March 2012 - Libraries

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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Changes to Typeable
by Simon Peyton-Jones 27 Nov '12

27 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: add Functor instance to Control.Exception.Handler
by Patrick Palka 04 Jul '12

04 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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Proposal: Add newUniqueSTM to Data.Unique
by Edward Kmett 28 Apr '12

28 Apr '12

Data.Unique's newUnique uses atomically internally to update a TVar. Consequently attempting to "unsafeIOToSTM newUnique" to allocate a Unique within an STM transaction blows you sky high. The proposal is to split the definition of newUnique into: newUnique = atomically newUniqueSTM and expose newUniqueSTM. Alternately, since bug #3838 has been resolved, we could revert to using an IORef, which would avoid the wart of having us have an overtly IO action detonate because of an internal implementation detail of using STM. Since, there are two paths forward, I haven't put forward a patch, but wanted to open a discussion. Discussion Period: 2 weeks. -Edward Kmett

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documentation patch for process
by Evan Laforge 17 Apr '12

17 Apr '12

I recently burned some time because I didn't realize that System.Process.createProcess doesn't report when the binary isn't found. I guess I was used to that behaviour from python's subprocess module, which goes to some effort to report the error from the fork()ed child when exec() fails. After a failed solution, I did some poking around in the source to see exactly what haskell's process does. In the interests of saving someone else that time, I submit a patch to process to add some documentation. I'm not sure how to do a 'darcs send' with git, but here's the patch: % git log | head commit 5a46937961b3400fa58ccffc785d0de8c1c01d94 Author: Evan Laforge <qdunkan(a)gmail.com> Date: Wed Feb 8 13:25:16 2012 -0800 More detailed haddock for createProcess: document what happens if the binary is not found commit c8b30a6f1d493ab724f2da05dbc49496b119a051 Author: Simon Marlow <marlowsd(a)gmail.com> Date: Mon Jan 16 16:25:34 2012 +0000 % git diff HEAD\^ diff --git a/System/Process.hs b/System/Process.hs index f3a8f9b..a372228 100644 --- a/System/Process.hs +++ b/System/Process.hs @@ -234,9 +234,18 @@ To create a pipe from which to read the output of @ls@: To also set the directory in which to run @ls@: > (_, Just hout, _, _) <- -> createProcess (proc "ls" []){ cwd = Just "\home\bob", +> createProcess (proc "ls" []){ cwd = Just "/home/bob", > std_out = CreatePipe } +Note that this does /not/ throw an error if the binary is not found, +or not executable. Instead, the process will fail with the exit code 127. +But you can't rely on it failing quickly, because it may e.g. hang up +searching the PATH on a slow NFS mount. Since there's no way to tell when +the exec() has succeeded, the only way to reliably know if the createProcess +failed is to 'waitForProcess'. + +If the CreateProcess record's 'cwd' directory doesn't exist, the exit code +will be 126. -} createProcess :: CreateProcess diff --git a/System/Process/Internals.hs b/System/Process/Internals.hs index 11de1f4..2a6c047 100644 --- a/System/Process/Internals.hs +++ b/System/Process/Internals.hs @@ -199,7 +199,8 @@ data CmdSpec = ShellCommand String -- ^ a command line to execute using the shell | RawCommand FilePath [String] - -- ^ the filename of an executable with a list of arguments + -- ^ The filename of an executable with a list of arguments. + -- The PATH is searched for the executable. data StdStream = Inherit -- ^ Inherit Handle from parent

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Proposal: add clearEnv and setEnvironment to System.Posix.Env
by Paolo Capriotti 05 Apr '12

05 Apr '12

Hi all, System.Posix.Env doesn't currently define a binding for the POSIX function clearenv. I propose to add: clearEnv :: IO () setEnvironment :: [(String, String)] -> IO () 'clearEnv' is a direct wrapper for the underlying function, while 'setEnvironment' is a higher-level utility function which mirrors the interface of the existing 'getEnvironment'. See attached patches here: http://hackage.haskell.org/trac/ghc/ticket/5648. Discussion deadline: 24 Mar 2012. BR, Paolo

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I plan to usurp plugins, speak now or forever hold your peace.
by Jeremy Shaw 01 Apr '12

01 Apr '12

Hello all, Don Stewart seems to have abandon plugins and has not (as far as I know) responded to any recent inquiries about it. It needs a few patches to build under GHC 7.0 and 7.4. Since there has been no response for several months, I intend to usurp the project and upload a new version to hackage in a few days, unless I hear any strong objections. My repository lives here at the moment: http://src.seereason.com/plugins/ I will likely migrate it to patch-tag. However, patch-tag only supports darcs 2 format, and plugins is darcs 1 format. There is no way to share patches between darcs 1 and darcs 2 repos -- so I am not going to make the change quite yet. - jeremy

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Proposal: add traverseWithKey to Data.Map
by Max Bolingbroke 31 Mar '12

