- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] do we have something like isDefined or isNull in Haskell?
by Bryan Burgers 15 Jun '06

15 Jun '06

Vladimir, I think you forgot to put Haskell-cafe as a recipient of this email, so first I'll repost what you wrote. On 6/15/06, Vladimir Portnykh <vportnykh(a)hotmail.com> wrote: > many thanks. i have the follwoing code: > > module MyType (DataContainer(..)) where > import Maybe > data DataContainer = > MyType { > date :: [Maybe Double], > weight ::[Maybe Double], > height ::[Maybe Double] > } > deriving (Eq, Ord, Show) > > myType = MyType { > date = > [38548,38562,38576,38590,38604,38618,38632,38646,38660,38674], > weight = [], > height = [] > } > > reportProblem = if isJust (height myType !! (length (date myType) - 1)) > then 0 else 1 > > > it failed to compiled saying "No instance for (Num (Maybe Double)) arising > from the literal 38674 . After reading your code for a while, I still don't know if I quite understand. Can you explain exactly what you are trying to do? My thought is that maybe you just want a list of objects, since you are indexing height at the same index as date. Then you just want a list of the structures that Duncan described. myObjects = [default {weight = 3.2}, default {weight = 7.9, date=38458},...] If not, hopefully somebody a little more knowledgable than I can help you. Bryan

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Receiving multicasts
by Rich Neswold 15 Jun '06

15 Jun '06

Hello, Has anyone figured out a way to receive multicasts in a Haskell program? It doesn't appear that Network.Socket.setSocketOption provides enough information to join a multicast address. Any information would be appreciated. -- Rich AIM : rnezzy ICQ : 174908475

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how to apply a function which returns IO() to a list?
by developer＠imaginando.net 14 Jun '06

14 Jun '06

Hi, I have this function which write a file: writeHtml b x y = do writeFile (b ++ ".html") (htmlCode b x y) i need to apply to each member of a given list: the way i always try to codify functions in haskell is allways the same, the most primitive one and i think i cant use it here. writeList [] = ??? writeList (x:xs) = (writeHtml x) ??? How should i do this? Many thx, Nuno SAntos

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Closure trace?
by Michael Marte 14 Jun '06

14 Jun '06

Hello *, frequently we are seeing messages like "List.head: empty list" or "Maybe.fromJust: Nothing". It is clear what happened and where the messages were triggered yet the real cause is usually VERY hard to find unless the program is small and simple. I came to the conclusion that functions like head and fromJust are best to be avoided because their use may render large programs unmaintainable. Instead I use irrefutable pattern matching like (x : _) = l and Just bla = maybeBla whenever possible because when a pattern match fails the ghc runtime system gives me a nice error message naming the module, line and column. However, this procedure may become tedious when the error occurs in a call to a third-party library function. I wonder whether it is possible to print a "closure trace" similar to a stack trace in procedural programming. Say we have two modules A and B: module A where import B a = b module B b = c where c = error "go to hell" I would like to see something like this: sh$ ghci A <ghci startup messages> > a Runtime error "go to hell" in module B at line 2 Trace: B.b.c B.b A.a This way it would be easy to find the reason that actually caused, say, head to fail. I guess it would be sufficient to complement closures with information on the precise place of their definition. Michael

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Teaching Haskell -- any advice?
by David House 14 Jun '06

14 Jun '06

Hi all, Next year I will have the opportunity to teach Haskell at my school! I am very excited about this and hope to offer something beyond the VB6 and Pascal that is currently available with the school's Computing course. The time slots will be very tight -- one 25 minute session a week. However I will have a year's worth of weeks to teach it in, so I hope to cover most of the major topics with Haskell, including monads (which I appreciate some of the more pedestrian courses tend to skip :)). My question is whether anyone has any advice as to: 1) How to structure the course as a whole? 2) How to structure the lessons (theory/practical balance)? 3) How ambitious should I be? 4) Which pratical projects work well? Thanks in advance. -- -David House, dmhouse(a)gmail.com

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DBDirect trouble
by Ferenc Wagner 14 Jun '06

