January 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

14 35

Proposal: Add newUniqueSTM to Data.Unique
by Edward Kmett 27 Apr '12

27 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

6 12

Something's weird about System.Directory.removeDirectoryRecursive
by John Millikin 09 Feb '12

09 Feb '12

http://hackage.haskell.org/packages/archive/directory/1.1.0.1/doc/html/Syst… The documentation says that removeDirectoryRecursive follows symlinks. However, the implementation doesn't (in most cases, see below). ------------------------------------------------------------------------------------------------------------ $ mkdir test-removeDirectoryRecursive $ cd test-removeDirectoryRecursive $ mkdir a b $ touch a/a.txt b/b.txt $ ln -s $PWD/b a/ $ ln -s $PWD/b/b.txt a/ $ ls -l a total 8.2k -rw-rw-r-- 1 john john 0 2012-01-27 23:33 a.txt lrwxrwxrwx 1 john john 65 2012-01-27 23:33 b -> /home/john/test-removeDirectoryRecursive/b lrwxrwxrwx 1 john john 71 2012-01-27 23:34 b.txt -> /home/john/test-removeDirectoryRecursive/b/b.txt # OK, a/ has a normal file and two symlinks in it. Let's recursively remove a/ and see what happens. $ ghci Prelude> import System.Directory Prelude System.Directory> removeDirectoryRecursive "a" Prelude System.Directory> Leaving GHCi. $ ls -l a b ls: cannot access a: No such file or directory b: total 0 -rw-rw-r-- 1 john john 0 2012-01-27 23:33 b.txt # a/ was removed -- good! # # b/ and its contents are untouched, good, but goes against the docs ------------------------------------------------------------------------------------------------------------ Now, there is one case where this function *will* follow symlinks. However, I believe it is a bug because it produces odd behavior: ------------------------------------------------------------------------------------------------------------ $ sudo mkdir a [sudo] password for john: $ sudo ln -s $PWD/b a/ $ ls -l a b a: total 4.1k lrwxrwxrwx 1 root root 65 2012-01-27 23:38 b -> /home/john/test-removeDirectoryRecursive/b b: total 0 -rw-rw-r-- 1 john john 0 2012-01-27 23:33 b.txt # Now a/ has a symlink, which cannot be deleted, because its containing directory is read-only to the current user. $ rm a/b rm: remove symbolic link `a/b'? y rm: cannot remove `a/b': Permission denied # What happens if removeDirectoryRecursive is called now? $ ghci Prelude> import System.Directory Prelude System.Directory> removeDirectoryRecursive "a" *** Exception: a/b: removeDirectory: permission denied (Permission denied) Prelude System.Directory> Leaving GHCi. $ ls -l a b a: total 4.1k lrwxrwxrwx 1 root root 65 2012-01-27 23:38 b -> /home/john/test-removeDirectoryRecursive/b b: total 0 # a/ is untouched, but b/ has been emptied! ------------------------------------------------------------------------------------------------------------ So what is the expected behavior of this function? What should it do in the presence of symlinks? IMO, the function should be documented as *not* following symlinks, and the directory check should be changed so that it returns False for symlink-to-directory.

3 2

Template Haskell: dataToExpQ and constructor mis-match
by Conal Elliott 08 Feb '12

08 Feb '12

I'm looking for a reliable way to turn values into Template Haskell expressions. I thought dataToExpQ from Language.Haskell.TH.Quote was the ticket, but it appears to produce constructors different from those produced by [| ... |], including [], (:), (), and (,). Here's an example run for (). I'm using template-haskell-2.5.0.0: GHCi, version 7.0.4: http://www.haskell.org/ghc/ :? for help Prelude> import Data.Data Prelude Data.Data> import Language.Haskell.TH Prelude Data.Data Language.Haskell.TH> :set -XTemplateHaskell Prelude Data.Data Language.Haskell.TH> import Language.Haskell.TH.Quote Prelude Data.Data Language.Haskell.TH Language.Haskell.TH.Quote> u1 <- runQ (dataToExpQ (const Nothing) ()) Prelude Data.Data Language.Haskell.TH Language.Haskell.TH.Quote> u2 <- runQ [| () |] Prelude Data.Data Language.Haskell.TH Language.Haskell.TH.Quote> u1 ConE () Prelude Data.Data Language.Haskell.TH Language.Haskell.TH.Quote> u2 ConE GHC.Unit.() Prelude Data.Data Language.Haskell.TH Language.Haskell.TH.Quote> u1 == u2 False Any suggestions? -- Conal

2 1

Improve System.Environment haddocks
by Gwern Branwen 07 Feb '12

07 Feb '12

17:52:34 < goodgrue> Is there a way to change the environment variables associated with Xmonad.Prompt.Shell at runtime? e.g. I'd like to be able to modify $http_proxy and have that affect subsequently launched processes. 17:55:31 < gwern> goodgrue: that'd be kind of hard. you could prepend a mpodification of $HTTP_PROXY to each command 17:56:10 < gwern> ideally you'd modify xmonad's own environment 17:56:33 < goodgrue> gwern: yeah, the latter is what I was thinking of. is that possible? 17:57:08 < gwern> shell uses `getEnv` from System.Environment but there's no setEnv! 17:57:21 < gwern> if there were, you could probably bind it to a key and use a prompt 17:58:44 < goodgrue> Hm...sounds like this may be a nice piece of missing functionality. is there any reason you can think of that i wouldn't be able to implement setEnv myself? 17:59:04 < gwern> http://hackage.haskell.org/trac/ghc/ticket/3587 17:59:14 < gwern> looks like 'putEnv' may be the real name -- gwern http://www.gwern.net

