Re: [Haskell-cafe] [Haskell] Hackage 2 now available for beta testing
On Mon, Sep 09, 2013 at 07:23:59PM +0100, Duncan Coutts wrote: Well-Typed and the Industrial Haskell Group (IHG) are very pleased to announce that Hackage 2 is now available for public beta testing. The plan is to do the final switchover in late September, to coincide with ICFP. http://beta.hackage.haskell.org/ What's the story with haddock documentation? I see that some packages have docs imported from the old server, some have newly generated docs, and some have none, but no indication whether a bot has tried to build it or not. There's mention of maintainer uploads of docs as a fallback, but I couldn't find where one would do that. (It would also need to document the flags needed to get the links right.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Best practices for Arrows?
On Sat, Jun 22, 2013 at 07:05:09PM +0100, Tom Ellis wrote: On Sat, Jun 22, 2013 at 03:36:15PM +0200, Ertugrul Söylemez wrote: If the interface is not under your control, make yourself comfortable with the complete arrow syntax, most notably how it handles operators, combinators and the `(| banana bracket notation |)`. This is very valuable information. Interesting. I hadn't noticed the `(| banana bracket notation |)` on the GHC Arrows page[1] before, but just saw it when I went back to check. The banana brackets can be handy when you have operations on your arrow type beyond the ones in Arrow and ArrowLoop. But beware that the types of the operations you can use with it will be changing in the next release of GHC. The change is the first one described on http://hackage.haskell.org/trac/ghc/wiki/ArrowNotation ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't ArrowChoice a parent of of ArrowApply?
On Thu, Jun 20, 2013 at 09:02:48AM +0200, Petr Pudlák wrote: In Control.Arrow we have: leftApp :: ArrowApply a = a b c - a (Either b d) (Either c d) Any instance of ArrowApply can be made into an instance of ArrowChoice by defining left = leftApp. So why isn't ArrowChoice a parent of ArrowApply? Probably because it wasn't in John Hughes's original paper. I agree that it would make sense. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: rematch, an library for composable assertions with human readable failure messages
On Tue, Apr 16, 2013 at 10:17:48AM +0100, Tom Crayford wrote:
The core API is very simple:
data Matcher a = Matcher {
match :: a - Bool
-- ^ A function that returns True if the matcher should pass, False if it
should fail
, description :: String
-- ^ A description of the matcher (usually of its success conditions)
, describeMismatch :: a - String
-- ^ A description to be shown if the match fails.
}
How about combining match and describeMismatch as a single function
of type a - Match? Then you wouldn't need the precondition on
describeMismatch.
Defining a Monoid instance for Match might also be useful.
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Re: [Haskell-cafe] arrow notation
On Mon, Feb 11, 2013 at 09:32:25AM +0100, Petr Pudlák wrote: While the implementation of Applicative can be defined without actually using `delay`: newtype ArrowApp a b c = ArrowApp (a b c) instance Arrow a = Functor (ArrowApp a b) where fmap f (ArrowApp a) = ArrowApp (a ^ f) instance ArrowDelay a = Applicative (ArrowApp a b) where pure x = ArrowApp $ arr (const x) (ArrowApp af) * (ArrowApp ax) = ArrowApp $ (af ax) ^ uncurry ($) I believe it only satisfies the laws only if the arrow satisfies delay/force laws. This is a reader, which always satisfies the applicative laws. What ArrowDelay does is pick out the arrows that are equivalent to the static arrow, i.e. F(b-c), of some applicative functor F. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] arrow notation
On Sun, Feb 10, 2013 at 09:28:12PM +0100, Petr Pudlák wrote: 2013/2/9 Conal Elliott co...@conal.net wrote: What I have in mind is a small collection of methods including fst snd (and similarly for sums) that could be defined via arr but could instead form the basis of translating restricted arrow notation for (pseudo-)arrows that don't support arr. I also support this idea, I'd appreciate such a generalization. As an example, where it would be useful: One of my students was working on a (very nice) project where he used Haskell as a DSL for generating a FRP-like javascript code. The arrow notation without arr would be ideal for this situation. He couldn't implement arr as it would require to translate an arbitrary Haskell function to JS. So having a more general variant of Arrow without arr and with a collection of methods sufficient for the arrow notation would be quite helpful. (I wonder what methods would have to be included in the collection.) Let's try to break this down. Suppose we split arr :: forall b c. (b - c) - a b c into two primitives (^) :: forall b c d. (b - c) - a c d - a b d id :: forall b. a b b The contravariant functor (^) is essential to arrow notation, but I suspect the issue is the universally quantified b in the type of id (or equivalently returnA). One might instead use a variant of id constrained to an ADT with just the operations you want. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] arrow notation
The proposed changes are described here: http://hackage.haskell.org/trac/ghc/wiki/ArrowNotation ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] arrow notation
I'd like to hear from anyone who's using arrow notation as supported by GHC, because I'm planning a couple of changes to parts of it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] arrow notation
On Thu, Feb 07, 2013 at 02:49:40PM -0800, Conal Elliott wrote: I make some use of arrow notation, though sadly I often have to avoid it because my (pseudo-)arrows don't have arr. I'd love to see a variant that has restricted expressiveness in exchange for arr-freeness. It's hard to imagine arrow notation without arr (or at least contravariance in the first argument of the arrow) because forming expressions using the local environment is so central to it. That is, I can't imagine what things you are trying to write in that situation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] catamorphisms and attribute grammars
On Sun, Jan 27, 2013 at 12:20:25AM +, Roman Cheplyaka wrote:
Very nice! This can be generalized to arbitrary arrows:
{-# LANGUAGE ExistentialQuantification #-}
import Prelude hiding (id)
import Control.Arrow
import Control.Applicative
import Control.Category
data F from to b c = forall d . F (from b d) (to d c)
instance (Arrow from, Arrow to) = Functor (F from to b) where
fmap f x = pure f * x
instance (Arrow from, Arrow to) = Applicative (F from to b) where
pure x = F (arr $ const x) id
F from1 to1 * F from2 to2 =
F (from1 from2) (to1 *** to2 arr (uncurry id))
You only require that from b is Applicative, so that in turn can be
generalized:
data F g to c = forall d . F (g d) (to d c)
instance (Applicative g, Arrow to) = Functor (F g to) where
fmap f x = pure f * x
instance (Applicative g, Arrow to) = Applicative (F g to) where
pure x = F (pure x) id
F from1 to1 * F from2 to2 =
F ((,) $ from1 * from2) (to1 *** to2 arr (uncurry id))
I wonder what's a categorical interpretation of F itself.
It's a variety of left Kan extension (cf section 5 of Constructing
Applicative Functors at MPC'2012).
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Re: [Haskell-cafe] foldr (.) id
On Fri, Oct 26, 2012 at 07:41:18PM +0100, Greg Fitzgerald wrote: I've recently found myself using the expression: foldr (.) id to compose a list (or Foldable) of functions. It's especially useful when I need to map a function over the list before composing. Does this function, or the more general foldr fmap id, defined in a library anywhere? I googled and hoogled, but no luck so far. Alternatively: flip (foldr id) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Sliding Window functional data structure
On Fri, Aug 31, 2012 at 05:45:27AM +0100, Richard O'Keefe wrote:
Consider the following interface
type Ord k = Sliding_Window k v
entries :: Sliding_Window k v - [(k,v)]
The cost is expected to be linear in the length of
the result. The pairs are listed in increasing
order of k.
add :: Ord k = k - v - Sliding_Window k v - Sliding_Window k v
precondition: all ( k) [k' | (k',_) - entries q]
The cost should be at most O((log . length . entries) q).
post: entries (add k v q) = entries q ++ [(k,v)]
since :: Ord k = k - Sliding_Window k v - [(k,v)]
answers [(k',v) | (k',v) - entries q, k' k]
The cost should be at most O((log . length . entries) q
+ length result)
purge :: Ord k = k - Sliding_Window k v - Sliding_Window k v
answers q' such that entries q' = [(k',v) | (k',v) - entries q,
k' k]
The cost should be at most O((log . length . entries) q
+ length [k' | (k',v) - entries q,
k' = k])
Any search tree implementation will do add and purge in O(log n) time.
