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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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Re: 2 points on language
by D. Tweed 24 Aug '01

24 Aug '01

On Fri, 24 Aug 2001, S.D.Mechveliani wrote: > D. Tweed <tweed(a)compsci.bristol.ac.uk> writes > > On a purely pragmatic note ( :-) ) these optimisations can't necessarily > > be safely used in three of the four most common cases, namely Int, Float > > and Double because of the restricted range of intermediates. > Int, Float, Double > are not supposed to be provided with the CommutativeRing instance. > And most common cases are, to my mind, different: > Integer, Rational, (Integer, Rational), Polynomial Integer ... Ah, I must admit I'd rather missed the point that you were only intending them to apply to things that were of the algebraic class `Commutative Ring'. I was thinking that it'd be something where a particular funciton over a (possibly polymorphic) type could be declared associative, commutative, etc and then those more elementary classes came together in the particular example of the CommutativeRing. For example, the operator ++ on lists is associative, there are useful computational savings to be made utilising this (IIRC Lennart Augustsson had this specal cased in the hbc compiler), but it's difficult to see how you gain anything by trying to pin down any of the other properties of lists that would figure in the mathematically standard classes (there's no inverse as far as I can see so we don't even make it as far as a group). I think from the point of view of lobbying compiler implementors to put in the spade work to make this happen you're more likely to get support if the elegant algebraic classes come as a neat combination of more low-level, `unstructured' features. Don't get me wrong, I like the idea of extending the power of the Haskell compilers, I was just pointing out that the obvious thought that associative- and commutative-rewrite based optimisation `will also be very helpful for programs that are doing "everyday arithmetic" operations (say running a Kalman filter over a sequence of data represented using Doubles)' isn't necessarily true, and it certainly wasn't obvious to me: it was only happening upon the reference in the MetaFont book that made me realise it. ___cheers,_dave________________________________________________________ www.cs.bris.ac.uk/~tweed/pi.htm |tweed's law: however many computers email: tweed(a)cs.bris.ac.uk | you have, half your time is spent work tel: (0117) 954-5250 | waiting for compilations to finish.

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Braces vs. Whitespace
by Ashley Yakeley 22 Aug '01

22 Aug '01

At 2001-08-21 20:31, Dean Herington wrote (on the Haskell list): >Now realizing that the "where" keyword is optional when there are no >accompanying declarations, I think it would be preferable to omit the >"where" keyword to indicate that the absence of declarations is >intentional. Personally I prefer the explicit braces/semicolons syntax rather than worrying about whitespace: module DeepSeq where { class DeepSeq a where { deepSeq :: a -> b -> b; deepSeq = seq; -- default, for simple cases }; ... instance DeepSeq Ordering; instance DeepSeq Integer; instance DeepSeq Int; instance DeepSeq Float; instance DeepSeq Double; } ...or if you prefer... module DeepSeq where { class DeepSeq a where { deepSeq :: a -> b -> b; deepSeq = seq; -- default, for simple cases }; ... instance DeepSeq Ordering; instance DeepSeq Integer; instance DeepSeq Int; instance DeepSeq Float; instance DeepSeq Double; } etc. Basically you put a braced block after every 'do', 'in' and 'where', and semicolons after every declaration. When I was first learning Haskell, I got massively confused by the whitespace rules, especially with mixed tabs/spaces etc. I reckon the braces style is more readable to anyone used to C/C++/Java etc. -- Ashley Yakeley, Seattle WA

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