2 3

Add meaningful instances to ZipList
by Daniel Peebles 01 Feb '12

01 Feb '12

I noticed that the ZipList newtype in Control.Applicative has no derived instances at all. We can't show a ZipList, check it for equality, or do anything to it except unpack it or treat it like an Applicative or Functor. It seems fairly uncontroversial to suggest adding some instances for it, but there are some minor questions to address: the obvious instances to add might be Eq, Ord, Show, Read, and possibly Typeable and/or Data. Are there any others I've missed? For the Read/Show instances, do we want manual ones that don't use the verbose record-style output? Since it's just defined as data ZipList a = ZipList { getZipList :: [a] } to get an easy accessor (does anyone actually use getZipList as a field for record updates?), it might be easier to write the projection manually and change the definition to data ZipList a = ZipList [a] with a separate getZipList function, so the deriving mechanism can give us a less noisy Show instance. Discussion period: 2 weeks, I guess - Dan

6 9

printf too strict?
by Conal Elliott 27 Jan '12

27 Jan '12

I'm seeing more strictness than I'd expect for printf: > printf "foo %s\n" (show ([1..10] ++ undefined)) foo *** Exception: Prelude.undefined In contrast, *Utils.Fabprim.ToHaskell> "foo " ++ show ([1..10] ++ undefined) ++ "\n" "foo [1,2,3,4,5,6,7,8,9,10*** Exception: Prelude.undefined Known issue? -- Conal

2 1

Proposal: more general unionWith for Data.Map
by Christian Sattler 26 Jan '12

26 Jan '12

2012/1/24 Johan Tibell <johan.tibell(a)gmail.com> > > On Tue, Jan 24, 2012 at 9:35 AM, Christian Sattler > <sattler.christian(a)gmail.com> wrote: > > There are some high-level operations on maps which take two tree traversals > > only because the interface fails to expose sufficiently general functions. > > This proposal is concerned with an analogue of unionWithKey of type Ord k => > > (k -> a -> a -> Maybe a) -> Map k a -> Map k a -> Map k a, with the intended > > semantics that if a key is present in both maps and the operation applied to > > the key and corresponding values returns Nothing, the key is deleted from > > the result. > > Is union really an appropriate name here? I expect the following to hold: > > forall k ∊ (keys m1) ∪ (keys m2) => k ∊ (m1 ∪ m2) > > This means that keys must not be deleted by the union operator. > Perhaps 'merge' is a better name. > > Cheers, > Johan I don't care much about the naming, but note that the analogous property already fails for the generalized intersectionWithKey in the development version.

5 11

Proposal: more general unionWith for Data.Map
by Christian Sattler 25 Jan '12

25 Jan '12

Hi all, There are some high-level operations on maps which take two tree traversals only because the interface fails to expose sufficiently general functions. This proposal is concerned with an analogue of unionWithKey of type Ord k => (k -> a -> a -> Maybe a) -> Map k a -> Map k a -> Map k a, with the intended semantics that if a key is present in both maps and the operation applied to the key and corresponding values returns Nothing, the key is deleted from the result. Example use: sparse vectors are represented as maps from indices to non-zero coordinates. Often, a highly sparse vector u needs to be a added to a rather dense sparse vector v, i.e. we have m << n where m and n are the counts of non-zero elements in u and v, respectively. I need this operation to be O(m + n) as well as O(m * log(n)), and it should be as efficient as reasonable possible. This seems like a straightforward use case for unionWith since its hedge-union algorithm guarantees O(m * log(n)). The only problem occurs when u[i] = x and v[i] = -x for some index i and non-zero x, resulting in a zero entry I would rather have deleted from the result (this is a common case when dealing with matrices with small integer entries). The same problems occurs for coordinate-wise vector multiplication over non-integral domains, this time with intersectionWith. Remarks: - It would align the types of unionWith and intersectionWith with differenceWith. - The concept of allowing operations with return type Maybe a to avoid an additional high-level operation is already expressed in functions like alter or mapMaybeWithKey. - It would enable an efficient implementation of symmetricDifference = unionMaybeWithKey (const Nothing). - This concerns Data.Map, Data.IntMap, Data.Set, Data.IntSet and the With[Key] versions of union/intersection/difference - At least for Data.Map.unionWithKey, modifying the existing implementation seems trivial, requiring a merge instead of a join in the new case. I apologize if some of these points have been brought up before. Christian

6 13

Broken Packages (and Maintainers) for poppler, hsc2hs
by Thomas DuBuisson 24 Jan '12

24 Jan '12

All, Sorry if I'm a bit bitter - just ran into breakage of poppler, hsc2hs, and zlib in a 20 minute stretch. If the maintainers are here: 1) Please set a proper e-mail address (one that isn't a protected ML). 2) Please at least make sure the installation works (or the file at least parses!) via cabal-install. Cheers, Thomas

2 1