A finger tree will do add in O(1) and purge in O(log(min(r, n-r))) time,
where r in the length of the result.
{-# LANGUAGE MultiParamTypeClasses #-}
module SlidingWindow where
import Data.FingerTree
import Data.Foldable
import Data.Monoid
data Entry k v = Entry k v
data Max k = Bot | Lift k
deriving (Eq, Ord)
instance Ord k = Monoid (Max k) where
mempty = Bot
mappend = max
instance Ord k = Measured (Max k) (Entry k v) where
measure (Entry k _) = Lift k
newtype SlidingWindow k v = SW (FingerTree (Max k) (Entry k v))
entries :: SlidingWindow k v - [(k,v)]
entries (SW t) = [(k, v) | Entry k v - toList t]
emptySW :: Ord k = SlidingWindow k v
emptySW = SW empty
add :: Ord k = k - v - SlidingWindow k v - SlidingWindow k v
add k v (SW t) = SW (t | Entry k v)
since :: Ord k = k - SlidingWindow k v - [(k,v)]
since k = entries . purge k
purge :: Ord k = k - SlidingWindow k v - SlidingWindow k v
purge k (SW t) = SW (dropUntil ( Lift k) t)
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Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Mon, Aug 27, 2012 at 07:39:39PM +0100, Erik Hesselink wrote: If we do agree that we want to prevent this problem for a while (which I'm not sure about), we should probably do it by preventing uploads for packages like this. That way, package maintainers will know what is going on, just like with the other 'package quality' issues hackage enforces. The place to check is Distribution.PackageDescription.Check.checkPackage (in Cabal). If this returns anything other than PackageDistSuspicious, hackage will reject the upload. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Mon, Aug 27, 2012 at 09:03:18PM +0100, Brent Yorgey wrote: On Mon, Aug 27, 2012 at 10:52:59AM -0700, Bryan O'Sullivan wrote: On Mon, Aug 27, 2012 at 9:57 AM, Erik Hesselink hessel...@gmail.com wrote: I'm seeing this again, on abstract-deque-0.1.6. Ross, can you fix it again? Hang on a second. The reason you're seeing build breakage is that the .cabal files of the broken packages were edited in-place without communicating with any of the package authors. I understand that the collective intentions around this were good, but by fixing things without telling anyone, package maintainers have no way to know that anything has happened. Now we are seeing the problem begin to recur as people issue new releases that don't incorporate those changes. For the record, abstract-deque was neither one of the packages fixed previously, nor does its .cabal file even contain a test section at all, much less one with a conditional. It did a couple of hours ago. But I agree with Bryan in principle that we need a more principled approach. Yes, and Cabal is the place to test for this. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Mon, Jul 30, 2012 at 01:46:24PM +0100, Niklas Broberg wrote: On Wed, Jul 25, 2012 at 12:22 PM, Ross Paterson r...@soi.city.ac.uk wrote: As I understand it, the plan is to modify the following packages in hackage in-situ to remove the test sections (which contain the troublesome conditionals): HUnit-1.2.5.0 bloomfilter-1.2.6.10 codemonitor-0.1 codemonitor-0.2 fixhs-0.1.4 leksah-server-0.12.0.3 leksah-server-0.12.0.4 leksah-server-0.12.0.5 pqc-0.5 pqc-0.5.1 Does anyone object? No objections, but some impatience. ;-) OK, done. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Reddy on Referential Transparency
On Fri, Jul 27, 2012 at 01:08:31PM +0100, Chris Dornan wrote: For those who haven’t seen it Uday Reddy has a comprehensive answer to a request to explain referential transparency on Stack Overflow. http://stackoverflow.com/questions/210835/what-is-referential-transparency/9 859966#9859966 For good measure he finishes with a rather scathing assessment of functional programmers’ claim to ownership of RT: Functional programmers don't know much of this research. Their ideas on referential transparency are to be taken with a large grain of salt. Marvellous! 'Tis rare to find such a robust assessment combined with understanding (and it certainly makes me think a bit). So a language is referentially transparent if replacing a sub-term with another with the same denotation doesn't change the overall meaning? But then isn't any language RT with a sufficiently cunning denotational semantics? Or even a dumb one that gives each term a distinct denotation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Reddy on Referential Transparency
On Fri, Jul 27, 2012 at 07:19:40PM +0100, Chris Dornan wrote: So a language is referentially transparent if replacing a sub-term with another with the same denotation doesn't change the overall meaning? Isn't this just summarizing the distinguishing characteristic of a denotational semantics? Right, so where's the substance here? My understanding is that RT is about how easy it is to carry out _syntactical_ transformations of a program that preserve its meaning. For example, if you can freely and naively inline a function definition without having to worry too much about context then your PL is deemed to possess lots of RT-goodness (according to FP propaganda anyway; note you typically can't freely inline function definitions in a procedural programming language because the actual arguments to the function may involve dastardly side effects; even with a strict function-calling semantics divergence will complicate matters). Ah, but we only think that because of our blinkered world-view. Another way of looking at it is that the denotational semanticists have created a beautiful language to express the meanings of all those ugly languages, and we're programming in it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Applicative functors with branch/choice ?
On Wed, Jul 25, 2012 at 09:22:23PM +0100, Евгений Пермяков wrote: So, it seems for me, that Applicative API should be extended with typeclass for making choice what actions to execute depending on result of some test (pattern matching). Is there any reasonable definition of such typeclass or clear explanation, why such typeclass is unneeded? The possible extension may look somehow like this: class Applicative a = Branching a where branch :: a (Either b c) - (a b - a d) - (a c - a d) - a d Do you have any instances in mind? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Wed, Jul 25, 2012 at 09:43:48AM +0100, Simon Hengel wrote: On Fri, Jul 20, 2012 at 10:02:18AM +0100, Ross Paterson wrote: On Fri, Jul 20, 2012 at 09:34:16AM +0100, Simon Hengel wrote: Hi Ross, can you fix this on Hackage? My suggested solution is to again just remove the test-suite sections from the cabal file, if that is fine with Richard. I'll modify the packages in-place if there's a consensus on what to do. I think Richard gave his consent. Is there still anything we need to sort out? BTW: Here is a reddit story on the issue: http://www.reddit.com/r/haskell/comments/x16h7/hackage_b0rked_for_cabal_0102/ As I understand it, the plan is to modify the following packages in hackage in-situ to remove the test sections (which contain the troublesome conditionals): HUnit-1.2.5.0 bloomfilter-1.2.6.10 codemonitor-0.1 codemonitor-0.2 fixhs-0.1.4 leksah-server-0.12.0.3 leksah-server-0.12.0.4 leksah-server-0.12.0.5 pqc-0.5 pqc-0.5.1 Does anyone object? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Fri, Jul 20, 2012 at 09:34:16AM +0100, Simon Hengel wrote: Hi Ross, can you fix this on Hackage? My suggested solution is to again just remove the test-suite sections from the cabal file, if that is fine with Richard. I'll modify the packages in-place if there's a consensus on what to do. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5
