- Beginners - Haskell.org

polymorphism vs. overloading
by Eugene Perederey 17 Dec '10

17 Dec '10

Haskell supports parametric polymorphism in the sense as, for example, operations on lists. Thus function List.length::[a]->Int is polymorphic. The difference between polymorphism and overloading is that polymorphism basically means that there is one algorithm for different types of operands whereas overloading means that for one symbol there are different implementations. Haskell compiler converts an overloaded function in such way that the function is passed an additional argument -- a dictionary that contains implementation of operations defined in the class. Consider function square: square :: Num n => n -> n square x = x*x class Num a where (+) :: a -> a -> a (-) :: a -> a -> a negate :: a->a ...etc... instance Num Int where (+) = intPlus a b (*) = intMult a b negate a = intNeg a ...etc... where functions intPlus, intMult, intNeg are defined somewhere else. So at compile time the type of square :: Num n => n -> n is replaced with square :: numDict n -> n -> n where numDict a is a parametric data type with defined accessor functions that return implementation of operations defined in the class. On 16 December 2010 19:34, Dennis Raddle <dennis.raddle(a)gmail.com> wrote: > I looked up the difference between polymorphism and overloading (speaking of > all languages in general), and apparently polymorphism is a general term for > an operation that can accept arguments of different types. Overloading is a > form of "ad-hoc polymorphism" in which the compiler chooses specific code to > be executed by inspecting the arguments at compile time. Apparently "true > polymorphism" means exactly the same code is executed no matter what the > argument types, such as implemented by inheritance in C++ (*) Now my > understanding of Haskell's classes is that the compiler chooses the right > instance of the class at compile time. Does that mean that Haskell > implements only ad-hoc polymorphism? > > > (*) Another question: if someone says "What feature of C++ makes > polymorphism possible?" What is the correct answer? Inheritance? Virtual > methods? Some combination? > > -D > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > -- Best, Eugene Perederey -- Best, Eugene Perederey

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mapping over newtypes
by Tim Baumgartner 15 Dec '10

15 Dec '10

Hello Haskellers, I defined newtype Banana = Banana Something When I work with it, I sometimes use a function mapBanana f (Banana b) = Banana (f b) which is of course discarded by the compiler. Is this usual practice or are there any better solutions? Regards Tim

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haven
by jean verdier 15 Dec '10

15 Dec '10

I was reading http://www.haskell.org/haskellwiki/Applications_and_libraries/Graphics and i'm interested in haven but the link is dead: http://haskell.org/haven/ Can someone give me some some pointer ?

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Type unions
by Russ Abbott 15 Dec '10

15 Dec '10

Is there a way to get this to work? data A = Aconstructor Int data B = Bconstructor Int data AorB = A | B f :: Int -> AorB f x | even x = Aconstructor x | otherwise = Bconstructor x I get this diagnostic. Couldn't match expected type `AorB' against inferred type `A' Since AorB is A or B, why is this not permitted? If instead I write data AorB = Aconstructor Int | Bconstructor Int everything works out ok. But what if I want separate types for A and B? Thanks, * -- Russ *

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Re: [Haskell-beginners] Equivalence of Inheritance
by Russ Abbott 14 Dec '10

