- Haskell-Cafe - Haskell.org

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.

1 0

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

08 Jul '10

WHUSSUP?

1 0

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

26 40

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

2 2

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.

1 0

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

1 0

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

9 18

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

1 0

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

3 3

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.

12 24