On Wed, Jul 18, 2012 at 09:35:52AM +0100, Erik Hesselink wrote: I don't think you can install this package on 7.4. As Andres said, it requires containers 0.5, but ghc 7.4's base libraries (in this case, template-haskell) use containers 0.4, and can't be reinstalled. I guess your best bet is to use sbv-2.1, which depends on containers = 0.3, or to unpack it and see if you can loosen the containers dependency and see if it still works with 0.4 So the moral of this story is that libraries should avoid depending on features of the containers package that have not yet been released with GHC. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5
On Wed, Jul 18, 2012 at 06:50:31AM +0100, Andres Löh wrote: Using --avoid-reinstalls blindly or as a default flag is also unfortunately not a good idea in general. There are simply too many cases where installing older versions of packages (which is often the only thing that helps) is not really the solution you want. That's also the reason why it's not enabled by default. I need a combination of flags that I can use blindly with the greatest chance of success. The default doesn't work on packages like this one: % cabal --version cabal-install version 0.14.0 using version 1.14.0 of the Cabal library % cabal install sbv-2.2 Resolving dependencies... In order, the following would be installed: HUnit-1.2.4.3 (new package) containers-0.5.0.0 (new version) random-1.0.1.1 (new package) strict-concurrency-0.2.4.1 (new package) syb-0.3.7 (new package) template-haskell-2.7.0.0 (reinstall) changes: containers-0.4.2.1 - 0.5.0.0 QuickCheck-2.5 (new package) transformers-0.3.0.0 (new package) mtl-2.1.2 (new package) sbv-2.2 (new package) cabal: The following packages are likely to be broken by the reinstalls: ghc-7.4.1 Use --force-reinstalls if you want to install anyway. So should I be blindly using --force-reinstalls? Each build is in a clean environment, so breaking the installation isn't so serious. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5
On Wed, Jul 18, 2012 at 09:35:52AM +0100, Erik Hesselink wrote: I don't think you can install this package on 7.4. As Andres said, it requires containers 0.5, but ghc 7.4's base libraries (in this case, template-haskell) use containers 0.4, and can't be reinstalled. I guess your best bet is to use sbv-2.1, which depends on containers = 0.3, or to unpack it and see if you can loosen the containers dependency and see if it still works with 0.4 I'm talking about unattended automated builds, so tweaking isn't an option. On the other hand breaking the package environment isn't so bad, because I'm throwing it away after each build. So in short, no combination of flags will work in this case, I think. Failure is the best option. Actually --force-reinstalls does work in this case, and this thread began with Levent being unhappy with the failure option for his package, so I'm tempted to use that flag on all hackage builds. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5
On Wed, Jul 18, 2012 at 10:37:31AM +0100, Gregory Collins wrote: Is there any reason QuickCheck specifically requires containers = 0.5? Perhaps its lower bound could be relaxed. It is sbv-2.2 that has that constraint. On Wed, Jul 18, 2012 at 11:29 AM, Ross Paterson r...@soi.city.ac.uk wrote: Actually --force-reinstalls does work in this case, and this thread began with Levent being unhappy with the failure option for his package, so I'm tempted to use that flag on all hackage builds. On the other hand, if installing a package is going to break people's package environments, maybe documenting the package on hackage isn't so important. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal install fails due to recent HUnit
On Wed, Jul 18, 2012 at 05:16:19PM +0100, Simon Hengel wrote: CCing: Ross Paterson and Richard G. On Wed, Jul 18, 2012 at 05:54:44PM +0200, Martijn Schrage wrote: On 18-07-12 17:37, Erik Hesselink wrote: Hi Martijn, Yes, upgrading will obviously fix things (we do use 0.14 on our development machines) Well, to me it wasn't entirely obvious that upgrading to Cabal-1.10.2.0 fixes the problem for cabal-install-0.12, and I still think this is a good solution for most people that use version 2012.2.0.0 of the platform. I'd suggest you ask Duncan to patch the hackage repository, and maybe contact the maintainer of HUnit to prevent future problems. This also breaks all travis-ci builds. I think it is critical to refrain from using conditionals in test-suite stanzas for some time + fix the broken release on Hackage. Is there a way to make this issue more well-know. Would a warning on the upload page help? Other packages in hackage with conditionals in test-suites: fixhs-0.1.4 bloomfilter-1.2.6.10 pqc-0.5 pqc-0.5.1 leksah-server-0.12.0.3 leksah-server-0.12.0.4 leksah-server-0.12.0.5 codemonitor-0.1 codemonitor-0.2 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5
With ghc 7.4.1, cabal-install 0.13.3 and Cabal 1.14.0, % cabal install --avoid-reinstalls sbv-2.2 fails to find a plan without reinstalls, and recommends --solver=modular. % cabal install --solver=modular --avoid-reinstalls sbv-2.2 reinstalls template-haskell-2.6.0.0, which breaks the GHC installation. I've added the suggested --constraint='template-haskell==2.7.0.0' option as a workaround, but it seems the --avoid-reinstalls option is being ignored. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fwd: hackage compile failure with QuickCheck 2.5
On Wed, Jul 18, 2012 at 12:14:12AM +0100, Antoine Latter wrote: Cabal doesn't play well with version constraints on the template-haskell package - it doesn't know it can't reinstall template-haskell. The workaround is to figure out why QuickCheck has version constraints on template-haskell and solve that problem in the QuickCheck package a different way - perhaps with CPP conditonal compilation macros - and then remove the problematic version constraints. QuickCheck's constraint is template-haskell = 2.4, which doesn't explain why cabal wanted to install 2.6.0.0 when 2.7.0.0 was already present. Also, I'd expect --avoid-reinstalls to stop it reinstalling anything, but apparently it doesn't do that with the modular solver. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Plain lambda inside banana brackets in the arrow notation
On Sun, Jul 15, 2012 at 06:51:07PM +0100, Tsuyoshi Ito wrote: Thank you for the response. This sounds exciting, but sadly, I must admit that it is a little (?) above my head, and I cannot relate this extension to my original question…. Sorry about that -- I got a bit side-tracked. The combinator you wanted to use was repeat :: Int - (Int - MyArr e a) - MyArr e a That won't be possible, but with this extension you could use repeat' :: Int - StaticArrow ((-) Int) MyArr e a - MyArr e a The definition of StaticArrow (in the arrows package) is a wrapper newtype StaticArrow f a b c = StaticArrow (f (a b c)) so StaticArrow ((-) Int) MyArr e a ~= Int - MyArr e a. Now you could write test2 :: MyArr [Double] String test2 = proc xs - do let y = func1 xs z - job1 - xs (|(repeat' 100) (StaticArrow (\i - job3 (i * 2)) - xs !! y + z)|) which isn't quite what you wanted, because i wouldn't be in the environment, but we could put it there as you did in your original post, or something like test2 :: MyArr [Double] String test2 = proc xs - do let y = func1 xs z - job1 - xs (|(repeat' 100) (do i - StaticArrow (arr . const) - () StaticArrow (\i - job3 (i * 2)) - xs !! i + y + z)|) I did say it would be clunky, but at least there's no dumping the tuple and picking it up again. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Plain lambda inside banana brackets in the arrow notation
Silly me -- that code works with the current GHC (module attached).
I still think the generalization is worth doing, though.
