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Here's why functions should return functions
by Costello, Roger L. 31 Jul '12

31 Jul '12

Hi Folks, Recently I had a small epiphany: when creating functions, design them to return functions rather than non-function values (Integer, Bool, list, tuple, etc.). Here's why: Consider a function that returns, say, the integer four ( 4 ). The type of the value returned by the function is this: 4 :: Num a => a That is, the value returned is not a function, it is a number. However, there are advantages to returning a function rather than a number. Recall the composition operator ( . ) Its operands are functions, e.g. (+1) . (*3) In programming, one school of thought is that programs should be written as a chain of function compositions: a . b . c . d . e . f Composition is intimately connected to a branch of mathematics called Category Theory: Category theory is based on composition as a fundamental operation in much the same way that classical set theory is based on the 'element of' or membership relation. ["Category Theory for Computing Science" by Michael Barr and Charles Wells] By designing programs in this fashion--as a chain of compositions--you have Category Theory's vast body of knowledge to help you and give rigor to your programs. Let's revisit the function mentioned above, the one that returns the integer four ( 4 ). If the function were to return the four cloaked in a function then that returned value could be used in a composition. That would very useful. Here's a data type that lifts non-function values to functions: data Lift a = Function a deriving (Show) The constructor ( Function ) is a function, as its type signature shows: Function :: a -> Lift a So rather than returning 4, return Function 4. Here's a function that converts any value to a function: lift :: a -> Lift a lift = Function Thus, lift 4 returns Function 4 Given the Lift data type and the lift function we can now start chaining functions together. Here the value four is lifted and then composed with a successor function: (successor . lift) 4 returns Function 5 where successor is defined as: successor :: Num a => Lift a -> Lift a successor (Function a) = lift (a + 1) Notice that successor returns a function, not a non-function value. Consequently, the result of successor can also be used in a composition. For example, here the value four is lifted, composed with successor, and then square is applied: (square . successor . lift) 4 returns Function 25 where square is defined as: square :: Num a => Lift a -> Lift a square (Function a) = lift (a * a) Once again notice that square also returns a function, not a non-function value. Consequently, the result of square can be used in a composition. At some point we are finished manipulating the value four and want to just see the result, not the result wrapped in a constructor. So we can create a function to return the non-function value: value :: Lift a -> a value (Function a) = a Here's an example: (value . square . successor . lift) 4 returns 25 Comments welcome. /Roger

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Fwd: Here's why functions should return functions
by Antoine Latter 29 Jul '12

29 Jul '12

Oops, I forgot to "reply-all" ---------- Forwarded message ---------- From: Antoine Latter <aslatter(a)gmail.com> Date: Sun, Jul 29, 2012 at 1:03 PM Subject: Re: [Haskell-beginners] Here's why functions should return functions To: Jay Sulzberger <jays(a)panix.com> On Sun, Jul 29, 2012 at 12:00 PM, Jay Sulzberger <jays(a)panix.com> wrote: > > > There is, in most sub-systems of mathematics, whether like recent > type theory or not, a general function, let us call it mcf which > in Scheme notation may be defined by executing > > (define mcf > (lambda (a) > (lambda (x) a))) > > Now in Haskell I know that one, perhaps the, parallel definition > must result in a polymorphic function. > > What is this definition? How polymorphic is it? What implicit > constraints are on a? Does "lazy vs eager" come in here? Are > there options to ghc which might modify how Haskell handles the > definition? > I think this is it: http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht… It's definitions is: const x _ = x Antoine > Of course, my questions are too many and I hope just for some > indications of the first things a beginner should study. > > oo--JS. > > > PS. Below is a short Scheme session showing some of the behavior of Scheme. > > SCM version 5d9, Copyright (C) 1990-2002 Free Software Foundation. > SCM comes with ABSOLUTELY NO WARRANTY; for details type `(terms)'. > This is free software, and you are welcome to redistribute it > under certain conditions; type `(terms)' for details. > ;loading /usr/local/lib/slib/require > ;done loading /usr/local/lib/slib/require.scm >> >> (define mcf > > (lambda (a) > (lambda (x) a))) > #<unspecified> >> >> (define const-17 (mcf 17)) > > #<unspecified> >> >> (const-17 "abc") > > 17 >> >> (define const-1+ (mcf 1+)) > > #<unspecified> >> >> (const-1+ "abc") > > #<CLOSURE <anon> "/usr/local/lib/scm/Init5d9.scm": (n) (+ n 1)> >> >> ((const-1+ "abc") 200) > > 201 >> >> ((const-1+ 56) 200) > > 201 >> >> (quit) > > ;EXIT > > Process scheme finished > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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cabal / haddock: view source with API
by Christopher Howard 28 Jul '12

28 Jul '12

Hi. I've been familiarizing myself with cabal (great system, btw) and learned that I could get haddock documentation built with each package with --enable-documentation (or adjusting the config file). I think that is great because I prefer having the API on my own system rather than having to surf to hackage every time. However, one difference I've noticed is that at hackage, I can click on the "source" buttons and see the source for each element, whereas on the local documentation this functionality is not available. Is it possible to configure my local cabal / haddock to do this as well? -- frigidcode.com indicium.us

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Re: [Haskell-beginners] List syntax
by Lyndon Maydwell 27 Jul '12

