- Haskell-Cafe - Haskell.org

RE: Eager IO function
by Simon Marlow 13 Feb '02

13 Feb '02

> > How should I modify the function below so that characters > are printed > > out as they arrive? > > > > printchar :: Handle -> IO () > > printchar handle = do c <- hGetChar handle > > putChar c > > Try using > hSetBuffering handle NoBuffering > hSetBuffering stdout NoBuffering > from the IO library, before you call printchar. If the input handle > is attached to a terminal, you may also need to adjust the terminal > emulation into `cbreak' mode, e.g. with > System.system "stty cbreak" > Don't forget to reset the terminal afterwards! GHC does the "stty cbreak" step for you, if the handle on which you set the buffering mode is a terminal. Note also that not all systems have an 'stty' command. Cheers, Simon

1 0

Eager IO function
by Tom Bevan 13 Feb '02

13 Feb '02

How should I modify the function below so that characters are printed out as they arrive? Thanks. Tom printchar :: Handle -> IO () printchar handle = do c <- hGetChar handle putChar c

2 1

RE: (no subject)
by Jay Allen 11 Feb '02

11 Feb '02

Ask yourself what the type of "head s, last s" is in the exp portion of the comprehension. In fact, it's not a valid Haskell expression, unless you convert it into a list or a tuple. What you actually want is: function seg = nub (concat([[head s, last s] | s <- seg]) -J- Jay Allen Technical Lead, DSV Internet Client "The good news is that in 1995 we will have a good operating system and programming language; the bad news is that they will be Unix and C++." - Richard Gabriel -----Original Message----- From: Phil Haymen [mailto:phaymen@yahoo.com] Sent: Saturday, February 09, 2002 11:09 AM To: haskell-cafe(a)haskell.org Subject: (no subject) hi,I have a function, using list comprehension to pick out the head and last elements from a list of lists and output this into a list without duplicates. It doesn't work. I want to know what is the error. function :: [[Int]] -> [Int] function seg = nub (concat([head s, last s | s <- seg]) __________________________________________________ Do You Yahoo!? Send FREE Valentine eCards with Yahoo! Greetings! http://greetings.yahoo.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Re: syntax across languages
by C.Reinke 11 Feb '02

11 Feb '02

> > encryptionTooSimple d = > > "This encryption key is too simple (must be at least " > > ++(show (d::Integer))++ characters long)" > > - i don't like separating the string from the code for simple strings if you don't extract the string, how can it be a parameter of/variable in your application? > - do you really suggest giving this translators? You can be sure it won't > compile any more ;p actually, this version seems to come from an out-of-date editor buffer (missing quote).. it won't compile!-) <GRIPE> one thing I really don't like about Haskell is it's lacking support for HERE documents, as you'll find them in shells or in Perl. Am I the only one who's constantly bitten by this? </GRIPE> <ROUGH SUGGESTION> Something like: encryptionTooSimple :: Integer -> String encryptionTooSimple d = <<HERE; This encryption key is too simple (must be at least $d characters long) HERE with "..$var.." translated to ".."++(toString var)++".." and toString somewhat like this (just to avoid spurious quotes): class Show a => ToString a where toString :: a -> String instance Show a => ToString a where toString = show instance ToString Char where toString c = [c] instance ToString String where toString s = s </ROUGH SUGGESTION> As some of our implementers seem to be in fast-track-response mode, perhaps this has a chance of slipping in?-) Claus

2 1

Re: syntax across languages
by C.Reinke 11 Feb '02

11 Feb '02

> > I'm not sure if "iterate" counts as loop forever? > > I don't think there can be a "loop forever" construct in haskell? why not? > in OCaml, it is simply: > let loop(f) = f() ; loop(f) > and is only useful together with exceptions and side-effects. and in Haskell, it is simply: loop f = f >> loop f or, as there is little difference between control structures and other data structures, you can convert the latter into the former, using the list version of loop, called repeat: loop f = mapM_ id $ repeat f or simply: loop = foldr1 (>>) . repeat example (get ready to hit that interrupt key;-): loop (putStrLn "hi") > > I can't remember any %-based sprintf off the top of my head, but hey, > > who'd want one anyway? > > me! I've always wondered how you internationalize a program without it? > > eg: > "This encryption key is too simple (must be at least %d characters long)" > "The package %s needs to be installed. Do you want to install it?" > > (I agree you don't need the full power of sprintf for this) indeed not. you only want to factor out the strings, so that they can be collected in a single space for the translators/customizers convenience. functional abstraction allows you to just do that (then put the functions into a module per language, or collect them in tuples/records, and link or select as appropriate): encryptionTooSimple d = "This encryption key is too simple (must be at least " ++(show (d::Integer))++ characters long)" packageMissing s = "The package "++s++" needs to be installed. Do you want to install it?" but if you absolutely want printf-style formatting, Danvy's solution is very nice and simple (once someone had the idea;-): int k r x = k (r++(show (x::Integer))) -- an Integer str k r x = k (r++x) -- a String s x k r = k (r++x) -- format String fragment nl = s "\n" -- newline format p = p id "" (similar continuation-based functions for other types) Now you can try it in Hugs: Main> :t format $ s "age: " . int . s " name: " . str . nl format $ s "age: " . int . s " name: " . str . nl :: Integer -> [Char] -> [Char] Main> format ( s "age: " . int . s " name: " . str . nl ) 10 "IGS" "age: 10 name: IGS\n" See: RS-98-12 Olivier Danvy. Functional Unparsing. May 1998. 7 pp. This report supersedes the earlier report BRICS RS-98-5. Extended version of an article to appear in Journal of Functional Programming. http://www.brics.dk/RS/98/Ref/BRICS-RS-98-Ref/BRICS-RS-98-Ref.html#BRICS-RS… > i don't put any non-simple functions, the main aim is not to show how it can > be done, but what are the various names used in various languages. but it is one of the major features of functional languages that it is so easy to compose/reuse functionality! this is why Haskell doesn't need a name for everything - composing the functions can be as simple as (and more flexible than) composing the names. Claus

