- Beginners - Haskell.org

Representing a Set using a boolean function
by Robert Weisser 31 Dec '11

31 Dec '11

I am working through Thompson's "Haskell The Craft of Functional programming", 2nd edition. I am not doing this as part of a class. It is just for my own enjoyment. I am having a problem with Exercise 16.38. Here is the exercise: The abstract data type Set a can be represented in a number of different ways. Alternatives include arbitrary lists (rather than ordered lists without repetitions) and Boolean valued functions, that is elements of the type a -> Bool. Give implementations of the type using these two representations. Note: the book previously showed a representation of Set a using ordered lists without repetitions. I had no problem representing Set a with arbitrary lists. My problem came when I tried to represent Set a with boolean functions. My first question was this: which of the following was Thompson asking for? newtype Set a = Set (a -> Bool) (version 1) newtype Set a = Set [a -> Bool] (version 2) I decided on version 1. Version 2 did not seem to have any advantage over version 1. Also, version 1 resembled something earlier in the book (an associative list implemented as a boolean function). I had no trouble with functions in the Module export list (see code below) which did not need to know the contents of the set. However, when I got to eqSet, I was stumped. I don't see how any function can see inside the set. I don't know enough Haskell to be certain, but it looks impossible to me. Is there something I am missing? > module Set ( > Set, > empty, -- Set a > sing, -- a -> Set a > memSet, -- Ord a => Set a -> a -> Bool > union, -- Ord a => Set a -> Set a -> Set a > inter, -- Ord a => Set a -> Set a -> Set a > diff, -- Ord a => Set a -> Set a -> Set a > symmDiff, -- Ord a => Set a -> Set a -> Set a > eqSet, -- Eq a => Set a -> Set a -> Bool > subSet, -- Ord a => Set a -> Set a -> Bool > makeSet, -- Ord a => [a] -> Set a > mapSet, -- Ord b => (a -> b) -> Set a -> Set b > filterSet, -- (a -> Bool) -> Set a -> Set a > foldSet, -- (a -> a -> a) -> a -> Set a -> a > showSet, -- (a -> String) -> Set a -> String > card) -- Set a -> Int > where > > import Data.List hiding (union) > > instance Eq a => Eq (Set a) where > (==) = eqSet > > instance Ord a => Ord (Set a) where > (<=) = leqSet > > newtype Set a = Set (a -> Bool) > > empty :: Set a > empty = Set $ \_ -> False > > sing :: Ord a => a -> Set a > sing y = Set $ \x -> x == y > > memSet :: Ord a => Set a -> a -> Bool > memSet (Set f) x = f x > > union :: Ord a => Set a -> Set a -> Set a > union s1 s2 = Set $ \x -> memSet s1 x || memSet s2 x > > inter :: Ord a => Set a -> Set a -> Set a > inter s1 s2 = Set $ \x -> memSet s1 x && memSet s2 x > > diff :: Ord a => Set a -> Set a -> Set a > diff s1 s2 = Set $ \x -> memSet s1 x && not (memSet s2 x) > > symmDiff :: Ord a => Set a -> Set a -> Set a > symmDiff s1 s2 = union (diff s1 s2) (diff s2 s1) > > -- Not exported: > contents :: Ord a => Set a -> [a] > contents (Set f) = undefined > > eqSet :: Eq a => Set a -> Set a -> Bool > eqSet (Set xs) (Set ys) = undefined > > -- Not exported: > leqSet :: Ord a => Set a -> Set a -> Bool > leqSet (Set xs) (Set ys) = undefined > > subSet :: Ord a => Set a -> Set a -> Bool > subSet (Set xs) (Set ys) = undefined > > makeSet :: Ord a => [a] -> Set a > makeSet xs = foldl addElem empty xs > > -- Not exported: > addElem :: Ord a => Set a -> a -> Set a > addElem (Set f) y = Set g > where > g x = if x == y then True > else f x > > mapSet :: Ord b => (a -> b) -> Set a -> Set b > mapSet f (Set xs) = undefined > > filterSet :: (a -> Bool) -> Set a -> Set a > filterSet p (Set xs) = undefined > > foldSet :: (a -> a -> a) -> a -> Set a -> a > foldSet f x (Set xs) = undefined > > showSet :: (a -> String) -> Set a -> String > showSet f (Set xs) = undefined > > card :: Set a -> Int > card (Set xs) = undefined For testing, I used the following. I checked individual values for membership in the union, intersection, etc. > list1 = [1..4] :: [Int] > list2 = [3..6] :: [Int] > > set1 = makeSet list1 > set2 = makeSet list2 > > setUnion = union set1 set2 > setInter = inter set1 set2 > setDiff = diff set1 set2 > setSymmDiff = symmDiff set1 set2 　　 Robert Weisser 　

