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-your-hands-in.html
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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