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An IO Question from a Newbie
by Matt O'Connor 14 Sep '03

14 Sep '03

Hello all. I'm new to functional programming and Haskell, but have been programming in C and Java for a while. I've been going through the tutorials and whatnot on haskell.org. I've read from the Gentle Introduction to Haskell about IO and some of the other stuff and I have a question about it. main = do putStr "Type Something: " str <- getLine putStrLn ("You typed: " ++ str) When compile and run this code the "Type Something: " isn't displayed until the putStrLn. So there is no prompt. The output looks like this. s Type Something: You typed: s But if I change the putStr "Type Something: " to a putStrLn or put a \n at the end of the string it displays the text immediately (ie, when I want it to). Is there a good reason for this? Am I doing something wrong? Or do I need to call some kind of standard output flush? Thanks. Oh, I'm using ghc. Matt -- "Not everone knows Josiah Royce's definition of a liar as a man who willfully misplaces his ontological predicates, but everyone who has ever told a lie will recognize its accuracy." Ch. 94: Truth - "The Great Ideas: A Synopticon of Great Books of the Western World"

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YAHT Problem
by Mark Espinoza 14 Sep '03

14 Sep '03

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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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Simple IO Monad problem
by kuq32tr02＠sneakemail.com 12 Sep '03

12 Sep '03

Hello, I'm starting to use Haskell for writing actual programs using monads and I'm already lost. I have the following script: #!/usr/bin/runhugs > module Main where > import System(getArgs) > main = do putStr "Hello, World\n" > strs <- getArgs > map putStrLn strs Which gives the following error: runhugs: Error occurred Reading file "./mailalias.lhs": Reading file "/usr/lib/hugs/lib/System.hs": Reading file "./mailalias.lhs": Type checking ERROR "./mailalias.lhs":5 - Type error in final generator *** Term : map putStrLn strs *** Type : [IO ()] *** Does not match : IO a Can someone please explain what I'm doing wrong? Thanks!

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Re: Standalone Parser for Haskell Grammar
by Peter Robinson 10 Sep '03

10 Sep '03

Thanks to both of you, you helped me a lot! :-) Peter On Wednesday 10 September 2003 14:56, you wrote: > You may find an error correcting combinator based parser in the > directory uust, to be found at cvs.cs.uu.nl > > Just check out the whole tree and follow the building and installing > instructions. The parser etc can be found in the UHC section. It makes > use of the parser combinators found elsewhere in the tree, > > Doaitse Swierstra > > On woensdag, september 10, 2003, at 12:33 PM, Peter Robinson wrote: > > Hello! > > Does anyone know a reasonable standalone Parser for the Haskell > > Grammar? The > > only one i found was hsparser but it's still an alpha release and i > > get a few > > errors during compiling. I know i could write one using Happy but i > > don't > > want to reinvent the wheel... > > regards > > Peter > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Standalone Parser for Haskell Grammar
by Peter Robinson 10 Sep '03

10 Sep '03

Hello! Does anyone know a reasonable standalone Parser for the Haskell Grammar? The only one i found was hsparser but it's still an alpha release and i get a few errors during compiling. I know i could write one using Happy but i don't want to reinvent the wheel... regards Peter

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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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Why not haskell?
by Nick Name 09 Sep '03

09 Sep '03

Inspired by the various replies to the "haskell for non-haskell sake" I wonder why so much people uses haskell "only for prototipying" or producing code in other languages. I am just curios to hear from people who do not use haskell for project releases, or just think it's not suitable for a mature project, what exactly they find bad in current haskell implementations, or perhaps in the standard. Vincenzo

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Library documentation
by Konrad Hinsen 05 Sep '03

05 Sep '03

Is there any place where I can find an introductory documentation for the Haskell libraries? To be more concrete, say I am interested in using Data.Array.ST.STUArray. I would need some explanation of the ST monad, and then an explanation of how I can use an STUArray in an ST monad. Plus ideally examples. But the same applies to most of the libraries. I can't find any documentation other than the very terse reference or the parts that happen to be covered by textbooks. Konrad.

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

05 Sep '03

> I wonder whether keeping the names of data constructors in info tables > is expensive? > > I would have guessed not. (more debugging info such as > function names and src locations is likely to be more > expensive however). > > Anyway, would it be possible/feasible for GHC to keep names > in info tables > all the time, not just for profiling? At least for data-constructors? That's a distinct possibility. If you send patches we'll incorporate them. Cheers, Simon

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