December 2014 - Libraries

PROPOSAL: Add instances to Data.Monoid newtype wrappers
by Sean Leather 13 Dec '14

13 Dec '14

As of base 4.7.0.1, none of the newtype wrappers in Data.Monoid have instances for: - Typeable - Data - Functor It looks like the base library repository has AutoDeriveTypeable, so I presume that takes care of the Typeable instances. It also looks like First and Last now have Functor instances, amongst others. In particular, I would like to add Functor instances for all parameterized types, where applicable (i.e. Dual, Sum, Product). In general, I think we should add whatever other instances make sense. Data came to mind. Others? Discussion period: 2 weeks (or perhaps until after the beginning of the new year) Regards, Sean

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Proposal: move (group|sort)With to Data.List
by amindfv＠gmail.com 11 Dec '14

11 Dec '14

GHC.Exts contains a couple of really useful functions: groupWith :: (Ord b) => (a -> b) -> [a] -> [[a]] sortWith :: (a -> b) -> [a] -> [a] There seems to be no reason why they wouldn't be -- more sensibly -- in Data.List. I propose we move them (and "the" while we're at it). Tom

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MonadPlus instance for WrappedMonad?
by Fumiaki Kinoshita 10 Dec '14

10 Dec '14

Hello, I'm glad that WrappedMonad is Monad since base-4.7.0.0, but I found out that MonadPlus instance is still missing. Is there any reason not to make MonadPlus m => MonadPlus (WrappedMonad m)? Fumiaki

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Proposal: relax type of asProxyTypeOf
by Daniel Wagner 07 Dec '14

07 Dec '14

In base, the new Data.Proxy module has this handy function: asProxyTypeOf :: a -> Proxy a -> a asProxyTypeOf = const This is nice for restricting the phantom type variable of a Proxy that you have lying around. This use could be generalized to other types with phantom type variables by making the type slightly more polymorphic: asProxyTypeOf :: a -> proxy a -> a Since phantom type variables are a very common pattern, this would make asProxyTypeOf more generally useful. Possible downside: it may be that somebody used asProxyTypeOf to restrict both the phantom type and the "Proxy" type, and that the more polymorphic type for asProxyTypeOf would cause that code to break. I find this pretty unlikely. Perhaps the folks behind the "tagged" package could mention whether this complaint ever came up when it made a similar generalization. ~d

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Data.Map.unzip?
by Joachim Breitner 06 Dec '14

06 Dec '14

Hi, did anyone else ever wanted to have a function Data.Map.unzip :: Map k (a, b) -> (Map k a , Map k b) and possibly unzipN for N >= 3? Greetings, Joachim -- Joachim “nomeata” Breitner mail(a)joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata(a)joachim-breitner.de • GPG-Key: 0xF0FBF51F Debian Developer: nomeata(a)debian.org

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Conversion between STRef RealWorld and IORef
by Wolfgang Jeltsch 03 Dec '14

03 Dec '14

Hi, I need to convert values of types STRef RealWorld a to IORef a in order to attach finalizers to them. For now, I am using GHC.IORef for this. Is there a better way to achieve what I want? All the best, Wolfgang

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Implement `calloc{,Bytes,Array,Array0}` allocators
by Oleksandr Petrov 02 Dec '14

02 Dec '14

Hi everyone, There is a proposal [1] (with implemented revision) to implement zero-initialising versions of malloc{,Bytes,Array,Array0} - Add calloc and callocBytes to Foreign.Marshal.Alloc. - Add callocArray and callocArray0 to Foreign.Marshal.Array. The benefit is that you can allocate required amount of memory and be certain that it will be cleared (set with zeroes) before you use it. Original discussion took a place in `fillBytes` [1] and was publicised on that mailing list [2]. Please share your thoughts. Thank you. [1] https://phabricator.haskell.org/D527 [2] https://phabricator.haskell.org/D461#11541 [3] https://www.haskell.org/pipermail/libraries/2014-November/024251.html -- alex p

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Proposal unix. Change to `IO (Maybe String)`: getLoginName, getUserEntry* and getGroupEntry*
by Thomas Miedema 01 Dec '14

01 Dec '14

In the unix package, in System.Posix.User, we have the following functions [1]: * getLoginName (getlogin) * getUserEntryForID (getpwuid_r) * getGroupEntryForID (getgrgid_r) * getUserEntryForName (getpwnam_r) * getGroupEntryForName (getgrnam_r) They have signature `IO String`, and use `throwErrnoIfNull` to call the c functions listed in parenthesis. ## Proposal Change the signature of the functions listed above to `IO (Maybe String)`. The new semantics would be: * If the c function returns a NULL pointer with errno set to ENOENT, meaning the given user or group could not be found, the result is Nothing. * If another error occured, an error is thrown (no change). * Otherwise the result is Just "result". ## Motivation At least `getlogin` and `getgrgid` are unreliable on Linux. It is possible for them to return NULL pointers [2] even when the user is logged in and has an associated user entry in /etc/passwd and group entry in /etc/group. Examples: * when `getLoginName` is called using a terminal emulator that doesn't write login records to /var/run/utmp, or inside screen/tmux, `getlogin` tends to return NULL pointers [3]. * `getGroupEntryForID` can throw a NULL pointer exception inside chroots or other environments where the information in /etc/groups is not to be considered reliable [4]. Since we can, let's give the above Haskell functions a type safe(r) api. If the proposal gets rejected, I will just add a warning message to the docstrings of the functions in question and permanently disable their tests from the unix testsuite, since this is currently causing problems [5,6]. Discussion period: 2 weeks. ## Links [1] https://github.com/haskell/unix/blob/master/System/Posix/User.hsc [2] http://linux.die.net/man/3/getlogin [3] https://github.com/haskell/unix/blob/cad1ef27bb0a51bc68ebadb1297de1ae05bee9… [4] https://ghc.haskell.org/trac/ghc/ticket/8293 [5] http://www.haskell.org/pipermail/ghc-devs/2014-September/006420.html [6] https://ghc.haskell.org/trac/ghc/ticket/1487#comment:23 ## Details * Another function in the same module that needs to change is `getEffectiveUserName`, because it calls the function `getUserEntryForID` mentioned before. * The following functions don't need to change, since they return the empty list if no entries are found: - getGroups (getgroups) - getAllGroupEntries (getgrent) - getAllUserEntries (getpwent) * The rest of the functions in the module don't need to change either, since they are not expected to return NULL pointers, according to their respective man pages: - getRealUserID (getuid) - getRealGroupID (getgid) - getEffectiveUserID (geteuid) - getEffectiveGroupID (getegid)

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