- Haskell-Cafe - Haskell.org

Order of Declaration
by PR Stanley 29 Sep '07

29 Sep '07

Hi in C type languages a function must be declared before its application. Would I be right in thinking that this isn't the case in Functional languages? For example: transmit :: String -> String transmit = decode . channel . encode channel :: [[Int]] -> [[Int]] channel = id Cheers, Paul

3 2

Text.Html
by Bob 29 Sep '07

29 Sep '07

Hi everybody! Where can I find a documentation or a tutorial for html combinator library Text.Html?

2 1

Rewriting char
by PR Stanley 29 Sep '07

29 Sep '07

Hi Any advice on writing intToChar? I'm not looking for the code; only sound advice on writing the function in good FP style. Thanks, Paul

2 1

Rewriting Char.ord
by PR Stanley 29 Sep '07

29 Sep '07

Hi ord :: Char -> Int ord c = sum [1 | x <- ['\0'..'\255'], x < c] Any comments? Any alternatives? Cheers, Paul

4 4

agda v. haskell
by brad clawsie 29 Sep '07

29 Sep '07

dons has been posting some links regarding agda on reddit. fairly interesting, a quick glance and you think you are reading haskell code. does anyone have any insights on the major differences in these languages?

4 3

some simple proof exercises
by Tim Newsham 29 Sep '07

29 Sep '07

Here are two small problem sets of proofs of Haskell functions. They are aimed at people who do not have experience writing proofs and are not necessarily well versed in Haskell. Feedback is appreciated. http://www.thenewsh.com/%7Enewsham/formal/problems/set1.html http://www.thenewsh.com/%7Enewsham/formal/problems/set2.html Tim Newsham http://www.thenewsh.com/~newsham/

2 1

Newb: List of nodes in a graph - is there a prettier way?
by Torsten Otto 28 Sep '07

28 Sep '07

Howdy, I'm working towards Dijkstra's algorithm here and I have a feeling that I could do without the helper function nodesInternal in the following code, if I only could figure out how. Any hints would be appreciated. nodes::Graph->[Id] should (and actually does) return a list of all nodes in the graph. Thanks a bunch in advance. Regards, Torsten Otto >module Route where Datatypes for the representation of the graph: >type Id = Int >type Weight = Int >type Edge = (Id,Id) >type Graph = [ (Edge, Weight) ] >graph::Graph >graph = [ ((0,1),1), > ((0,2),3), > ((0,4),6), > ((1,2),1), > ((1,3),3), > ((2,0),1), > ((2,1),2), > ((2,3),1), > ((3,0),3), > ((3,4),2), > ((4,3),1), > ((5,2),9)] >data Cost = Finite Weight | Infinity > deriving (Eq, Ord, Show) >type PathCost = (Cost, Id) Return the number of edges in the graph: >edges :: Graph -> Int >edges graph = length graph Calculate the sum of all weights: >weightTotal::Graph -> Weight >weightTotal ((edge, weight):xs)| xs == [] = weight > | otherwise = weight + (weightTotal xs) List all the nodes in the graph: >nodes::Graph -> [Id] >nodes graph = nodesInternal [] graph >nodesInternal::[Id]->Graph->[Id] >nodesInternal list (((id1,id2),weight):xs) > | (elem id1 list) && (elem id2 list) = nodesInternal list xs > | (elem id1 list) && (not (elem id2 list)) = nodesInternal (id2:list) xs > | (not (elem id1 list)) && (elem id2 list) = nodesInternal (id1:list) xs > | (not (elem id1 list)) && (not (elem id2 list)) = nodesInternal (id1:id2:list) xs >nodesInternal list [] = list Function for adding costs so that we can make use of Infinity for impossible routes: >addCosts::Cost -> Cost -> Cost >addCosts Infinity Infinity = Infinity >addCosts Infinity (Finite x) = Infinity >addCosts (Finite x) Infinity = Infinity >addCosts (Finite x) (Finite y) = Finite (x + y) Return the cost of a given edge: >lookUp::Edge -> Graph -> Cost >lookUp (id1,id2) (((id1x,id2x),weightx):xs) > | (id1==id1x && id2==id2x) = Finite weightx > | xs==[] = Infinity > | otherwise = lookUp (id1,id2) xs

3 3

Dynamic choice of "reverse" implementation
by Krzysztof Kościuszkiewicz 28 Sep '07

28 Sep '07

Fellow Haskellers, I wanted to experiment a bit with lists and sequences (as in Data.List and Data.Sequence), and I got stuck. I wanted to dynamically select processing function depending on cmdline argument: > main = do > args <- getArgs > let reversor = case args of > ["sequence"] -> reverseList > ["list"] -> reverseSeq > _ -> error "bad args" > input <- getContents > let output = reversor $ lines $ input > mapM_ putStrLn output In my oppinion reversor would have type > reversor :: (Foldable f) => [a] -> f b but I couldn't get this to work. I've tried typeclass approach: > class (Foldable f) => Reversor f where > reverse' :: [a] -> f a > > instance Reversor ([]) where > reverse' = Data.List.reverse > > instance Reversor ViewR where > reverse' = viewr . foldr (<|) empty > > reverseList = reverse' :: (???) > reverseSeq = reverse' :: (???) but now in order to differentiate between "reverse'" functions I'd have to provide different type annotations, and then "reversor" won't typecheck... Similar problem surfaced with this try: > data Proc = SP | LP > reverseList = reverse' LP > reverseSeq = reverse' SP > > reverse' :: (Foldable f) => Proc -> [a] -> f a > reverse' LP = Data.List.reverse > reverse' SP = viewr . foldr (<|) empty So now I'm looking for some suggestions how should I approach the problem... Regards, -- Krzysztof Kościuszkiewicz Skype: dr.vee, Gadu: 111851, Jabber: kokr(a)jabberpl.org Mobile IRL: +353851383329, Mobile PL: +48783303040 "Simplicity is the ultimate sophistication" -- Leonardo da Vinci

6 8

Opengl and Haskell GLdouble/GLfloat vs. Double/Float
by bbrown 28 Sep '07

28 Sep '07

I am going to be doing a lot of opengl stuff in haskell and so far one thing has irked me. Why does haskell keep the GLFloat and GL types and not just the Haskell types. -- Berlin Brown [berlin dot brown at gmail dot com] http://botspiritcompany.com/botlist/?

3 2

Newb question about map and a list of lists
by Chuk Goodin 28 Sep '07

28 Sep '07

I have a list of lists of pairs of numeric Strings (like this: [["2","3"],["1","2"],["13","14"]] etc.) I'd like to change it into a list of a list of numbers, but I'm not sure how to go about it. If it was just one list, I could use map, but map.map doesn't seem to work. Any suggestions, or pointers to a reference online? thanks in advance, -- chuk

5 4