31 Mar '12

Hi Haskellers, I propose to add a new function, traverseWithKey, to Data.Map: """ -- | /O(n)/. -- @'traverseWithKey' f s == 'fmap' 'fromList' ('traverse' (\(k, v) -> fmap ((,) k) (f k v)) ('toList' s))@ -- That is, behaves exactly like a regular 'traverse' except that the traversing -- function also has access to the key associated with a value. -- -- > traverseWithKey (\k v -> if k + v < 10 then Just (v + 1) else Nothing) (fromList [(1, 2), (5, 4)]) == Just (fromList [(1, 3), (5, 5)]) -- > traverseWithKey (\k v -> if k + v < 10 then Just (v + 1) else Nothing) (fromList [(5, 5)]) == Nothing traverseWithKey :: Applicative t => (k -> a -> t b) -> Map k a -> t (Map k b) """ This is a rather useful function, and if we define it in SATed style and with an INLINE pragma (as in my attached patch), GHC can generate really good code for it at use sites. While the utility of a traversal function like this is clear, we can also use it to define other useful combinators such as mapWithKeyM/mapMWithKey. It can also be used to define efficient right/left with-key-folds on Map that avoid a problem in GHC's strictness analyser. For example, this function: """ c m = M.foldlWithKey (\k v (a, b) -> if k + v > 2 then (a, b) else (b, a)) (0, 1) m """ Generates a loop which allocates a new pair on every iteration. (The situation is the same with foldrWithKey.). If we have traverseWithKey, we can work around it by defining an optimised fold for accumulators that are pairs: """ newtype State2L s1 s2 a = State2L { unState2L :: s1 -> s2 -> (s1, s2) } instance Functor (State2L s1 s2) where fmap _ = State2L . unState2L instance Applicative (State2L s1 s2) where pure _ = State2L (,) mf <*> mx = State2L $ \s1 s2 -> case unState2L mf s1 s2 of (s1, s2) -> unState2L mx s1 s2 -- NB: left side first {-# INLINE foldl2WithKey' #-} foldl2WithKey' :: ((a1, a2) -> k -> v -> (a1, a2)) -> (a1, a2) -> M.Map k v -> (a1, a2) foldl2WithKey' f (a1, a2) kvs = unState2L (traverseWithKey (\k v -> State2L $ \a1 a2 -> f (a1, a2) k v) kvs) a1 a2 """ The loop resulting from foldl2WithKey' does not allocate any pairs when compiled with current GHCs. Of course, traverseWithKey is also sufficiently general that it can implement many functions already exported from Data.Map (with varying degrees of efficiency), such as find{Max,Min}, mapWithKey, fold{r,l}WithKey, withAccumWithKey and mapAccumL. There is precedence for the "traverseWithKey" name: 1. "unordered-containers" has a function with exactly this name and compatible type, used for HashMaps [1] 2. The "keys" package defines a type class TraversableWithKey with a compatible type [2] This is an API addition so breakage from the change should be low. Overall this should be a low risk addition to the interface which adds a lot of flexibility. What do you think? Deadline: 2 weeks (i.e. 28th March) Patch is attached, assuming it doesn't get stripped by the mailing list manager. Cheers, Max [1] http://hackage.haskell.org/packages/archive/unordered-containers/0.1.1.0/do… [2] http://hackage.haskell.org/packages/archive/keys/0.1.0/doc/html/src/Data-Ke…

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Fwd: Tiny little bug in Network.URI
by Graham Klyne 28 Mar '12

28 Mar '12

It's just been brought to my attention that there's a small bug in the Network.URI code I wrote many years ago. I'll happily fix this, but I'm not sure what are the current mechanisms for accessing/updating the library repositories these days. Can anyone offer a pointer? (I've checked the error report against RFC 3986, and confirm that the proposed change seems correct. Ideally, there should be a test case for this.) Thanks to Eugen Kuksa for pointing this out. [[ In http://hackage.haskell.org/packages/archive/network/latest/doc/html/src/Net… at ipvFuture :: URIParser String ipvFuture = do { char 'v' ; h <- hexDigitChar ; char '.' ; a <- many1 (satisfy isIpvFutureChar) ; return $ 'c':h:'.':a } there should be ; return $ 'v':h:'.':a ]] #g --

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ANNOUNCE: options, an easy-to-use command-line option parser
by John Millikin 25 Mar '12

25 Mar '12

Hackage: http://hackage.haskell.org/package/options Home page: https://john-millikin.com/software/haskell-options/ API reference: https://john-millikin.com/software/haskell-options/reference/haskell-option… The options package lets library and application developers easily work with command-line options. The following example is a full program that can accept two options, --message and --quiet: ---------------------------------------------------------- {-# LANGUAGE TemplateHaskell #-} import Options defineOptions "MainOptions" $ do stringOption "optMessage" "message" "Hello world!" "A message to show the user." boolOption "optQuiet" "quiet" False "Whether to be quiet." main :: IO () main = runCommand $ \opts args -> do if optQuiet opts then return () else putStrLn (optMessage opts) ---------------------------------------------------------- ---------------------------------------------------------- $ ./hello Hello world! $ ./hello --message='ciao mondo' ciao mondo $ ./hello --quiet $ ---------------------------------------------------------- In addition, this library will automatically create documentation options such as --help and --help-all: ---------------------------------------------------------- $ ./hello --help Help Options: -h, --help Show option summary. --help-all Show all help options. Application Options: --message A message to show the user. --quiet Whether to be quiet. ----------------------------------------------------------

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