- Haskell-Cafe - Haskell.org

What's the problem with iota's type signature?
by michael rice 28 May '09

28 May '09

Still exploring monads. I don't understand why the type signature for double is OK, but not the one for iota. Michael ================= --double :: (Int a) => a -> Maybe b --double x = Just (x + x) iota :: (Int a) => a -> [b] iota n = [1..n] --usage: [3,4,5] >>= iota --should produce [1,2,3,1,2,3,4,1,2,3,4,5]

5 4

DEFUN09: Final Call for Talks & Tutorials (co-located w/ ICFP09)
by Matthew Fluet (ICFP Publicity Chair) 28 May '09

28 May '09

Call for Talks and Tutorials ACM SIGPLAN 2009 Developer Tracks on Functional Programming http://www.defun2009.info/ Edinburgh, Scotland, September 3 and 5, 2009 The workshop will be held in conjunction with ICFP 2009 http://www.cs.nott.ac.uk/~gmh/icfp09.html Important dates Proposal Deadline: June 5, 2009 Notification: June 19, 2009 This is a second invitation to submit talk and tutorial proposals for DEFUN 2009, the ICFP 2009 Developer Tracks. The deadline for submissions is next Friday, June 5. We want to know about your favorite programming techniques, powerful libraries, and engineering approaches you've used that the world should know about and apply to other projects. We want to know how to be productive using functional programming, write better code, and avoid common pitfalls. DEFUN is contiguous with CUFP 2009, the goal of which is to build a community for users of functional programming languages and technology. DEFUN provides the more technical, teaching-oriented parts of functional programming, while CUFP focuses on the commercial, management, and software engineering aspects. For more details of the kinds of proposals we would like to see, and how to submit, please see our original call for proposals at http://www.defun2009.info/blog/call-for-talks-and-tutorials/

1 0

trying to understand space leaks....
by Daryoush Mehrtash 28 May '09

28 May '09

In Section 2.5 of "Generalizing Monads to Arrows" paper (link<http://www.cs.chalmers.se/%7Erjmh/Papers/arrows.ps>) John Huges talks about the space leak inherit in monadic implementation of backtracking parsers. newtype Parser s a = P( [s] => Maybe (a, [s])) instance MonadPlus Parser where P a mplus P b = P (\s -> case a s of Just (x, s') -> Just (x, s') Nothing -> b s) The problem (as I understand it) is that to implement the backtracking, the monad plus implementation needs to keep the state (the computation that returns the next token) around in case the parser fails (so that it can be passed to the other side of mplus). I am having hard time to understand... a)what exactly gets saved on the heap between the mplus calls? b)I am assuming the computation to get the next character for parsing to be an "IO Char" type computation, in that case, what would be the size of the heap buffer that is kept around in case the computation result needs to be reused? c) Assuming Pa in the above code reads n tokens from the input stream then fails, how does the run time returns the same token to the P b? Thanks, Daryoush

2 4

platform/cabal problems
by Johannes Waldmann 27 May '09

27 May '09

Hi all. I've written some Haskell program, and I wanted to give it to a friend, in source form, so he can run and modify it, and learn some Haskell while doing so. I was using some cabalized extra packages but hey, this looks like the typical use case for the haskell-platform. So: I told him to install the newest stable ghc and the current platform release and this happens: la:/home/b/appl.linux/haskell-platform-2009.2.0# ./configure ... checking for hsc2hs... /usr/local/bin/hsc2hs checking version of ghc... 6.10.3 checking version of ghc-pkg matches... yes checking ghc actually works... yes checking the ghc core packages are all installed... no configure: error: The core package editline-0.2.1.0 is missing. It should have been distributed with 6.10.2 ^^^^^^^^^^^^^^^^^^^^^^^^ Well, editline is available from hackage, but to access it, one needs cabal, which is included in the platform, which cannot be built, because, see above. Yes, I know a handfull of workarounds - but they all require some previous knowledge, experience, and/or extra time. Really, if the platform fails in this very simple setting (a fresh install with current stable releases) then it is not much help in advertising Haskell. What's the particular reason for the failure? editline was distributed with ghc-6.10.2, but not with 6.10.3? I wouldn't expect an incompatible change when (only) the patchlevel is bumped. The platform web site should probably replace "works with ghc-6.10.x" by "works with ghc-6.10.2 only". Best regards, Johannes.

