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Re: ~ symbol
by Marcin 'Qrczak' Kowalczyk 29 Aug '01

29 Aug '01

Wed, 29 Aug 2001 12:06:37 +0300, Cagdas Ozgenc <co19(a)cornell.edu> pisze: > What is the use of ~ symbol in Haskell? For example in prelude > function unzip > > unzip = foldr (\(a,b) ~(as,bs) -> (a:as, b:bs)) ([], []) When a lambda abstraction is applied to arguments, objects passed as arguments are matched against patterns in the lambda. The pattern (a,b) causes evaluation of the top-level constructor of the matched value (the constructor must be (,) because of the type) and binds a and b to arguments of the constructor. Matching against the pattern ~(as,bs) succeeds without evaluation of the value. Names as and bs are bound to expressions which cause evaluation of the top-level constructor of the matched value and select appropriate components. So with \(a,b) the constructor is evaluated when the lambda is applied, before evaluation of its body. With \ ~(as,bs) it's evaluated when as or bs are evaluated inside the body. The effect is that laziness is improved: parts of the result are available before the input is consumed. The first argument of foldr should better be lazy in its second argument, so foldr can return something without waiting for the result of the recursive call on the tail of the list being traversed. With this definition unzip ((foo, bar):whatever) can return (foo:whatever', bar:whatever'') where evaluation of whatever' and whatever'' causes further traversal of whatever. It doesn't wait until 'unzip whatever' returns the pair but it returns an object whose components cause evaluation of 'unzip whatever'. There is no ~ on (a,b). This implies that unzip (spam:eggs) does evaluate spam to obtain a pair before returning anything. It delays only the recursive call of 'unzip eggs'. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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RE: String matching
by Simon Marlow 29 Aug '01

29 Aug '01

> I'm adding a search function to my www forum implemented in Haskell. > > Is there a Prelude/Library function that will efficiently > find a pattern > in a String? Nothing in modules PreludeList or List seem to > fit the bill. If you're using GHC, then there's a pretty good interface to the GNU regex library in module RegexString (see http://www.haskell.org/ghc/docs/latest/set/regexstring.html) Cheers, SImon

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String matching
by Andrew Rock 29 Aug '01

29 Aug '01

Hi Haskellers: I'm adding a search function to my www forum implemented in Haskell. Is there a Prelude/Library function that will efficiently find a pattern in a String? Nothing in modules PreludeList or List seem to fit the bill. Thanks, Rock. -- Andrew Rock -- arock(a)cit.gu.edu.au -- http://www.cit.gu.edu.au/~arock School of Computing and Information Technology Griffith University, Nathan, Brisbane, Queensland 4111, Australia

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~ symbol
by Cagdas Ozgenc 29 Aug '01

29 Aug '01

Hello, What is the use of ~ symbol in Haskell? For example in prelude function unzip unzip = foldr (\(a,b) ~(as,bs) -> (a:as, b:bs)) ([], []) It works fine without the tilda. Is the use of tilda specific to lambda expression? Thanks

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RE: Numeric literals
by Chris Angus 28 Aug '01

28 Aug '01

this is a bit ugly but would allow you to do if (x `isA` string) ... or (x `isA` int) ... int = int::Int string = string::String typeBind :: a -> ReadS a -> ReadS a typeBind _ x = x isA :: Read a => String -> a -> Bool isA s x = not (null (typeBind x (readsPrec 0) s)) -----Original Message----- From: Simon Marlow [mailto:simonmar@microsoft.com] Sent: 28 August 2001 15:53 To: Mark Carroll; haskell-cafe(a)haskell.org Subject: RE: Numeric literals > I also haven't yet worked out how to tell if a string is "read"able or > not, yet - if (read "45")::Integer or whatever gives an error > (e.g. if I'd > put "af" instead of "45"), it seems to be pretty uncatchable, > which is a > pain. How do I tell if an instance of reading will work, or > catch that it > didn't? In GHC, you can do this: import Exception do result <- catch (evaluate (read "foo" :: Int)) (\error -> ... ) but unfortunately read doesn't raise a useful exception (just error "Prelude.read: no parse"), so you can't filter it very easily. However I don't think that read is likely to raise any other exceptions. Cheers, Simon _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: Easy syntax question
by Marcin 'Qrczak' Kowalczyk 27 Aug '01

27 Aug '01

Mon, 27 Aug 2001 15:02:24 -0400 (EDT), Mark Carroll <mark(a)chaos.x-philes.com> pisze: > Generally, this is a question about where multiple cases lead to the same > thing, maybe even in the middle of a function. (Like C's "case 1: case 2: > case 3: foo; break;".) Apart from what others said, if the common expression is big, the common solution is to bind it outside using 'let' or 'where'. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Numeric literals
by Mark Carroll 27 Aug '01

27 Aug '01

Today's other question has to do with numeric literals. How do I write literals for interesting IEEE values, e.g. Prelude> x Infinity Prelude> :type x Double Prelude> (read "Infinity") :: Double *** Exception: Ratio.%: zero denominator Prelude> ...? If I'm just missing something obvious in the standard documentation, please feel free to just give me a citation. I've already tried sections 2.5 and 6.4.1 of the Haskell Report, etc. though. Thanks. -- Mark

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Easy syntax question
by Mark Carroll 27 Aug '01

27 Aug '01

I can write a function x :: Integer -> Integer that returns 5 if I give it 2, 5 or 9, or 7 otherwise. Say, x 2 = 5 x 5 = 5 x 9 = 5 x _ = 7 Generally, this is a question about where multiple cases lead to the same thing, maybe even in the middle of a function. (Like C's "case 1: case 2: case 3: foo; break;".) Does it get any better than this, though? I can't convince 'case' to do something like, case n of 2,5,9 -> 5 otherwise -> 7 Am I missing some syntax somewhere? I'm lost in the grammar in the Haskell report. -- Mark

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Re: GHC Mandrake problem
by Pixel 24 Aug '01

24 Aug '01

Hampus Ram <d00ram(a)dtek.chalmers.se> writes: > > Loading package std ... ghc-5.00.2: can't find .o or .so/.DLL for: m (lib.so: cannot open shared object file: No such file or directory) > > > > Can anyone tell me what's missing, or how to fix this?? > > A _very_ ugly "fix" for this problem is to make a symbolic link to > libm.so and call it lib.so. Maybe a nicer fix:

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GHC Mandrake problem
by phaywood＠mail.iprimus.com.au 24 Aug '01

24 Aug '01

Hello, I tried to install version ghc 5.00.2 & prof from mandrake (vers 8.0 with current updates). It installed fine, but i get this message when i try to run ghci: Loading package std ... ghc-5.00.2: can't find .o or .so/.DLL for: m (lib.so: cannot open shared object file: No such file or directory) Can anyone tell me what's missing, or how to fix this?? TIA -------------------------------------------------------------------- Mail2Web - Check your email from the web at http://www.mail2web.com/ .

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