- Beginners - Haskell.org

Polymorphism question from an OO-speaking newbie
by Joel Neely 04 May '09

04 May '09

Short version: Is it possible/reasonable to define a single function that accepts a single string or a list of strings (in which case it maps the single-string "flavor" over the list)? Longer version: Thus far I know that Haskell allows me to define a function on a single string, then define another function that maps the first one across a list of strings, as in: *Main> let quote1 s = "\"" ++ s ++ "\"" *Main> "Quux said " ++ quote1 "foo" ++ " loudly" "Quux said \"foo\" loudly" *Main> let quote = map quote1 *Main> quote ["foo", "baz", "bletch"] ["\"foo\"","\"baz\"","\"bletch\""] (BTW, is it standard terminology to say that quote lifts quote1 to lists?) In the above I have different names for the two functions. OO languages such as Java allow me to overload quote to accept either String or List<String> (and return the same type). AFAICT, Haskell's parametric polymorphism allows me to define functions (e.g. length) which deal with "listness" concepts indifferent to the type contained by the list. Am I missing something, or should I admit to OO wrongheadedness and accept that my inability to write a single declaration unifying: quote :: String -> String quote :: [String] -> [String] is an emphatic clue that I should change my expectations? Thanks in advance for any enlightenment shed my way! -jn- -- Beauty of style and harmony and grace and good rhythm depend on simplicity. - Plato

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Re: [Haskell-beginners] Parsec tutorial
by Keith Sheppard 03 May '09

03 May '09

Hi Patrick, This isn't what you asked for but it may be useful to you. I've created a parsec based SQL parser for my "TxtSushi" command-line program. It's still pretty early in development but it does parse a subset of the SELECT grammar. The project's page is http://keithsheppard.name/txt-sushi/index.html Keith

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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 02 May '09

02 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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Simplifying a 'split' function
by Ben Sanders 30 Apr '09

30 Apr '09

In looking through a merge sort implementation example, I came across this code: split :: [a] -> [[a]] split [] = [] split (x:xs) = [x] : split xs I would have written the same code as split :: [a] -> [[a]] split = map (: []) Is there any particular difference here (other than explicit recursion)? And is there any other nicer way to write it? Thanks, Ben Sanders

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List Function
by Nathan Holden 30 Apr '09

30 Apr '09

I don't know what you'd call it. Is there a function in any of the basic functions that does this something like this: Sending two lists, [1,2,3] and [2,3,4] it would return [(1,4),(1,5),(1,6),(2,4),(2,5),(2,6),(3,4),(3,5),(3,6)]. I managed to code my way into returning a list of lists, which works. But it seemed like a very basic list/matrix function, so I honestly believe that the Haskell designers probably would've put it in.

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Parsec tutorial
by Patrick LeBoutillier 30 Apr '09

30 Apr '09

Hi all, I'm looking for a Parsec tutorial that can get me started in writing a simple SQL parser. I can't seem to find a lot using Google. Does anyone have some resources that could be helpful? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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P.S. "Haskell: Craft of Functional Programming"
by Michael P Mossey 30 Apr '09

30 Apr '09

Oh, and I got HCFP at the local Border's. Apparently it's selling well enough to include it in a general bookstore. Look out Stephen King---Simon Thompson is coming!

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