- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Re: Functional progr., infinity, and the Universe
by voigt.16734551＠bloglines.com 23 Jun '06

23 Jun '06

--- paul.hudak(a)yale.edu wrote: voigt.16734551(a)bloglines.com wrote: > 1:_|_ is certainly finite. In what sense is it not? I see that point. I have been using "finite" as, by convention, equal to "total and finite". And so have others. As always with convention, one can argue. I won't, of course. > Sorry, see my reply to Bill Wood. Yes, saw that later. > distinguishes what are called "interesting" elements from other elements > (if I recall the terminology correctly). Interesting ;-) Ciao, Janis.

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sortBy2 for List library
by Serge D. Mechveliani 23 Jun '06

23 Jun '06

Consider the following suggestions for the standard Haskell library `List'. ------------------------------------------------------------------------ import List (intersprerse, insertBy) compose :: [a -> a] -> a -> a -- just appropriate name compose = foldr (.) id insertBy2 :: (a -> b) -> Comparison b -> a -> [a] -> [a] insertBy2 f cp a xs = map fst $ insertBy (\ p p' -> cp (snd p) (snd p')) (a, f a) [(x, f x) | x <- xs] Similar are mergeBy2, sortBy2. type ComparisonValue = LT | GT | Eq deriving ... -- better name than `Ordering' type Comparison a = a -> a -> ComparisonValue ------------------------------------------------------------------------ Motivation example for `compose': myShowsList :: Show a => [a] -> String -> String myShowsList xs = compose $ intersperse (", "++) $ map shows xs myShowsList ("; "++) [1,2,3] --> "1; 2; 3" Motivation example for insertBy2: Consider the program insertBy (\ n m -> compare (f n) (f m)) 100 [0 .. 1000], where f :: Integer -> Integer is expensive to compute. We see that the program insertBy2 f compare 100 [1 .. 1000] spends 2 times less of `f' invocations, and looks natural. ----------------- Serge Mechveliani mechvel(a)botik.ru

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Polymorphic type
by Sara Kenedy 23 Jun '06

23 Jun '06

Hello all, Now I am trying with the function of polymorphic type: This function returns the Nth element of list with type a. I try it as below. getNthElem :: Int -> [a] -> Maybe a getNthElemt _ [] = Nothing getNthElem 0 _ = Nothing getNthElem n s | n > length s = Nothing | otherwise = Just ((drop (n-1) (take n s))!!0) >getNthElem 2 ["a","b","c"] Just "b" However, I do not satisfy with this function because I want to return the Nth element of type a, not (Maybe a). For example, I want this function: getNthElem :: Int -> [a] -> a But, I do not know how to define the empty element of type a. getNthElemt _ [] = ???? getNthElem 0 _ = ???? If you have some ideas about this, please give me some clues. Thanks a lot. S.

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neural network code
by John Meacham 22 Jun '06

22 Jun '06

I was curious if there was any code floating around out there for dealing with neural networks in haskell? John -- John Meacham - ⑆repetae.net⑆john⑈

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Principal type in Haskell
by william kim 22 Jun '06

22 Jun '06

Hi All, I am confused by the notion of principal type in Haskell with type classes. Consider a simple example: f x y = [x] == [y] GHCi yields type f :: (Eq [a]) => a -> a -> Bool. But according to the paper "Type classes: an exploration of the design space", predicate Eq [a] should be reduced to Eq a. Is this reduction performed here? What should be the principal type of f? Thanks. william _________________________________________________________________ Get an advanced look at the new version of MSN Messenger. http://messenger.msn.com.sg/Beta/Default.aspx

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user type declarations in Haskell
by Vladimir Portnykh 22 Jun '06

22 Jun '06

I am trying to define the following types data MyStringType a = String deriving (Eq, Ord, Show) data QADouble a = Double deriving (Eq, Ord, Show) data HType a = QADouble a| DDTraceType a deriving (Eq, Ord, Show) So HType can represent strings or doubles. later I want to do something like the following: let a1 =QADouble 1 let a2 =QADouble 2 let a3 = a1 + a2 First, it is not working because Haskell complains about a3. it does not know how to calculate it. Is it a way to give him a hint? QADouble is Double...am I doing something absolutely wrong and silly? many thanks, vladimir _________________________________________________________________ Are you using the latest version of MSN Messenger? Download MSN Messenger 7.5 today! http://join.msn.com/messenger/overview

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Re: [Haskell-cafe] Re: Functional programming for processing oflargeraster images
by voigt.16734551＠bloglines.com 22 Jun '06

22 Jun '06

--- Matthias Fischmann <fis(a)wiwi.hu-berlin.de wrote: > On Thu, Jun 22, 2006 at 09:22:34AM +0100, Simon Peyton-Jones wrote: > > To: Brian Hulley <brianh(a)metamilk.com>, Joel Reymont <joelr1(a)gmail.com> > > Cc: haskell-cafe(a)haskell.org > > From: Simon Peyton-Jones <simonpj(a)microsoft.com> > > Date: Thu, 22 Jun 2006 09:22:34 +0100 > > Subject: RE: [Haskell-cafe] Re: Functional programming for processing > > oflargeraster images > > > > http://haskell.galois.com/cgi-bin/haskell-prime/trac.cgi/wiki/BangPatterns > > > > Bang patterns make it much more convenient to write a strict function. > > E.g > > f (x, !y) = ... > > is strict both in the pair (of course) but also in the second component > > of the pair, y. > > i am ecstatic to hear that :). Well, you shouldn't be too enthusiastic, but rather follow the above link ... > if it really means that 'y' will be fully evaluated (not top level > normal form, but whatsthenameforthis, in the way ocaml evaluates > expressions), it's something i have been missing so much that i was > thinking of switching back to a strict language again. ... to find out that that's exactly not what bang patterns will do for you. They are compiled into uses of seq, which means evaluation to weak head normal form. Ciao, Janis.

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ghc-pkg on Windows XP
by Jeff Polakow 22 Jun '06

22 Jun '06

Hello, I have installed ghc-6.4.2 on a windows XP machine. However, the machine refuses to execute ghc-pkg (thus preventing me from using Cabal) and complains that "C:\ghc\ghc-6.4.2\bin\ghc-pkg.exe is not a valid Win32 application." Is there something obvious I might have overlooked while installing? Also, I am new to using ghc on windows and was wondering what are the trade-offs of using cygwin or minsys versus just living in a emacs shell? thanks, Jeff -- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Template haskell and scoping
by C Rodrigues 22 Jun '06

22 Jun '06

The (.&.) in the splice is out of scope according to GHC. If I use [||] then it works, but for my purposes it's easier to use the constructors. How should I refer to that variable? import Data.Bits import Language.Haskell.TH main = print $ $(return $ VarE $ mkName ".&.") 7 (14 :: Int)

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Re: [Haskell-cafe] Functional progr., images, laziness and all the rest
by Piotr Kalinowski 22 Jun '06

22 Jun '06

On 22/06/06, Brian Hulley <brianh(a)metamilk.com> wrote: > 1) Someone reading the code needs to do a lot of work to try to recover the > original equation That's what comments are for, I suppose. > 2) Wouldn't an imperative loop, using the original equation directly, have > made everything much simpler? > 3) Therefore laziness has lead to obfuscated code. In fact, according to your point 2, it is not the laziness that brought us here, but being functional. Just my 2 cents. Regards, Piotr Kalinowski -- Intelligence is like a river: the deeper it is, the less noise it makes

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