June 2012 - Libraries

Proposal: Use time rather than old-time in directory
by Ian Lynagh 16 Sep '14

16 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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A more useful Monoid instance for Data.Map
by Daniel Peebles 13 Mar '13

13 Mar '13

Hi all, Currently Data.Map has a Monoid instance, but it's rather lossy and not as general as it could be: instance (Ord k) => Monoid (Map k v) where mempty = empty mappend = union mconcat = unions The instance would be much nicer if it required a Monoid on v and used unionWith mappend instead of just union. The current behavior could be emulated by using First/Last as the Monoid (or ideally a semigroup, but that's or another discussion), but other more interesting Monoid instances could also be used for the values. I realize that changing instances could break code, but I'd be curious to see how many people even use the current monoid instance. Does anyone have any system for testing hypotheses like this (by typechecking a large randomized chunk of hackage or something)? It looks like the idea has been around for at least five years: http://hackage.haskell.org/trac/ghc/ticket/1460 but the proposal was abandoned, so I wanted to see if I could get people to start talking about it again. Thanks, Dan

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Changes to Typeable
by Simon Peyton-Jones 26 Nov '12

26 Nov '12

Friends (Ian pointed out that I should have sent this to the libraries list): The page describes an improved implementation of the Typeable class, making use of polymorphic kinds. Technically it is straightforward, but it represents a non-backward-compatible change to a widely used library, so we need to make a plan for the transition. http://hackage.haskell.org/trac/ghc/wiki/GhcKinds/PolyTypeable Comments? You can fix typos or add issues directly in the wiki page, or discuss by email Simon

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Proposal: Remove Control.Concurrent{QSem,QSemN,,SampleVar,mergeIO,nmergeIO} from base
by Ian Lynagh 12 Nov '12

12 Nov '12

Hi all, I propose that we remove Control.Concurrent.QSem Control.Concurrent.QSemN Control.Concurrent.SampleVar Control.Concurrent.mergeIO Control.Concurrent.nmergeIO from base. Rationale: There is no real need for them to be in base, and they are not being well-maintained there (e.g. see [3160]). Chris Kuklewicz has already made a SafeSemaphore package that provides fixed versions of the 3 modules. Patch is attached (but basically it just removes the above). Suggested discussion deadline: Fri 22 June 2012. [3160] http://hackage.haskell.org/trac/ghc/ticket/3160 Thanks Ian

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Haskell Platform proposal: Add the vector package
by Johan Tibell 08 Oct '12

08 Oct '12

Hi, I am, with Roman's support, making a formal proposal to have the vector package included in the Haskell Platform: http://trac.haskell.org/haskell-platform/wiki/Proposals/vector See the wiki page for details, and a list of open issues for discussion. The vector package itself can be found on Hackage: http://hackage.haskell.org/package/vector I have set the deadline to 13 August 2012 (~2 months' time), with comments due by 13 July 2012 (~one month). There's plenty of time before the next platform release anyway (scheduled for 12 November 2012). -- Johan

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Proposal: unsnoc, unsafeInit and unsafeLast for the bytestring library
by Mikhail Vorozhtsov 28 Aug '12

28 Aug '12

Hi. I propose to add the missing counterparts of `uncons` and `unsafeHead/Tail` functions to the bytestring library. Discussion deadline: 2 weeks The patch is attached.

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Proposal: add an order-reversing newtype to Data.Ord
by ARJANEN Loïc Jean David 18 Jul '12

18 Jul '12

Hello all, I recently noticed that despite conversations about that in this mailing-list, there isn't an order-reversing newtype in Data.Ord. That newtype would be quite useful, e.g. for reverse-sorting a list or a set, so I propose we add a newtype defined thusly: newtype Reverse a = Reverse { fromReverse :: a } deriving (Eq) instance Ord a => Ord (Reverse a) where compare (Reverse x) (Reverse y) = compare y x I took the implementation from the Down newtype in GHC.Exts, so we could perhaps just re-export it. This will cause breakage to code already defining a type named Reverse, so I don't know if the damage will be important. Of course, the name and definition are open to discussion. Deadline: 12 July 2012. P.S. : A conversation about that took place here: http://haskell.1045720.n5.nabble.com/A-Down-newtype-in-Data-Ord-td5005373.h… -- ARJANEN Loïc Jean David http://blog.luigiscorner.com --- "Computer science is no more about computers than astronomy is about telescopes, biology is about microscopes, or chemistry is about beakers and test tubes. Science is not about tools. It is about how we use them, and what we find out when we do." Michael R. Fellows and Ian Parberry

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Proposal: Control.Concurrent.Async
by Simon Marlow 10 Jul '12

10 Jul '12

I'd like to add a higher-level concurrency API to the base package. This API has grown while I've been working on examples for my book, and I'm convinced that we should have something like this in the standard libraries. Here's the API I propose: http://community.haskell.org/~simonmar/async-stm/Control-Concurrent-Async.h… In fact it already exists in at least two packages on Hackage: 'async' and 'threads'. My version is a superset of both of these, except for a few small differences (which are up for discussion, of course). One good reason to have this package is that it avoids some common pitfalls, such as forgetting to handle exceptions in your child threads. In the Async API, you get to choose whether to (a) receive the result as 'Either SomeException a', or (b) have the exception re-thrown in the waiting thread. You don't get to ignore it altogether. Another common pitfall avoided by this library is accidentally leaving threads running in the background. This is handled by withAsync: withAsync :: IO a -> (Async a -> IO b) -> IO b the child thread is always cancelled (i.e. killed) when the second argument returns or throws an exception, if it hasn't finished already. I'm opening this up for discussion so that we can tidy up any inconsistent details and add any functions that people feel are missing. Naming is obviously up for discussion too. Cheers, Simon

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Proposal: Make digitToInt and intToDigit work up to base 36
by Twan van Laarhoven 05 Jul '12

05 Jul '12

Hello List, I propose to change the functions digitToInt and intToDigit in Data.Char to digitToInt :: Char -> Int digitToInt c | isDigit c = ord c - ord '0' | c >= 'a' && c <= 'z' = ord c - ord 'a' + 10 | c >= 'A' && c <= 'Z' = ord c - ord 'A' + 10 | otherwise = error ("Char.digitToInt: not a digit " ++ show c) intToDigit :: Int -> Char intToDigit i | i >= 0 && i <= 9 = toEnum (fromEnum '0' + i) | i >= 10 && i <= 35 = toEnum (fromEnum 'a' + i - 10) | otherwise = error "Char.intToDigit: not a digit" (and equivalently for the Ghc specialized version in Data.Show) Right now the functions only work for c <= 'f' and i <= 15, i.e. only up to hexadecimal. But I can think of no reason why that should be the case. There is the problem of compatibility with the Haskell98/2010 report. But since this proposed change only reduces the number of cases that are errors, I think that is not a big concern. Discussion period: 2 weeks, deadline July 13. Twan

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Proposal: add Functor instance to Control.Exception.Handler
by Patrick Palka 03 Jul '12

03 Jul '12

A Functor instance for Handler could be useful: e.g. foo :: IO (Maybe a) handlers :: [Handler ()] foo `catches` map (Nothing <$) handlers Proposed instance definition: instance Functor Handler where fmap f (Handler h) = Handler (fmap f . h) Deadline: 2 weeks

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