July 2010 - Haskell-Cafe

Something that kind of resembles OO
by Michael Mossey 09 Jul '10

09 Jul '10

I notice in algebraic data defined with named fields, you can use the same name inside different constructors and then apply it to any data of that type. data Vehicle = Car { ident :: String, wheel :: Circle } | Plane { ident :: String, stick :: Line } f :: [Vehicle] -> [String] f = map ident This is a little like fields in a base class. I also see that a wrong use of accessor functions will compile but give a runtime error: test = wheel (Plane "foo" (Line 1)) Will give a runtime error.

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does () match with a??
by Hector Guilarte 09 Jul '10

09 Jul '10

Hey everyone, I'm making a Game Monad for an assignment (yes, homework) . Here's a little explanation of what I need to do (I can't use anything from Control.Monad.*, I need to do everything myself): I need to define my newtype Game and make it's Monad instance I need to make the function : runGame :: Game a -- A particular game -> Int -- Initial amount of lives -> Maybe (a , Int ) -- Result and remaining lives. I need to make the instance for this class: class Monad m = > GameMonad m where extraLife :: m () getLives :: m Int checkPoint :: m a -> m a die :: m a now what I've done (I was inspired by the State Monad) I defined my Game type as follows: > newtype Game r = Game { execGame :: Int -> Maybe (r,Int) } My Monad instance like this: > instance Monad Game where > return a = Game $ \r -> Just (a,r) > m >>= k = Game $ \r -> let x = execGame m r > in case x of > Just (a, r') -> execGame (k a) r' > Nothing -> Nothing and my GameMonad instance: > instance GameMonad Game where > extraLife = Game $ \l -> Just ((),l+1) > getLives = Game $ \l -> Just (l,l) > die = do > n <- getLives > Game $ \_ -> Just ((),n-1) -- Here's the problem so, what's bothering me? Look at the type signature of die in the GameMonad class, it's supposed to return something of type (m a), but I don't know what to return in that case, and whatever I try to return it doesn't work, because when I try to compile it says that it couldn't match expected type 'a' against infered type 'whatever' ('whatever' being anything, from (), to string, or a number). shouldn't 'a' match with anything I put there? Note that I can't change the signatures because they were giving to me that way and I already checked with teacher if they were right. (I fixed it returning Maybe (Maybe a, Int) instead, but I can't change the signature of the function runGame) Thanks you, Hector Guilarte

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Re: [Haskell-cafe] Multidimensional Matrices in Haskell
by John Lato 09 Jul '10

09 Jul '10

Hello, There are a lot of options. The "array" package, which is included with GHC, provides both mutable and immutable arrays of arbitrary dimensions. A quick scan of hackage shows ArrayRef, ix-shapable, and judy as alternatives, among others. Immutable arrays can be pure, but all mutable array interfaces will require a monad of some type. I usually think ST-based mutability is easiest to work with. John > From: Mihai Maruseac <mihai.maruseac(a)gmail.com> > > Hi, > > A friend of mine wanted to do some Cellular Automata experiments in > Haskell and was asking me what packages/libraries are there for > multidimensional matrices. I'm interested in both immutable and > mutable ones but I don't want them to be trapped inside a monad of any > kind. > > Any hints? > > -- > MM >

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use of modules to save typing
by Michael Mossey 08 Jul '10

