On 08/09/05, Yoel Jacobsen
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
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You could have a large record configuration type capable of storing all possible necessary information (say, using Maybe to indicate that fields are optional), and use combinators to build more complex configurations out of simpler ones. If there are a very large number of potential fields and you only care about chopping the strings apart, a (Map String String) might be useful. You may want something more complicated though where the numbers and dates etc. are all parsed. For that, you basically need to figure out how your data are structured beforehand. (There is also the possibility of including the type in the config file, deriving Data, and using Data.Generics in the program to manipulate the records, but if you can specify the possible fields beforehand, it will probably be much easier to write your program.) Further, it looks like your configurations are just the parameters describing the format in which to parse strings. You might consider just using Parsec and just building the parsers directly in the configuration file, perhaps writing a few specific combinators to help with this specific app. - Cale