May 2009 - Beginners - Haskell.org

Selecting a GUI toolkit
by Daniel Carrera 02 May '09

02 May '09

Hello, This question is just idle curiosity. I don't actually plan to write a desktop GUI program. With that said, what library would you recommend to someone who wants to write a cross-platform desktop app with Haskell? Consider the following requirements: 1) Cross-platform. Windows, Linux and preferably OS X too. 2) Maintained, stable, mature, documented, etc. 3) Easy to use. Notice that this has much to do with the Haskell bindings. For example, Qt might be a fabulous toolkit, but qtHaskell might be immature (it looks like it started in Dec 2007). wxHaskell looks good. I like the idea behind wxWidgets, but I have never used it (or any GUI toolkit for that matter). Thank you for your input. Cheers, Daniel.

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Re: [Haskell-beginners] gcd
by Steve 01 May '09

01 May '09

On Fri, 2009-05-01 at 16:19 +0200, Peter Verswyvelen wrote: > I suggest you forward this to the Haskell Cafe, since it's not really > a beginners question, it's a really good question :) Thanks for the advice, I've reposted the question to the Haskell Cafe. Steve > On Fri, May 1, 2009 at 4:19 PM, Steve <stevech1097(a)yahoo.com.au> > wrote: > I had a look at the gcd definition in GHC 6.10.1 > ghc-6.10.1/libraries/base/GHC/Real.lhs > > -- | @'gcd' x y@ is the greatest (positive) integer that > divides both > @x@ > -- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) > (-6)@ = @3@, > -- @'gcd' 0 4@ = @4@. @'gcd' 0 0@ raises a runtime error. > gcd :: (Integral a) => a -> a -> a > gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined" > gcd x y = gcd' (abs x) (abs y) > where gcd' a 0 = a > gcd' a b = gcd' b (a `rem` b) > > Why is gcd 0 0 undefined? > > http://en.wikipedia.org/wiki/Greatest_common_divisor says: > "It is useful to define gcd(0, 0) = 0 and lcm(0, 0) = 0 > because then the > natural numbers become a complete distributive lattice with > gcd as meet > and lcm as join operation. This extension of the definition is > also > compatible with the generalization for commutative rings given > below." > > An added advantage, for haskell, of defining gcd 0 0 = 0 is > that gcd > would change from being a partial function to a total > function. > > Regards, > Steve > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > >

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gcd
by Steve 01 May '09

01 May '09

I had a look at the gcd definition in GHC 6.10.1 ghc-6.10.1/libraries/base/GHC/Real.lhs -- | @'gcd' x y@ is the greatest (positive) integer that divides both @x@ -- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) (-6)@ = @3@, -- @'gcd' 0 4@ = @4@. @'gcd' 0 0@ raises a runtime error. gcd :: (Integral a) => a -> a -> a gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined" gcd x y = gcd' (abs x) (abs y) where gcd' a 0 = a gcd' a b = gcd' b (a `rem` b) Why is gcd 0 0 undefined? http://en.wikipedia.org/wiki/Greatest_common_divisor says: "It is useful to define gcd(0, 0) = 0 and lcm(0, 0) = 0 because then the natural numbers become a complete distributive lattice with gcd as meet and lcm as join operation. This extension of the definition is also compatible with the generalization for commutative rings given below." An added advantage, for haskell, of defining gcd 0 0 = 0 is that gcd would change from being a partial function to a total function. Regards, Steve

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gcd
by Steve 01 May '09

01 May '09

I had a look at the gcd definition in GHC 6.10.1 ghc-6.10.1/libraries/base/GHC/Real.lhs -- | @'gcd' x y@ is the greatest (positive) integer that divides both @x@ -- and @y@; for example @'gcd' (-3) 6@ = @3@, @'gcd' (-3) (-6)@ = @3@, -- @'gcd' 0 4@ = @4@. @'gcd' 0 0@ raises a runtime error. gcd :: (Integral a) => a -> a -> a gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined" gcd x y = gcd' (abs x) (abs y) where gcd' a 0 = a gcd' a b = gcd' b (a `rem` b) Why is gcd 0 0 undefined? http://en.wikipedia.org/wiki/Greatest_common_divisor says: "It is useful to define gcd(0, 0) = 0 and lcm(0, 0) = 0 because then the natural numbers become a complete distributive lattice with gcd as meet and lcm as join operation. This extension of the definition is also compatible with the generalization for commutative rings given below." An added advantage, for haskell, of defining gcd 0 0 = 0 is that gcd would change from being a partial function to a total function. Regards, Steve

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