3 2

Ord
by Stanisław Findeisen 30 Dec '11

30 Dec '11

Hi Could anyone please explain to me what is going on here? ------------------------------------------------------------------------ -- | The 'Ord' class is used for totally ordered datatypes. -- -- Instances of 'Ord' can be derived for any user-defined -- datatype whose constituent types are in 'Ord'. The declared order -- of the constructors in the data declaration determines the ordering -- in derived 'Ord' instances. The 'Ordering' datatype allows a single -- comparison to determine the precise ordering of two objects. -- -- Minimal complete definition: either 'compare' or '<='. -- Using 'compare' can be more efficient for complex types. -- class (Eq a) => Ord a where compare :: a -> a -> Ordering (<), (<=), (>), (>=) :: a -> a -> Bool max, min :: a -> a -> a compare x y = if x == y then EQ -- NB: must be '<=' not '<' to validate the -- above claim about the minimal things that -- can be defined for an instance of Ord: else if x <= y then LT else GT x < y = case compare x y of { LT -> True; _ -> False } x <= y = case compare x y of { GT -> False; _ -> True } x > y = case compare x y of { GT -> True; _ -> False } x >= y = case compare x y of { LT -> False; _ -> True } -- These two default methods use '<=' rather than 'compare' -- because the latter is often more expensive max x y = if x <= y then y else x min x y = if x <= y then x else y http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-Classes.… ------------------------------------------------------------------------ AFAIU this is the definition of the Ord type class in ghc. But what is this <= function that is used in the definition of compare? Here: else if x <= y then LT -- This e-mail address is invalid, see: http://people.eisenbits.com/~stf/public-email-note.html . OpenPGP: E3D9 C030 88F5 D254 434C 6683 17DD 22A0 8A3B 5CC0

5 5

Amanda
by Christiaan Kras 30 Dec '11

30 Dec '11

Hello list, First time I'm posting here. I've been interested in Haskell for about a year now, but sadly haven't done too much with it yet. I decided to get my bachelors degree in Computer Science/Engineering (it's a bit of a mixed course at my university) after having worked for over 4.5 years. One of the classes I've got to follow is discrete math. A first glimpse on the study material made me think "Cool! They're using Haskell!". This is however not the case. Instead, we're using Amanda. Amanda was written by Dick Bruin, who as far as I know used to teach at my university, but has now moved on to another university. I was told Amanda was being used at my university, NHL Leeuwarden (Netherlands) and the University of Twente (Netherlands). (strictly my university isn't a university, but high school means something different in English than it does in Dutch :-)) The reason I'm posting here is because Amanda seems extremely heavily influenced by Haskell. I think it was written in either Delphi or Pascal, but I've got to verify that with one of the teachers. It's quite old as well, as it was developed between 1990 and 2000. A lot of stuff written in Amanda can easily be converted to Haskell with a few small changes. Which makes me wonder why they aren't using Haskell now. List comprehensions use the same syntax, but use a semicolon instead of a comma for separating generators and terms. Operators such as +,/,* etc. are functions, just like they are in Haskell. For what I can tell Amanda is more or less a stripped down version of Haskell. The thing I was wondering though, is if anyone on this list has ever heard of Amanda before. If so, where did you got in contact with it? -- Christiaan Kras http://blog.htbaa.com

3 3

Program built on Ubuntu 11.10 does not run on previous versions
by AbdulSattar Mohammed 30 Dec '11

30 Dec '11

Ubuntu 11.10 comes with this library libgmp.so.10 and previous versions come with libgmp.so.3. When you make ghc --make app.hs, it dynamically links libgmp.so.10. This won't run on 11.04 or earlier with the error message libgmp.so.10 not found. So, I compiled using ghc -static -optl-static -optl-pthread --make app.hs. ( http://stackoverflow.com/questions/7344744/haskell-program-built-on-ubuntu-… got it running). I even ran ldd to check dependencies: ldd app not a dynamic executable Now, it fails with app: mkTextEncoding: invalid argument (Invalid argument) I'm trying to deploy to Heroku. Here's the repository: https://github.com/abdulsattar/hkonhk. Here's the heroku app: http://hkonhk.herokuapp.com/ And here's the output of heroku log (after the push): 2011-12-30T07:13:58+00:00 heroku[slugc]: Slug compilation started 2011-12-30T07:14:06+00:00 heroku[web.1]: State changed from crashed to created 2011-12-30T07:14:06+00:00 heroku[web.1]: State changed from created to starting 2011-12-30T07:14:06+00:00 heroku[slugc]: Slug compilation finished 2011-12-30T07:14:08+00:00 heroku[web.1]: Starting process with command `./app` 2011-12-30T07:14:08+00:00 app[web.1]: app: mkTextEncoding: invalid argument (Invalid argument) 2011-12-30T07:14:10+00:00 heroku[web.1]: State changed from starting to crashed 2011-12-30T07:14:10+00:00 heroku[web.1]: Process exited 2011-12-30T07:14:11+00:00 heroku[router]: Error H10 (App crashed) -> GET hkonhk.herokuapp.com/ dyno= queue= wait= service= status=503 bytes= I've asked about this many times on #haskell, but I couldn't get it working. Help would be highly appreciated. -- Warm Regards, AbdulSattar Mohammed

