- Haskell-Cafe - Haskell.org

Why was this function that fast ?
by Romain Gehrig 29 Nov '15

29 Nov '15

Hi, I implemented a function to count how many steps it takes for a number to be reduced to 1 using the Collatz Conjecture (also known as the 3n + 1 problem) and I was very surprised to see it working instantaneously for integers as big as 10^100000. The said function: collatzSteps :: Integer -> Integer collatzSteps n = steps 1 n where steps acc 1 = acc steps acc n' | even n' = steps (acc+1) (n' `div` 2) steps acc n' = steps (acc+1) (n' * 3 + 1) You can try in GHCi (apparently with an upper limit of 10^199669): GHCi> collatzSteps 10^199668 [big num...] GHCi> length $ show $ collatzSteps 10^199668 168740 GHCi> collatzSteps 10^199669 -- Oops [1] 36128 bus error ghci Notice the number 168740: it is the number of *digits* for the number of steps. It implies that the `acc` is ~= *10^168740* when the function ends. Without optimisation at some point, it would mean there were as much (acc+1) calls ! My understanding completely stops there; I can't comprehend why a function that shouldn't return anything even long after the end of the universe just finished before my eyes. Can someone shed some light on how this peculiar function was optimised by GHC? Many thanks, (still dazed) Romain

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Info about List with terminal element
by Silvio Frischknecht 27 Nov '15

27 Nov '15

Hello, I'm interested in learning more about a list-like structure with a terminal element. I.e. data List t e = Cons e (List t e) | Null t or maybe data List t e = Cons e (List t e) | Null | Terminal t An practical application could be lazy reading from a file readFile :: String -> List Error String Obviously there is a Functor instance. There is even a Monad instance. However, it is not defined as obviously because it's not clear what the terminal element should be. There is a straight forward fold like structure. My questions are. Are there more interesting generalizations of List? What rules do they follow? How should the Monad work? Why? I know this is related to the whole Iteratee discussion (I'm not completely familiar with it). Cheers Silvio

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Could not deduce Monad m => MonadReader r (ReaderT r m) ??
by David Turner 26 Nov '15

26 Nov '15

Porting some code from GHC 7.8.4 to GHC 7.10.2 and have encountered this rather odd failure: Could not deduce (MonadReader (PqColumn m -> m B.ByteString) (ReaderT (PqColumn m -> m B.ByteString) m)) arising from a use of ‘ask’ from the context (Monad m) bound by the type signature for readFieldByteString :: Monad m => PqColumn m -> RowReader m B.ByteString at src/EventStore/LibPq.hs:164:24-72 In a stmt of a 'do' block: readField <- ask In the expression: do { readField <- ask; lift $ readField col } In an equation for ‘readFieldByteString’: readFieldByteString col = do { readField <- ask; lift $ readField col } I wondered if it was because the environment type (PqColumn m -> m B.ByteString) itself was an instance of MonadReader and the typechecker was getting confused, so I put the environment in a newtype and it was indeed happy. Unfortunately I can't share enough of this code to get the problematic fragment to compile, and my initial attempts at a small reproduction have been unsuccessful. It's something like the following, but this compiles just fine. class MyClass m where type MyType m defaultValue :: m (MyType m) l :: Monad m => MyType m -> ReaderT (MyType m -> m Int) m Int l n = do f <- ask lift $ f n Before I put more effort into reproducing this, does anyone recognise this failure or know what might be going on here? Cheers, David

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Re: [Haskell-cafe] Type-level lazy bool operations
by Oleg 26 Nov '15

26 Nov '15

Dmitry Olshansky wrote: > There are some type-level boolean operation (If, &&, ||, Not) in > Data.Type.Bool. Are they lazy enough? > > I faced with problem and it seems that the reason is non-lazy "If". > I.e. both (on-True and on-False) types are trying to be calculated. > Does it work in this way really? Could it be changed? Just a remark: if (type-level) computations are pure, then whether they are done lazily or in any other way should not matter. The evaluation strategy invariance is the main characteristic, if not the definition, of purity. Type-level computations are not pure however: they may have an effect of divergence, or generating a type-error. Thus we really need a type-level If, which evaluates only one of the two conditional branches but never both. It can be easily implemented using the standard approach of delaying evaluation with a thunk. Please see http://okmij.org/ftp/Haskell/TTypeable/TTypeable.hs and search for ORELSE (near the end of the file). See http://okmij.org/ftp/Haskell/typeEQ.html#TTypeable for background.