-
{-# LANGUAGE Arrows #-}
module ArrowTest where
import Control.Applicative
import Control.Arrow
import Control.Category
import Prelude hiding (id, (.), repeat)
-- copied from Control.Arrow.Transformer.Static (in the arrows package)
newtype StaticArrow f a b c = StaticArrow (f (a b c))
instance (Category a, Applicative f) = Category (StaticArrow f a) where
id = StaticArrow (pure id)
StaticArrow f . StaticArrow g = StaticArrow ((.) $ f * g)
instance (Arrow a, Applicative f) = Arrow (StaticArrow f a) where
arr f = StaticArrow (pure (arr f))
first (StaticArrow f) = StaticArrow (first $ f)
newtype MyArr b c = MyArr (b - c)
instance Category MyArr
instance Arrow MyArr
repeat :: Int - (Int - MyArr e a) - MyArr e a
repeat = undefined
func1 :: [Double] - Double
func1 = undefined
job1 :: MyArr [Double] Double
job1 = undefined
job3 :: Int - MyArr Double String
job3 = undefined
repeat' :: Int - StaticArrow ((-) Int) MyArr e a - MyArr e a
repeat' n (StaticArrow f) = repeat n f
test2 :: MyArr [Double] String
test2 = proc xs - do
let y = func1 xs
z - job1 - xs
(|(repeat' 100) (do
i - StaticArrow (arr . const) - ()
StaticArrow (\i - job3 (i * 2)) - xs !! i + y + z)|)
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Re: [Haskell-cafe] Plain lambda inside banana brackets in the arrow notation
On Thu, Jul 12, 2012 at 02:47:57PM +0100, Ross Paterson wrote: Though one possibility that might get us most of the way there would be to refactor the Arrow class as class PreArrow a where premap :: (b - b') - a b' c - a b c class (Category a, PreArrow a) = Arrow a where arr :: (b - c) - a b c arr f = premap f id first :: a b c - a (b,d) (c,d) I've done this and the associated GHC changes locally; it yields a simple rule for determining which instances are needed, based on the keywords used: * all commands (proc and operator arguments) need PreArrow * do needs Arrow * rec needs ArrowLoop * case or if need ArrowChoice I'm warming to it as a worthwhile generalization (though not exactly what was asked for). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Plain lambda inside banana brackets in the arrow notation
On Thu, Jul 05, 2012 at 10:55:07PM +0100, Tsuyoshi Ito wrote: In a program, I have an arrow MyArr and a combinator called repeat of the following type: repeat :: Int - (Int - MyArr e a) - MyArr e a My problem is that the code becomes messy when I use this combinator inside the arrow notation, and I am looking for a way to write the code in a more readable way. [...] It does not seem possible to use banana brackets here because the type of the subcomputation does not meet the requirements stated in http://www.haskell.org/ghc/docs/7.4.2/html/users_guide/arrow-notation.html#id686230. How can I use combinators like repeat, which takes a plain function as an argument, in the arrow notation in a more readable way? Or am I trying to do an impossible thing? Unfortunately the arrow notation doesn't support this. There's no semantic reason why it wouldn't work with arguments of the form f (a (...(e,t1), ... tn) t) for any functor f, or even g (...(e,t1), ... tn) for any contravariant functor g. The limitation is due to Haskell's structural matching of types. Though one possibility that might get us most of the way there would be to refactor the Arrow class as class PreArrow a where premap :: (b - b') - a b' c - a b c class (Category a, PreArrow a) = Arrow a where arr :: (b - c) - a b c arr f = premap f id first :: a b c - a (b,d) (c,d) instance PreArrow (-) where premap f g = g . f instance PreArrow (Kleisli m) where premap f (Kleisli g) = Kleisli (g . f) instance (PreArrow a, Functor f) = PreArrow (StaticArrow f a) where premap f (StaticArrow g) = StaticArrow (fmap (premap f) g) The PreArrow class would be sufficient for the low-level translation (i.e. excluding if, case and do). You'd need to fiddle with newtypes to use it in your example, though. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] hackage hackage accounts
On Tue, May 15, 2012 at 08:11:05PM +0100, Dmitry Malikov wrote: 1) Did anyone successfully get their hackage account in last 2 months? About 40 of them. But I slipped up with two of you, I think. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Fail-back monad
On Wed, Mar 28, 2012 at 01:34:29AM +0100, Alberto G. Corona wrote: In my package MFlow [1] I program an entire web navigation in a single procedure. That happened in the good-old WASH web application framework. The problem is the back button in the Browser. To go back in the code to the previous interactions when the data input does not match the expected because the user pressed the back button one or more times, i came across this Monad specimen,: that solves the problem. This definition does not satisfy the right identity law (m = return = m) included in the monad definition: *FailBack BackT [BackPoint 1] = return BackT [NoBack 1] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hackage 2 maintainership
On Mon, Feb 13, 2012 at 11:44:18PM +, Ben Gamari wrote: Those of you who follow the Haskell subreddit no doubt saw today's post regarding the status of Hackage 2. As has been said many times in the past, the primary blocker at this point to the adoption of Hackage 2 appears to be the lack of an administrator. It seems to me this is a poor reason for this effort to be held up. Having taken a bit of time to consider, I would be willing to put in some effort to get things moving and would be willing to maintain the haskell.org Hackage 2.0 instance going forward if necessary. Hurrah! After 5 years, 1 month and 7 days of doing this, I'm keen to stop. (I've been waiting since Hackage 2 was announced in the Nov 2008 HCAR.) It seems that this process will go something like this, 1) Bring Hackage trac back from the dead 2) Bring up a Hackage 2 instance along-side the existing hackage.haskell.org 3) Enlist testers 4) Let things simmer for a few weeks/months ensuring nothing explodes 5) After it's agreed that things are stable, eventually swap the Hackage 1 and 2 instances Sounds like a good plan. I expect there are a number of small things that still need doing to make the new server a complete replacement. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is the haddock generator on Hackage down?
On Tue, Jan 03, 2012 at 07:14:58PM +, Bardur Arantsson wrote: No Haddock documentation seems to have been generated on Hackage in the past few days. The machine's down, I think. I'll give it a kick tomorrow. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Splitting off many/some from Alternative
The current definition says that some and many should be the least solutions of the equations some v = (:) $ v * many v many v = some v | pure [] We could relax that to just requiring that they satisfy these equations (which I think is what John wants). In that case there would be another possible definition for Maybe: some Nothing = Nothing some (Just x) = Just (repeat x) many Nothing = Just [] many (Just x) = Just (repeat x) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Splitting off many/some from Alternative
On Thu, Dec 15, 2011 at 02:19:34AM +, Gregory Crosswhite wrote: On Dec 15, 2011, at 12:03 PM, Ross Paterson wrote: The current definition says that some and many should be the least solutions of the equations some v = (:) $ v * many v many v = some v | pure [] We could relax that to just requiring that they satisfy these equations (which I think is what John wants). In that case there would be another possible definition for Maybe: some Nothing = Nothing some (Just x) = Just (repeat x) many Nothing = Just [] many (Just x) = Just (repeat x) That is a really good idea! In fact, this behavior was exactly what my intuition had at first suggested to me that these methods should do. But the part that still confuses me is: why are these not considered the least solutions of the equations? It has to do with the termination partial ordering -- the least solutions give a denotational description that's equivalent to what recursion computes. In this case, the least solutions are some Nothing = Nothing some (Just x) = _|_ many Nothing = Just [] many (Just x) = _|_ It's easy to verify that these are solutions, and that they're less defined than the versions above. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Splitting off many/some from Alternative
On Thu, Dec 15, 2011 at 02:36:52AM +, Antoine Latter wrote: This seems to generalize to list: some [] = [] some xs = [cycle xs] many [] = [[]] many xs = [cycle xs] More like some [] = [] some (x:xs) = repeat (repeat x) many [] = [[]] many (x:xs) = repeat (repeat x) Although I'm still not sure why I would be using these operations in maybe or list. That's true. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cabal upload failure
Version skew problem -- I hope it's fixed now. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hackage build failure
On Tue, May 10, 2011 at 10:46:53AM +0200, Jonas Almström Duregård wrote: Is there something wrong with the Hackage build system? It fails to build BNFC-meta: http://hackage.haskell.org/package/BNFC-meta It complains about a missing dep even though it (the dependency) has been built successfully. Also the dependencies are exactly the same as in the previous version (which configures fine but fails to build because of some bug in HsColor). happy-meta is one of those packages that won't build again after you clean them (it has files under dist/build to work around a bug in Happy). Fixed now. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] how to force hackage to use ghc 6.12.3
On Mon, May 09, 2011 at 11:38:47AM +0200, Daniel Fischer wrote: On Monday 09 May 2011 08:54:49, Michal Konečný wrote: I was hoping hackage will try also ghc 6.12.3 but it does not. Is there some way I can change the cabal file to help hackage to compile it with 6.12.3 and generate haddock? The package requires base = 4 so I cannot try base 4. base = 4 4.3 should do it That will stop users from building it with ghc 7.0, but I'm afraid the build client only uses the latest version, so these won't be fixed until ghc 7.2 is released. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: unordered-containers - a new, faster hashing-based containers library
On Wed, Feb 23, 2011 at 08:45:47AM +, Max Bolingbroke wrote: 3. Some map combining algorithms work more efficiently when one of their two arguments are smaller. For example, Data.Map.union is most efficient for (bigmap `union` smallmap). If you don't care about which of the two input maps wins when they contain the same keys, you can improve constant factors by testing the size of the map input to size (in O(1)) and flipping the arguments if you got (smallmap `union` bigmap) instead of the desirable way round. This is a most unfortunate feature of Data.Map, forcing users to bend the logic of their application to suit the library. Ideally you'd want binary operations that are symmetric and associative performance-wise, freeing users to follow the structure of their problem domain. The documentation (and the original paper) doesn't quantify the gain for such unions, but hopefully it achieves the optimum: O(m log(n/m)), where m and n are the sizes of the smaller and larger trees respectively. Then building a tree costs O(n log(n)), regardless of the way you do it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] upgrading mtl1 to mtl2
On Tue, Feb 15, 2011 at 07:46:29PM -0800, Evan Laforge wrote: Do I really have to add (Functor m) to the 300 or so functions with (Monad m) on them? Or just not use fmap or applicative? If you're using Monad m to get Functor or Applicative instances for a functor built from m, then I'm afraid you will need to add (Functor m) or (Applicative m) to the constraints in most cases. So I thought if it's going to be this much of a hassle I might as well just port to transformers, which I gather is supposed to be the future anyway. But transformers is lacking the classes, and I gather they're in monads-tf and monads-fd. But monads-fd says it's now deprecated because of the existence of mtl-2. So what's the story? Has transformers now turned around and been deprecated in favor of mtl-2? monads-fd is deprecated is favour of mtl-2, but transformers isn't. Now you have a choice: use the portable transformers or the functional dependencies of mtl. Either choice is compatible with other packages, because both ultimately rest on transformers. http://haskell.org/haskellwiki/Monad_Transformers Says that the only benefit of 'transformers' is that it's haskell 98 and thus more portable, but doesn't that come with the caveat that only if you don't use classes and do all the lifting manually? Good point -- I've added such a caveat. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Byte Histogram
On Mon, Feb 07, 2011 at 11:05:53AM +, John Lato wrote: From: Ivan Lazar Miljenovic ivan.miljeno...@gmail.com On 7 February 2011 12:30, wren ng thornton w...@freegeek.org wrote: On 2/6/11 4:53 PM, Ivan Lazar Miljenovic wrote: On 7 February 2011 08:12, Alexey Khudyakovalexey.sklad...@gmail.com ?wrote: Also there is a container-classes package which provide set of type class for containers. [1] http://hackage.haskell.org/package/container-classes Don't use that package: it's broken. ?I did start on a re-write that works (and there are similar libraries around that I believe do work), but gave up because it was a) too fiddly, and b) I decided that the use case I had for them wasn't worth it (as it would just make that library more complicated and difficult to use, possibly also less efficient). ?If anyone wants it I can certainly send them what I've got though. Any chance you could push a 0.0.0.1 version which tells people this, so that they know to avoid it? Didn't think about that; I'll try to do so tonight. Perhaps a better approach would be to ask the hackage maintainer (Ross Patterson, if I'm not mistaken) to deprecate container-classes. Done. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Incrementially updating an array
On Tue, Dec 28, 2010 at 08:46:24PM +0800, Robert Clausecker wrote:
I have the following problem. As code is most time easier to understand,
let give an example what I try to do in C:
unsigned int i,j;
unsigned int r[100];
for (i = 0; i 100; i++) {
j = makeResult(); //j is known to be smaller than 100.
