history of companies making
proprietary extensions to the language in order to require you to buy
their compiler, and they would most certainly pursue legal action if
someone else copied the features. This is why GCC is as big a coup for
the free/open-source movement as Linux is--- long before GCC changed its
name and focus to being a compiler collection.
The languages which are open-source are in close correspondence with the
languages which have a free/open-source implementation. There are a lot
of them, including the vast majority of recent languages. But don't be
seduced into thinking that a language is a predicate on acceptable
strings, a transducer from those strings into computer behaviors, or
that such predicates and transducers are public domain.
Sigh. Yes, you are right, of course. All this is true, sadly. There are
stupid people who think that they can own a programming language. I hope
they will go the way all the other mis-adapted creatures have gone and just
die out.
Still, Haskell is an open source product doesn't sound right to me.
Even Haskell is open source (without the product) has a bad ring
because source is short for source code and source code is not
something a programming language has.
I agree that non-proprietary is a valid and important characterization of
the language. This should be mentioned where we speak about libraries and
community, since the active and friendly community is the motor behind the
growing set of libraries, and you get this sort of participation only with
a free/non-proprietary language. This applies not only to individuals but
to companies as well, maybe even more.
I anticipate the objection that potential commercial users might be scared
off by the terms non-proprietary or free, whereas the term open
source has been coined to (and probably actually does) sound more commerce
friendly. To countermand such an effect, we can point out that most
libraries have non-copyleft licenses and that there are a number of
companies who have done and still do a lot to support and advance Haskell.
Cheers
Ben
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I am somewhat sympathetic to your argument, but I care far less overall.
Nevertheless, perhaps this would appease:
Haskell is an open standard with a robust open source implementation.
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[Haskell-cafe] Network.HTTP, BasicAuth+Headers
I'm trying to send a simple request using Network.HTTP and getting a bit lost in all the possibilities. I've experimented with both Network.HTTP and Network.Browser but have been unable to come up with something specific to my needs, which are quite simple. I'd like to send a request equivalent to the following curl: curl --basic -u user:pass -H Accept: application/xml -H Content-type: application/xml https://host/path; It seems with Network.Browser I cannot send headers, but with Network.HTTP I cannot see how to send the BasicAuth information. Any tips are appreciated. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Unwrapping newtypes
I think you might want -XGeneralizedNewtypeDeriving http://haskell.org/ghc/docs/6.12.2/html/users_guide/deriving.html#id659906 On 08/09/10 22:01, Kevin Jardine wrote: I have a generic object that I want to wrap in various newtypes to better facilitate type checking. For example, newtype Blog = Blog Obj newtype Comment = Comment Obj newtype User = User Obj Unlike Obj itself, whose internal structure is hidden in a library module, the newtype wrappings are purely to facilitate type checking. It is no secret that each is just a wrapper around Obj. It is obvious how to construct the various wrapper objects. It is not so obvious how to extract the Obj they contain in a reasonably generic way however. What I want is a getObj function that works on all of them. Of course this could work if someone using the library wrote an instance for each wrapper object: instance GetObject Blog where getObj (Blog obj) = obj but this is a pain in the neck to write for each newtype. I discovered that Foldable defines a handy toList function that extracts content from generic Foldable structures. So that I could write: toObj :: Foldable thing = thing Obj - Obj toObj w = head $ toList w Slightly kludgy but it works. Even better, recent versions of GHC will allow you to automatically derive Foldable. Unfortunately, newtype Blog = Blog Obj deriving Foldable returns a kind error. What does work is: newtype BlogF a = Blog a deriving Foldable type Blog = BlogF Obj After having spent close to a day on this, I am a bit baffled that such a seemingly trivial problem seems so hard to do. I am wondering if I am missing something really, really obvious. Any suggestions? Or is there perhaps a more Haskelly way to place type constraints on a more generic type? Kevin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Unwrapping newtypes
On 08/09/10 22:19, Kevin Jardine wrote:
Hi Tony,
I stared at that specific section for at least half an hour earlier
today but could not figure out how it applied in my specific case. The
only examples I have see are for deriving Num. Do you have any more
detail on how I could use that extension?
Here is an example:
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
class C a where
c :: a - Int
data G = G
instance C G where
c _ = 7
newtype H = H G deriving C
--
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[Haskell-cafe] lambdacats
Hello, does anyone happen to have the lambdacats page cached? The domain ( arcanux.org) and server have disappeared and the wayback machine doesn't have the images. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] lambdacats
I wonder if the original site is recoverable from this, but I suspect there are some missing. On Fri, Aug 6, 2010 at 2:33 PM, Don Stewart d...@galois.com wrote: ivan.miljenovic: On 6 August 2010 14:12, Tony Morris tonymor...@gmail.com wrote: Hello, does anyone happen to have the lambdacats page cached? The domain (arcanux.org) and server have disappeared and the wayback machine doesn't have the images. Plenty of stuff shows up on google: http://images.google.com/images?q=lambdacats -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Monad transformers, design
Hello I have a question regarding monad transformers and how to design
an API with a transformer. I have a narrowed code example of the
question. Please see the questions in the comments below.
import Data.Monoid
import Control.Monad
-- Suppose some data type
newtype Inter a = Inter (Int - a)
-- and a monad transformer for that data type.
newtype InterT m a = InterT (m (Inter a))
-- It's easy to implement this type-class
instance (Monoid a) = Monoid (Inter a) where
mempty = Inter (const mempty)
Inter a `mappend` Inter b = Inter (a `mappend` b)
-- and for the transformer too by lifting into the monad
instance (Monad m, Monoid a) = Monoid (InterT m a) where
mempty = InterT (return mempty)
InterT a `mappend` InterT b = InterT (liftM2 mappend a b)
-- But what about this type-class?
class Ints a where
ints :: a - Int - Int
-- Seems easy enough
instance (Integral a) = Ints (Inter a) where
ints (Inter a) n = fromIntegral (a n)
-- OH NO!
