April 2012 - Libraries

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

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Concurrent collections
by Simon Peyton-Jones 24 Apr '12

24 Apr '12

I went to Aleksandar Prokopec's talk about Concurrent Tries at the Scala day earlier this week. I thought it was pretty cool. Here's the paper. http://infoscience.epfl.ch/record/166908/files/ctries-techreport.pdf http://lampwww.epfl.ch/~prokopec/ctries-snapshot.pdf Maybe we should have a Haskell version? Maybe we already do? Simon

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Re: [Haskell] Types of when and unless in Control.Monad
by Henning Thielemann 22 Apr '12

22 Apr '12

moving to libraries, again ... On Sun, 22 Apr 2012, Julian Gilbey wrote: > On Sat, Apr 21, 2012 at 08:28:27PM -0500, Strake wrote: >> On 21/04/2012, Andreas Abel <andreas.abel(a)ifi.lmu.de> wrote: >> > to avoid silly "return ()" statements like in >> > >> > when cond $ do >> > monadicComputationWhoseResultIWantToDiscard >> > return () >> >> (when cond ∘ void) monadicComputationWhoseResultIWantToDiscard >> or >> when cond $ () <$ monadicComputationWhoseResultIWantToDiscard > > How is that simpler than > > when cond monadicComputationWhoseResultIWantToDiscard > > which it would be with the suggested new type? The original poster wanted to make it simpler, others want to let it be safer.

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Re: [Haskell] Types of when and unless in Control.Monad
by Henning Thielemann 21 Apr '12

21 Apr '12

On Sat, 21 Apr 2012, Andreas Abel wrote: > While your comments about not ignoring results of functions may be right, in > the particular case of when and unless the situation is different. Since > both are basically an if-then without an else, the expression *cannot* yield > a result, just execute the monadic effect. Thus, confusion is unlikely. I don't understand this argument. The current type of 'when' is > when :: (Monad m) => Bool -> m () -> m () thus it makes sure, that the enclosed action has no result that could be ignored by accident. If the function would be > when_ :: (Monad m) => Bool -> m a -> m () then you could write > when_ b (system "command") and thus miss the ExitCode of 'system' by accident. > I have defined myself when_ and unless_ but I think it is silly, the library > functions could be more general. Since you think that the current 'when's type is silly, I would like to know what are your examples where you want to ignore the result of the enclosed action?

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Problem Installing ad (Automatic Differentiation) Package
by Dominic Steinitz 21 Apr '12

21 Apr '12

Hi, I was trying to install ad (Automatic Differentiation) but got the following error. I guess the answer is to upgrade my ghc (perhaps someone could confirm) but I am using the Haskell Platform and would rather stick with that. I see from http://hackage.haskell.org/platform/ that the next release is due out in May. Will that be early May? in which case I may well wait a week or will it be late May? in which case I might bite the bullet and upgrade my ghc. Thanks, Dominic Building template-haskell-2.7.0.0... [1 of 7] Compiling Language.Haskell.TH.Syntax.Internals ( Language/Haskell/TH/Syntax/Internals.hs, dist/build/Language/Haskell/TH/Syntax/Internals.o ) [2 of 7] Compiling Language.Haskell.TH.Syntax ( Language/Haskell/TH/Syntax.hs, dist/build/Language/Haskell/TH/Syntax.o ) [3 of 7] Compiling Language.Haskell.TH.PprLib ( Language/Haskell/TH/PprLib.hs, dist/build/Language/Haskell/TH/PprLib.o ) Language/Haskell/TH/PprLib.hs:55:10: Illegal instance declaration for `Show Doc' (All instance types must be of the form (T t1 ... tn) where T is not a synonym. Use -XTypeSynonymInstances if you want to disable this.) In the instance declaration for `Show Doc' Updating documentation index /Users/Test/Library/Haskell/doc/index.html cabal: Error: some packages failed to install: ad-1.3.0.1 depends on template-haskell-2.7.0.0 which failed to install. template-haskell-2.7.0.0 failed during the building phase. The exception was: ExitFailure 1 The Glorious Glasgow Haskell Compilation System, version 7.0.3

