October 2005 - Haskell-Cafe

Endian conversion
by Joel Reymont 07 Oct '05

07 Oct '05

Folks, Are there any endian conversion routines for Haskell? I'm looking to build binary packets on top of NewBinary.Binary but my data is coming in little-endian whereas I'll need to send it out big endian. Thanks, Joel -- http://wagerlabs.com/idealab

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Alternatives to hs-plugins?
by John Goerzen 07 Oct '05

07 Oct '05

I'm thinking of porting my OfflineIMAP program from Python to Haskell. In the configuration file for OfflineIMAP, I permit people to use a Python lambda expression for some things. For instance: folderfilter = lambda foldername: not re.search('(^Trash$|Del)', foldername) So I think it would be useful to be able to do something similar in Haskell. hs-plugins immediately occured to me. But there are several problems with it: 1. Only works with GHC (no Hugs) 2. Only as portable as ghci. So quite a few platforms that are important to me would be excluded (Alpha, ARM, AIX, etc.) I'm wondering if there are any alternatives? Can a Haskell program using Hugs call Hugs to evaluate an arbitrary hunk of code? Is there any portable Haskell interpreter written in Haskell? Thanks, -- John

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New to Monads
by Tom Hawkins 07 Oct '05

07 Oct '05

I'm just starting to feel comfortable working inside IO; now I am trying to wrap my head around Control.Monad.State. Looking at the implementation, I came across some unfamiliar syntax... class (Monad m) => MonadState s m | m -> s where What is the meaning of "| m -> s"? I found no mention of it in the tutorial. And slight logical dilemma... With IO, I understand one your in it, you can't get out. I.e., any function that conducts IO must have IO _ in the type signature. But this must not apply to all monads, otherwise the type signature for "main" would be rather messy (IO + State + every other monad used in a program). But I don't see how a monad could escape this fate since bind and return only produce more monads with the same type constructor. Thanks for your help! -Tom

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Working inside the fold
by Greg Buchholz 07 Oct '05

07 Oct '05

Recently I've been browsing some of Oleg Kiselyov's articles entitled "Towards the best collection traversal interface"... http://okmij.org/ftp/Computation/Continuations.html#enumerator-stream A programming language system gives us typically one of the two interfaces to systematically access elements of a collection. One traversal API is based on enumerators -- e.g., for-each, map, filter higher-order procedures -- of which the most general is fold. The second approach relies on streams, a.k.a. cursors, lazy lists. ...where he argues that (since they're both intraconvertible) a default enumerator/fold choice is better than the lazy-list approach. I was sufficiently intrigued I thought I'd take up the challenge and see what could be done working inside the fold. This message is literate Haskell source, for those of you playing at home. The first thing to do is define a few folds which will take the place of lists. I'll start out with a finite and infinite one... >numsTo10 f unit = foldr f unit [1..10] >nats f unit = foldr f unit [1..] If you want the sum of the first ten naturals you'd invoke it as... *Main> numsTo10 (+) 0 55 ...and to see that you can convert back to a list... *Main> numsTo10 (:) [] [1,2,3,4,5,6,7,8,9,10] That's nice, but the first thing you'll notice is that while there are numerous list processing functions in the prelude, there are none for specifically working inside a fold. Let's try and define a few... >map_ :: (a -> b) -> (b -> c -> d) -> a -> c -> d >map_ f g a b = g (f a) b ...so if we want the sum of the first ten squares... *Main> numsTo10 (map_ (^2) (+)) 0 385 ...or the actual list... *Main> numsTo10 (map_ (^2) (:)) [] [1,4,9,16,25,36,49,64,81,100] ...Filter is also a nice function... >filter_ p f a b = if p a then f a b else b *Main> numsTo10 (filter_ odd (:)) [] [1,3,5,7,9] ...and we can also play nice with infinity... >takeWhile_ unit p f a b = if p a > then f a b > else unit *Main> nats (takeWhile_ [] (<15) (:)) [] [1,2,3,4,5,6,7,8,9,10,11,12,13,14] ...That's a little bit ugly because you have to supply the unit value (in this case "[]") for the fold to takeWhile_. The dropWhile_ function is also a little quirky, since it uses the tupling trick from... "A Tutorial on the Universality and Expressiveness of Fold" http://citeseer.ist.psu.edu/hutton93tutorial.html >dropWhile_ p f a (ys, xs) =((if p a > then ys > else f a xs), f a xs) *Main> numsTo10 (dropWhile_ (<5) (:)) ([],[]) ([5,6,7,8,9,10],[1,2,3,4,5,6,7,8,9,10]) *Main> numsTo10 (dropWhile_ (<5) (+)) (0,0) (45,55) ...where the first member of the pair is the desired answer. If you want to do more than one thing inside the fold, "fork" might be the solution (is there a better name?) >fork f g z (x,y) = (f z x, g z y) *Main> numsTo10 (fork (:) (+)) ([],0) ([1,2,3,4,5,6,7,8,9,10],55) *Main> numsTo10 (fork (filter_ odd (:)) (fork (+) (*))) ([],(0,1)) ([1,3,5,7,9],(55,3628800)) ...And you can mostly compose these operations, although for infinite lists you'll hit bottom if anything other than takeWhile_ is the first function... >c = nats (takeWhile_ ([],(0,1)) (<20) > (filter_ even (map_ (^2) (fork (:) (fork (+) (*)))))) > ([],(0,1)) *Main> c ([4,16,36,64,100,144,196,256,324],(1140,34519618525593600)) ...So a few generic munging functions can be defined for inside the fold. I couldn't think of how to define "take" or "drop" or figure out what a zipWith would mean. Are there any other interesting functions that could be defined? Is there a better way define these functions? Is there anything other than curiosity which would motivate someone to use these functions? FWIW, Greg Buchholz