{-
instance (Monad m, Integral a) = Ints (InterT m a) where
ints (InterT a) n = error OH NO!
-}
-- We could try this
class Copointed f where
copoint :: f a - a
-- but it seems rather impractical
instance (Copointed m, Integral a) = Ints (InterT m a) where
ints (InterT a) = ints (copoint a)
{-
So it seems that for some type-classes it is possible to implement
for both the data type and the transformer, but not all type-classes.
Is there a general approach to this problem?
-}
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Re: [Haskell-cafe] Monad transformers, design
gah you're right, @mtl had confuzzled me. Well that changes things then, thanks. Ross Paterson wrote: On Sat, Jul 31, 2010 at 10:56:31PM +1000, Tony Morris wrote: -- Suppose some data type newtype Inter a = Inter (Int - a) -- and a monad transformer for that data type. newtype InterT m a = InterT (m (Inter a)) The monad transformer should be Inter (m a). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type-Level Programming
http://www.cs.nott.ac.uk/~txa/publ/ydtm.pdf Andrew Coppin wrote: Liam O'Connor wrote: It means that not only can values have types, types can have values. Uh, don't types have values *now*? An example of the uses of a dependent type would be to encode the length of a list in it's type. Oh, right. So you mean that as well as being able to say Foo Bar, you can say Foo 7, where 7 is (of course) a value rather than a type. (?) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Retrospective type-class extension
Ivan Miljenovic wrote: On 20 May 2010 14:42, Tony Morris tonymor...@gmail.com wrote: We all know that class (Functor f) = Monad f is preferable but its absence is a historical mistake. We've all probably tried once: instance (Functor f) = Monad f where Do you mean the reverse of this (instance (Monad m) = Functor m where) ? Yes. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Retrospective type-class extension
I've compared and clearly the former is significantly superior :) I'm rather interested if there are any sound suggestions to resolve the general issue of retrospective type-class extension. Miguel Mitrofanov wrote: That won't be a great idea; if I just want my monad to be declared as one, I would have to write instance Functor MyMonad where fmap = ... instance Pointed MyMonad where pure = ... instance Applicative MyMonad where (*) = ... instance Monad MyMonad where join = ... Compare this with instance Monad MyMonad where return = ... (=) = ... and take into account that (=) is usually easier to write than join. Limestraël wrote: Then it would be: class Functor f where fmap :: (a - b) - f a - f b class (Functor f) = Pointed f where pure :: a - f a class (Pointed f) = Applicative f where (*) :: f (a - b) - f a - f b class (Applicative f) = Monad f where join :: f (f a) - f a This would be a great idea, for the sake of logic, first (a monad which is not a functor doesn't make sense), and also to eliminate redudancy (fmap = liftM, ap = (*), etc.) 2010/5/20 Tony Morris tonymor...@gmail.com mailto:tonymor...@gmail.com Ivan Miljenovic wrote: On 20 May 2010 14:42, Tony Morris tonymor...@gmail.com mailto:tonymor...@gmail.com wrote: We all know that class (Functor f) = Monad f is preferable but its absence is a historical mistake. We've all probably tried once: instance (Functor f) = Monad f where Do you mean the reverse of this (instance (Monad m) = Functor m where) ? Yes. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org mailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Retrospective type-class extension
We all know that class (Functor f) = Monad f is preferable but its absence is a historical mistake. We've all probably tried once: instance (Functor f) = Monad f where ... However, is there a type system extension (even proposed but not implemented) that allows me to retrospectively apply such a notion? Ideally something like this would be handy if it could somehow be retrospectively applied: Monad - Applicative - Pointed - Functor -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What is the meaning of tilde (~) symbol
Stephen Tetley wrote: Hi Evan Singleton (aka wrap) would be nice - isn't it called Pointed in the typeclassopedia but not otherwise existent? I suppose its missing by historical accident rather than design. I frequently use Semigroup (append but no zero) - there is one on Hackage without any instances: http://hackage.haskell.org/package/algebra I do too. I also wish there was an associative: class F f where k :: f a - f a - f a without the zero component. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Lazy language on JVM/CLR
I have hypothesised a pure, lazy language on the JVM and perhaps the .NET CLR with FFI to .NET/Java libraries. I foresee various problems but none that are catastrophic; just often requiring a compromises, sometimes very unattractive compromises. I have authored several libraries in the same vain as pure, lazy programming to run on the JVM in Java and Scala programming languages. I expect others have forethought and perhaps even experimented with such a language. Are there any dangers to be wary of that undo the entire endeavour? Thanks for any insights. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maybe, maybe not.
