April 2013 - Libraries

Proposal: merge either into transformers
by Roman Cheplyaka 03 Jan '17

03 Jan '17

I propose to add the sole module of the 'either' package[1], Control.Monad.Trans.Either, to the transformers package. It provides EitherT, a very basic and fundamental data type. The difference between EitherT and ErrorT is that the latter has an Error constraint, which is used to imlement 'fail'. Note that 'either' depends on the 'semigroupoids' and 'semigroup' packages to provide appropriate instances. The proposal is not to add those instances to 'transformers' to avoid additional dependencies. The instances can then be left in the 'either' package or moved to the 'semigroupoids' and 'semigroup' packages respectively. ('semigroupoids' already depends on 'transformers', while 'semigroups' does not). Compared to the 'either' package, Show, Read, Eq and Ord instances will be dropped to keep the code Haskell2010 (those instances require FlexibleInstances, FlexibleContexts, and UndecidableInstances). The patch is attached. [*] [*] against transformers-0.3.0.0, because the darcs version is not buildable (Control/Monad/Signatures.hs is not in the repository). Roman

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Proposal: Use time rather than old-time in directory
by Ian Lynagh 17 Sep '14

17 Sep '14

Hi all, This proposal is to change directory to use the time package rather than the old-time package. This will hopefully be a significant step towards actually making the transition in general. I'm not sure if POSIXTime or UTCTime is the better type to use. Does anyone know? I started off with UTCTime, then switched to POSIXTime as UTCTime didn't have a Show instance. I've since found that actually UTCTime has both Show and Read instances, but they are orphaned. So actually, if UTCTime is the better type then switching back to UTCTime would allow us to fix this comment in hpc: -- We would rather use ClockTime in Mix, but ClockTime has no Read instance and simplify the code slightly. Attached are patches for directory, hpc and ghc. Suggested discussion period: Until 5th Nov 2011. Thanks Ian

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Re: Proposal: Add hasBitSize to Data.Bits.Bits
by John Lato 19 Sep '13

19 Sep '13

> From: Edward Kmett <ekmett(a)gmail.com> > > On Fri, Jul 27, 2012 at 3:42 PM, Ian Lynagh <igloo(a)earth.li> wrote: > >> On Fri, Jul 27, 2012 at 03:06:04PM -0400, Edward Kmett wrote: >> > There is currently no way to know whether or not calling >> > Data.Bits.bitSizewill crash your program. >> > >> > I propose extending the Bits class to include: >> > >> > hasBitSize :: Bits b => b -> Bool >> > >> > such that it returns False for Integer and True for the other instances > > Would it be better to move bitSize into a separate class? > > > I don't think so. Ultimately, you should be able to know for every numeric > type if it has a fixed or variable number of bits. In a perfect world 'd > rather just have bitSize return a Maybe that way we don't have to truck > with partial functions in the API. > > However, the case for just adding hasBitSize is that it avoids breaking > existing code. Given that existing code using bitSize can already cause a crash when used with Integer or other infinite types, wouldn't it be better to break existing code so authors can fix (or at least examine) their use of bitSize? I'm in favor of changing the type of bitSize, although since none of my code uses that function it's an easy position for me to take. John L.

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Data instance for Data.Version
by David Lazar 13 Sep '13

13 Sep '13

Hi, Can we derive Data for Data.Version.Version? It would benefit Cabal. Thanks, David

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Re: [commit: unix] master: change notes (1461d21)
by Ian Lynagh 24 Aug '13

24 Aug '13

On Fri, Feb 08, 2013 at 01:56:28PM +0000, Simon Marlow wrote: > On 03/02/13 20:48, Simon Marlow wrote: > >On 02/02/13 14:11, Ian Lynagh wrote: > >> > >>The unix version number is still 2.6.1.0, but doesn't the new policy > >>http://hackage.haskell.org/trac/ghc/wiki/Repositories#Modifyinglocalpackages > >> > >>mean that it should be bumped to 2.7.0.0 now? Actually, having just tried to do this, I'm not sure how it's meant to work in practice. Bumping unix's version requires relaxing the dependencies in Cabal and haskeline on unix. One way to do this is to get upstream to relax the dependency and updating our lagging repo. That will mean a version of the library will be uploaded to hackage with the relaxed dependency. But we don't know what the release 2.7.0.0 of unix will look like yet; perhaps by the time it is released it will not work with the current Cabal/haskeline code. The other way is to make local ghc-only changes to relax the dependencies, but that would increase the risk that we'd release with a ghc branch of the library by mistake. Wouldn't it be better to go back to the previous scheme, where we didn't update version numbers until release time? Thanks Ian

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Re: new-typeable, new cast?
by José Pedro Magalhães 01 Aug '13

