December 2011 - Beginners

Haskell wants the type, but I only know the class.
by Amy de Buitléir 26 Mar '12

26 Mar '12

I'm trying to read something from a file, do something with it, and then write it out again. I don't know, or care about, the type of the object I'm reading, but I do know its class. Here's my code: ---------- import Data.Binary ( Binary, encode, decode ) import Data.ByteString.Lazy as B ( readFile, writeFile ) import Codec.Compression.GZip ( compress, decompress ) class ( Binary a, Show a, Eq a ) => Thing a where doSomething :: a -> IO a readThing :: ( Thing a ) => FilePath -> IO a readThing f = return . decode . decompress =<< B.readFile f writeThing :: ( Thing a ) => FilePath -> a -> IO () writeThing f = B.writeFile f . compress . encode main = do a <- readThing "file1.txt" a' <- doSomething a writeThing "file2.txt" a' ---------- And here's the compiler error I get. Amy.hs:18:3: Ambiguous type variable `a0' in the constraint: (Thing a0) arising from a use of `writeThing' Probable fix: add a type signature that fixes these type variable(s) In the expression: writeThing "file2.txt" a' In the expression: do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt"; a' <- doSomething a; writeThing "file2.txt" a' } Is there a way to fix my code? I tried adding a type signature, e.g., a <- readThing "file1.txt" :: (Thing t) => IO t a' <- doSomething a :: (Thing t) => IO t but then it says... Couldn't match type `t0' with `t' because type variable `t' would escape its scope This (rigid, skolem) type variable is bound by an expression type signature: Thing t => IO t The following variables have types that mention t0 a :: t0 (bound at Amy.hs:16:3) In a stmt of a 'do' expression: a' <- doSomething a :: Thing t => IO t In the expression: do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' } In an equation for `main': main = do { a <- readThing "file1.txt" :: Thing t => IO t; a' <- doSomething a :: Thing t => IO t; writeThing "file2.txt" a' }

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Iterating a monadic action with memoization
by Tim Cowlishaw 29 Jan '12

29 Jan '12

Hi all, I was wondering if any of you knew whether the following was possible, (I discussed this a little on #haskell at the weekend but didn't quite get to the bottom of it): Say I have a monadic action: > f :: a -> m a and an initial value of type a. I'd like to iterate this action and collect the results of each iteration in a list, as with 'iterate' on normal functions: > iterate (>>= f) . return $ a However, I would also like to memoize the intermediate results of the iteration, such that the entire iteration runs in O(n) time rather than O(n^2). This also implies a slight semantic difference, as side-effecting or non-deterministic actions will not be repeated in each iteration, while maintaining the laziness of the iteration function. Is it possible to do this? Many thanks, Tim

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Audio packages.
by Sourabh 31 Dec '11

31 Dec '11

Hello, I'm a Haskell beginner and am working on my first *serious* haskell project. After going through a few books, and working on the exercises in the books, I finally started writing a small 2D game in Haskell. I figured this would give me lots of exposure to Haskell, and also on how graphics, keyboard, sound, etc are done. Also, I'm working on windows, so unfortunately building the correct libraries has been a bit of a pain. In fact, my question pertains to this. I am currently at the stage where I can get various events to happen correctly in the game, but there is no sound. I started looking at various audio related packages earlier today, but I can't seem to find any documentation on any packages. To add to my woes, I just can't, for the life of me, get any packages to install correctly on windows. So far I have tried OpenAL, SDL-mixer, PortAudio on windows, and none of them seem to work. Cabal install outright fails and asks me to go the MinGW/msys route. When I try to go through this route (which worked successfully for me for graphics), I still can't get any libraries to build. The various errors I have encountered range from configure/build failing, to dependencies not installing correctly. Looking at the various hits on google, it seems that I am not alone in this. I finally managed to get SDL to install (but not SDL-mixer). Even though SDL has an Audio component to it, the lack of documentation on the API has left me with no way to figure out how to actually use this...? Link that worked for me: http://www.animal-machine.com/blog/2010/04/a-haskell-adventure-in-windows/ Finally, I have started getting this error every time I do a cabal install now: Setup.hs:1:8: Could not find module `Distribution.Simple': Use -v to see a list of the files searched for. Have I finally done something terrific and blown everything apart? Sadly, instead of spending time writing Haskell code, I find myself spending a lot of time getting libraries to install and work on Windows. Is this common, or am I doing something wrong? At this point I would very much like advice on a couple of things: 1. Does anyone know any good Audio packages, that would work on windows, along with some examples and documentation of the API? 2. What should I do to get cabal to start installing stuff again? I apologize if I sound a bit frustrated, because I guess I am. :) Thanks for any advice, and have a happy new year!

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basic data types
by Stanisław Findeisen 30 Dec '11

30 Dec '11

Hi What are those basic data type instances? For example here: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t:B… we have: Instances Bounded Bool Enum Bool Eq Bool Data Bool Ord Bool Read Bool Show Bool Ix Bool Typeable Bool Generic Bool Storable Bool What is the difference between, e.g., Bounded Bool and Enum Bool? -- 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

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Representing a Set using a boolean function
by Robert Weisser 30 Dec '11

30 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 　

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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

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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

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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

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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

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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

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