It might be more obvious by giving: fromMaybe :: a - (a - x, x) - x Ivan Miljenovic wrote: 2010/1/27 Edward Z. Yang ezy...@mit.edu: Excerpts from Daniel Peebles's message of Tue Jan 26 23:25:28 -0500 2010: There are actually only two (extensionally) possible total functions with that type, as far as I can see :) Is the other one... const? As far as I can tell, yes. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maybe, maybe not.
Ivan Miljenovic wrote:
2010/1/27 Tony Morris tonymor...@gmail.com:
It might be more obvious by giving:
fromMaybe :: a - (a - x, x) - x
I actually found this more confusing, and am not sure of its validity:
should that be Maybe a there at the beginning?
Sorry a mistake. Correction: fromMaybe :: a - ((a - x, x) - x) - x
{-# LANGUAGE RankNTypes #-}
data Maybe' a = M (forall x. (a - x, x) - x)
to :: Maybe' t - Maybe t
to (M f) = f (Just, Nothing)
from :: Maybe a - Maybe' a
from (Just a) = M (flip fst a)
from Nothing = M snd
--
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Re: [Haskell-cafe] Parse error
No, but there's a specific reason why GHC consistently refuses to accept your perfectly unreasonable code snippet :) GHC accepts the following perfectly reasonable code snippet: main = do putStrLn Line 1 putStrLn Line 2 let xs = do x - [1..10] y - [1..10] return (x+y) print xs Andrew Coppin wrote: Is there a specific reason why GHC consistently refuses to accept the following perfectly reasonable code snippet? main = do putStrLn Line 1 putStrLn Line 2 let xs = do x - [1..10] y - [1..10] return (x+y) print xs No matter which way I rearrange this, it *insists* that there's a parse error. This is very frustrating, given that it's utterly clear what I want... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Language simplicity
Andrew Coppin wrote: OK people, it's random statistics time! Haskell '98 apparently features 25 reserved words. (Not counting forall and mdo and so on, which AFAIK are not in Haskell '98.) So how does that compare to other languages? C: 32 C++: 62 Borland Turbo Pascal: ~50 [without the OOP extensions added later] Eiffel: 59 VB: The source I checked listed in excess of 120 reserved words, but I'm dubious as to how reserved they really are. (Is CInt really reserved? I doubt it!) It also depends wildly on which of the bazillion VB dialects you mean. Java: 50 JavaScript: 36 Smalltalk: 0 Lisp: AFAIK, there are no truly reserved words in Lisp, only predefined functions. (??) Python: 31 Ruby: 38 Tcl: Same analysis as for Lisp I believe. As you can see, this conclusively proves... something. Hmm, I wonder if there's some way to compare the size of the language specification documents? :-} PS. It comes as absolutely no surprise to me that C++ has the most keywords. But then, if I were to add AMOS Professional, that had well over 800 keywords at the last count... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Java has 53 reserved words. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ghc -e
Can I import a module when using ghc -e? e.g. ghc -e import Control.Monad; forM [[1,2,3]] reverse -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ghc -e
Gwern Branwen wrote: On Wed, Jan 6, 2010 at 7:23 PM, Tony Morris tonymor...@gmail.com wrote: ghc -e import Control.Monad; forM [[1,2,3]] reverse As of 6.10.2, the bug whereby the GHC API lets you use functions from anywhere just by naming them (Java-style) has not been fixed: $ ghc -e Control.Monad.forM [[1,2,3]] reverse package flags have changed, resetting and loading new packages... interactive:1:25: Warning: Defaulting the following constraint(s) to type `Integer' `Num t' arising from the literal `3' at interactive:1:25 In the expression: 3 In the expression: [1, 2, 3] In the first argument of `forM', namely `[[1, 2, 3]]' interactive:1:25: Warning: Defaulting the following constraint(s) to type `Integer' `Num t' arising from the literal `3' at interactive:1:25 In the expression: 3 In the expression: [1, 2, 3] In the first argument of `forM', namely `[[1, 2, 3]]' [[3],[2],[1]] it :: [[Integer]] (0.01 secs, 1710984 bytes) I see the same on GHC 6.10.4. $ ghc -e Control.Monad.forM [[1,2,3]] reverse [[3],[2],[1]] What would it be fixed to? What is wrong with how it is? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] (liftM join .) . mapM
Can (liftM join .) . mapM be improved? (Monad m) = (a - m [b]) - [a] - m [b] -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Children elements with HXT
I am trying to parse XML using HXT following
http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_XML
Here is my XML file (way.xml):
way id=27776903 visible=true timestamp=2009-05-31T13:39:15Z
version=3 changeset=1368552 user=Matt uid=70
tag k=access v=private/
tag k=highway v=service/
/way
The problem is when parsing, by reading the tag entries into the
list held by the Way data structure, I cannot get anything but an
empty list.
Here is my parsing code:
import Text.XML.HXT.Arrow
newtype Way = Way {
tags :: [Tag]
} deriving (Eq, Show)
xpWay :: PU Way
xpWay = xpElem way (xpWrap (Way, tags) (xpList xpTag))
data Tag = Tag {
k :: String,
v :: String
} deriving (Eq, Show)
xpTag :: PU Tag
xpTag = xpElem tag (xpWrap (uncurry Tag, k v) (xpPair (xpAttr
k xpText) (xpAttr v xpText)))
When I run, I get the following result:
Main run = runX (xunpickleDocument xpWay [] way.xml)
[Way {tags = []}]
Why is the tags list empty instead of holding two entries?
