- Haskell-Cafe - Haskell.org

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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newbie type signature question
by Brock Peabody 13 Jun '06

13 Jun '06

Please excuse my newbiness, but in this snippet: data (Monad m) => DataType m = DataType { f :: Char -> m () } test_function :: (Monad m) => DataType m -> m () ^^^^^^^^^^^^ test_function d = f d 'C' Why is "(Monad m) =>" required, when the definition of DataType already implies it? Is there an easier way to do this or will I have to have it in all signatures containing DataType? Thanks, Brock

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Re: [Haskell-cafe] Learning C after Haskell
by Chad Scherrer 12 Jun '06

12 Jun '06

Thanks, Minh. So are things like recursion and memory sharing typically out the window? Also, I don't see how thinking about type classes will help, without the benefits of polymorphism. -Chad ---------- Forwarded message ---------- From: minh thu <noteed(a)gmail.com> Date: Jun 12, 2006 12:23 PM Subject: Re: [Haskell-cafe] Learning C after Haskell To: Chad Scherrer <chad.scherrer(a)gmail.com> hi, C is very different from Haskell. * you'lle have to manage explicitly memory de/allocation. * c programming is a bit like haskell io monad programming (but without the functionnal part) : you'lle use "=" in place of "<-" and the left hand side can be reassigned multiple times. * imperative programming (c is imperative) involve states (a lot !). (see how to write the equivalent of mapM print [1..10]); imperative programmers use loops, not much recursion * things you can keep in your mind : the way you organize things (module, type classes (but there is no polymorphism in c), have functions and not a huge amount of code lines...), the way you use self documenting names. well, not sure it helps, but here you are :) mt 2006/6/12, Chad Scherrer <chad.scherrer(a)gmail.com>: > Ok, so I'm doing things somewhat backward. I've been using Haskell for a > while now, whenever I get a chance to. But in order to become more involved > in high-performance computing projects at my work, I need to learn C. > > I've heard a lot of people say that experience in Haskell can improve one's > abilities in other languages, but I also wonder how different the C "way of > doing things" is different from Haskell's. > > My question is, as I learn C, are there any particular Haskell concepts I > should keep in the back of my mind, or is it better to approach C from > scratch? > > Thanks in advance! > > Preparing for a foot-shooting, > Chad > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Newbie: safe program termination in Windows
by Alberto G. Corona 12 Jun '06

12 Jun '06

Hi, I have a program that handle cached data. I don´t find the way to store the data when the program is killed by other program. There is nothing similar to signal in the Windows implementation and no exception appears to trigger when the program is killed by a service handler or by the Task manager.... Anyone knows how to deal with that?.

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