July 2010 - Haskell-Cafe

Suggestions for an MSc Project?
by John Smith 08 Jul '10

08 Jul '10

My MSc requires a project dissertation, which is expected to take about 800 hours. I would like to work on something which is of use to the Haskell community. Any suggestions? My interests are in higher-level application programming, rather than low-level libraries or compiler hacking. (This is also the focus of my course.) I don't know whether Haskell is suitable for this (the only significant Haskell programs seem to be its own tool-set), but I had fun learning Haskell and I'd be interested in using it for my project if relevant. The details for the project's requirements are here http://www.ljmu.ac.uk/cmp/onlinemsc/students/97939.htm

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Re: Adding new entry to list
by Johnny Morrice 08 Jul '10

08 Jul '10

Hi there Jack, I have modified your program. However, where you were attempting to use direct recursion to compute totalSales, I have used functions from the prelude. I have also provided a recursive definition totalSalesRec. Here is a lesson on recursion from the Haskell wikibook http://en.wikibooks.org/wiki/Haskell/Recursion Also, what you have is an associative array which pairs (Artist, Title) with Sold. A linked list [(Artist, Title, Sold)] is an inappropriate data type for this purpose. Consider using Map from Data.Map in the form Map (Artist, Title) Sold. I have included comments which I hope you will find useful in understanding the code. Also note my use of comments like: -- | This is a function that does blah These are annotations for the Haskell documention generator, Haddock http://www.haskell.org/haddock/ What I'd do to run haddock on this file (I use linux and I have saved as artist.hs) is mkdir doc && haddock artist.hs -o doc -h Now you can look at documentation for your program in a web browser while fickering with it in GHCi. Be aware that my copy pasting this file onto here may well have mangled its syntax! Have fun, Johnny module Main (main ,testDatabase ,totalSales ,totalSalesRec ,printNames ,mainLoop) where -- | Title of a record type Title = String -- | An artist, who makes records type Artist = String -- | Number of copies of a record sold type Sold = Int type Sales = [(Title, Artist, Sold)] -- | A small sales database testDatabase :: Sales testDatabase = [("Kids", "MGMT", 3), ("This Charming Man", "The Smiths", 5), ("Gimme Shelter", "The Rolling Stones", 7)] -- Notice there are no explicit function arguments here. Look up 'partial application'. -- | Total sales of all records in the database. totalSales :: Sales -> Sold totalSales = -- The sum of all the sales sum . map sale where sale (_,_,s) = s -- | Recursive version of totalSales totalSalesRec :: Sales -> Sold totalSalesRec ((_,_,s):ss) = s + totalSalesRec ss totalSalesRec _ = 0 -- | Print out the contents of a sales database printNames :: Sales -> IO () printNames testDatabase = mapM_ print testDatabase -- Main application loop mainLoop :: Sales -> IO () mainLoop testDatabase = do putStrLn "1 - Show all tracks in database" putStrLn "2 - Show the total sales" putStrLn "3 - Add a new entry" putStrLn "4 - Exit" putStrLn "" putStrLn "Please select an option:" input <- getLine case read input of 1 -> do banner "Show All Tracks" printNames testDatabase putStrLn "" mainLoop testDatabase 2 -> do banner "Total Sales" print $ totalSales testDatabase putStrLn "" mainLoop testDatabase 3 -> do banner "New entry" putStrLn "Enter artist name" a <- getLine putStrLn "" putStrLn "Enter title" t <- getLine putStrLn "" putStrLn "Enter number of sales" -- Look up functors to understand fmap s <- fmap read getLine putStrLn "" -- 'cons' the new element to the start of the linked list testDatabase -- Look up data constructors. mainLoop $ (a,t,s) : testDatabase -- No 'do' is required for only one statement. Look up monads and how 'do' is syntactic sugar. 4 -> return () where -- Print banner banner s = do putStrLn "------------------" putStrLn s putStrLn "------------------" -- | Run main :: IO () main = mainLoop testDatabase

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Build problems on Hackage server
by Emil Axelsson 08 Jul '10

08 Jul '10

Hello Last week I uploaded new versions of feldspar-language and feldspar-compiler. Both packages build just fine on our local machines with GHC 6.10 and 6.12. But Hackage reports the following build failure: > cabal: dependencies conflict: ghc-6.12.2 requires array ==0.3.0.1 however > array-0.3.0.1 was excluded because ghc-6.12.2 requires array ==0.3.0.0 Any idea of what might be causing this? Thanks! / Emil

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checking types with type families
by Evan Laforge 08 Jul '10