01 Aug '13

Hi, On Mon, Mar 4, 2013 at 4:51 PM, Richard Eisenberg <eir(a)cis.upenn.edu> wrote: > Unless I'm missing something, this all looks very straightforward. The > implementation is there, already, in the guise of (gcast Refl). We would > just shuffle the definitions around a bit. Am I indeed missing something? > I think that is the case indeed. Though I agree that it would be a nice addition to Data.Typeable. Cheers, Pedro > > As for the name, I agree that :~: is awkward to type. But, I think == > should be saved for type-level Boolean equality, as a closer analogue to > its term-level meaning. I don't have a better suggestion than :~:. Or, if > we just allow Constraint/* polymorphism, than we can simply use ~, as that > *already* is a GADT wrapping around an even more primitive denotation of > equality. (Just kidding about the Constraint/* polymorphism…) > > Richard > > On Mar 4, 2013, at 10:30 AM, Simon Peyton-Jones wrote: > > > ok good. Since this is in the intersection of singletons and Typeable, I > wonder if Pedro and Richard might together lead on driving this to a > conclusion and implementing it? > > > > Simon > > > > | -----Original Message----- > > | From: Stephanie Weirich [mailto:sweirich@cis.upenn.edu] > > | Sent: 04 March 2013 14:24 > > | To: Shachaf Ben-Kiki > > | Cc: Simon Peyton-Jones; libraries(a)haskell.org; Richard Eisenberg > > | (eir(a)cis.upenn.edu) Dimitrios Vytiniotis; Iavor Diatchki; Conor > > | McBride; Pedro Magalhães > > | Subject: Re: new-typeable, new cast? > > | > > | Nice! I agree that having a (poly-kinded!) function of type > > | > > | >> eqT :: forall (a :: k) (b :: k). (Typeable a, Typeable b) => Maybe > (a > > | >> :~: b) > > | > > | around is The Right Way to Go. I'd call this function "eqT" or > > | "eqTypeable", after the similar operation in Data.Type.Equality. Thanks > > | for pointing out that we can get such a function so easily from gcast. > > | (Note that gcast itself can be polykinded now, I'm writing down the > > | kinds to make them obvious.) > > | > > | gcast :: forall (a :: k) (b::k) (c::k ->*). (Typeable a, Typeable b) -> > > | c a -> Maybe (c b) > > | > > | WRT to gcast1 and gcast2, these functions can be subsumed by eqT, but > > | not by polykinded gcast due to the restrictions on higher-order > > | polymorphism. > > | > > | gcast1 can convert [b Int] to [Maybe Int], but gcast cannot. eqT can do > > | so with a pattern match. This behavior does seem rather special-purpose > > | though---eqT is much more flexible. > > | > > | With singletons, we could imagine two different polykinded operations, > > | that differ only in whether their arguments are explicit/implicit. > > | > > | eqTypeable :: (Typeable a, Typeable b) => Maybe (a :~: b) > > | > > | eqSing :: Sing a -> Sing b -> Maybe (a :~: b) > > | > > | Again, Sing should be kind-indexed, so that we can have Sing True, Sing > > | 0, and (eventually) Sing Int. > > | > > | --Stephanie > > | > > | > > | On Mar 4, 2013, at 8:46 AM, Shachaf Ben-Kiki wrote: > > | > > | > On Mon, Mar 4, 2013 at 4:24 AM, Simon Peyton-Jones > > | > <simonpj(a)microsoft.com> wrote: > > | >> | λ> data a :~: b where Refl :: a :~: a > > | >> | λ> :t gcast Refl > > | >> | gcast Refl :: forall a b. (Typeable a, Typeable b) => Maybe > > | >> | ((:~:) * a b) > > | >> > > | >> That is a ferociously-unintuitive use of 'gcast'! It too me dome > > | while to convince myself that it was right. Though it does indeed > > | work. I'd see (gcast Refl) as the primitive, with 'gcast' itself as a > > | simple derived function. > > | >> > > | >> I'm not sure what name to give the new cast function > > | >> ?? :: Typeable a, Typeable b) => Maybe (a :~: b) > > | >> > > | >> > > | >> Do we need gcast1 and gcast2 any more, in our new polykinded world? > > | Can they be depreceated? > > | >> > > | >> Simon > > | >> > > | > > > | > A couple of people have suggested "same". (I wonder if this would > also > > | > subsume eqSingNat/eqSingSym?). > > | > > > | > By the way -- since (:~:) seems to be in HEAD now, but not yet > > | > released, is there a chance of renaming it? ":~:" is pretty awkward > to > > | > type, given all the new TypeOperators options these days; how about > > | > "=="? > > | > > > | > Shachaf > > | > > > > > > >

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Proposal: Strict scanl, scanl1 and mapAccumL
by Bas van Dijk 19 Jul '13