14 Dec '10

That's good. (It's more or less the way I was doing it.) What I wanted to avoid was this. getGenderSpecificCondition ( Man _ _ cond) = cond getGenderSpecificCondition (Woman _ _ cond) = cond I know it seems like a small thing, but I would like to be able to write it like this. getGenderSpecificCondition p | p == (Man _ _ cond) = cond | p == (Woman _ _ cond) = cond But that's not legal syntax. A pattern can't appear in that context. But this does the job. getGenderSpecificCondition :: Person -> Condition getGenderSpecificCondition p | isMan p = prostateCondition p | isWoman p = ovaryCondition p isMan ( Man _ _ cond) = True isMan _ = False isWoman (Woman _ _ cond) = True isWoman _ = False That works! prostateCondition and ovaryCondition are both defined on Person. (I'm surprised to see that.) *Person> Group [Man "Harry" 32 OK, Woman "Sally" 29 Good] Harry(32, OK) Sally(29, Good) Also *Person> prostateCondition (Woman "Sally" 29 Good) *** Exception: No match in record selector prostateCondition *Person> prostateCondition (Man "Harry" 29 Good) Good *-- Russ * * * On Tue, Dec 14, 2010 at 12:31 PM, Michael Katelman <katelman(a)uiuc.edu>wrote: > Perhaps this? > > https://gist.github.com/741048 > > -Mike > > On Tue, Dec 14, 2010 at 2:27 PM, Russ Abbott <russ.abbott(a)gmail.com> > wrote: > > What I'm after is a version of my example that compiles. Can you make > one? > > > > -- Russ > > > > > > On Tue, Dec 14, 2010 at 12:18 PM, Antoine Latter <aslatter(a)gmail.com> > wrote: > >> > >> Sorry, I really don't know enough about what you're after to attempt > that. > >> > >> But you'll need to change you're signatures of the form: > >> > >> > function :: Person -> Foo > >> > >> to something of the form: > >> > >> > function :: Person p => p -> Foo > >> > >> Because again, a type class can not be used as a type. > >> > >> Antoine > >> > >> On Tue, Dec 14, 2010 at 2:12 PM, Russ Abbott <russ.abbott(a)gmail.com> > >> wrote: > >> > What got fouled up is all the adjustments I had to make to the other > >> > declarations. > >> > Can you complete the example so that it compiles using > >> > > >> > class Person p where ... > >> > > >> > I'd very much like to see an example that actually compiles. > >> > > >> > Thanks. > >> > -- Russ > >> > > >> > On Tue, Dec 14, 2010 at 11:58 AM, Antoine Latter <aslatter(a)gmail.com> > >> > wrote: > >> >> > >> >> On Tue, Dec 14, 2010 at 1:52 PM, Russ Abbott <russ.abbott(a)gmail.com> > >> >> wrote: > >> >> > If gender is a field in a Person type, then a Person must have both > >> >> > an > >> >> > ovaryCondition and a prostateCondition. That seems awkward. > >> >> > Regarding > >> >> > class Person p where > >> >> > I started down that path but got completely fouled up. > >> >> > >> >> How did this get fouled up? Every class declaration must take > >> >> arguments - here, 'p' is the argument for the class. > >> >> > >> >> Thanks, > >> >> Antoine > >> > > >> > > > > > > > _______________________________________________ > > Beginners mailing list > > Beginners(a)haskell.org > > http://www.haskell.org/mailman/listinfo/beginners > > > > >

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GUI library for beginners?
by Venanzio Capretta 14 Dec '10

14 Dec '10

Hi, I've been trying to find out what is the best GUI library to use for a Haskell beginner. At first I adopted gtk2hs and it was going well. But the web page for it has disappeared during the Haskell site migration. So now I don't have access to the documentation and the examples any more. Next I tried with wxHaskell, but unfortunately it doesn't install on Ubuntu (I checked on the mailing list: other people had the same problem and I don't understand the solution). Is there a library for GUI in Haskell that is simple to use and well supported, something that is user friendly for a beginner Haskell programmer? Best wishes, Venanzio

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Re: [Haskell-beginners] Equivalence of Inheritance
by Russ Abbott 14 Dec '10