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Emacs Haskell Mode: Indenting after "where"
by Joel Reymont 07 Oct '05

07 Oct '05

Folks, Does anyone know how to make the Haskell mode (Emacs) indent the line after "where" for this type of look: peek = if be then peek0 else peekR where peek0 a = fmap BigEndian $ peek (castAway a) peekR a = peekByteOff a 0 instead of this: peek = if be then peek0 else peekR where peek0 a = fmap BigEndian $ peek (castAway a) peekR a = peekByteOff a 0 Thanks, Joel -- http://wagerlabs.com/idealab

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Re: [Haskell-cafe] New to Monads
by Wolfgang Jeltsch 07 Oct '05

07 Oct '05

Am Freitag, 7. Oktober 2005 06:32 schrieb Cale Gibbard: > That | m -> s reads "where m determines s", and means that there can > be at most one s (here, the state type) for a given m (the monad > type). This is used in type inference and checking to ensure that the > type of state being carried around by a monad in MonadState is well > defined, and to prevent you from having to annotate the operations in > MonadState with type signatures. (To tell which type s to use.) What you can express via this syntax is called functional dependencies (in analogy to functional dependencies in database theory). One should note that functional dependencies and even multi-parameter type classes are an extension to Haskell 98 and not part of Haskell 98 which is why you haven't read about them in the tutorial. > [...] Best wishes, Wolfgang

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Confused about types
by Lanny Ripple 06 Oct '05

06 Oct '05

I thought I was doing ok with haskell until I tried to program in it. :/ I figured the first thing to tackle was something easy so I hacked up newton's method. Worked great for Double's. Then I figured I would extend it to work with different types of Fractionals. What a pain. I finally got that working for Floats and Rationals but I'm drawing a blank on Complex. I've got: <Newton.hs> import Data.Complex import Data.Ratio import Debug.Trace main = putStrLn $ "answer = " ++ show (newton myd $ -5) newton :: (Fractional a, Ord a) => (a -> a) -> a -> a newton f x = newton_h f x 1.0e-6 newton_h, next_x_h, dy_h :: (Fractional a, Ord a) => (a -> a) -> a -> a -> a newton_h f x h = until ((<= h) . abs . f) (next_x_h f h) x next_x_h f h x = trace ("next_x = " ++ show foo) foo where foo = x - (f x) / (dy_h f x h) dy_h f x h = ( (f $ x + h) - f x ) / h -- roundTo :: Fractional a => a -> a -> a -- roundTo x eps = ((fromRational . toRational) (round $ x / eps)) * eps myd :: Double -> Double myd x = ((x/100 + 1) * x + 1) * x - 10 myf :: Float -> Float myf x = ((x/100 + 1) * x + 1) * x - 10 myr :: Rational -> Rational myr x = approxRational myr' 1e-6 where myr' = ((x/100 + 1) * x + 1) * x - 10 myc :: RealFloat a => Complex a -> Complex a myc x = x * x + 1 </Newton.hs> When I try newton myc 5 in ghci I get a warning about newton wanting Ords. Complex aren't ordered. I figure I need to specify a newton_h that can handle complex newton_h f (Complex x) h = until ((<= h) . realPart . abs . f) (next_x_h f h) x laughs at me. So does newton_h f (:+ r im) h = until ((<= h) .realPart . abs . f) (next_x_h f h) (:+ r im) although I sure thought I had read that constructors could be used in pattern matching. I've been working on this for too long and getting frustrated. Can someone clue me in? Thanks, -ljr -- Lanny Ripple <lanny(a)cisco.com> CC Tools / Cisco Systems, Inc.

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RE: [Haskell-cafe] Papers from the 2005 Haskell Workshop (Tallinn)?
by Simon Marlow 06 Oct '05

06 Oct '05

On 05 October 2005 17:11, Dimitry Golubovsky wrote: > The papers presented at the Workshop are already available in the ACM > library which requires membership/subscription to read full text PDFs. > Are there any plans to make those papers available anywhere else on > the Web without subscription? The papers I co-authored are available from my web page: http://www.haskell.org/~simonmar/bib/bib.html Cheers, Simon

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Re: [Haskell-cafe] Papers from the 2005 Haskell Workshop (Tallinn)?
by Dimitry Golubovsky 06 Oct '05

06 Oct '05

Nils Anders Danielsson wrote: > > Most authors do put their papers on their web pages nowadays. > In particular, I would like to read the paper on "halfs" (haskell filesystem). Googling for <halfs haskell filesystem> gave nothing but the Workshop's schedule and ACM Library TOC. Dimitry Golubovsky Middletown, CT

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Define a data structure of a predicate
by Sara Kenedy 05 Oct '05

05 Oct '05

Hello you all, I am a newbie in Haskell. Now I am working on datatype structure of Haskell, especially on predicate. Today I try to search on Internet to find the reference document but I did not find the specific. If any of you know how to define a data structure for the abstract syntax of predicate logicI , for example : x^2 + 3*x + 2 > 0, if you don't mind, please share with me. Thanks in advance.

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