08 Jul '10

I have a parameterized data type: > data Val result = VNum Double | VThunk (SomeMonad result) > type Environ result = Map Symbol (Val result) I have a class to make it easier to typecheck Vals: > class Typecheck a where > from_val :: Val result -> Maybe a > > instance Typecheck Double where > from_val (VNum d) = Just d > from_val _ = Nothing Now I can write > lookup_environ :: (Typecheck a) => Symbol -> Environ result -> Maybe a Now of course there's a question of how to write Typecheck for VThunk. I would like to be able to call 'lookup_environ' expecting a 'SomeMonad result' and get Nothing or Just depending on if it's present. > instance Typecheck (SomeMonad result) where > from_val (VThunk f) = Just f But I need something to say that the 'result' from the instance head is the same as the 'result' from the class declaration, because otherwise I get Couldn't match expected type `result' against inferred type `result1' So maybe a type family? > class Typecheck ... > type ResultOf a :: * > from_val :: Val (ResultOf a) -> Maybe a > > instance Typecheck (SomeMonad result) where > type ResultOf (SomeMonad result) = result > ... Now the question is, what's the ResultOf a Double? Well, since the output doesn't depend on 'result' at all, I would like to be able to write 'type ResultOf Double = anything', and then I can look up a VNum in an 'Environ anything'. Unfortunately, that type declaration isn't legal because it wants 'anything' to be bound somewhere. I tried explicitly introducing 'anything' with a forall, but no dice. So there are a number of possible solutions to this problem. One is, is there another way to get from the expression language type VNum, VThunk, etc. to a haskell type that doesn't use typeclasses in this way? Or, is there a way to assert in the instance of 'Typecheck (SomeMonad result)' that its 'result' is equal to the 'result' in the class declaration without using type families in this way? Or, is there a way to use type families in the way I'm using them to leave 'result' polymorphic in the cases where it can be left free in the result (e.g. VNum 42 :: Val anything)? I know lots of people have used haskell to write little interpreters, so maybe someone has solved this problem before. Basically, I have a Val that is "semi-polymorphic", but even if I go the totally monomorphic / dynamic type route by making a Result type with all the different types, I still have trouble converting from haskell types and back when I need to write the "Result -> result" function. I suppose I could play the same Typecheck game there, but I like the idea of an "Environ X" that doesn't admit the wrong kind of VThunk. It seems like I should be able to express that in the type system and be safer as well as save some packing and unpacking.

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[C Binding] Turning the mutable to immutable?
by Yves Parès 08 Jul '10

08 Jul '10

Hello, I don't know if some of you are familiar with the SFML library (stands for Simple and Fast Multimedia Library) --> http://sfml-dev.org As SDL, SFML is a 2D graphics library, but conversely to SDL it provides a hardware-accelerated drawing, through OpenGL. Well, I'm currently writing its Haskell binding, and I'm stuck with design issues. What I'm heading to is a full IO binding, and that's what I'd like to avoid. I particularly like the way HGL works, with the Draw monad which (IMO) enables powerful composability of the drawings. But HGL is simpler in the way that, for instance, it doesn't deal with images/sprites. So that's why I would like to pick up advice concerning the design of such a binding. First, this is how SFML (in C/C++) works: The main types are RenderWindow, Drawable, Image, Sprite, Shape, Font and Text. An Image basically represents an array of pixels. It is just a resource, and is not directly drawable. An image is loaded from a file, and provides methods for accessing/altering its pixels. Its main purpose is to be bound as an OpenGL texture. A RenderWindow is the window on which the Drawables are... well... drawn. A Sprite, now, is a Drawable. One Sprite is linked to one Image. A Sprite NEVER alters its Image, it just provides positionning/rotation/resizing/colorizing methods that will be translated, when drawing on the RenderWindow, to glTranslate/glRotate/glScale/glColor, etc. So we can see a Sprite as an instance of an Image, since if a same Image is to be drawn 3 times on the same frame, we are advised to create three Sprites of it, which will differ by their position. A Font is loaded from a file, it is a simple resource, just like Image. Text and Shape are Drawables. So, now, questions: 1) For those who know HGL, can the monad Draw principle be adapted to my case? Or am I heading for disaster? 2) How would I handle images? SFML API enables the user to alter the pixels of the image, and I obviously don't wanna copy the entire Image each time a pixel is changed. 3) Is there another library on hackage that handles images in a functional way? (I mean not *all in IO*)

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monadic design patterns for the web -- a 2 day, hands-on workshop
by Greg Meredith 07 Jul '10