--
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Re: [Haskell-cafe] Children elements with HXT
Adding (a_remove_whitespace,v_1) as a parser option when running solves
it. Silly me.
Tony Morris wrote:
I am trying to parse XML using HXT following
http://www.haskell.org/haskellwiki/HXT/Conversion_of_Haskell_data_from/to_XML
Here is my XML file (way.xml):
way id=27776903 visible=true timestamp=2009-05-31T13:39:15Z
version=3 changeset=1368552 user=Matt uid=70
tag k=access v=private/
tag k=highway v=service/
/way
The problem is when parsing, by reading the tag entries into the
list held by the Way data structure, I cannot get anything but an
empty list.
Here is my parsing code:
import Text.XML.HXT.Arrow
newtype Way = Way {
tags :: [Tag]
} deriving (Eq, Show)
xpWay :: PU Way
xpWay = xpElem way (xpWrap (Way, tags) (xpList xpTag))
data Tag = Tag {
k :: String,
v :: String
} deriving (Eq, Show)
xpTag :: PU Tag
xpTag = xpElem tag (xpWrap (uncurry Tag, k v) (xpPair (xpAttr
k xpText) (xpAttr v xpText)))
When I run, I get the following result:
Main run = runX (xunpickleDocument xpWay [] way.xml)
[Way {tags = []}]
Why is the tags list empty instead of holding two entries?
--
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Re: [Haskell-cafe] Finding HP
Furthermore, when someone offers feedback designed to improve a page, and does so in a very non-threatening way: On Dec 2, 2009, at 2:26 PM, Andrew Coppin wrote: My suggestion is that if we really want people to grab the HP rather than download GHC directly, maybe we could make the link slightly more prominent? It also wouldn't hurt to mention it from the Implementations page, and maybe the GHC homepage? Just a suggestion... ... then in my own humble opinion, snapping back with Are you sure this isn't user error? is not a particularly nice response. When someone asks a question after being offered feedback designed to improve a page, and does so in a very non-threatening way: Are you sure this isn't user error? ... then in my own humble opinion, snapping back with \Are you sure this isn't a user error?\ is not a particularly nice response is not a particularly nice response. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Implicit newtype unwrapping
Isn't that the point of type-classes? Martijn van Steenbergen wrote: So here's a totally wild idea Sjoerd and I came up with. What if newtypes were unwrapped implicitly? What advantages and disadvantages would it have? In what cases would this lead to ambiguous code? Thanks, Martijn. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Working with multiple projects
I don't want to have to upload every time I make a minor change as I am working. Surely there is an easier way. Martijn van Steenbergen wrote: Tony Morris wrote: I have two projects that I intend to put on hackage soon. One depends on the other. I have cabaled both. I am wondering how others work with this kind of set up where changes are made to both libraries as they work. You just update and re-upload the packages as necessary. It really helps here if you follow the versioning guidelines: http://www.haskell.org/haskellwiki/Package_versioning_policy HTH, Martijn. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Working with multiple projects
I have two projects that I intend to put on hackage soon. One depends on the other. I have cabaled both. I am wondering how others work with this kind of set up where changes are made to both libraries as they work. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Applicative but not Monad
newtype Accy o a = Acc{acc :: o } -- Applicative Programming With Effects
Yusaku Hashimoto wrote:
Hello cafe,
Do you know any data-type which is Applicative but not Monad?
Cheers,
-~nwn
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Re: [Haskell-cafe] Cal, Clojure, Groovy, Haskell, OCaml, etc.
There is a significant difference between: * A $ function without a type system * A statically checked $ function * A $ keyword without static checking Curt Sampson wrote: On 2009-09-30 13:45 -0300 (Wed), namekuseijin wrote: The Perl call is spot on. Specially because Haskell has been incorporating so much syntatic sugar that it's almost looking Perlish noise already: [examples deleted] No, I disagree with your particular examples; they're bog-standard Haskell that don't use any syntatic sugar (. and $ are just library functions), and I find them perfectly fine to read. Note that nothing in there is inconsistent or interpreted in any sort of exceptional way, unlike many things that look like that in Perl. It does take time to learn to read that sort of stuff, but once you've got it, simplifying this sort of thing would only make it harder to read, because it would be more verbose without saying anything more. Haskell's concision is one of its most important strengths. (Incidently, a good exercise for learning to understand stuff like that might be to go thorugh it and convert it to use parens instead of $, full application instead of ., and so on.) cjs -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comments requested: succ Java
John A. De Goes wrote: write them yourself (at a cost of several to dozens of man years), Is that right? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cal, Clojure, Groovy, Haskell, OCaml, etc.
