June 2006 - Haskell-Cafe

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 21 Jun '06

21 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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haskell companies
by Andreas Kägi 21 Jun '06

21 Jun '06

hello i wonder if there are companies that use haskell in practice. i'm a computer science student from switzerland (europe) and need to do an internship. since i find haskell very interesting i thought these two things could maybe be combined.

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ACross
by Jenny678 20 Jun '06

20 Jun '06

Hello, Can somebody help me My Problem is to define a code for: >cross 7 -- means 7*7 >+++++++ >++ ++ >+ + + + >+ + + >+ + + + >++ ++ >+++++++ Another Example >cross 13 -- 13*13 >+++++++++++++ >++ ++ >+ + + + >+ + + + >+ + + + >+ + + + >+ + + >+ + + + >+ + + + >+ + + + >+ + + + >++ ++ >+++++++++++++ Can I use putString ??? Thanks for any help -- View this message in context: http://www.nabble.com/ACross-t1813549.html#a4943328 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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Newbie question on iterating over IO list
by Taro Ikai 20 Jun '06

20 Jun '06

Here's a code fragment I have from working with Hal Daume's "Yet Another Haskell Tutorial". I cannot figure out how to iterate over the result from askForNumbers that returns "IO [Integer]", and print them. The "<-" operator seems to take IO off of the results from actions like getContents, which returns type "IO String", but I cannot seem to use it to take IO off of lists with it. What am I supposed to do? Also, as a side question, what should I be returning from the do function? >>>code>>>> module Main where import IO main = do hSetBuffering stdin LineBuffering s <- askForNumbers {-- This compiles --} map show s {-- But this doesn't. Why? --} {-- What expression should I be returning from do? --} askForNumbers = do putStrLn "Give me a number (or 0 to stop):" guess <- getLine let number = read guess if number == 0 then return [] else do rest <- askForNumbers return (number : rest) <<<code<<<< Thanks in advance. Taro Ikai

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Efficient way to edit a file
by Maurício 20 Jun '06

20 Jun '06

Hi, I need to edit big text files (5 to 500 Mb). But I just need to change one or two small lines, and save it. What is the best way to do that in Haskell, without creating copies of the whole files? Thanks, Maurício

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