2 1

cabal option to specify the ghc version?
by Johannes Waldmann 27 May '09

27 May '09

Hi. How can I tell cabal (= the executable from cabal-install) to use a specific ghc version (and not the one that's currently linked to "ghc")? - Thanks, J.W.

3 2

Facebook puzzles allow for GHC solutions
by David Leimbach 27 May '09

27 May '09

Interesting: http://www.facebook.com/careers/puzzles.php So they use Haskell at Facebook? Dave

5 5

Updating HsJudy to work with the latest Judy
by Robin Green 27 May '09

27 May '09

I would like to use the HsJudy bindings to the Judy high-performance trie library (on hackage), but unfortunately they have bitrotted. I can have a go at mending them but I have no experience with FFI. Any tips? -- Robin

3 2

Timing pure functions?
by Magnus Therning 27 May '09

27 May '09

Yesterday I spent about 5 minutes trying to time a single function in haskell (after having spent about 30 minutes on the timeit module in Python). I found timeit[1] on Hackage but it only times an IO computation once, what I'd like to do is time a pure function several times. Timing it once was no problem, passing `return $! myPureFunc` to `timeIt` did that[2]. My feeble attempt at collecting several timings failed though. import System.CPUTime import qualified Codec.Binary.Base64 as B64 import System.IO import qualified Data.ByteString as BS import Control.Monad timeIt times ioa = let timeOnce = do t1 <- getCPUTime a <- ioa t2 <- getCPUTime let t = fromIntegral (t2-t1) * 1e-12 return t in sequence $ take times $ repeat timeOnce main = do fh <- openBinaryFile "/dev/urandom" ReadMode d <- liftM BS.unpack $ BS.hGet fh 100000 t <- timeIt 10 $ return $! B64.encode d print t Running this on my machine produces the output [2.3331e-2,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0,0.0]. I.e. the first time the data is encoded, but the following 9 times it's not. I suspect that it all comes from `B64.encode d` being pure, hence the encoding happens only once. Now I _really_ want the encoding to happen 10 times, is there some easy way to achieve this? /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/timeit [2]: -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

5 5

Bidirectional programming in Haskell
by Shin-Cheng Mu 27 May '09

27 May '09

Hi, On Tue May 26 01:21:28 EDT 2009, Artyom Shalkhakov wrote: > Are there any libraries for "bidirectional" [1] programming > in Haskell? Any related work? Some of the early work in the PSD project (closely related to the "lenses") were developed in Haskell, http://www.iis.sinica.edu.tw/~scm/2007/inv/ It is not in active maintenance now, but if you are interested in doing something with it, I'd be happy to help. :) Ref: [1] S-C. Mu, Z. Hu and M. Takeichi, An injective language for reversible computation. In Mathematics of Program Construction 2004, LNCS 3125, pp. 289-313, July 2004. [2] Z. Hu, S-C. Mu and M. Takeichi, A programmable editor for developing structured documents based on bidirectional transformations. In Partial Evaluation and Semantics-Based Program Manipulation, pp. 178-189. August 2004

2 1

The essence of my monad confusion
by Paul Keir 27 May '09

27 May '09

On the wiki page for Applicative Functors (http://www.haskell.org/haskellwiki/Applicative_functor) a familiar characteristic of monads is quoted; that they "allow you to run actions depending on the outcomes of earlier actions". I feel comfortable with Functors and Applicative Functors, but I don't yet get that "extra power" that monads provide. An example immediately follows that quotation on the wiki: do text <- getLine if null text then putStrLn "You refuse to enter something?" else putStrLn ("You entered " ++ text) For simplicity's sake, I modified it to avoid using the IO monad; the "text" binding is now provided by the first parameter, and (=<<) is used due to its similarity to fmap: bar :: Monad m => m String -> m String bar as = (=<<) (\a -> if null a then return "nothing" else return "something") as This works fine, so bar ["Blah"] gives ["something"], and bar (Just "") gives ["nothing"]. But, I can get the same effect using a Functor (replacing (=<<) with fmap): bar2 :: Functor f => f String -> f String bar2 as = fmap (\a -> if null a then "nothing" else "something") as Can anyone help me out of the swamp? Cheers, Paul

4 4