I think one must distinguish what it means for a language to exist and be practical. Counter-example: Java fails catastrophically at all three and it most certainly exists; boy do I know it. Casey Hawthorne wrote: I think a language needs the following to exist: - a community - good library - a package manager Thoughts? -- Regards, Casey ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Library function for map+append
Dusan Kolar wrote: Hello all, During a small project I'm trying to develop a small application. It becomes quite often that I need a function mapapp: mapapp _ [] ap = ap mapapp f (a:as) ap = f a : map f as ap I tried hoogle to find such a function with no success. Is there any function/functions built-in standard libraries that could easily satisfy the functionality with the same or even better (?) efficiency? Of course, (map f list) ++ append would do the same as mapapp f list append but with less efficiency. Or am I wrong? Thanks Dusan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe mapapp = ((++) .) . map Reasoning about efficiency in a pure lazy language is different. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monoid wants a (++) equivalent
David Leimbach wrote: On Tue, Jun 30, 2009 at 11:54 AM, Brent Yorgey byor...@seas.upenn.edu mailto:byor...@seas.upenn.edu wrote: On Tue, Jun 30, 2009 at 09:45:45AM -0700, Bryan O'Sullivan wrote: I've thought for a while that it would be very nice indeed if the Monoid class had a more concise operator for infix appending than a `mappend` b. I wonder if other people are of a similar opinion, and if so, whether this is worth submitting a libraries@ proposal over. +1. IIRC Jules Bean has proposed using (+) for this purpose, which I like. It has the advantages of (a) not clashing with any other (common) operators, (b) making more obvious the fact that mappend is not necessarily commutative, and (c) providing the obvious (+) for 'flip mappend' which is sometimes useful. I actually think this proposal is pretty excellent. I happen to agree. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] List spine traversal
Is there a canonical function for traversing the spine of a list? I could use e.g. (seq . length) but this feels dirty, so I have foldl' (const . const $ ()) () which still doesn't feel right. What's the typical means of doing this? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] I need a hint in list processing
nub . concat ? Richard O'Keefe wrote: On 14 Jun 2009, at 8:06 pm, Fernan Bolando wrote: Hi all If I have a number of list example list1 = [2,3] list2 = [1,2] list3 = [2,3,4] list4 = [1,2,3] I want to create a list from the list above with n elements, non-repeating and each elements index represents 1 of the elements from the corresponding list so for the above input I would get. a = [3,2,4,1] I have been staring at this off and on all day, and I haven't the faintest idea what you want. What is n. What is it that doesn't repeat? How does the index of an element represent 1 element? Which list corresponds to what? I'm beginning to suspect that what you want is a choice function: f [s1,...,sn] = [x1,...,xn] when each xi is an element of the corresponding si and no two xs are the same. Instead of finding one answer, let's find them all. all_choices :: Eq a = [[a]] - [[a]] all_choices [] = [[]] all_choices (set:sets) = [x:xs | xs - all_choices sets, x - set, not(x `elem` xs)] The test case all_choices [[2,3], [1,2], [2,3,4], [1,2,3]] has the answer [[3,2,4,1], [3,1,4,2], [2,1,4,3]] and you probably want to use it something like case all_choices sets of [] - there are no such choices (first_choice:_) - first_choice is one such choice For inputs like [[1,2],[2,1],[1]] there is of course no such choice function. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] I need a hint in list processing
Just guessing. How do you know it's an accident? Richard O'Keefe wrote: On 15 Jun 2009, at 4:26 pm, Tony Morris wrote: Prelude Data.List nub . concat $ [[2, 3], [1, 2], [2, 3, 4], [1, 2, 3]] [2,3,1,4] In this particular case. But that's a lucky accident.\ -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] When folding is there a way to pick out the last point being processed?
By swapping from foldl to foldr? Care to provide more detail? Casey Hawthorne wrote: When folding is there a way to pick out the last point being processed? The first point can easily be picked out with (x:xs) but last xs crawls down the list. -- Regards, Casey ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Non Empty List?
I note that you didn't address the suggestion of a zipper. Günther Schmidt wrote: Dan Weston schrieb: Unless I'm missing something in your description, why not data Container a = Single a | Many a a [a] Hi Dan, the above solution would still allow to construct, for instance, Many 5 42 [] :: Container Int The reason why I'm trying to find the design for a data structure in which this would not even be possible is to be able to avoid writing additional bookkeeping code into the functions that operate on the structure, ie. the lookups, inserts, delete etc. Günther ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] instance Monad (Except err)
Martijn van Steenbergen wrote: Hello, Mr. McBride and mr. Paterson define in their Applicative paper: data Except e a = OK a | Failed e instance Monoid e = Applicative (Except e) where ... Sometimes I'd still like to use = on Excepts but this feels wrong somehow, because it doesn't use monoids nicely like the Applicative instance does. Are there any good reasons such a Monad instance shouldn't be defined? Does it violate any laws, for example? Thanks, Martijn. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe I'm sure you could, but then ap /= (*). This seems related to a question that I once asked http://www.haskell.org/pipermail/haskell-cafe/2009-January/054139.html -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] instance Monad (Except err)
Yes, however, ap is not equal to (*) for the given Applicative. I don't believe such a monad is possible. Neil Brown wrote: Martijn van Steenbergen wrote: Hello, Mr. McBride and mr. Paterson define in their Applicative paper: data Except e a = OK a | Failed e instance Monoid e = Applicative (Except e) where ... Sometimes I'd still like to use = on Excepts but this feels wrong somehow, because it doesn't use monoids nicely like the Applicative instance does. Are there any good reasons such a Monad instance shouldn't be defined? Does it violate any laws, for example? Isn't the Except type just Either by another name? OK = Right, Failed = Left. Therefore the monad is just the same as the Either monad, and is useful as an error monad: instance Monad (Except e) where (OK x) = f = f x Failed e = _ = Failed e return = OK This obeys all the monad laws. Thanks, Neil. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combining computations
michael rice wrote: If you look at this stuff long enough it almost begins to make sense. Maybe. ;-) I've been messing around with MonadPlus and I understand its usage with the Maybe and List monads. Since one use of Monads is combining computations, how can I combine a Maybe with a List? let m1 = Nothing let m2 = [1] let m3 = m1 `mplus` m2 == [1]--if the Maybe is Nothing, do nothing let m1 = Just 1 let m2 = [] let m3 = m1 `mplus` m2 == [1] --if the Maybe is not Nothing, add it to the list Or am I misunderstanding combining computations? Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Hi Michael, You'll want the Data.Maybe.listToMaybe and Data.Maybe.maybeToList functions. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] name for monad-like structure?
