September 2003 - Haskell-Cafe

RE: An IO Question from a Newbie
by Simon Marlow 15 Sep '03

15 Sep '03

> > But there's one significant difference between C and > Haskell, which is > > applicable in the case of Matt's program. In C, any line-buffered > > output streams are automatically flushed when a read from an > > unbuffered or line-buffered stream can't be satisfied from > its buffer. > > Interesting. I didn't know this. Maybe we should match this > behaviour, or > provide a write-string-and-flush function. It seems like this issue > is causing an undue amound of trouble. I wrote GHC's IO library, and deliberately didn't include this feature. The previous version of the library did have such a feature, specifically for stdin/stdout. Note that the report doesn't say we must do this. The reason I didn't include the feature is because I can't see a way to do it right. Flushing *all* line-buffered handles (the ANSI C way) doesn't seem right. Flushing stdout just because we read from stdin is not right, because the two streams might refer to completely different I/O objects. Perhaps we should attempt to detect when there are two streams connected to the same I/O object (file, pipe, tty, whatever) and enable the magic flushing then. But now do I have to explain to people how this works? I suppose we could take the view that extra flushing is basically harmless, so it doesn't matter that we flush a bunch of Handles more often than we need to. The advantage of the current scheme is that it is easy to understand and explain; the library doesn't try to do clever stuff behind your back. The disadvantage is that it catches people out, and it sometimes requires you to import IO in an otherwise Prelude-only program. I'm more-or-less agnostic - if there's a way to avoid catching people out without introducing too much overhead or complicated rules that we have to explain, then I'll happily implement it. Cheers, Simon

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threaded red-black tree
by Lex Stein 15 Sep '03

15 Sep '03

Hi, No one responded to my question several weeks ago about a purely functional implementation of a threaded, red-black tree. My message was sent about the same time as that flurry of silly emails about "how to respond to homework questions". Was my message not responded to because it was classified as a homework question? I assure you this is officework, not homework. I ended up porting Okasaki's red-black tree implementation; hacking it apart with a bunch of mutation for the threading of the list through the tree. However, I'm still missing a deletion function and I haven't been able to find a prototype (Okasaki's red-black tree module lacks delete). My study of the RB-tree deletion routine in CLR hasn't yet yielded an adaptation for a functional environment. Any advice would be much appreciated. Thanks, Lex -- Lex Stein http://www.eecs.harvard.edu/~stein/ stein(a)eecs.harvard.edu cell: 617-233-0246

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Instance checking and phantom types
by Nick Name 15 Sep '03

15 Sep '03

Hi all, I have an example wich I don't understand: ---------------begin class C t data T = T instance C T data C t => T1 t = T1 f1 :: T1 () f1 = T1 data C t => T2 t = T2 t f2 :: T2 () f2 = T2 () ----------------end The first function, f1, is accepted both by hugs and ghc, unlike the second wich is rejected. Why does this happen? Shouldn't f1 be rejected with "no instance C ()" Thanks for help Vincenzo

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Read file problems
by Frederic BELLOC 15 Sep '03

15 Sep '03

HI, I have a recurrent probleme with function readFile and Monad : it is that i try to get a line, transform it and stock it in a list and what i want is that function return me the list. But hugs say me that in readFile my_file >>= \s -> map cons_line (lines s) readFile is a IO String type but is used here as [[Char]] type... and i don't know what to do... Ghc say me that is the lambda abstraction that fails with "map cons_line (lines s)" , he couldn't match IO against [] ! Help me please, i understand well (i hope) how Monad work but here i don't see a solution. Sorry for my poor english but i'm already a student. Thanks for all. -- C, socks and sun ;-) and haskell bugs :-(

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zlib binding providing Handle to compressed file.
by David Roundy 14 Sep '03

14 Sep '03

I've just created a binding to the gzopen function of zlib which causes it to create a Handle. The code currently only supports ReadMode and WriteMode, and hFileSize won't work properly when reading a file. In fact, pretty much nothing but plain old reading and writing will work, but such is life. Anyhow in case anyone is interested, I'm attaching the code. It creates a pipe using pipe(2) and spawns a thread to pass the data between the pipe and gzread or gzwrite. It's not pretty, but it's better than any other solution I could think of. Suggestions or criticisms are welcome. -- David Roundy http://www.abridgegame.org/darcs

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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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