- Haskell-Cafe - Haskell.org

Safe way to parse arguments?
by Xiao-Yong Jin 23 Jun '08

23 Jun '08

Hi, I'm wondering how usually you parse command line arguments list safely. If the given argument is wrong, the program can still print out some error information instead of giving something like Prelude.read: no parse Let's make the discussion concrete. Suppose we have the following code. -->8----------8<-- import System.Environment data Conf = Conf { number :: Int } parseArgs [] = error "need at least one argument" parseArgs (x:_) = Conf { number = read x } work (Conf { number = n }) = n * (n-1) `div` 2 main = print . work . parseArgs =<< getArgs -->8----------8<-- If the "read x" fails in line 8, the program dies with Prelude.read: no parse What is the standard way to make the error message useful? Xiao-Yong -- c/* __o/* <\ * (__ */\ <

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Lambda and closures in PHP -- could someone please comment?
by Karoly Negyesi 23 Jun '08

23 Jun '08

Hi, The PHP community is discussing the adding of closures and lambdas to the language, see the proposal at http://wiki.php.net/rfc/closures If someone with knowledge of both languages could take a quick look it would be great. Thanks a lot Karoly Negyesi Ps. I am not a member of the PHP internals team, I am just a PHP developer but I am very very interested in getting these in my favourite language.

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INVITATION
by Haskell Hall 22 Jun '08

22 Jun '08

Haskell Hall is up and running. Haskell Hall is a mailing list, a forum, where you can discuss Haskell, functional programming and anything related, freely and openly with fellow enthusiasts. We welcome people of all abilities and know-how. So, if you fancy a change from what you get on Haskell Cafe send an email to haskell-request(a)freelists.org with the word subscribe in the subject field.The first 10 members will have access to the wine cellar. :-) _________________________________________________________________ The i’m Talkathon starts 6/24/08. For now, give amongst yourselves. http://www.imtalkathon.com?source=TXT_EML_WLH_LearnMore_GiveAmongst

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another FFI question
by Galchin, Vasili 22 Jun '08

22 Jun '08

Hello, I am reading the FFI spec. The following is unclear to me. Say I have data Bonzo = A | B |C and I want to "poke" Bonzo to an Int A -> 0 B -> 1 C -> 4 I have made Bonzo an instance of Storable. How do I write the poke functions and call them? Thanks, Vasili

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ieee 754
by Sean McLaughlin 22 Jun '08

22 Jun '08

Hello, I'm considering using Haskell for a numerical application. However, I need to rely on IEEE 754 standards being implemented correctly. What is the current state of 754 in Haskell? The definition has this paragraph, which makes me suspect Haskell is not appropriate for this application: "The default floating point operations defined by the Haskell Prelude do not conform to current language independent arithmetic (LIA) standards. These standards require considerably more complexity in the numeric structure and have thus been relegated to a library. Some, but not all, aspects of the IEEE floating point standard have been accounted for in Prelude class RealFloat." Thank you, Sean

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Testing
by Haskell Hall 22 Jun '08

22 Jun '08

Testing Testing _________________________________________________________________ Need to know now? Get instant answers with Windows Live Messenger. http://www.windowslive.com/messenger/connect_your_way.html?ocid=TXT_TAGLM_W…

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Scripting OpenOffice with Haskell
by Henning Thielemann 22 Jun '08

22 Jun '08

Has someone already tried to communicate with OpenOffice via Haskell? An interface seems to exist: http://framework.openoffice.org/scripting/index.html For instance, I like to mark some cells in oocalc, preprocess them and write them to a database.

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message passing style like in Haskell?
by jinjing 22 Jun '08

22 Jun '08

Hi guys, This is my second attempt to learn Haskell :) Any way here's the code: module Dot where import Prelude hiding ( (.) ) (.) :: a -> (a -> b) -> b a . f = f a infixl 9 . So for example, 99 questions: Problem 10 (*) Run-length encoding of a list. comparing: encode xs = map (\x -> (length x,head x)) (group xs) to encode xs = xs.group.map token where token x = (x.length, x.head) I found starting with data and working my way to a solution seems to be easier to think with, or maybe it's just me ... What is your thought? Jinjing

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Pretty little definitions of left and right folds
by George Kangas 22 Jun '08

22 Jun '08

Hi, Cafe, For no practical purpose, I wrote new definitions of list folds as chains of partially applied functions, joined by composition. I took the liberty of rearranging the type signature. > foldright :: (a -> b -> b)) -> [a] -> b -> b > foldright f = chain where > chain (a:as) = (f a).(chain as) > chain _ = id > foldleft :: (a -> b -> b) -> [a] -> b -> b > foldleft f = chain where > chain (a:as) = (chain as).(f a) > chain _ = id These definitions are point free, with respect to the "initializer" argument (which is now the last argument). Also, you can see how similar they are to each other, with the difference boiling down to the order of the composition, e.g.: foldright (+) [1, 2, 3] 0 == ( (1 +).(2 +).(3 +).id ) 0 foldleft (+) [1, 2, 3] 0 == ( id.(3 +).(2 +).(1 +) ) 0 We can also see the following identities: foldright f as == foldright (.) (map f as) id foldleft f as == foldright (flip (.)) (map f as) id I like that second one, after trying to read another definition of left fold in terms of right fold (in the web book "Real World Haskell"). The type signature, which could be written (a -> (b -> b)) -> ([a] -> (b -> b)), suggests generalization to another type constructor C: (a -> (b -> b)) -> (C a -> (b -> b)). Would a "foldable" typeclass make any sense? Okay, it goes without saying that this is useless dabbling, but have I entertained anyone? Or have I just wasted your time? I eagerly await comments on this, my first posting. very truly yours, George Kangas

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an example from YAHT doesn't run
by trifod＠gmail.com 21 Jun '08

21 Jun '08

Hello, On pdf page 43 of YAHT it provides a code example. I typed this into a file named "Guess.hs" and tried unsuccessfully to load it into ghci. I am running ghci version 6.8.2 This is the output it gave me: " [1 of 1] Compiling Main ( Guess.hs, interpreted ) Guess.hs:19:12: parse error on input `doGuessing' Failed, modules loaded: none. " I've attached the code module. As far as I can tell it is identical to the example in YAHT. Did I type it wrong, is the tutorial wrong, or something else? thanks again, Jared Langson

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