source = "#123456789012345678901234567890123456789012345678901234567890\n\ \SVCLFOWLER 10101MS0120050313.........................\n\ \SVCLHOHPE 10201DX0320050315........................\n\ \SVCLTWO x10301MRP220050329..............................\n\ \USGE10301TWO x50214..7050329..............................." type ConfigLine = (String, [(String, (Int, Int))]) type Configuration = [ConfigLine] type KeyVal = (String, String) type Header = String type Entry = (Header, [KeyVal]) config :: Configuration config = [("SVCL", [("CustomerName", (4, 18)), ("CustomerID", (19, 23)), ("CallTypeCode", (24, 27)), ("DateOfCallString", (28, 35))]), ("USGE", [("CustomerID", (4, 8)), ("CustomerName", (9, 22)), ("Cycle", (30, 30)), ("ReadDAte", (31, 36))])] getRange :: Int -> Int -> [a] -> [a] getRange a b l = take (b-a+1) $ drop a l lineToFields :: String -> [(Int,Int)] -> [String] lineToFields line points = map (fieldFromTo line) points where fieldFromTo line (x,y) = getRange x y line lineType :: String -> String lineType = getRange 0 3 applyConfig :: String -> ConfigLine -> [KeyVal] applyConfig line (ckey, cval) = if lineType line == ckey then zip names $ lineToFields line points else [] where part = unzip cval names = fst part points = snd part parseLine :: Configuration -> String -> Entry parseLine cnf line = (header, parsed) where rawData :: [[KeyVal]] rawData = map (applyConfig line) cnf parsed :: [KeyVal] parsed = head $ dropWhile null rawData header = lineType line parse :: Configuration -> [String] -> [Entry] parse cnf lines = map (parseLine cnf) lines run = parse config $ filter noComment $ lines source where noComment = \x -> (head x) /= '#' ====== Output *Main> run [("SVCL",[("CustomerName","FOWLER "),("CustomerID","10101"),("CallTypeCode","MS01"),("DateOfCallString","20050313")]),("SVCL",[("CustomerName","HOHPE "),("CustomerID","10201"),("CallTypeCode","DX03"),("DateOfCallString","20050315")]),("SVCL",[("CustomerName","TWO x"),("CustomerID","10301"),("CallTypeCode","MRP2"),("DateOfCallString","20050329")]),("USGE",[("CustomerID","10301"),("CustomerName","TWO x"),("Cycle","7"),("ReadDAte","050329")])] *Main> Yoel Jacobsen wrote:
It seems that Martin Fowler's article "Language Workbenches: The killer-App for Domain Specific Languages?" - http://www.martinfowler.com/articles/languageWorkbench.html - has generated some nice dynamic solution where a configuration file is written in the same language as the program. Notable examples are lisp - http://lispm.dyndns.org/news?ID=NEWS-2005-07-08-1 and python - http://billionairebusinessman.blogspot.com/2005/09/drop-that-schema-and-put-...
I'm trying to create an _elegant_ solution in Haskell. But I'm stuck. Since the native record-like access in Haskell syntax is using labelled fields in datatype decleration but the later are strictly compile-time. Therefore, if I compile my program and add a field in the configuration file (written in Haskell, using, for instance hs-plugins), I'll need to recompile the data declaration as well.
Further, what is the type of the parser? Consider the following implementation:
source = "#123456789012345678901234567890123456789012345678901234567890\n\ \SVCLFOWLER 10101MS0120050313.........................\n\ \SVCLHOHPE 10201DX0320050315........................\n\ \SVCLTWO x10301MRP220050329..............................\n\ \USGE10301TWO x50214..7050329..............................."
data Configuration = Config String [(String, Int, Int)]
config = [ Config "SVCL" [("CustomerName", 4, 18), ("CustomerID", 19, 23), ("CallTypeCode", 24, 27), ("DateOfCallString", 28, 35)],
Config "USGE" [("CustomerID", 4, 8), ("CustomerName", 9, 22), ("Cycle", 30, 30), ("ReadDAte", 31, 36)]]
-- parse takes the configuration, a line from the source string and generate a record
parse :: Configuration -> String -> Record
What is the type of Record?
Anyway, any elegant solution or a hint towards one are most welcome.
Thanks, Yoel