r[j] = r[j] + 1;
}
(My C is poor, so please don't laugh)
Currently I'm using a Data.Map for this, but as the result is bounded in
this area, an array seems to be more appropreate. I don't want to mess
with mutable arrays, but I can't find anything which does the same like
Data.Map.insertWith for arrays in a reasonable way.
In my example, the input comes from a lazy list and is than folded into
the Map by insertWith (+) j 1 r. Is there any nice, quick way to do this
with Arrays without using mutable ones?
accumArray is designed for situations like this.
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Re: [Haskell-cafe] List of numbers to list of ranges
On Thu, Dec 23, 2010 at 10:57:43PM +0530, C K Kashyap wrote: Here's my attempt to convert a list of integers to a list of range tuples - Given [1,2,3,6,8,9,10], I need [(1,3),(6,6),8,10)] import Data.Function import Data.List ranges ns = [(head gp, last gp) | gp - map (map fst) $ groupBy ((==) `on` snd) $ zip ns (zipWith (-) ns [1..])] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problem with class Control.Monad.Error noMsg usage
On Tue, Dec 21, 2010 at 03:28:22PM +, Aaron Gray wrote: I have been trying to build the Scheme in 24 Hours on WikiBooks :- http://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours http://jonathan.tang.name/files/scheme_in_48/code/listing10.hs But I am getting an error :- scheme.hs:289:6: `noMsg' is not a (visible) method of class `Error' scheme.hs:290:6: `strMsg' is not a (visible) method of class `Error' I don't get that here. Perhaps you're using an old version of monads-fd (0.1.0.0). I'd recommend installing mtl-2.0.1.0 instead. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Rendering of hask in new wiki (MSIE6)
On Wed, Dec 15, 2010 at 09:01:49AM -0500, Dimitry Golubovsky wrote: HTML (a small piece of it): provides the classes div class=inline-codediv dir=ltr style=text-align: left;div class=source-haskell style=font-family: monospace;MonadTrans/div/div/div Words MonadTrans, MonadIO, StateT etc are enclosed in hask tags. They show up in monospace, each starting a new line. Is this only MSIE6, or this is how it is supposed to render? They're inline in FireFox. That text-align: seems unnecessary. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hackage down?
On Sat, Dec 04, 2010 at 02:01:44PM +, Ozgur Akgun wrote:
http://downforeveryoneorjustme.com/http://hackage.haskell.org
Apparently {darcs,hackage}.haskell.org is out for the day -- announced
on Reddit but not here.
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Re: [Haskell-cafe] category-extras clash with transformers
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
Re: [Haskell-cafe] cabal update error
On Sat, Nov 13, 2010 at 08:33:18PM +1100, Ivan Lazar Miljenovic wrote: I would guess that it's related to http://www.reddit.com/r/haskell/comments/e5db7/alert_hackage_downtime_tomorrow_0600_1200_pst/ That would be 14:00 - 20:00 UTC, wouldn't it? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] new version of the mtl package
After lengthy discussion on the libraries list, the mtl package has been updated to depend on the transformers package, from which mtl re-exports the monad transformers and the MonadTrans and MonadIO classes. This makes the two packages compatible. The monads-fd package is now a deprecated stub re-exporting the mtl modules. Ths transformers/mtl-2 combination is under consideration for inclusion in the Haskell Platform: http://trac.haskell.org/haskell-platform/wiki/Proposals/transformers Discussion is on the libraries list. So far there have been no objections. The new version has a few incompatibilities with the old, which will break a few packages, though the great majority build unchanged. The above web page lists the incompatibilities and suggested fixes. You might also consider whether the portable transformers package suffices for your package. As packages move to the new version, there will be increasing dependency clashes with packages that currently specify a bound on their mtl dependency excluding the new version. In some cases the dependency can be simply relaxed with no other change, or with small changes obtain compatibility with both versions. In other cases one must choose a version, or use cpp hackery to support both. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What do you call Applicative Functor Morphism?
On Fri, Nov 05, 2010 at 11:49:27PM -0400, rocon...@theorem.ca wrote: An applicative functor morphism is a polymorphic function, eta : forall a. A1 a - A2 a between two applicative functors A1 and A2 that preserve pure and *: eta (pure c) = pure c eta (f * x) = eta f * eta x What do you guys call such a thing? My leading candidate is idomatic transformation. An applicative functor is a functor with some extra structure. Such a function is a natural transformation between the underlying functors that preserves the extra structure. So applicative transformation seems a logical name. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is Curry alive?