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regex-posix broken on 64 bit windows.
by Gershom B 26 Nov '15

26 Nov '15

Here’s a ticket reporting the issue: https://github.com/haskell/haskell-platform/issues/144 The ticket is on the platform site, but it doesn’t really belong there. On the other hand, there’s no tracker for regex-posix anywhere else. This problem has probably existed for a _long_ time, and I guess has to do with 64-bit incompatibilities in the cbits shipped with the package. I’ve cc’d Chris Kuklewicz as he is author and maintainer. But I know that he has taken a step back from these packages some time ago. Generally, I would be happy to consider them stable-ish and let the community just migrate away from them or not as open source typically evolves. But regex-posix is pretty widely used still and is in the platform, so I think it is unfortunate that it is broken in this way. It would be great if some interested volunteer were to step up and fix this issue and maybe even offer to take over maintainership (to which I’m sure Chris would have no objection?). (Relatedly, it would be good to have a future discussion on which regex library, if any, is suitable for inclusion in the platform. At this point my inclination would be regex-tdfa-text, although my sense is nearly all the libs are relatively unmaintained at this point?) —Gershom

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[ANN] Linode package
by Sebastian 26 Nov '15

26 Nov '15

Hello everybody, I'm happy to announce the initial release of the Linode package. It contains bindings to the Linode API and a few user friendly routines to setup and destroy VPS instances. The next major release will cover private IPs and load balancing. Hackage: http://hackage.haskell.org/package/linode Github: https://github.com/Helkafen/haskell-linode A working example is provided in the docs. It creates one linode instance and performs a dummy install: https://hackage.haskell.org/package/linode-0.1.0.4/docs/Network-Linode.html Use cases include: - Running some daily batch, with one or several instances - Dynamically adding some resources to a long running process - Anything you would normally do with a VPS Ideas, contributions and feedback are welcome! -- Sebastian de Bellefon

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Run time module definition file location information?
by David Banas 25 Nov '15

25 Nov '15

Hi all, How can I get a running Haskell program to tell me where it’s looking, in order to satisfy “import …” statements? Thanks, -db

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Comparison between fields of each record
by Jeon-Young Kang 25 Nov '15

25 Nov '15

Dear All. I'd like to compare fields of each record. here is my record. data Person = Person {name:: String, age:: Int } deriving(Show) data Relations = Friend | Older | Younger class Comparison a where compare:: a -> a -> Relations instance Comparison Person where compare Person a b Person a b | b1 == b2 = Friend | b1 > b2 = Older | b1 < b2 = Younger How can I fit it? Sincerely, Jeon-Young Kang

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ANN: haskell-tor 0.1
by Adam Wick 24 Nov '15

24 Nov '15

Howdy - Galois is pleased to announce an initial release of haskell-tor. Haskell-tor is intended to be a full-featured, drop-in implementation of the Tor onion routing protocol. This release provides full support for resolving names and building connections via anonymized channels, as well as (less tested) support for running relay and exit nodes. There are still many tasks left to do, however, if you're interested in learning about and working on a Tor implementation, including support for hidden services, proper flow control, directory support, etc. So if you're interested, jump in! We welcome your patches. You can find haskell-tor on: GitHub: https://github.com/GaloisInc/haskell-tor Hackage: https://hackage.haskell.org/package/haskell-tor Haskell-tor is HaLVM-ready. - Adam

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darcs 2.10.2 release
by Guillaume Hoffmann 23 Nov '15

23 Nov '15

The darcs team is pleased to announce the release of darcs 2.10.2 ! # Downloading # The easiest way to install darcs 2.10.2 from source is by first installing the Haskell Platform (http://www.haskell.org/platform) If you have installed the Haskell Platform or cabal-install, you can install this release by doing: $ cabal update $ cabal install darcs-2.10.2 Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.10.2.tar.gz and build it by hand as explained in the README file. The 2.10 branch is also available as a darcs repository from http://darcs.net/releases/branch-2.10 # What's new in 2.10.2 (since 2.10.1) # * optimize patch apply code memory use * make patch selection lazier in presence of matchers * switch patches retrieval order when using packs * switch from dataenc (deprecated) to sandi * finish updating help strings with new command names * clean contrib scripts * disable mmap on Windows * enhance darcs send message * fix quickcheck suite * shorter README with quickstart instructions * fixed the following bugs: * 2457: fix darcs-test command line options * 2463: building darcs on powerpc * 2444: added default interactivity parameter to isInteractive # Feedback # If you have an issue with darcs 2.10.2, you can report it on http://bugs.darcs.net/ . You can also report bugs by email to bugs(a)darcs.net, or come to #darcs on irc.freenode.net.

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