1 0

The numeric type stack
by Mike Meyer 29 Dec '11

29 Dec '11

While haskell's type system is usually a delight to work with. However, every once and a while I need to do mixed mode programming in spite of Kernighan and Plauger's advice, and wind up cursing the numeric type stack. I was wondering it there was a writeup on it somewhere? Preferably one aimed at practical programming. A chapter in Real World Haskell would have been ideal, but it doesn't seem to exist. Thanks, <mike

3 4

Better error messages
by Peter Hall 29 Dec '11

29 Dec '11

If I have something like this main = do args <- getArgs let file = args !! 0 -- etc... And I run it without any arguments, I get an error message like this: "Prelude.(!!): index too large". What's the best way to handle an error like this to give a better message back to the user? Peter

4 4

question about any
by Manfred Lotz 29 Dec '11

29 Dec '11

Hi there, might be trivial but anyway. I have a usage of 'any' and 'all' but I only have a predicate p :: a -> IO Bool. I wrote my own functons for this: mor :: Monad m => [m Bool] -> m Bool mor = liftM or . sequence mand :: Monad m => [m Bool] -> m Bool mand = liftM and . sequence or' :: Monad m => ( a -> m Bool) -> [a] -> [m Bool] or' _ [] = [] or' p (x:xs) = p x : or' p xs and' :: Monad m => ( a -> m Bool) -> [a] -> [m Bool] and' _ [] = [] and' p (x:xs) = p x : and' p xs myany :: Monad m => (a -> m Bool) -> [a] -> m Bool myany p = mor . or' p myall :: Monad m => (a -> m Bool) -> [a] -> m Bool myall p = mand . and' p which seems to do what I want. Question: Is there any libray function I could use to do this? -- Thanks, Manfred

5 7

list range
by Stanisław Findeisen 28 Dec '11

28 Dec '11

Hi Is list range (for example: [1..10]) a language construct or a function? What type does it have? -- This e-mail address is invalid, see: http://people.eisenbits.com/~stf/public-email-note.html . OpenPGP: E3D9 C030 88F5 D254 434C 6683 17DD 22A0 8A3B 5CC0

5 6

Getting the current system date outside the IO Monad
by AbdulSattar Mohammed 27 Dec '11

27 Dec '11

I'm still learning Haskell and I've started learning Yesod. I pretty much built a blog before I came this silly roadblock. Whenever I create a new post, I want it take the current system time as its creation time. I understand it's an impure operation and it must go into the IO Monad to be performed. But, in Yesod, your handler is of the type Handler RepHtml. I don't get how you could get the impure value into a pure function without main playing any part. Here's the link for my blog: http://hpaste.org/55757#line85 -- Warm Regards, AbdulSattar Mohammed

2 1

gtk2hs and threadDelay
by gabre＠caesar.elte.hu 26 Dec '11

26 Dec '11

Hello! I managed to do it with timeoutAdd: --code below is the onClicked function: drawProgram gui preprocPrg = do let drArea = (drawWin gui) drw <- widgetGetDrawWindow drArea -- let list = compileToList preprocPrg -- renderWithDrawable drw (drawFromList list) renderWithDrawable drw (myDraw 200 200) -- drawWindowProcessUpdates drw True timeoutAdd ((\_ -> do renderWithDrawable drw (myDraw2 200 200) return False) 5) 500 return () But "sometimes" it gives the following error: (<interactive>:23929): Gdk-WARNING **: Using Cairo rendering requires the drawable argument to have a specified colormap. All windows have a colormap, however, pixmaps only have colormap by default if they were created with a non-NULL window argument. Otherwise a colormap must be set on them with gdk_drawable_set_colormap (<interactive>:23929): Gdk-CRITICAL **: IA__gdk_cairo_region: assertion `region != NULL' failed <interactive>: user error (NULL pointer) <interactive>: interrupted <interactive>: warning: too many hs_exit()s What can cause this? I think the other parts of my program are correct, and I cant understand, how can ghci "randomly give" this error. (every 5. try) To be honest, my goal is to make a "turtle graphics" program, so I ll have to write a function, which draws a line on the GUI in slow motion: there will be even more problems with timing, because I ll have to draw the line pixel by pixel.

1 0