On Thu, Nov 04, 2010 at 06:06:40PM -0400, Dan Doel wrote: Implementing type inference can be very easy in a logic language, because most of the work in a non-logic language is implementing unification: Provided the implementation includes the occurs check. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Red links in the new haskell theme
On Sat, Oct 30, 2010 at 05:26:33PM +0200, Henning Thielemann wrote: Two notes on the Classic style: The package field names like Version, Dependencies, License are centered, which was not the case in the original style. Also in the original style the table cells had a grey background, what I found quite useful for recognizing the table structure. This refers to the hackage package pages using the haddock style files, which have no provision for two-tone tables. The alignment is fixed now, though. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Red links in the new haskell theme
On Sat, Oct 30, 2010 at 06:39:00PM +0200, Henning Thielemann wrote: It's still vertically centered, which makes it difficult to see, e.g. where the dependency list starts and where it ends if it is several lines long. Not any more. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HackageDB: Haddock parse failure
On Mon, Sep 27, 2010 at 06:22:09PM +0200, Jonas Almström Duregård wrote: Also, perhaps HackageDB should ignore errors in building the documentation? Currently the reported build failure propagates to the dependencies of the library... The only reason it's building is to generate the documentation, and to do that you need the interface files haddock creates for the pre-requisite packages. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: happstack-auth-0.2
On Sat, Sep 18, 2010 at 08:19:33PM +0200, Nils Schweinsberg wrote: Am 17.09.2010 22:06, schrieb Nils Schweinsberg: [1] http://hackage.haskell.org/package/happstack-auth Hackage fails to build this package: http://hackage.haskell.org/packages/archive/happstack-auth/0.2/logs/failure/ghc-6.12 However, Crypto == 4.* should be on hackage: http://hackage.haskell.org/package/Crypto-4.2.1 Is there anything I can do with my package to get this to build? Unfortunately Crypto-4.2.1 was broken by the latest version of QuickCheck (2.3), which introduced Arbitrary instances for the Word types: http://hackage.haskell.org/packages/archive/Crypto/4.2.1/logs/failure/ghc-6.12 A new release of Crypto, without those instances, is needed. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A new cabal odissey: cabal-1.8 breaking its own neck by updating its dependencies
On Sat, Sep 11, 2010 at 12:17:27PM -0700, Jason Dagit wrote: From the FAQ linked by Paolo: http://www.haskell.org/cabal/FAQ.html#dependencies-conflict To avoid this problem in the future, avoid upgrading core packages. The latest version of cabal-install has disabled the upgrade command to make it a bit harder for people to break their systems in this way. It's not always possible. In particular, random-1.0.0.2 (shipped with GHC 6.12.*) depends on the time package, of which more recent versions have been released. That can trigger rebuilding of random-1.0.0.2, and thus haskell98-1.0.1.1. It might help if the release of random with GHC 7.0 had a tight dependency on the version of the time package shipped with it. Maybe all the core packages need tight dependencies. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Combining applicative with arrows?
On Fri, Sep 10, 2010 at 10:34:04AM +0200, Nils Schweinsberg wrote: I just wondered if you can define Applicative instances for arrows? Basicly what I thought of is: I have a type for my arrow which is CollectA (using HXT here): type CollectA a = SomeArrow XmlTree a If you do import Control.Applicative type CollectA = WrappedArrow SomeArrow XmlTree then CollectA will be an instance of Applicative. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Is bumping the version number evil, if it's not mandated by the PVP?
On Sat, Aug 14, 2010 at 11:13:19AM +0200, Sebastian Fischer wrote: I wonder whether (and how) I should increase the version number of a library when the API does not change but the implementation gets more efficient. Should I bump a.b.C or even a.B to signal that it's worth using the new version or should I bump only a.b.c.D such that packages that depend on a.b get installed with the new version automatically? When bumping only a.b.c.D, the new version is not installed as a dependency if the old version already is installed (unless the new version is explicitly demanded.) It seems bumping a.b.c.D has advantages for some users and disadvantages for others. How would bumping the major version change that? Let's suppose a package depending on your declares a version constraint a.b.* (which would be good practice). If you bump the patch level, anyone installing the other package with get the latest version of your package; if you bump the major version, they won't, until the maintainer of the other package releases a new version to update their dependencies. (Some maintainers might be annoyed by that; if you annoy maintainers too much, they might look for less annoying dependencies.) Whenever you bump a version number, you are signalling that the new version is worth using. If you bump the patchlevel, you're also signalling that the new version is compatible with the old, so using it is a no-brainer. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] dear traversable
On Fri, Jul 30, 2010 at 08:13:43PM -0700, Ben wrote: dear traversable geniuses -- i am looking for better implementations of unzipMap :: M.Map a (b, c) - (M.Map a b, M.Map a c) unzipMap m = (M.map fst m, M.map snd m) unliftMap :: (Ord a) = M.Map a (b - c) - M.Map a b - M.Map a c unliftMap mf ma = M.mapWithKey (\k v - mf M.! k $ v) ma the first is obviously inefficient as it traverses the map twice. the second just seems like it is some kind of fmap. The second one assumes that every key in ma is also in mf. A generalization without that assumption is unliftMap = intersectionWith id As Wren said, it looks like a * operator, but in this case there's no corresponding pure function. To make the laws work, pure x would have to be a map that took every key to x. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monad transformers, design
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
Re: [Haskell-cafe] Weird behavior with arrow commands
On Sat, Jul 24, 2010 at 01:10:56PM +0200, Jürgen Doser wrote: This seems to be a bug in ghc. First, let's fix bar to give the full three arguments (Int, Float, Double) to g: bar f g = proc x - do (f - x) `foo` (\n m k - g - (n,m,k)) ghc infers the type: bar :: (t - String) - ((Double, Float, Int) - String) - t - String and we see that the argument order in the second argument to bar is reversed. But the arguments are still given to bar in the order (Int, Float, Double). For example, the 6.0 in foo is interpreted as an Int and outputs a 0 (the first 32 bits in such a small double are zeros). When one varies the numbers in foo, one can see the effects in bar. Can someone from GHC HQ confirm my understanding, or is this just not supposed to work with multiple arguments? You're right: it's getting the argument order wrong, so -dcore-lint fails on this example and all bets are off. Definitely a bug. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] cabal, Setup.lhs example
On Wed, Jul 21, 2010 at 09:43:16AM -0700, Rogan Creswick wrote: On Wed, Jul 21, 2010 at 3:00 AM, Magnus Therning mag...@therning.org wrote: I am successfully using hooks with the following in my .cabal file: Build-Type : Simple I've been unable to reproduce this -- flipping the build type to Custom has been necessary in every configuration I've tried. I'd like to see what I'm doing differently -- is this used in a publicly available package I could take a look at? Magnus is building by directly running the Setup.hs himself, which ignores the Build-Type. To get cabal-install to use his Setup.hs, the Build-Type must be set to Custom. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] cabal, Setup.lhs example
On Thu, Jul 22, 2010 at 11:31:21AM +0100, Magnus Therning wrote: On Thu, Jul 22, 2010 at 10:59, Ross Paterson r...@soi.city.ac.uk wrote: Magnus is building by directly running the Setup.hs himself, which ignores the Build-Type. To get cabal-install to use his Setup.hs, the Build-Type must be set to Custom. Oh, why*2? Why is the header there if it's not used by Cabal, and why does cabal care? The field allows cabal to avoid compiling the Setup.hs in this case. It might also be used by other tools, e.g. one might only trust Simple packages. Not all fields are used by all tools, and several of them do not affect the operation of the library (e.g. Home-page). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Transformers versus monadLib versus...