27 Jul '12

On Fri, Jul 27, 2012 at 10:52 PM, Stayvoid <stayvoid(a)gmail.com> wrote: > Thanks. > > P.S. I think that you should send your answer to the list to show the > others that this issue is solved. Ah, I thought I replied to all. Well, here is my reply for the record :-) Here's what I'm describing in a GHCi session: > λ [LT..GT] > > <interactive>:2:2: > Failed to load interface for `LT' > Use -v to see a list of the files searched for. > > <interactive>:2:2: > A section must be enclosed in parentheses thus: (LT.. GT) I'm assuming that this was the error that you encountered when you tried to use the syntax [LT..GT]. In my example I used a renamed module, but this also makes sense for a module imported the usual way. I renamed the module so you could see directly how an ambiguity could occur for 'LT'. If you import a module you can rename it by using the 'as' keyword, in this case, 'LT'. There is now an ambiguity between 'LT' = "less than" and 'LT' = "Data.List". > λ import qualified Data.List as LT > > λ :i LT.map > map :: (a -> b) -> [a] -> [b] -- Defined in `GHC.Base' Here is a demonstration of the renamed module in action. > λ :i LT.. > > Top level: > Failed to load interface for `LT' > Use -v to see a list of the files searched for. And here is what I think is causing your error to be thrown - The symbol '.' is being searched for in module LT in the same manner that 'map' was in the previous example, now given that there is no LT module, it is failing. This ambiguity might seem annoying, but it's not too bad since you can just put some spaces in to resolve it.

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List syntax
by Stayvoid 27 Jul '12

27 Jul '12

Hi, Why can't we do this: [LT..GT]? The following works: ['a'..'e'] ['a' .. 'e'] [LT .. GT] Cheers

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Can't square zero as a complex number?
by Scott N. Walck 26 Jul '12

26 Jul '12

Hi, Is there a reason why I can't use ** to square zero as a complex number? Prelude> :m Data.Complex Prelude Data.Complex> (0:+0) ** 2 NaN :+ NaN Prelude Data.Complex> (0 :: Complex Double) ** 2 NaN :+ NaN Using GHC 7.4.1. Thanks, Scott

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Very little (golfing) question...
by Corentin Dupont 26 Jul '12

26 Jul '12

Hi, I always write functions like this: addPlayer :: PlayerInfo -> State Game Bool addPlayer pi@(PlayerInfo {playerNumber = pn}) = do pls <- gets players case find (\(PlayerInfo {playerNumber = myPn}) -> pn == myPn) pls of Nothing -> do modify (\game -> game { players = pi : pls}) return True otherwise -> return False It simply adds a new PlayerInfo to a list contained in Game, with the condition that it doesn't exists already. Do you see a more elegant way to do this? Best, Corentin

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A Question about withFile
by Bin Shi 25 Jul '12

25 Jul '12

Hello, I wrote a simple test program as main = do withFile "a.txt" ReadMode (\h -> do c <- hGetContents h putStrLn c) then I got my expected results: I'm a.txt but if I changed to main = do c <- withFile "a.txt" ReadMode hGetContents putStrLn c I got just a empty line. Where am I wrong? Thanks.

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simple sound playback (Gnu/Linux)
by Christopher Howard 22 Jul '12

22 Jul '12

Hi. I've been playing around with gloss and its game modeling module. However, I am wondering what package to use to get simple sound playback. (For example, playing an explosion sound when a bomb lands.) There seem to be lots of modules in the sound category at hackage but I'm not sure what would be appropriate and simplest to use. (Only Gnu/Linux compatibility matters, if that helps.) -- frigidcode.com indicium.us

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Pb with toggleActionNew
by Pierre Michallet 22 Jul '12

22 Jul '12

I am trying to become acquainted with both haskell and Gtk thru the writing of small pieces of code exercising some Gtk functions. Having had problems with the ToggleEntry function (unable to get the checked/non checked status - refer to chapter 7.2 of the Gtk2HS tutorial) I have tried to understand the toggleAction thing. So I have written this : import Graphics.UI.Gtk.ActionMenuToolbar.ToggleAction ... togls::[ToggleAction] togls = let stone = toggleActionNew "STON" "Stones number" Nothing Nothing (myTog stone) deste = toggleActionNew "DEST" "Destination" Nothing Nothing (myTog deste) state = toggleActionNew "STAT" "Board status" Nothing Nothing (myTog state) in [stone,deste,state] myTog::ToggleAction->IO() myTog ta = putStrLn ("The name of the action is " ++ (toggleActionName ta)) The compiler is not happy with this and sends back the following: ------------------------------------------------------------------------------------------------------------------------------------------- The function 'toggleActionNew' is applied to five arguments but its type 'String ->String ->Maybe String -> Maybe stockId ->IO ToggleAction has only four. ------------------------------------------------------------------------------------------------------------------------------------------- To me, stone has 4 parameters ("STON", "Stones number", Nothing,Nothing) and a result (myTog stone) as in the definition given by the compiler. I am using ghc-7.0.3 gthk+-bundle_2.16.2-20090601_win32 gtk-0.12.3.1 documentation I am certainly missing something obvious to everybody but I could not solve it , so I would appreciate any help on this topic. Thanks in advance p.michallet(a)free.fr

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