August 2003 - Haskell-Cafe

Why no findM ? simple Cat revisited
by Ahn Ki-yung 11 Nov '03

11 Nov '03

Simple Cat (revisitied) \begin{code} import IO findM f [] = return Nothing findM f (x:xs) = do { v <- x; if f v then return (Just v) else findM f xs } isLeft (Left _) = True isLeft _ = False main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h \end{code} This is my answer for the question of my own, which is posted a couple There are mapM, filterM in the Haskell 98 Standard Library. But why no findM there ? As you can see from simple cat, it seems quite useful. I think fildM should be added to the module Monad. -- Ahn Ki-yung

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No safety in numbers
by Konrad Hinsen 14 Sep '03

14 Sep '03

I am trying to write a larger piece of code using only type constraints for all the numbers, not specific types, in order to be able to choose the precision as late as possible. This works rather well (something I can't say of many other languages), but one problem I keep running into is constants. Something I need frequently is, for example, the Boltzman constant, 0.0083144708636327096 in my unit system. I can certainly type 0.0083144708636327096 everywhere in the code and make things work, literals have the nice property of being overloaded. But for the sake of readibility, I prefer to give this beast a name and have the explicit value only once in my code. So I create a module "Constants" with something like k_B = 0.0083144708636327096 The trouble is that k_B then becomes "Double" by default (or any other type I declare it to be). And this ruins all the code where I make a reference to it, with Hugs telling me "Inferred type is not general enough" because all my "generic" types become unified with Double. The only solution I can see is to write a function "fromDouble" in analogy to "fromInteger" and put this in front of all named constants. But how could I implement such a function? I might also declare all my constants to be "Rational" and use "fromRational", but I don't know much about the "Rational" type. Do I have to worry about insufficient or compiler-dependent precision? Konrad.

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Prefix and postfix operators
by Eugene Nonko 09 Sep '03

09 Sep '03

Hello, Are there any plans to extend Haskell parser to support prefix and postfix operators? It will be just great for domain-specific languages. It always kind of bothered me that unary minus is special. Thanks, Eugene

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RE: haskell reify, was sequencing data structures
by Simon Marlow 05 Sep '03

05 Sep '03

> - There needs to be some support from the > compilers/interpreters. Hugs > already has this. Ghc has some of it, but I abuse the > profiling system > in order to get the names of constructors to be present > on the heap. > I'm not happy with this. Under normal compilation GHC > doesn't keep > constructor names around, as far as I know. However, the > new debugger > in GHC must do something to get names of things, so perhaps the > -prof hack is no longer needed? I believe the debugger that Robert Ennals is working on tries to infer constructor names from the symbol table of the binary. Personally, I'd like to see debugging info placed in a separate segment of the binary, so it won't be loaded with the code but can be used by a debugger. Cheers, Simon

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Poll: How to respond to homework questions
by Tom Pledger 02 Sep '03

02 Sep '03

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

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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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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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