On Fri, Jul 09, 2010 at 03:13:19PM -0400, Edward Kmett wrote: On Thu, Jul 8, 2010 at 12:37 PM, Yitzchak Gale g...@sefer.org wrote: btw, does this overhaul include adding Applicative instances, perchance? They are already part of monads-fd and would, I presume, come along for the ride when an mtl = monads-fd + transformers release occurs. =) Actually they're in transformers with the type constructors. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Build problems on Hackage server
On Wed, Jul 07, 2010 at 10:22:25AM +0200, Emil Axelsson wrote: Last week I uploaded new versions of feldspar-language and feldspar-compiler. Both packages build just fine on our local machines with GHC 6.10 and 6.12. But Hackage reports the following build failure: cabal: dependencies conflict: ghc-6.12.2 requires array ==0.3.0.1 however array-0.3.0.1 was excluded because ghc-6.12.2 requires array ==0.3.0.0 Any idea of what might be causing this? I think the problem was that containers-0.3.0.0 had been rebuilt (using array-0.3.0.1, which released at the same time as GHC 6.12.3). Upgrading to GHC 6.12.3 has fixed this (until the next GHC release). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type classes
On Sun, Jul 04, 2010 at 09:55:53PM +1000, Ivan Lazar Miljenovic wrote: Andrew Coppin andrewcop...@btinternet.com writes: In summary, I think we need to devise a way of better-documenting class instances. Haddock 2.7 supports documenting instance implementations; I don't know how this works, but according to the Changelog it's available. Now we need to go round and document our instances. Hmm, it seems only partial: documentation attached to an instance is shown in the list of instances under a type, but not the list under a class. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type classes
On Sun, Jul 04, 2010 at 02:32:35PM +0200, Daniel Fischer wrote: On Sunday 04 July 2010 14:07:03, Ivan Lazar Miljenovic wrote: Ross Paterson r...@soi.city.ac.uk writes: Hmm, it seems only partial: documentation attached to an instance is shown in the list of instances under a type, but not the list under a class. I'm guessing that's to reduce noise... I'm guessing it might have something to do with the fact that often the module containing the class definition isn't processed together with the module containing the instance declaration. It could be either way: sometimes you define a new class with instances for existing types, and with the current implementation that produces no documentation. (I tested with type, class and instance in the same package.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Call for comments: neither package
On Tue, Jun 29, 2010 at 02:56:18PM -0500, Jeremy Shaw wrote: On Jun 29, 2010, at 6:02 AM, Stephen Tetley wrote: The Applicative Programming with Effects Paper has the monodial accumulating applicative instance on a sum type Conor McBride and Ross Paterson call Except: data Except err a = OK a | Failed err The applicatives-extra package defines a type: type ErrorMsg = String data Failing a = Failure [ErrorMsg] | Success a Which is a less general version of that type. There's also a generalization, which I was going to put in transformers, but couldn't decide on the best name: data Sum p a = Return a | Others (p a) instance Functor p = Functor (Sum p) where fmap f (Return x) = Return (f x) fmap f (Others m) = Others (fmap f m) instance Applicative p = Applicative (Sum p) where pure = Return Return x * Return y = Return (x y) Return x * Others my = Others (x $ my) Others mx * Return y = Others (mx * pure y) Others mx * Others my = Others (mx * my) instance Alternative p = Alternative (Sum p) where empty = Others empty Return x | _ = Return x Others _ | Return y = Return y Others mx | Others my = Others (mx | my) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO (Either a Error) question
On Sat, May 08, 2010 at 07:49:57AM +1000, Ivan Lazar Miljenovic wrote: Limestraël limestr...@gmail.com writes: 2010/5/1 John Millikin jmilli...@gmail.com You might want to make a local version of ErrorT in your library, to avoid the silly 'Error' class restriction. This is pretty easy; just copy it from the 'transformers' or 'mtl' package. Yes, I wonder why mtl is not updated so as to remove this restriction. Presumably because its in maintenance mode (i.e. it only gets changed/updated to reflect changes in GHC that might affect it and the API is frozen). The API isn't frozen -- it can be changed with a library proposal, if you can get people to agree to it. As Ryan said, the Error constraint is there to support a definition of the fail method in the Monad instance for ErrorT. (Personally I think fail is a terrible wart, and should be shunned.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO (Either a Error) question
On Sat, May 08, 2010 at 01:54:21AM +0200, Limestraël wrote:
Personally I think fail is a terrible wart, and should be shunned.
So do I.
I can't understand its purpose since monads which can fail can be implemented
through MonadPlus.
It was introduced to implement pattern match failure in do-notation,
in Section 3.14 of the Haskell Report:
do {p - e; stmts} = let ok p = do {stmts}
ok _ = fail ...
in e = ok
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Re: [Haskell-cafe] The instability of Haskell libraries
On Mon, Apr 26, 2010 at 06:47:27AM +1000, Ivan Lazar Miljenovic wrote: My main problem with this is if you want a custom variant of that instance. Let's take FGL graphs for example with instances for QuickCheck's Arbitrary class. Maybe you want arbitrary graphs that are simple, or maybe multiple edges are fine. Even when considering Arbitrary instances for something like String you may wish to have a custom variant that makes sense for what you're testing. Being able to use different instances in different contexts would be handy, and there have been some proposed extensions to allow this. But in the current state of Haskell, orphan instances are unworkable -- they invariably lead to trouble sooner or later. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The instability of Haskell libraries
On Sat, Apr 24, 2010 at 08:21:04PM +1000, Ivan Lazar Miljenovic wrote: Mads Lindstrøm mads.lindstr...@gmail.com writes: But that would be an API change. It is of cause a question of how strict we want to be. I have seen several times that commercial software developers denies changes bugs, as the fix would constitute API changes. We obviously do not want to go there, but we could bump the version number as it would require little work. I would argue that it would involve bumping a minor version number, not a major version number (which indicates an API change) since it's just a bug fix. I think we need a simpler, declarative, definition from which these details can be worked out, e.g. Same minor version: any code that worked with the old version will work with the new version, in the same way. Same major version: any code using fully explicit imports that worked with the old version will work with the new version, in the same way. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] The instability of Haskell libraries
On Sat, Apr 24, 2010 at 08:48:07PM +1000, Ivan Lazar Miljenovic wrote: Ross Paterson r...@soi.city.ac.uk writes: Same major version: any code using fully explicit imports that worked with the old version will work with the new version, in the same way. By this you mean that additions have been made? According to these definitions, adding exports would require a new minor version, while adding instances of already exported classes and types would require a new major version. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: haskell-src-exts 1.9.0
On Thu, Apr 15, 2010 at 10:52:51AM +0200, Sean Leather wrote: Any idea why the Haddock docs have not been generated for this version? There's also no built on available. Is it an issue with the server? A glitch when I was switching the builds to use cabal install --dry-run. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: GSoC: Hackage 2.0
On Fri, Apr 09, 2010 at 09:23:51PM -0400, Matthew Gruen wrote: The proposal as I submitted it is here: http://docs.google.com/View?docid=0Afa5MxwyB_zYZGhjanNrdjNfMjkzZjloOWNienYpageview=1hgd=1hl=en [...] The work on bringing hackage-server up to feature parity is primary. Absolutely. I would go further: all the new features (1-3) should be deferred until the new server is not just deployable, but in service. (After all, that's the only way to be sure it really is deployable.) If you can do that, you'll get much more input on the new features when you come to them. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Hackage accounts and real names
On Tue, Apr 06, 2010 at 10:10:09AM +0100, David House wrote: 3. Privacy issues. Some people simply cannot reveal their real names. I've already said I was prepared to make exceptions in such cases, but perhaps you missed that. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why does Haskell not infer most general type?
On Tue, Apr 06, 2010 at 03:56:32PM -0400, Job Vranish wrote: f _ = undefined where _ = y :: Int - Int y x = undefined where _ = f x Because f and y are mutually recursive, their types are inferred together, so y gets the type Int - Int (as given), which forces f :: Int - a. If you add the type signature f :: a - b, you break the cycle: that type is used in inferring the type of y (namely a - b), which is then used in checking the typeof f. Ditto if you add y :: a - b instead. (This is not Haskell 98, but the implementations have done this for years, and it will be in Haskell 2010.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why does Haskell not infer most general type?
On Tue, Apr 06, 2010 at 05:18:34PM -0400, Job Vranish wrote: So in Haskell 98, would the added constraints result in a type error? Yes, because the types of the mutually recursive identifiers would be inferred together without using the type signatures, and then would fail to match the declared types. But then there aren't any implementations of Haskell 98 to test this on. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hackage accounts and real names
On Sun, Apr 04, 2010 at 10:28:26PM +0100, David House wrote: An issue came up on #haskell recently with Hackage accounts requiring real names. The person in question (who didn't send this email as he's wishing to remain anonymous) applied for a Hackage account and was turned down, as he refused to offer his real name for the username. Those of us in the conversation thought this a bit of an odd policy, and were wondering where this came from. It also emerged that a couple of other people had been held back from getting Hackage accounts because of this reason. Basically http://meatballwiki.org/wiki/RealNameUserAdvantages, especially simplicity, trust and recognizability. At root, I find it convenient to run username allocation that way. I am prepared to make exceptions for privacy concerns, and have done so once, but that wasn't the case with the person you're referring to. Their real name is all over the Internet, and they wanted to use their first name only, which is easily linked to their full name. I don't recall anyone else refusing to use their real name, though a number of people have not responded to enquiries I made of them. Of course some may have been put off by the User accounts page. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANN: data-category, restricted categories
On Tue, Mar 30, 2010 at 11:26:39PM +0100, Conor McBride wrote: Getting back to the question, whatever happened to empty case expressions? We should not need bottom to write total functions from empty types. Empty types? Toto, I've a feeling we're not in Haskell anymore. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] breadth first search one-liner?