14 Dec '10

I'm also confused about how to do the equivalence of inheritance in Haskell. Here is a complete example below. It doesn't compile. The error message is Class `Person' used as a type If I write "(Person p) =>" instead, I get other diagnostics. I would very much appreciate seeing how this should be done in Haskell. ---------- Example (multiple files) ------------ --- Group.hs --- module Group where import Person data Group = Group { members :: [Person] } instance Show Group where show group = unlines $ map show $ members group --- Person.hs --- module Person ( Condition(Bad, OK, Good) , Person ) where class Person where age :: Person -> Int name :: Person -> String getGenderSpecificCondition :: Person -> Condition instance Show Person where show p = name p ++ "(" ++ age p ++ ", " ++ getGenderSpecificCondition p ++ ")" data Condition = Bad | OK | Good --- Man.hs --- module Man ( age , name , Man (Man) ) where import Person data Man = Man { name :: String , age :: Int , prostateCondition :: Condition } instance Person Man where getGenderSpecificCondition :: Person -> Condition getGenderSpecificCondition m = prostateCondition m --- Woman.hs--- module Woman ( age , name , Woman (Woman) ) where import Person data Woman = Woman { name :: String , age :: Int , ovaryCondition :: Condition } instance Person Woman where getGenderSpecificCondition :: Person -> Condition getGenderSpecificCondition w = ovaryCondition w ---------- End example (multiple files) ------------ Thanks *-- Russ * On Tue, Dec 14, 2010 at 12:11 AM, <beginners-request(a)haskell.org> wrote: > Date: Mon, 13 Dec 2010 22:09:25 -0600 > From: Antoine Latter <aslatter(a)gmail.com> > Subject: Re: [Haskell-beginners] Equivalent of inheritance in Haskell > To: C K Kashyap <ckkashyap(a)gmail.com> > Cc: beginners(a)haskell.org > Message-ID: > <AANLkTinE30iTwWs8qBvWAcDoqGvy2T2_qpQqbXZKrCcm(a)mail.gmail.com> > Content-Type: text/plain; charset=UTF-8 > > On Mon, Dec 13, 2010 at 9:10 PM, C K Kashyap <ckkashyap(a)gmail.com> wrote: > >> > >> But there is not a way to easily say (in Haskell) "type A is > >> everything that type B is plus these other things here ...". Haskell > >> is not an OO language. > > > > This captures what I had in mind. Using compound types seems ok but > > I'd still need to do some mechanical stuff if I had to provide a > > function that works on the compound type which is actually defined for > > a component type. > > > > If I understand you right .. you'd build a 'Man' type and 'Woman' type > > by using a 'Person' type. Lets say, there is a function called getName > > that is Person -> String > > I'd have to mechanically define a function getName :: Man -> String - > > that extracts the person inside and calls getName on it - did I > > understand it right? > > Or would you typically write extract functions that'll return the > > components and then the user could call the method on the component? > > As in .... getPerson :: Man -> Person ... then call getName on that. > > > > How do you deal with situations like that? > > > > Well, in this case I might just have a person type with a 'gender' > field :-) Then I get the polymorphism and code-reuse for free! > > But what you're talking about is something that OO-style programming > is particularly aligned towards, and functional programming generally > is not. > > One thing people do is use type-classes - this would be a bit like > having 'Car' and 'Truck' implement the same interface. The simple > building blocks would be duplicated, but the complex application-level > functionality could be written against the typeclass. > > Another approach is with functional lenses - these are libraries that > aim to make updating complex compound types easier. Off the top of my > head I know of fclabels[1], but I know there are others. If you're > interested in this approach you might be able to email the -cafe > mailing list to ask for more. > > Is there a particular problem you're trying to solve? we might be able > to take the conversation in a less speculative direction. > > Antoine

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Is this scope range problem?
by Sok H. Chang 14 Dec '10

14 Dec '10

Hello, everyone. Thank you so much to previous great answer! Now, I don't know how can I fix this. The error is, Haskell>ghc --make Temp.hs -o Temp.exe [1 of 1] Compiling Main ( Temp.hs, Temp.o ) Temp.hs:19:27: Not in scope: `inpStr' The code is, import System.IO import System.Random sentenceAry = [] main :: IO () main = do inh <- openFile "c:\\Documents and Settings\\shaegis\\inputFile.txt" ReadMode outh <- openFile "c:\\Documents and Settings\\shaegis\\outputFile.txt" WriteMode mainloop inh outh hClose inh hClose outh mainloop :: Handle -> Handle -> IO () mainloop inh outh = do ineof <- hIsEOF inh if ineof then do hPutStrLn outh inpStr return () else do inpStr <- hGetLine inh let sentenceAry = inpStr : [] mainloop inh outh I'm try to change like below, but I failed. mainloop :: Handle -> Handle -> IO () mainloop inh outh = do ineof <- hIsEOF inh if not ineof then do inpStr <- hGetLine inh let sentenceAry = inpStr : [] mainloop inh outh else do hPutStrLn outh inpStr return () I wonder why... Thank you. Sincerely, Sok Chang

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Re: [Haskell-beginners] GUI library for beginners?
by . 14 Dec '10

