- Haskell-Cafe - Haskell.org

IO StateTransformer with an escape clause
by Thomas L. Bevan 01 Sep '03

01 Sep '03

Hi, I'd like some help building an IO StateTransformer which can be escaped midway through without losing the state accummulated up to that point. I tried constructing a StateT s MaybeIO a monad but the state is lost when the rest of the monad collapses. Tom

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Haskell 6.0 install
by Mark Espinoza 01 Sep '03

01 Sep '03

2 1

Re: Haskell for non-Haskell's sake
by Nick Name 30 Aug '03

30 Aug '03

Alle 13:33, sabato 30 agosto 2003, Alastair Reid ha scritto: > - Knit (http://www.cs.utah.edu/flux/knit/) > > A component language for C Are there component languages wich work on multilanguage projects? Something like "my project has a module written in haskell, one in C and one in Ocaml, and I don't want an haskell 'list a' to pass through C nor corba to get into Ocaml". Is there any hope in this sense? V.

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Type classes question
by Eugene Nonko 29 Aug '03

29 Aug '03

Hello, I am trying to define class Dual and few instances as follows: === class Dual a where dual :: a -> a instance Dual Bool where dual = not instance (Dual a, Dual b) => Dual (a -> b) where dual f = dual . f . dual instance Dual a => Dual [a] where dual = reverse . map dual instance Num a => Dual a where dual = negate === For some reason Hugs does not lake the last definition saying 'ERROR dual.hs:13 - syntax error in instance head (constructor expected)'. What am I doing wrong? -- Eugene

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Monads and Maybe
by Konrad Hinsen 29 Aug '03

29 Aug '03

I have been following the recent "Monad tutorial" discussion with interest, and even read the tutorial, which is a useful addition to the existing Haskell documentation. So useful in fact that it raises a question... The whole monad mechanism seems to geared towards functions of one argument, plus eventually state, that get chained together. How about functions with several arguments? As an example, I'll use the Maybe monad. Suppose I want to write code to handle experimental data, in which there might be missing values. I might then decide to represent measurements by data of type "Maybe Double", with missing values represented by "Nothing". I could then go on to define functions on missing values, which would return "Nothing" when their argument is "Nothing", and I could string these functions together via the monad mechanism. Fine. But how would I handle e.g. addition of two such values? The result should be "Nothing" when either of its arguments is "Nothing". Is there any mechanism to handle that? Konrad.

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Debugging
by Konrad Hinsen 29 Aug '03

29 Aug '03

My Haskell experiments have reached a size in which debugging tools would be more than welcome, so I looked around, and was very disappointed. I tried Hood, which is a pain to use (lots of editing of the code required), I looked at Buddha but didn't want to downgrade to GHC 5 for trying it (nor is my code Haskell 98, because of multi-parameter classes), and all that seems left to try is Hat, whose Web site I can't reach at the moment. So what are you Haskell programmers using for debugging in real life? Konrad.

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threaded balanced tree
by Lex Stein 28 Aug '03

28 Aug '03

Hello, Does anyone have a purely functional haskell implementation of a threaded, balanced tree? I modified Okasaki's SML red-black tree implementation, but my modifications mutate. I would be very interested in seeing an approach that doesn't use mutability. Thanks Lex -- Lex Stein http://www.eecs.harvard.edu/~stein/ stein(a)eecs.harvard.edu cell: 617-233-0246

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RE: New to Haskell
by Hal Daume 28 Aug '03

28 Aug '03

the first line says that 'fname' is a function which takes a list and returns a string. the list is of type '[([Char],a)]'. this means that it's a list of ([Char],a)s. these are pairs of [Char]s and as. a [Char] is a string (a string is a list of characters) and an a is a something -- anything you want. the second line tells you the value of fname when its argument list is empty ([]). the result in this case is the string "Result_Blank". the third line tells you what to do when the input is not empty. when you have a non-empty list, it's composed of a head and a tail, written (head:tail). since our list is of pairs, the head has the form (a,b). putting this together, we have that the list is of the form ((x,xs):ys). here, x is our a, xs is our b and ys is the tail. (x,xs) is our head. it then says that the result of this is just the value 'x'. this is simply the [Char] part (the first half) of the first pair in the list. HTH - Hal -- Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume > -----Original Message----- > From: haskell-cafe-admin(a)haskell.org > [mailto:haskell-cafe-admin@haskell.org] On Behalf Of Matthew Morvant > Sent: Thursday, August 28, 2003 8:44 AM > To: 'Haskell (haskell-cafe(a)haskell.org)' > Subject: New to Haskell > > > Hello, > I am very new to haskell. I was hoping someone could > explain something > to me > > fname :: [([Char],a)] -> String > fname [] = "Result_Blank" > fname ((x,xs):ys) = x > > What exactly [use little words :-)] does this do? > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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New to Haskell
by Matthew Morvant 28 Aug '03

28 Aug '03

Hello, I am very new to haskell. I was hoping someone could explain something to me fname :: [([Char],a)] -> String fname [] = "Result_Blank" fname ((x,xs):ys) = x What exactly [use little words :-)] does this do?

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AW: Poll: How to respond to homework questions
by Markus.Schnell＠infineon.com 28 Aug '03

28 Aug '03

OK: D E Not OK: A B C I have never seen anybody asking more than one "do my homework" question. I would encourage students to give it a try, even if it's not working, and give some advice on that. Pointing to a web page like "do your homework yourself - it will benefit you most that way" is a very good idea. Markus -- Markus Schnell > -----Ursprüngliche Nachricht----- > Von: Tom Pledger [mailto:Tom.Pledger@peace.com] > Gesendet: Mittwoch, 27. August 2003 23:43 > An: haskell-cafe(a)haskell.org > Betreff: Poll: How to respond to homework questions > > > Hi. > > I'm curious about what the people on this list consider appropriate, > as responses to homework questions. Even if there isn't a consensus, > it may be interesting to see how opinion is divided. > > Please consider the following. > > (A) Give a perfect answer. > (B) Give a subtly flawed answer. > (C) Give an obfuscated answer. > (D) Give a critique of what the questioner has tried so far. > (E) Give relevant general advice without answering the > specific question. > > I mean them to include giving references, e.g. a Wiki URL to relevant > general advice would count as (E). > > Which of those do you approve of? Which do you disapprove of? > > Please email me a concise vote, e.g. > > OK: B D E > Not OK: A > > and I'll post a collated result next week. > > - Tom > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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