- Beginners - Haskell.org

Exception back trace
by Russ Abbott 06 Dec '10

06 Dec '10

Hi, I am in the midst of debugging and got an exception: *** Exception: Prelude.minimum: empty list That's all it said. There was no information about where the exception occurred. Is it possible to ask for more information, preferably including a stack trace? Thanks. * -- Russ *

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Re: [Haskell-beginners] defining 'init' in terms of 'foldr'
by Hein Hundal 05 Dec '10

05 Dec '10

I am trying to understand the differences between two different solutions to the problem of expressing init in terms of foldr. The two solutions are called heinInit and chaddiInit. The heinInit does not work on infinite lists. The chaddiaiInit function does work on infinite lists. heinInit v = fromJust $ foldr f Nothing v where f a Nothing = Just [] f a (Just v) = Just (a:v) chaddaiInit v = fromJust $ foldr f Nothing v where f x xs = Just (case xs of Nothing -> []; Just ys -> x:ys) Here is my guess at the "trace" of the chaddaiInit code. 1) head (chaddaiInit [1..]) 2) head (fromJust $ foldr f Nothing [1..]) 3) head (fromJust $ f 1 (foldr f Nothing [2..])) 4) head (fromJust $ Just (case (foldr f Nothing [2..]) of Nothing -> []; Just ys -> 1:ys)) 5) head (case (foldr f Nothing [2..]) of Nothing -> []; Just ys -> 1:ys) 6) head (case (f 2 (foldr f Nothing [3..])) of Nothing -> []; Just ys -> 1:ys) 7) head (case (Just (case (foldr f Nothing [3..]) of Nothing -> []; Just ys -> 2:ys)) of Nothing -> []; Just ys -> 1:ys) 8) head (1:(case (foldr f Nothing [3..]) of Nothing -> []; Just ys -> 2:ys)) 9) 1 Here is my guess at the "trace" of the heinInit code. myinit v = fromJust $ foldr f Nothing v where f a Nothing = Just [] f a (Just v) = Just (a:v) 1) head (heinInit [1..]) 2) head (fromJust $ foldr f Nothing [1..]) 3) head (fromJust $ f 1 (foldr f Nothing [2..]) 4) head (fromJust $ f 1 (f 2 (fold f Nothing [3..]) 5) head (fromJust $ f 1 (f 2 (f 3 (fold f Nothing [4..]) And it just keeps going. Is this the right idea? Are my "traces" a reasonable estimate of how Haskell evaluates those expressions? Cheers, Hein

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Named Pipes Et Cetera
by matthew coolbeth 05 Dec '10

05 Dec '10

Hello, all. Three questions: 1> What, if anything, can I do in a Haskell program to do blocking reads from a Linux FIFO? For the curious, there is a small program below that doesn't block, but immediately terminates because of an EOF 2> How could I have found the answer to this question on my own? (The documentation that Google found for me was not completely adequate) 3> Is this sort of question generally regarded as too unixy or not haskelly enough to be appropriate for this mailing list? Thanks. --BEGIN PROGRAM-- module Main () where import Control.Monad import System.IO import System.Posix.Files pipeString = "/tmp/dld.fifo" main :: IO () main = do createNamedPipe pipeString $ unionFileModes ownerReadMode ownerWriteMode pipe <- openFile pipeString ReadMode forever (hGetLine pipe >>= putStrLn) -- mac

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Re: [Haskell-beginners] defining 'init' in terms of 'foldr'
by Hein Hundal 05 Dec '10