Michael Vanier wrote: I've stumbled upon a structure that is like a weaker version of a monad, one that supports return and but not =. Has anyone seen this before, and if so, does it have a standard name? Mike ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Are you sure it supports () :: m a - m b - m b and not mplus :: m a - m a - m a ? -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Getting the x out
You mean, the x out of *Maybe* x even. In the very literal sense, the assumption that there is an x in Maybe x is false -- there may not be one since it is maybe, but not necessarily, x. IT's a bit like the use of null that you might have seen in other languages where you might have a value or you might have null. What you can do however, is say give me the x if there is one, otherwise, use this value. This is the fromMaybe function. Prelude Data.Maybe let safeDivision x y = if y == 0 then Nothing else Just (x/y) Prelude Data.Maybe 3 + (42 `fromMaybe` safeDivision 10 5) 5.0 Prelude Data.Maybe 3 + (42 `fromMaybe` safeDivision 10 0) 45.0 michael rice wrote: How do I get the x out of Just x? Michael = safeDivision :: Float - Float - Maybe Float safeDivision x y = if y == 0 then Nothing else Just (x/y) *Main Data.List safeDivision 10 5 Just 2.0 *Main Data.List 3 + (safeDivision 10 5) interactive:1:0: No instance for (Num (Maybe Float)) arising from a use of `+' at interactive:1:0-22 Possible fix: add an instance declaration for (Num (Maybe Float)) In the expression: 3 + (safeDivision 10 5) In the definition of `it': it = 3 + (safeDivision 10 5) *Main Data.List ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad explanation
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Gregg Reynolds wrote: The point being that the metalanguage commonly used to describe IO in Haskell contains a logical contradiction. A thing cannot be both a value and a function, but e,g, getChar behaves like a function and has the type signature of a value. getChar has the signature RealWorld - (RealWorld, Char) - -- Tony Morris http://tmorris.net/ * * Anteromedial Heterotopic Osseous Impingement Syndrome * * http://www.ajronline.org/cgi/content/full/178/3/601 can result in chronic ankle pain, especially in athletes and the younger population (15-40 years old) http://radiographics.rsnajnls.org/cgi/content/figsonly/22/6/1457 Soft-tissue and osseous impingement syndromes of the ankle can be an important cause of chronic pain, particularly in the professional athlete. -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFJj/bcmnpgrYe6r60RAicqAJ9z3f+aM/k+gDv8d5yAaNSCFf9NVQCfX3Qo ItFqQSWPDUE2h9WS+axAXV8= =c8Nw -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad explanation
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 You're right - my statement is inaccurate. Implementation details aside, I am referring specifically to the statement getChar ... has the type signature of a value. It clearly does not. Lennart Augustsson wrote: Not it doesn't. getChar has the type signature IO Char. The IO type is abstract. GHC happens to implement it by a state monad. But in, e.g., hbc it is implemented in a totally different way, more like a continuation monad. Peeking inside an implementation of IO can be illuminating, but one must remember that IO is abstract. -- Lennart On Mon, Feb 9, 2009 at 10:26 AM, Tony Morris tmor...@tmorris.net wrote: Gregg Reynolds wrote: The point being that the metalanguage commonly used to describe IO in Haskell contains a logical contradiction. A thing cannot be both a value and a function, but e,g, getChar behaves like a function and has the type signature of a value. getChar has the signature RealWorld - (RealWorld, Char) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe - -- Tony Morris http://tmorris.net/ S, K and I ought to be enough for anybody. -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.9 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEARECAAYFAkmQAfkACgkQmnpgrYe6r61tmQCcCx42Cz1iunkD7JGubla/z2Pg uhAAoLk5rkjeHnrfc936IhYoBQYO/+0r =6xWk -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad explanation
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 I also agree it is a value. The original post was attempting to make a distinction that does not exist. I deliberately avoided that topic. A thing cannot be both a value and a function, but e,g, getChar My original intent was to hope the poster reconsidered the whole post. You've blown my cover :) Lennart Augustsson wrote: But an (IO Char) is a value. You can do all the things with it that you can do with values, e.g., pass it as an argument, stick it in a list, etc. It is a special kind of value, since if it ever gets in contact with the top level it will be executed. But the fact that IO types also behave as values makes Haskell a very powerful imperative language. On Mon, Feb 9, 2009 at 11:14 AM, Tony Morris tmor...@tmorris.net wrote: You're right - my statement is inaccurate. Implementation details aside, I am referring specifically to the statement getChar ... has the type signature of a value. It clearly does not. Lennart Augustsson wrote: Not it doesn't. getChar has the type signature IO Char. The IO type is abstract. GHC happens to implement it by a state monad. But in, e.g., hbc it is implemented in a totally different way, more like a continuation monad. Peeking inside an implementation of IO can be illuminating, but one must remember that IO is abstract. -- Lennart On Mon, Feb 9, 2009 at 10:26 AM, Tony Morris tmor...