07 Jul '10

Dear Haskellians, Biosimilarity and Stellar Scala Consulting will be running a 2 day workshop in Seattle in September on monadic design patterns for the web. You can get the details here <http://monadic.eventbrite.com/>. Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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Adding new entry to list
by Mrwibbly 07 Jul '10

07 Jul '10

Hi all I'm having a problem trying to add a new record to a list. I need to be able to add a new record to the database. My code is below, I also need to work out the total sales for everything in the Sales list. As you can see, I've tried to do it but I can't seem to work it out. type Title = String type Artist = String type Sold = Int --type Record = (Title, (Artist, Sold)) type Sales = [(Title, Artist, Sold)] testDatabase :: Sales testDatabase = [("Kids", "MGMT", 3), ("This Charming Man", "The Smiths", 5), ("Gimme Shelter", "The Rolling Stones", 7)] totalSales :: Sales -> Title -> Artist -> Sold totalSales [] title artist = 0 --totalSales ((t,a,n):testDatabase) title artist = title artist --addSale :: Sales -> Sales --addSale (Title, Artist, sold) = (title, artist, sold + 1) --newRecord :: String -> String -> Sales -> Sales --newRecord testDatabase title artist = (title, artist) ++ testDatabase --newRecordAction :: Sales -> IO() --newRecordAction testDatabase = do --putStrLn "Enter a new title: " --title <- getLine --putStrLn "Enter an artist: " --artist <- getLine --return $ newRecord title artist testDatabase getTitle :: Sales -> Title getTitle (title,_) = title printNames :: Sales -> IO() printNames testDatabase = mapM_ print testDatabase mainLoop :: Sales -> IO() mainLoop testDatabase = do putStrLn "1 - Show all tracks in database" putStrLn "2 - Exit" putStrLn "" putStrLn "Please select an option:" input <- getLine case read input of 1 -> do putStrLn "------------------" putStrLn "Show All Tracks" putStrLn "------------------" printNames testDatabase putStrLn "" mainLoop testDatabase 2 -> do return () main :: IO() main = mainLoop testDatabase Thank you, Jack -- View this message in context: http://old.nabble.com/Adding-new-entry-to-list-tp29099032p29099032.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] Re: [Haskell] analogous functions in Sound.File.Sndfile
by John Lato 07 Jul '10

07 Jul '10

> From: stefan kersten <sk(a)k-hornz.de> > > following up on haskell-cafe ... > > > i just noticed though that the lazy versions don't work correctly because they > don't take closing the file handle into account properly. i think they should be > removed from the API, for incremental processing an iteratee interface would > probably be more appropriate. Incidentally I've been meaning to write an iteratee interface to hsndfile for some time. It would be an excellent fit, and it should be quite simple to implement. This would be a good small project for someone interested in audio and iteratees. John

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reading and playing music openAL
by Maria Gabriela Valdes 07 Jul '10

07 Jul '10

Hi ! We have a question about about openAL. We would like to know if anybody knows how to read a WAV file by chunks of a determined size, and after doing some processing with a specific chunk send that same chunk back to the sound card so we can play the whole WAV continiously (just like a music player). Thanks in advance, -- Maria Gabriela Valdes G. Linux Registered User #485743

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Re: [Haskell-cafe] finding the right mathematical model
by John Lato 07 Jul '10

07 Jul '10

If you ignore the identity mappings (which all technically create trivial loops), these mappings would form a directed acyclic graph (DAG). I would look at some of the graph libraries, e.g. fgl, to see if they have anything appropriate. John > From: G?nther Schmidt <gue.schmidt(a)web.de> > Hi list, > > the problem I have stems from the app I had developed. What my app does > is to split the money a hospital receives for a case to the departments > involved in a fair way. > > An additional requirement however was to allow the users of the app to > re-map any revenue shares credited to certain departments to other > departments. Such cases are sometimes due to politics within the > hospital and also have more legitimate reasons, like saying the > radiology should not receive shares for surgical procedures but those > shares should be redirected to the "General surgery" department. > > The feature is already implemented, but I'm not pleased with it, > especially since I did not develop a mathematical model for it. > > Details: > > It boils down to model mappings, or rather what sort of data structure > would be suited for this kind of thing. > > Dept A is "mapped" to itself > A -> A > > Dept B is mapped to Dept C > B -> C > > Dept C is mapped to Dept C > C -> C > > Dept D is mapped to Dept A > D -> A > > It should not be possible to construct looping mappings, ie. > > 1. A -> B > 2. B -> C > 3. C -> A > > ...... > > > What sort of model would be suitable to describe this, some sort of matrix? > > Günther

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