On Mon, Mar 22, 2010 at 11:02:32AM +0100, Johannes Waldmann wrote: Dear all, there is this neat one-line BFS implementation bfs :: Eq a = ( a - [a] ) - a - [a] bfs next start = let xs = nub $ start : ( xs = next ) in xs but it has a problem: it only works for infinite graphs. This is fine: take 20 $ bfs ( \ x - [2*x, x+1] ) 1 but this is not: take 20 $ bfs ( \ x - filter (0) [ div x 2, x - 1 ] ) 10 Is there a nice way to repair this? bfs :: (a - [a]) - a - [a] bfs f s = concat $ takeWhile (not . null) $ iterate (= f) [s] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: breadth first search one-liner?
On Mon, Mar 22, 2010 at 10:30:32AM +, Johannes Waldmann wrote: Nice! - Where's the 'nub'? A bit longer: bfs :: Eq a = (a - [a]) - a - [a] bfs f s = concat $ takeWhile (not . null) $ map snd $ iterate step ([], [s]) where step (seen, xs) = let seen' = xs++seen in (seen', nub $ [y | x - xs, y - f x, notElem y seen']) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: breadth first search one-liner?
A bit closer to the original: bfs :: Eq a = (a - [a]) - a - [a] bfs f s = concat $ takeWhile (not . null) levels where levels = foldr trim [] $ [s] : map (nub . (= f)) levels trim xs xss = xs : map (\\ xs) xss ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Seen on reddit: or, foldl and foldr considered slightly harmful
On Thu, Feb 11, 2010 at 11:00:51AM +0100, Ketil Malde wrote: Johann Höchtl johann.hoec...@gmail.com writes: In a presentation of Guy Steele for ICFP 2009 in Edinburgh: http://www.vimeo.com/6624203 he considers foldl and foldr harmful as they hinder parallelism because of Process first element, then the rest Instead he proposes a divide and merge aproach, especially in the light of going parallel. In Haskell foldl/foldr apply to linked lists (or lazy streams, if you prefer) which are already inherently sequential, and gets a rather harsh treatment. I notice he points to finger trees, which I though was implemented in Data.Sequence. Direct URL for the slides: http://research.sun.com/projects/plrg/Publications/ICFPAugust2009Steele.pdf As he says, associativity is the key to parallelism -- an old observation, but still underappreciated. Even without parallelism, associativity also gives us greater freedom in structuring our solutions. The moral is that our datatypes need associative binary operations more than asymmetric ones. We use lists too much (because they're so convenient) and apart from the loss of parallelism it constrains our thinking to the sequential style criticised by Backus back in 1978. Regarding finger trees, he's just referring to the idea of caching the results of monoidal folds in nodes of a tree. That's crucial to the applications of finger trees, but it can be applied to any kind of tree. As he mentions, it's related to the Ladner-Fischer parallel prefix algorithm, which has an upward pass accumulating sums for each subtree followed by a downward accumulation passing each sum into the subtree to the right. But it's not just for parallelism: when you have these cached values in a balanced tree, you can compute the sum of any prefix in O(log n) steps. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: data-ordlist-0.2
Why not wrap lists as Set and Bag abstract datatypes? An added bonus is that you could make them instances of Monoid. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HList darcs repo missing?
On Mon, Jan 25, 2010 at 05:27:57PM -0500, Gwern Branwen wrote: It was there as of 15 September 2009 when I sent Oleg my last patch. Maybe some of the server changes since messed around with it? Most things that could be moved to community.haskell.org weren't moved across to the new machine: http://www.haskell.org/pipermail/haskell/2010-January/021861.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Non-termination due to context
On Fri, Jan 22, 2010 at 12:24:37PM +0100, Emil Axelsson wrote:
Consider the following program:
{-# LANGUAGE FlexibleInstances, OverlappingInstances, UndecidableInstances
#-}
class B a = A a
instance A Int
class Eq a = B a
instance (A a, Eq a) = B a
[...]
Although I don't know all the details of the class system, it seems
unintuitive that I can make a program non-terminating just by
changing the context of a function (regardless of
UndecidableInstances etc.).
Is this a bug or a feature?
I'm afraid you voided the warranty when you used UndecidableInstances.
You really do have a circularity between A and B here, so it's not
surprising that you get a loop. By changing the context, you demanded
more instances, undecidable ones in fact.
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Re: [Haskell-cafe] Non-ASCII characters in .cabal files break Haddock?
On Tue, Jan 19, 2010 at 01:00:07PM -0800, John Millikin wrote: I recently uploaded a package to Hackage[1], which contains a non-ASCII character in the .cabal file. That file is encoded with UTF-8, and it displays fine in Vim, Firefox, and even the Hackage package landing page. However, Haddock is failing to build it[2] -- apparently it can't handle UTF-8 encoded text properly. It seems that openTempFile writes the text without any encoding (#3832). Haddock quite reasonably fails because it expects encoded text. In documentation sections, Haddock supports special syntax for escaping non-ASCII. Is there any equivalent syntax for Cabal files? The Description field is haddock markup, so that should be a workaround. (or you could use more naive spelling) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parse error
On Sun, Jan 17, 2010 at 10:33:45AM +, Andrew Coppin wrote: Tony Morris wrote: main = do putStrLn Line 1 putStrLn Line 2 let xs = do x - [1..10] y - [1..10] return (x+y) print xs Urg, but that's *ugly*. Is there no way I can reduce the amount of indentation to something more reasonable? Sure: start a new line and indentation level after every where, let, do or of. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: amqp-0.1
On Sun, Jan 17, 2010 at 12:35:08PM +0100, Holger Reinhardt wrote: Get it here: http://hackage.haskell.org/package/amqp For some reason it doesn't compile on Hackage. It says At least the following dependencies are missing: network-bytestring =0.1.2. Is there a way to solve this? Sorry, that was a glitch in the upgrade to GHC 6.12.1. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hackage strangeness
On Fri, Jan 15, 2010 at 03:45:56PM +0100, Roel van Dijk wrote: I wrote a small cronjob to update my hackage reverse-deps. When checking if it worked I noticed that my hackage appears to be more up-to-date than the real thing! [...] Is this a case of data loss or something else? It's in the process of moving to a new (and hopefully more reliable) host. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hackage strangeness
On Fri, Jan 15, 2010 at 03:08:36PM +, Ross Paterson wrote: It's in the process of moving to a new (and hopefully more reliable) host. Done. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haddock question: hiding internal instances
On Fri, Jan 15, 2010 at 09:16:03PM +0100, Niklas Broberg wrote: The question I have is this: How can I get Haddock to omit listing instances of package-internal data types for exported classes? It's a known limitation: http://trac.haskell.org/haddock/ticket/37 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HackageDB Question
On Sat, Jan 09, 2010 at 01:44:03PM -0800, Gregory Crosswhite wrote: How does HackageDB generate the documentation for uploaded packages? Specifically, if I am using my own build system rather than Cabal, then what do I need to do (e.g., what interface do I need to supply) for HackageDB to correctly generate the documentation? I assume you're using Cabal, but you mean a custom Setup.hs. It relies on your Setup.hs doing the same as the simple one (as documented in the Cabal User's Guide) for setup configure --haddock-option=... setup haddock --html-location=... --hyperlink-source setup haddock --hoogle ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: error-message
On Sun, Dec 06, 2009 at 03:50:55PM -0500, Brent Yorgey wrote: So what we have here, it seems, is a type with at least two reasonable Applicative instances, one of which does *not* correspond to a Monad instance. My argument is that it is very strange (I might even go so far as to call it a bug) to have a type with an Applicative instance and a Monad instance which do not correspond, in the sense that pure = return (*) = ap There are several of these. Another is the possible Applicative instance for lists with pure = repeat (*) = zipWith id In these cases we usually make the Applicative instance match the Monad one and define an equivalent type for each other Applicative instance (e.g. ZipList in Control.Applicative). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] New Hackage category: Error Handling
On Sat, Dec 05, 2009 at 05:52:11PM +0200, Michael Snoyman wrote: For the record, I find this pedanticism misplaced, ... I think you'll find that's pedantry. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] module export question
On Tue, Dec 01, 2009 at 05:11:42PM -0500, Sean McLaughlin wrote: The problem is that I explicitly didn't export 't' as an element of T (by not writing T(..)). Am I just misunderstanding how exports work? I couldn't figure out what the correct behavior should be by looking at the 98 report. Exports (and imports) merely enumerate names, not their relationships, so the original export is equivalent to T(T, t) or T(..). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