2 1

Re: syntax across languages
by Pixel 11 Feb '02

11 Feb '02

Patrick M Doane <patrick(a)watson.org> writes: [...] > I find sprintf to be not powerful enough for internationalization. The > specification of actual values is order dependant which may change from > language to language. not true with advanced printf's: There is one last vital consideration for output. The order of words in a sentence is different in various languages, for example, ``yellow flower'' becomes ``la flor amarilla'' en Espanbsp;nol. The differences between English and German are equally dramatic. To solve this problem, the standard printf() allows the order of the arguments to be handled in variable order in the format string. For example, if we have printf(fmt, month, date, who); we can use an fmt of %s %d is %s's birthday in English to produce ``April 26 is James's birthday'' and %3$s's geburtstag ist %2$d. %1$s in German to generate ``James's geburtstag ist 26. April.'' Notice that qualifiers like 2$ allows us to reorder the parameters to account for differing word order in different languages. (from http://www.alumni.caltech.edu/~copeland/work/i18n-b.html)

1 0

Re: efficiency question
by Jorge Adriano 11 Feb '02

11 Feb '02

On Monday 11 February 2002 02:10, Hal Daume III wrote: > So instaed of storing 1 and 2 in the list, it stores pointers to > them. The reason it does this is so that it makes it easy to generate > code for polymorhpic functions. (,) being boxed means that instead of > storing a pair of element, you're storing a pair of pointers. That means > you get worse performance because you have to chase more pointers. > > (at least that's my impression) Thanks, I knew the concept but not the name. On Sunday 10 February 2002 18:48, Kirsten Chevalier wrote: > I'd guess that it's not just that you have to apply the (,) constructor -- > it also has to do with the fact that the tuples it's constructing here are > boxed. This brings another question to my mind, isn't it toupling a standard technique used in functional programming? I'm pretty sure I've seen it focused in some papers/text books. I for one would not expect that folding the list twice would be more efficient... J.A.

1 0

Jan Skibinski's numeric-quest code
by Mike Thomas 11 Feb '02

11 Feb '02

Hi there. Does anyone know where to get Jan's Haskell math/physics Haskell examples from since www.numeric-quest.com went down? An open-source LU matrix factorisation function would also be handy. Cheers Mike Thomas.

3 4

(no subject)
by Phil Haymen 09 Feb '02

09 Feb '02

hi,I have a function, using list comprehension to pick out the head and last elements from a list of lists and output this into a list without duplicates. It doesn't work. I want to know what is the error. function :: [[Int]] -> [Int] function seg = nub (concat([head s, last s | s <- seg]) __________________________________________________ Do You Yahoo!? Send FREE Valentine eCards with Yahoo! Greetings! http://greetings.yahoo.com

1 0

RE: efficiency question
by Konst Sushenko 09 Feb '02

09 Feb '02

(moved to haskell-café) I ran Hal's code on my computer, and with test2 I get a stack overflow (so I had to use +RTS option for it to finish). test1 does not overflow stack (of standard size, I mean without +RTS). Which implies that test2 uses more stack space than test1. why would it use more stack if not because of laziness? konst > -----Original Message----- > From: Hal Daume III [mailto:hdaume@ISI.EDU] > Sent: Friday, February 08, 2002 4:35 PM > To: Jorge Adriano > Cc: Konst Sushenko; haskell(a)haskell.org > Subject: Re: efficiency question > > > I agree that it's the overhead of (,), but I don't see why > there would be > any overhead for doing this. > > -- > Hal Daume III > > "Computer science is no more about computers | hdaume(a)isi.edu > than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume > > On Sat, 9 Feb 2002, Jorge Adriano wrote: > > > On Friday 08 February 2002 23:52, Hal Daume III wrote: > > > I've tried using a strict fold: > > > > > > foldl' f a [] = a > > > foldl' f a (x:xs) = (foldl' f $! f a x) xs > > > > > > but that has no effect (or minimal effect). > > > > That wouldn't work even if if laziness is the problem > because that would only > > cause the elements of the list to be evaluated to head > normal form, the > > elements of the pair would not be evaluated so you'd have a > 'suspension of > > (minus and plus) operations'. > > > > instead of > > > (\x (a,b) -> (x+a,x-b)) > > try > > > (\x (a,b) -> (((,) $! x-a)$! x-b) ) > > > > I just noticed that you were the one who sent me the DeepSeq module. > > This is the kind of place where I want to use it. > > Instead of $!, try $!!. > > > > > > And Konst Sushenko wrote: > > >>My guess is that it is due to the laziness of the > addition/subtraction > > >>in (,) > > > > Seems to me like lazyness is not the right guess because > both functions Hall > > first posted were lazy. So I think it's just the overhead > of applying (,) > > besides (+) and (-) in each step. Do I make sense or am I > missing something? > > > > J.A. > > > >

2 2