05 Dec '10

> From: Daniel Fischer <daniel.is.fischer(a)googlemail.com> > Subject: Re: [Haskell-beginners] defining 'init' in terms > of 'foldr' > On Saturday 04 December 2010 23:20:51, Paul Higham wrote: > > Not sure if this thread is still active but I also > > struggled with this > > same exercise. I offer the following solution as > > a thing to shoot at: > > > > myInit :: [a] -> [a] > > myInit ys = foldr snoc [] $ (\(x:xs) -> xs) $ foldr snoc [] ys > > where snoc = (\x xs -> xs ++ [x]) > > init === reverse . tail . reverse > only holds for finite lists, for infinite lists xs, reverse > xs = _|_, but > init xs = xs. > Also, it's inefficient, but that's not the point of the > exercise. Here is my incomplete beginner solution: --Start Code import Data.Maybe myinit v = fromJust $ foldr f Nothing v where f a Nothing = Just [] f a (Just v) = Just (a:v) --End Code This worked for the finite lists that I tried, but it did not work for infinite lists. I was surprised that foldr works with infinite lists. If I set v = take 5 $ foldr (\x xs -> (x*x):xs) [] [1..] then the value of v is [1,2,9,16]. Is there another way to create the init function with foldr that works for infinite lists? Hein

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Maybe perform an action
by Tim Baumgartner 05 Dec '10

05 Dec '10

Hi Haskellers, I have another simple question in order to improve my coding style. I'm using the following function, e.g. when a dialog returned Maybe NiceStuff: performMaybe :: Monad m ⇒ Maybe a → (a → m b) → m () performMaybe x action = when (isJust x) (action (fromJust x) >> return ()) example use: main = performMaybe (Just "Hello") print Now I wonder if others use such a function as well, if it's already defined in the standard libraries (didn't find it using Hoogle) and perhaps how to implement it a little nicer. Regards Tim

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Error building internal Scion server, how to solve it?
by Guofeng Zhang 04 Dec '10

04 Dec '10

Hi, Installed Haskell Platform 2010.2.0.0 on Windows, then install EclipseFP 2 Latest Release 2.0.2 on Helios 3.6.1. JDK 1.6.0_21. But the internal Scion server cannot be built, so I cannot compile my Haskell code. How to solve it? The following error message is displayed on the Eclipse console when I launch Eclipse: Resolving dependencies... selecting cabal.exe: cannot configure scion-0.1.0.6. It requires AttoJson >=0.5.2, binary ==0.5.*, derive -any, ghc-paths ==0.1.*, ghc-syb -any, ghc-syb-utils -any, hslogger ==1.0.*, list-tries -any, multiset >=0.1 && <0.3 and uniplate -any There is no available version of AttoJson that satisfies >=0.5.2 There is no available version of binary that satisfies ==0.5.* There is no available version of derive that satisfies -any There is no available version of ghc-paths that satisfies ==0.1.* There is no available version of ghc-syb that satisfies -any There is no available version of ghc-syb-utils that satisfies -any There is no available version of hslogger that satisfies ==1.0.* There is no available version of list-tries that satisfies -any There is no available version of multiset that satisfies >=0.1 && <0.3 There is no available version of uniplate that satisfies -any

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Parser composition
by Angelos Bimpoudis 04 Dec '10

04 Dec '10

Hi all, This is my first post to this list. Nice to meet you! I study Functional Programming with Haskell, and I have stuck at p.77 (the first checkpoint in FP, as far as I understand). Passing this, I feel that I make steps towards understanding monadic binding! So I would like to ask about whether or not this: P1>>=λu1 -> P2>>=λu2 -> P3>>=λu3 -> P4>>=λu4 -> return (f u1 u2 u3 u4) can be rewritten to: P1>>=(λu1 -> P2>>=(λu2 -> P3>>=(λu3 -> P4>>=(λu4 -> return (f u1 u2 u3 u4))))) I am trying to understand what is the f in p>>=f in each one of above sequencing applications? So >>= gets you in this structure (is this the monad?), and return is the gate out of it? Best regards. These are my first steps to FP! -- Angelos Bimpoudis Doctoral Researcher, Pervasive Computing [p-comp.di.uoa.gr] SDE, Nessos IT S.A. [www.nessos.gr] (m): +306942075153 <http://gr.linkedin.com/in/aggelosmp> Description: Description: Linkedin <http://www.facebook.com/aggelosmp> Description: Description: Facebook <http://www.studentguru.gr/blogs/grnemo/> Description: Description: Blog RSS <http://twitter.com/aggelosmp> Description: Description: Twitter