14 Jun '06

Hi! Let me repeat my question from IRC here, too... I installed HaskellDB and all the necessary stuff (I hope) as local (not system-wide) packages: $ ghc-pkg list /usr/lib/ghc-6.4.1/package.conf: rts-1.0, base-1.0, haskell98-1.0, template-haskell-1.0, unix-1.0, Cabal-1.0, parsec-1.0, haskell-src-1.0, network-1.0, QuickCheck-1.0, HUnit-1.1, mtl-1.0, fgl-5.2, X11-1.1, HGL-3.1, stm-1.0, readline-1.0, (lang-1.0), (concurrent-1.0), (posix-1.0), (util-1.0), (data-1.0), (text-1.0), (net-1.0), (hssource-1.0) /home/wferi/.ghc/i386-linux-6.4.1/package.conf: NewBinary-0.1, Crypto-3.0.3, (Cabal-1.0), Cabal-1.1.4, hsql-1.7, hsql-mysql-1.7, haskelldb-0.9, plugins-1.0, haskelldb-dynamic-0.9, haskelldb-hsql-0.9, haskelldb-hsql-mysql-0.9 Still, DBDirect doesn't run: $ ~/haskell/installed/bin/DBDirect Test mysql server=myserver,db=test,uid=wferi,pwd=secret DB/Direct: Daan Leijen (c) 1999, HWT (c) 2003-2004, Bjorn Bringert (c) 2005 Connecting to database... DBDirect: user error (Couldn't load Database.HaskellDB.HSQL.MySQL.driver from package haskelldb-hsql-mysql-0.9) strace -eopen shows it reads /usr/lib/ghc-6.4.1/package.conf only, not the local one... Any ideas how to persuade it to use the locally installed packages? -- Thanks, Feri.

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Bug in WinHugs?
by Lyle Kopnicky 13 Jun '06

13 Jun '06

Hi folks, I just installed the May 2006 release of Hugs. When I use the command-line version, the '$$' symbol to reference the last expression works fine. It does not work in WinHugs, but yields 'ERROR - Syntax error in expression (unexpected symbol "$$")'. I typed ':set' to verify the '-r$$' option was set properly. Is this a known bug, or am I doing something wrong? Thanks, Lyle

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elementsinlist
by Jenny678 13 Jun '06

13 Jun '06

Hallo I search a code for >elements_in_List([1,2],[1,2]). >True >elements_in_List([1,8],[1,2,3,4,8]). >True >elements_in_List([2,1],[1,2]). >True >elements_in_List([1,1],[1]). >False I have a code elements_in_List :: Eq a => [a] -> [a] -> Bool elements_in_List [] _ = True elements_in_List _ [] = False elements_in_List (x:xs) (y:ys) | x == y = elements_in_List xs ys | True = elements_in_List (x:xs) ys but it failed at >elements_in_List([2,1],[1,2]). >True I hope somebody can help me Please don't use built-in-Functions. Thanks for Help -- View this message in context: http://www.nabble.com/elementsinlist-t1782975.html#a4855615 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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Re: [Haskell-cafe] elementsinlist
by Bryan Burgers 13 Jun '06

13 Jun '06

On 6/13/06, Jenny678 <mestor1(a)gmx.de> wrote: > > Hallo > I search a code for > > >elements_in_List([1,2],[1,2]). > >True > >elements_in_List([1,8],[1,2,3,4,8]). > >True > >elements_in_List([2,1],[1,2]). > >True > >elements_in_List([1,1],[1]). > >False > > I have a code > elements_in_List :: Eq a => [a] -> [a] -> Bool > elements_in_List [] _ = True > elements_in_List _ [] = False > elements_in_List (x:xs) (y:ys) > | x == y = elements_in_List xs ys > | True = elements_in_List (x:xs) ys > > but it failed at > >elements_in_List([2,1],[1,2]). > >True > > I hope somebody can help me > Please don't use built-in-Functions. > > Thanks for Help I haven't looked too far into this, but how about something like this: elements_in_list a b = and $ map (\x -> element_in_list x b) a where element_in_list = ... Maybe if you don't want to use built-in functions, define your own 'and' as well. Bryan

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Re: [Haskell-cafe] Separate a string into a list of strings
by Donn Cave 13 Jun '06

13 Jun '06

Quoth Clifford Beshers <clifford.beshers(a)linspire.com>: | Well, I couldn't resist the puzzle. Here are solutions using foldr and | unfoldr. Don't know if they are cunning or not, but they were kind of fun. ... | splitByElem1 e xs = | foldr f [[]] xs | where f a b = if a == e then [] : b else (a : head b) : (tail b) This does the right thing with trailing separators, which is not to be taken for granted among Haskell split implementations. The splits I have been seeing in this thread are conservative, so if the separator is ':', then "::a" splits to ["", "", "a"]. Frequently however the implementation fails to deal with the trailing separator, so "a:" is ["a"], where it should be ["a", ""]. It's not something you run into right away. In a liberal split, "a " should indeed be ["a"], but that's a different matter. Neither of the two I've looked at seems to be shooting for a liberal "words" white space split. Donn

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