08 Jul '10

I'm fairly beginnerish to Haskell, and come from OO. I have a complaint about Haskell, but I think I found a good solution. Any suggestions welcome. I have RSI and like to minimize typing. The use of classes as name spaces helps to do that. Also I can use some Emacs abbreviation magic easily with OO and not so easily with Haskell. I'll explain in a second. In Haskell, when defining data for complex programs I like to use named fields to allow for changing data definitions without having to change all code. But named fields are top-level functions (I think). They must be chosen not to clash. My habit has been to prefix them with the name of the constructor. So in a program for playing back musical documents that needs to track some state, we have: data PlayState = PlayState { playState_cursor :: Int , playState_verts :: [Loc] , playState_len :: Int , playState_doc :: MusDoc } Note all these "playState_" prefixes. Lots of typing, which is not good. In OO, you could type x.cursor() In Haskell you have to type playState_cursor x which also, I feel, is harder to read. Now suppose I want to use PlayState with a State monad. -- Increment the cursor. incrCursor :: State PlayState () incrCursor = cur <- gets playState_cursor len <- gets playState_len let newCur = min (cur+1) (len-1) p <- get put $ p {playState_cursor = newCur} Okay, I'm sorry, that is just a lot of typing for what it is doing. Not good for people with RSI, and not all that readable. I could define a function to make modifying the state a little easier. playState_update_cursor :: Int -> PlayState -> PlayState playState_update_cursor i p = p {playState_cur=i} Then incrCursor would look like: incrCursor :: State PlayState () incrCursor = cur <- gets playState_cursor len <- gets playState_len let newCur = min (cur+1) (len-1) modify (playState_update_cursor newCur) Notice how often the characters "playState_" get typed. This would be a great situation for Emacs abbreviations. When you define an abbreviation in Emacs, such as defining "xps" to expand to "PlayState", emacs will watch for the characters xps. It will then replace "xps" with "PlayState" when you type a non-alphanumeric character following "xps". So if I type "xps." the moment I hit "." it changes to "PlayState." But I would have a hard time using this feature with "playState_" because it is always followed by an alphanumeric character. So my idea, now, is to put the definition of PlayState in its own module and import it qualified as PlayState. ---------------- module PlayState -------------- data PlayState = PlayState { cursor :: Int , verts :: [Loc] , len :: [Int] , doc :: MusDoc } update_cursor i p = p {cursor = i} ----------------------------------------------- I got rid of the "playState_" prefixes because I am not worried about using generic field names like "doc". They won't clash if I always import this qualified. And that reduces the necessary typing in the definition. Now my monad looks like testMonad = do cursor <- gets PlayState.cursor len <- gets PlayState.len let newCur = min (cur+1) (len-1) modify $ PlayState.update_cursor newCur Now I can define an abbreviation for PlayState. This is a big help. Also, I find this more readable. To me PlayState.cursor is more readable than playState_cursor For one thing, syntax highlighting helps in the former case. For another, the latter case requires that you recognize a naming convention, but the former case says clearly: "cursor is within the namespace PlayState, so this combination must be describing a cursor related to PlayState."

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Re: [Haskell-cafe] Merge hsql and HDBC -- there can only be one!
by Leonel Fonseca 08 Jul '10

08 Jul '10

On the subject of bridging relational persistence and fucntional programming I wish to communicate on what I'm working on (Tránsito Abierto Library) (or Open Transit): 1) Whole objective: To disminish to gap between functional and relational paradigms. 2) Specific objetive: Illustrate and explore the gap. 3) Narrow focus: I'm using Takusen and Oracle and Haskell (á la GHC). And Template Haskell (TH). 4) Hypothesis: There'll be always a gap because you're using distinct nature beasts, but you a bit more of convinience would yield great improvements. 4.1) Another thought on this is: A mapping always needs to be specified between data shapes (types/representations). 5) Findings/Achievements(?): 5.1) Generation of Haskell types and iterator code given the shape of relational table. This is not very useful itself. Because of (4.1), using SQL is an effective way to: (1) convey such mapping and (2) specify queries that involve more than one table. 5.2) Given a sql query text, use the facilities on Oracle database to describe the query. With such description, use Template Haskell to generate Haskell types and iterator code. Let us call Geco to this generated code that is built by means of an AST. 5.3) As 5.2 (spliced Geco) but saving the Geco AST. What for? It turns out that later, during program execution you can regenerate a fresh AST code and compare it to Geco AST. If there's a difference it means your database environment is changed and it is likely that your code is no longer safe. The idea here, your program reacts to the enviroment detecting if its unsafe to run. 5.4) Takusen covers many database datatypes but not all. 5.5) I was about to automate pl/sql bindings using a inspired Takusen wrapper found on the enumerator tests directory . It follows scheme 5.2. But it is uncomplete. 5.6) Takusen is limited to iterators up to 8 fields. By using TH, easily you can have any number of columns in your query. 5.7) I wrote a serialiser library on the DB side and a deserialiser library on Haskell side. The idea it's to push *any* pl/sql result onto a string and recover it on the Haskell side. Needs schemes 5.2 and 5.5 to work. Status: Not working. 6) I believe intelligent schema generation is possible following the scheme above. What you would get is a whole module that would enforce constraints (cardinality, restrictions, etc). 7) I haven't try the way Haskell -> DB. Neither the storing of functional values as it was done before in Persistent Haskell and others. Cheers, -- Leonel Fonseca.