@tmorris.net wrote: Gregg Reynolds wrote: The point being that the metalanguage commonly used to describe IO in Haskell contains a logical contradiction. A thing cannot be both a value and a function, but e,g, getChar behaves like a function and has the type signature of a value. getChar has the signature RealWorld - (RealWorld, Char) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe - -- Tony Morris http://tmorris.net/ S, K and I ought to be enough for anybody. -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.9 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEARECAAYFAkmQB5EACgkQmnpgrYe6r60L5QCfffj1Vy2Yg25adZLsLBReOk/K ZAoAoISEpzQH/9D0AzQOZdxJoxmoKeBj =+ZZx -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Function const (Binding)
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Maybe it will help with parentheses: (const id 1) 2 (const id 1) ignores the second argument and yields the id function so then id 2 which is just 2. TKM wrote: Hello, I've a small question about the function const. I'm a bit of confused about how it binds. Let me take the following expression as example: const id 1 2 If I execute this expression, I will get as answer 2 with Helium. Now is my question, why doesn't it give me 1 as the answer? Because the type of id would be: a - a. So first it would execute id 1 in my opinion. That gives us 1. And after executing const 1 2 it should give us 1. Can somebody explain to me why it does not bind as I expect? (I know I can do: const (id 1) 2 to get what I want) Thank you for your answers. Greetz TKM -- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe - -- Tony Morris http://tmorris.net/ * Anteromedial Osseous Impingement * http://www.ajronline.org/cgi/content/full/178/3/601 can result in chronic ankle pain, especially in athletes and the younger population (15-40 years old) http://radiographics.rsnajnls.org/cgi/content/figsonly/22/6/1457 Soft-tissue and osseous impingement syndromes of the ankle can be an important cause of chronic pain, particularly in the professional athlete. 1. Take any person with soft tissue and osseous joint impingement from trauma 2. Surgically tighten the joint ligaments particularly those in the area of impingement. 3. When the patient complains of incredible and permanent pain, shrug your shoulders. Outcome You'll find the patient at the mental hospital bordering psychosis. -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFJjhkomnpgrYe6r60RAmvtAKDBQxNAlc2tfN283vcvs5gkzXUEjwCgpw7s CcGAha7L6AgHESzwLSoD2XU= =szrP -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Applicative/Monad for Either
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Hash: SHA1
In the code below, the Applicative instance accumulates on the Left
constructor using Monoid/mappend.
Is it possible to write an equivalent Monad such that ap = (*) ? I'm
finding difficulty in proving to myself either way.
import Control.Monad.Instances
import Control.Applicative
import Data.Monoid
newtype Z e a = Z {
either :: Either e a
}
instance Functor (Z e) where
fmap f (Z e) = Z (f `fmap` e)
instance (Monoid e) = Applicative (Z e) where
pure = Z . Right
(Z (Left e1)) * (Z (Left e2)) = Z (Left (e1 `mappend` e2))
(Z (Left e1)) * (Z (Right _)) = Z (Left e1)
(Z (Right _)) * (Z (Left e2)) = Z (Left e2)
(Z (Right f)) * (Z (Right a)) = Z (Right (f a))
instance (Monoid e) = Monad (Z e) where
return = pure
(Z e) = f = error todo -- ?
- --
Tony Morris
http://tmorris.net/
S, K and I ought to be enough for anybody.
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0QXf533sWb3HyrL0pQNjMww=
=R36O
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Re: [Haskell-cafe] Function composition
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Check the type of (.) Prelude :t (.) (.) :: (b - c) - (a - b) - a - c Then the type of (.) not Prelude :t (.) not (.) not :: (a - Bool) - a - Bool Now try to apply (==) Prelude :t (.) not (==) -- not going to happen Won't happen. What do you want to happen? Oscar Picasso wrote: Hi, I can write: *Main let yes = not . not *Main :t yes yes :: Bool - Bool But not: *Main let isNotEqual = not . (==) interactive:1:23: Couldn't match expected type `Bool' against inferred type `a - Bool' Probable cause: `==' is applied to too few arguments In the second argument of `(.)', namely `(==)' In the expression: not . (==) Why? Oscar -- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe - -- Tony Morris http://tmorris.net/ S, K and I ought to be enough for anybody. -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFJVamwmnpgrYe6r60RAnerAKDAs4KKsGHN7/WnqUYAJcVJixQiCgCgkQTV CgeAJDTFEeKdAl4Ep3ibG88= =GcLE -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] About do notation.
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Magicloud wrote: Hi, As some articles say, do notation is expand to () and (=) when being compiled. So I want to know the details. Like: main = do a - getArgs b - getLine myFunc1 (head a) b myFunc2 b (head a) I cannot figure out what is the () and (=) way of this. Thanks. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe getArgs = (\a - getLine = (\b - myFunc1 (head a) b myFunc2 b (head a))) - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFI9WqJmnpgrYe6r60RAhAEAKCNvJaxziyZ3g9wGUOoJpcx4/MrtwCfVnlL ZflntZ5xrDOCv3kHgcuMP18= =21PH -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Having trouble with zip12..