19 Jul '13

Hi, Data.List exports strict versions of foldl an foldl1. I think it should also export these strict versions of scanl, scanl1 and mapAccumL: scanl' :: (a -> b -> a) -> a -> [b] -> [a] scanl' f q ls = q : (case ls of [] -> [] x:xs -> let q' = f q x in q' `seq` scanl f q' xs) scanl1' :: (a -> a -> a) -> [a] -> [a] scanl1' f (x:xs) = scanl' f x xs scanl1' _ [] = [] mapAccumL' :: (acc -> x -> (acc, y)) -> acc -> [x] -> (acc, [y]) mapAccumL' _ s [] = (s, []) mapAccumL' f s (x:xs) = (s'',y:ys) where (s', y ) = f s x (s'',ys) = s' `seq` mapAccumL' f s' xs Is there a good reason they're not included? Discussion deadline: 2 weeks (till Monday 26 November). Regards, Bas

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A single general modification function for Map and IntMap proposal
by Nikita Volkov 15 Jul '13

15 Jul '13

There is a list of problems with the current "Map" and "IntMap" modification interfaces: - they are filled with quirky and too specialized functions - they are not consistent in terms of how equally named functions behave: http://hackage.haskell.org/packages/archive/containers/latest/doc/html/Data… - they still don't cover some important scenarios of use Because of the above I have very often found myself in requirement for the following function: withItem :: (Ord k) => k -> (Maybe i -> (r, Maybe i)) -> Map k i -> (r, Map k i) withItem k f m = let item = Map.lookup k m (r, item') = f item m' = Map.update (const item') k m in (r, m') It covers all the imaginable scenarios of modification operations: delete, update, replace, - yet it also provides one with ability to extract the modified data and not only. The problem is that this implementation involves a repeated lookup for the same item: first with "lookup", then with "update" - but the "containers" library exposes no functionality to get around that. So I suggest to implement an efficient version of "withItem" in the library. This function turns out to be far more generalized than any of the currently present in the library, so it can become a basic building block for all sorts of modifying functions, including all the already existing ones, e.g.: alter :: Ord k => (Maybe a -> Maybe a) -> k -> Map k a -> Map k a alter f k = snd . withItem k (\i -> ((), f i)) delete :: Ord k => k -> Map k a -> Map k a delete k = snd . withItem k (const ((), Nothing)) updateLookupWithKey :: (Ord k) => (k -> a -> Maybe a) -> k -> Map k a -> (Maybe a, Map k a) updateLookupWithKey f k = withItem k $ \i -> case i of Just i -> case f k i of Nothing -> (Just i, Nothing) Just i' -> (Just i', Just i') _ -> (Nothing, Nothing) You can see how easy it makes to achieve any sort of specialized functionality. So, besides the evident benefits, this function can also become a replacement for a whole list of confusing specialized ones, thus greatly lightening the library. You might have also noticed how this function is based around the standard "a -> (b, a)" pattern of the "State" monad, thus making it easily composable with it using the "state" and "runState" functions. Summarizing, my suggestions are: 1. Implement an efficient version of "withItem" for lazy and strict versions of "Map" and "IntMap". 2. Change the order of parameters from "lambda -> key" to "key -> lambda". The "updateLookupWithKey" example implementation shows how this change can be benefitial. 3. Begin the deprecation process of the following functions: insertWith, insertWithKey, insertLookupWithKey, adjust, adjustWithKey, update, updateWithKey, updateLookupWithKey, alter. A deadline for discussion is set to 6 weeks. For a formatted version of this message please visit https://github.com/haskell/containers/issues/28.

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Proposal: When possible, give more useful exceptions from rawSystem and friends
by Ian Lynagh 02 Jun '13

02 Jun '13

Hi all, Currently, if you try to use rawSystem to run a program that doesn't exist, you will just get a bad error code as a result, e.g. this program: import System.IO.Error import System.Process main :: IO () main = do (rawSystem "/bin/true" [] >>= print) `catchIOError` \e -> putStrLn ("Exc: " ++ show e) (rawSystem "/bin/false" [] >>= print) `catchIOError` \e -> putStrLn ("Exc: " ++ show e) (rawSystem "/non/existent" [] >>= print) `catchIOError` \e -> putStrLn ("Exc: " ++ show e) putStrLn "Done" prints: ExitSuccess ExitFailure 1 ExitFailure 127 Done However, if we are on a platform that supports vfork, then we can pass information from the child process back to the parent process as they share address space. With the attached patch we instead get: ExitSuccess ExitFailure 1 Exc: resolveProcessHandle: does not exist (No such file or directory) (and it should be easy to also get "/non/existent" in the exception). If there are platforms that don't support vfork, then they will still give the old output. I haven't yet looked at whether we can also get good exceptions on Windows (the patch won't build on Windows yet; I still need to update the Windows-specific code). I think that despite the possibility that some platforms may not be able to support it, we should provide the better behaviour on platforms that do. What do you think? And do you have any other comments on the patch? (this proposal was inspired by http://hackage.haskell.org/trac/ghc/ticket/7859) Suggested discussion deadline: Mon 13 May 2013. Thanks Ian

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instance Arbitrary Data.Map
by Henning Thielemann 01 Jun '13

01 Jun '13

It seems like the 'containers' package uses QuickCheck for testing, but does not export an Arbitrary Data.Map instance. Is this intended?

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