14 Dec '10

Hi Venanzio, I found this to be working: <http://hackage.haskell.org/package/gtk> As far as GUI toolkits are concerned, I have used gtk2hs myself (also being still quite a haskell beginner). I have used gtkmm from c++ before, which sure helps in understanding what's going on (doing things in Haskell is much more fun and more pleasing though). Haskell rocks! SCNR Christian On Tue, 2010-12-14 at 12:00 +0100, beginners-request(a)haskell.org wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. GUI library for beginners? (Venanzio Capretta) > 2. Re: GUI library for beginners? (Tom Hobbs) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 14 Dec 2010 09:16:40 +0000 > From: Venanzio Capretta <vxc(a)Cs.Nott.AC.UK> > Subject: [Haskell-beginners] GUI library for beginners? > To: beginners(a)haskell.org > Message-ID: <4D0735F8.1010509(a)cs.nott.ac.uk> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > Hi, > I've been trying to find out what is the best GUI library to use for > a Haskell beginner. > At first I adopted gtk2hs and it was going well. But the web page for it > has disappeared during the Haskell site migration. So now I don't have > access to the documentation and the examples any more. > Next I tried with wxHaskell, but unfortunately it doesn't install on > Ubuntu (I checked on the mailing list: other people had the same problem > and I don't understand the solution). > Is there a library for GUI in Haskell that is simple to use and well > supported, something that is user friendly for a beginner Haskell > programmer? > Best wishes, > Venanzio > > > > ------------------------------ > > Message: 2 > Date: Tue, 14 Dec 2010 09:55:45 +0000 > From: Tom Hobbs <tvhobbs(a)googlemail.com> > Subject: Re: [Haskell-beginners] GUI library for beginners? > To: beginners(a)haskell.org > Message-ID: > <AANLkTimDxvCzJW77Wszwb9+S+kNm_iGDYora_4Diq0zG(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > Hi Venanzio, > > I found the following sites very helpful when trying to understand > OpenGL + Haskell. > > - http://blog.mikael.johanssons.org/archive/2006/09/opengl-programming-in-has… > - http://myawesomeblag.blogspot.com/2007/03/opengl-tetris-in-haskell.html > > My site contains the commands I used to get everything install on > Ubuntu, it also has a link to my code on it, but you'll find that the > code is very similar to Mikael's above. > > http://whatimean.wordpress.com/2010/10/19/simple-cell-automata-in-haskell/ > > Good luck. > > Tom > > On Tue, Dec 14, 2010 at 9:16 AM, Venanzio Capretta <vxc(a)cs.nott.ac.uk> wrote: > > Hi, > > ?I've been trying to find out what is the best GUI library to use for a > > Haskell beginner. > > At first I adopted gtk2hs and it was going well. But the web page for it has > > disappeared during the Haskell site migration. So now I don't have access to > > the documentation and the examples any more. > > Next I tried with wxHaskell, but unfortunately it doesn't install on Ubuntu > > (I checked on the mailing list: other people had the same problem and I > > don't understand the solution). > > Is there a library for GUI in Haskell that is simple to use and well > > supported, something that is user friendly for a beginner Haskell > > programmer? > > Best wishes, > > ?Venanzio > > > > _______________________________________________ > > Beginners mailing list > > Beginners(a)haskell.org > > http://www.haskell.org/mailman/listinfo/beginners > > > > > > ------------------------------ > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > > End of Beginners Digest, Vol 30, Issue 21 > *****************************************

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Cheap lookup tables
by Paul Sargent 14 Dec '10

14 Dec '10

I've just done some profiling on my application, and I've found that about 50% of my time and allocations is taken by two functions. All these are a functions which return constants from tables. At the moment they look something like this (I'm using the dimensional package to handle SI Units, hence the *~ one): buehlmannGradient :: (Fractional a) => Gas -> Int -> Dimensionless a buehlmannGradient Nitrogen 1 = 1.7928 *~ one buehlmannGradient Nitrogen 2 = 1.5352 *~ one [...] buehlmannGradient Nitrogen 15 = 1.0414 *~ one buehlmannGradient Nitrogen 16 = 1.0359 *~ one buehlmannGradient Helium 1 = 2.0964 *~ one buehlmannGradient Helium 2 = 1.7400 *~ one [...] buehlmannGradient Helium 15 = 1.0850 *~ one buehlmannGradient Helium 16 = 1.0791 *~ one This one causes 36% of my time. The other is similar: buehlmannZeroPoint :: (Fractional a) => Gas -> Int -> Int -> Pressure a buehlmannZeroPoint Nitrogen 1 mode = ([29.6, 29.6, 29.6] *~~ msw) !! mode - buehlmannGradient Nitrogen 1 * (10 *~ msw) [...] Again, 16 Cases times two gases. This one uses 14.5%. They tend to be called in pairs, and the first function is called about 100,000 times. The second about 50,000 times (which then causes 50,000 of the first function). What's my best strategy for optimising these functions? Is it the pattern match that's hurting me? Can I write them in a better way, or do I need to look at how I'm using them? Obviously calling them less often would help, but I'm not sure what I'd do to my code to achieve this. These constants are used in lots of fairly diverse areas of my code, and they're encapsulated into functions to stop magic numbers flying around. Thanks Paul

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