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Re: Haskell-Cafe Digest, Vol 83, Issue 25
by Jeff Rubard 08 Jul '10

08 Jul '10

WHUSSUP?

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What is Haskell unsuitable for?
by aditya siram 08 Jul '10

08 Jul '10

Hi all, Haskell is a great language and in a lot of ways it still hasn't found a niche, but that's part of what is great about it. But I wanted to ask people are more experienced with Haskell - what kinds of problems is it unsuited for? Have you ever regretted using it for something? Meaning if you could write the program over you would do it in another language. thanks ... -deech

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C9 video on monads and coordinate systems
by Greg Meredith 08 Jul '10

08 Jul '10

Dear Haskellians, You may be interested in this video i did with Brian Beckman<http://channel9.msdn.com/shows/Going+Deep/E2E-Whiteboard-Jam-Session-with-B…>on monads, location and coordinate systems. 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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Re: [Haskell-cafe] Merge hsql and HDBC -- there can only be one!
by Oscar Finnsson 08 Jul '10

08 Jul '10

I'd love to see this too. I've only used HDBC and I think it works well. A common problem in the Haskell-community is that some produce low-level bindings/libs and some produce high-level (extremely type safe) bindings/libs but there's seldom good support for switching between or combining the two approaches. My experience is that when it comes to real world scenarios involving databases/xml/json/&c you are seldom able to figure out *all* types compile time and are thus forces to use the low-level lib for everything even though you got the types (e.g. of a table/view/function/...) at compile time for 90% of your code. That's a shame and something that I wish a merging between the packages would be able to solve. In other words: a good package should support more than one way to interface with the db and the different ways should be possible to seamlessly combine. My wish-list: * low level capabilities a la HDBC * high level capabilities (matching result sets to data types, db functions <-> haskell type classes or similar) * sql 2003/2008-, pl/sql-quasi quoter * code generation given a db schema. * schema generation from code (i.e. the opposite from the above). * fixture generation (Haskell -> db, db -> Haskell) * syb-support * BSD3 or DWTFYWT * github :) I'm prepared to spend a couple of hours a week helping out and I got some experience developing database layers that's used in telecom, gaming and logistics. -- Oscar P.S. I'm not saying that there should be only one; competition is always good. It would however be lovely if the community made an effort to (a) produce an package that can cover mosts needs that (b) most developers/package-maintainers used that (c) other developers can build upon with new features.

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Re: [Haskell-cafe] use of modules to save typing
by John Lato 08 Jul '10

08 Jul '10

> From: Michael Mossey <mpm(a)alumni.caltech.edu> > > Now my monad looks like > > testMonad = do > cursor <- gets PlayState.cursor > len <- gets PlayState.len > let newCur = min (cur+1) (len-1) > modify $ PlayState.update_cursor newCur > > Now I can define an abbreviation for PlayState. This is a big help. > Also, I find this more readable. To me > > PlayState.cursor > > is more readable than > playState_cursor > > For one thing, syntax highlighting helps in the former case. For > another, the latter case requires that you recognize a naming > convention, but the former case says clearly: "cursor is within the > namespace PlayState, so this combination must be describing a cursor > related to PlayState." I do this as well, although I would probably import like this: import qualified PlayState as PS which then allows for "PS.cursor" John

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