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Michael Feathers wrote: I have some code that looks like this and I'm having trouble with it: zip12 ((tails . nub) flightPaths) wayPoints etopsPackets (hd geoCaches) groundSpeeds headings (map windShift headings) (regulations !! 2) (foldr (\|/) (tail pathDistances)) [ghy x | x - [1..], full x] (nub . nub) arrivalSchedule Hi Michael, Sorry to distract from your issue, but I note that (nub . nub) can be replaced with just 'nub' since the function nub is idempotent (f . f == f). dibblego @check \x - (nub . nub) x == nub x -- is nub idempotent? lambdabot OK, passed 500 tests. - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFIcS9LmnpgrYe6r60RAiDOAKCJlDaqNd5ssgxrUrrHee75WGzhbgCfftdn 70+4isXh4zaoYly0da2Gdk8= =ryfF -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] zlib, missing zlib.h
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 apt-get install zlib1g-dev Tony Morris http://tmorris.net/ Real-world problems are simply degenerate cases of pure mathematical problems. Thomas Hartman wrote: Tried to install http://hackage.haskell.org/cgi-bin/hackage-scripts/package/zlib which is required for http://darcs.haskell.org/cabal-branches/cabal-1.4 which is required for cabal-install. Got the following error. apt-get installed zlibc on a stab in the dark, but same result. advice? [EMAIL PROTECTED]:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4runghc Setup.hs configure Configuring zlib-0.4.0.4... [EMAIL PROTECTED]:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4runghc Setup.hs build out [EMAIL PROTECTED]:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4head out Preprocessing library zlib-0.4.0.4... Stream.hsc:74:18: error: zlib.h: No such file or directory Stream.hsc: In function 'main': Stream.hsc:254:0: error: 'z_stream' undeclared (first use in this function) Stream.hsc:254:0: error: (Each undeclared identifier is reported only once [EMAIL PROTECTED]:~/haskellInstalls/smallInstalls/cabal-install/zlib-0.4.0.4 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFIP8GgmnpgrYe6r60RAjGGAKDCpiIE9IGv5Madf2jMhEZCDuuEhgCdGljK ztH9XwTczFX7ABBrh3TuT8I= =jR/y -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] lambdabot on GHC 6.8.2
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 So after some effort, I managed to get lambdabot installed on a ubuntu machine using GHC 6.8.2, however, it fails when evaluating an expression: 1 + 1 setResourceLimit: invalid argument (Invalid argument) Googling around brings up a bug (#2038) against GHC 6.8.2. Am I hitting this bug? If so, can I get around it to get a working lambdabot? Thanks for any tips. - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFIMnFnmnpgrYe6r60RAp7DAJ9JhFsWwL05eyrcECf6C9Z/ML+h0gCgt5/I 3E7eOjVMm+RSXumbYiCsgR8= =bpjW -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Test.QuickCheck.Gen
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 I am trying to understand the QuickCheck source in a bit of depth and I have noted that a Gen x is a function Int - StdGen - x, however, some of the combinators can at times, fail to ever produce a result by throwing an error. I include an example using ghci: *Test.QuickCheck (f 0 (mkStdGen 0)) :: Int *** Exception: Prelude.(!!): negative index It seems this can be alleviated by changing Gen x to Int - StdGen - Maybe x and having the generator produce Nothing when a combinator fails. However, now the function promote cannot be written (I have tried and run into the conundrum of writing the function (a - Maybe b) - Maybe (a - b)), which is used to generate functions. So, I'm caught on the fence about whether there is a possible improvement here (for example, should the elements function throw the error?), or if the current scenario is acceptable. I'm seeking comments about this, thanks! - -- Tony Morris http://tmorris.net/ -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFIGjO6mnpgrYe6r60RAtohAKCBsl1lHxuNrBaLHuqwCN58PBHCIACbBALk YWkBkw9o9NUQbr+lgo0rXE0= =JQlH -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskellwiki and Project Euler
Daniel Fischer wrote: Hi all, I try not to be too rude, although I'm rather disgusted. I know there are several sites out on the web where solutions to PE problems are given. That is of course absolutely against the sporting spirit of Project Euler, but hey, not all people are sporting. I've found http://www.haskell.org/haskellwiki/Euler_problems irritating for a while, but wasn't overly annoyed by it while it only contained code for solving a few dozen problems. Today I learnt that it now contains code for all problems. Really bad! On top of that, the code for many problems isn't even Haskell, but C, WTF! Other code was submitted without consent of the author, copied from the PE fora, which are restricted access and so, even if perhaps not legally, but in spirit, do not fall under the legitimate resources for haskellwiki: You are also promising us that you wrote this yourself, or copied it from a public domain or similar free resource. DO NOT SUBMIT COPYRIGHTED WORK WITHOUT PERMISSION! To make matters worse still, there was a page containing nothing but the answers. That was changed, but Cale chose to reintroduce that crap. I just removed it again. Your turn, Cale. I call on the Haskell community to vote for immediate removal of these pages from the wiki! Show that you're a sporting bunch. Daniel Fischer ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe You're going the right way about having the answers published in more ways than just the Haskell wiki. I'm only making a prediction, not a threat. -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