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Re: [Haskell] Types of when and unless in Control.Monad
by Henning Thielemann 21 Apr '12

21 Apr '12

moving to libraries(a)haskell.org ... Andreas Abel schrieb: > In Control.Monad, when has type > > when :: Monad m => Bool -> m () -> m () > > I think this type should be generalized to > > when :: Monad m => Bool -> m a -> m () > > to avoid silly "return ()" statements like in > > when cond $ do > monadicComputationWhoseResultIWantToDiscard > return () You may use the new 'void' function for that purpose. Generally I prefer to not ignore monadic results by default. I also do not like that behavior in the C programming language. GHC now warns about unused non-() results in a do-block. But (>>) and mapM_ still ignore non-unit results of monadic action, and certainly have to remain this way because of Haskell-98 compatibility. If a monadic action has a result, then this has a reason. If I have to apply a 'void' then I think about whether it is correct to ignore the result. E.g. if the result is an ExitCode then it may be convenient to ignore it (like in C), but it is certainly not correct to do so. It was often stated that for parsers ignoring of the parsed results by default is a good idea. However since GHC warns about implicitly ignored monadic results in do-notation, I found that many warnings in parsers are due to not using the right parser combinators. E.g. I often found something like do char '(' x <- expression char ')' return x and this should certainly be rewritten to between (char '(') (char ')') expression That is, those warnings about implicitly ignored results helped me to improve my code. > P.S.: A more systematic solution would be to change the Haskell > language by either introducing a Top type which is the supertype of > everything and use it instead of (). There is the type Any, but I suspect it cannot be written in source code. But I think a type variable like 'a' is the right thing to use.

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Integrity checking for cabal
by M. Schulte 20 Apr '12

20 Apr '12

Hello, I'm pretty new to Haskell and I hope this list is the appropriate place to ask the following question: When I use cabal to install a libary or a program, are there any integrity checks done? Preferably, are there digital signatures checked? So far I couldn't find anything related to integrity checking in cabal -- but I found it a bit hard to believe that there is no such mechanism at all. If there is none, are there any plans on adding such a mechanism? For instance, one could configure cabal to complain whenever a package's signature could not be checked and implement some light-weigth signature checking via pgp-keys. Thanks, m.

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System.Random instances for tuples
by Dan Burton 19 Apr '12

19 Apr '12

Ever find yourself wanting to generate a tuple of random things? Instances are trivial to write in Haskell 98: instance (Random a, Random b) => Random (a, b) where randomR ((loL, loR), (hiL, hiR)) g = ((l, r), g'') where (l, g') = randomR (loL, hiL) g (r, g'') = randomR (loR, hiR) g' random g = ((l, r), g'') where (l, g') = random g (r, g'') = random g' Can we add instances like this to System.Random in the random package? -- Dan Burton

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documentation patch for process
by Evan Laforge 16 Apr '12

16 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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cabal-install: Conduits/IO concurrency
by Eyal Lotem 15 Apr '12

15 Apr '12

Hey, Currently it seems that cabal-install lacks IO concurrency in its various operations (i.e: download packages concurrently to their compilation). Specifically, I found it to be a major bottleneck during "cabal update" operations. The download, decompression, and use of the decompressed index are all serially executed, and all take quite a bit of time, and could probably be executed in parallel easily. To make "cabal update" execute the download and decompression concurrently, it would be nice to use a streaming decompressor on a stream download. The approach I had in mind was replacing the use Network.HTTP and zlib with use of the conduits counterparts. Additionally, use of conduits can probably ease addition of status reports (X% completed) by injecting progress reporters in the conduit pipeline. "cabal update" progress could be less opaque that way. My question is: Would patches that add conduit use and demonstrate performance benefits be welcome? I don't want to embark on this adventure if there's a good reason to avoid it in the first place. Eyal

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