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Finding the first successful Map.lookup
by John Obbele 02 Dec '10

02 Dec '10

Hi every one ! A long time ago, I got a problem about Map.lookup and lazyness (the programmer's one). Since it's still bugging me, I'd like to ask for some external advice. Let's say I've build a big documentation index (:: Map Key URL) where both Key and URL are Strings. The problem is that my keys have some random prefixes (ie the key "foo" could be store either as "foo", "a_foo", "b_foo", "g_foo", "gtk_foo" or "pango_foo"). How could I implement effectively in Haskell a function which return me the first (and only the first!) correct URL ? For now, I'm doing something like this: {{{ type Key = String type URL = String lookupDocumentation :: Map Key URL -> Key -> Maybe URL lookupDocumentation myMap key = let r = map (lookupSymbols myMap) [ key , "a_" ++ key , "b_" ++ key , "g_" ++ key , "gtk_" ++ key , "pango_" ++ key ] in case catMaybes r of [] -> Nothing (x:_) -> Just x }}} My problem is: will `lookupDocumentation` perform n=6 lookups in my Map, or will Haskell-built-in-lazyness only evaluate till an URL is found ? regards, /John

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explanation
by John Moore 01 Dec '10

01 Dec '10

Hi , Havent used haskell in a while can someone explain just whats happening here. I know what it does, I just can't workout how it does it. Only need to explain how the red area works and I can work out the rest myself Regards John module Rollback where data Expression = Val Double | Add Expression Expression | Subtract Expression Expression | Multiply Expression Expression | Divide Expression Expression deriving Show demo1 = (Add(Multiply(Divide(Subtract(Val 25)(Val 5))(Val 10))(Val 7))(Val 30)) evalStep :: Expression -> Expression evalStep (Val x)= (Val x) evalStep (Add x y) = case x of (Val a) -> case y of (Val b) -> Val (a+b) left -> Add x (evalStep y) right -> Add (evalStep x)y evalStep (Subtract x y) = case x of (Val a) -> case y of (Val b) -> Val (a-b) left -> Subtract x (evalStep y) right -> Subtract (evalStep x)y evalStep (Multiply x y) = case x of (Val a) -> case y of (Val b) -> Val (a*b) left -> Multiply x (evalStep y) right -> Multiply (evalStep x)y evalStep (Divide x y) = case x of (Val a) -> case y of (Val b) -> Val (a/b) left -> Divide x (evalStep y) right -> Divide (evalStep x)y type Stack = [Expression] evaluate :: Expression -> IO () evaluate exp = do stk <- evalWithStack exp [exp] putStrLn "End of Equation" evalWithStack :: Expression -> Stack -> IO Stack -- Base case evalWithStack (Val a) stk = return stk -- Recursive case evalWithStack e stk = do putStrLn "Evaluating one more step" let e' = (evalStep e) putStrLn ("Result is "++(show e')) putStrLn "Do another step (y/n) or rollback (r)? :" c <- getLine case c of "y" -> evalWithStack e' (e':stk) "r" -> let (a,stk') = stackBack stk in evalWithStack a stk' "n" -> do { putStrLn ("Ok you said :" ++ show c ++ "so that's it " ++ "You went as deep as " ++ show (getCount stk) ++" levels") ; return (e': stk) } stackBack :: Stack -> (Expression,Stack) stackBack [a] = (a,[a]) stackBack (a:as) = (a,as) stackBack [] = error "Nothing there" getCount :: Stack -> Int getCount stk = length stk

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Timer Events?
by matthew coolbeth 30 Nov '10

30 Nov '10

I'd like to write a Haskell program that executes an IO action every 50 milliseconds. It would be most helpful if someone could point me in the right direction. -- mac

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