On 07/16/2013 08:41 PM, John Lato wrote:
The suggestion of parameterizing on a functor would be good, however there's another approach I've often seen (although it's not quite what you've asked for). You can leave your config datatype alone, but instead of making it a monoid have your configuration parsers return functions with the type (Cfg -> Cfg). You can wrap these functions in Endo to get a monoid, combine them together, and then apply that function to the default configuration.
I'm using cmdargs for the command-line parsing, and I think (if I don't want to abandon its magic entirely) that I'm stuck filling a data structure automatically. I settled on using (Maybe Foo) so that the default value returned by cmdargs will be Nothing if the user doesn't supply that option; if I use a plain Cfg object, and the user doesn't pass --verbose, I'll get False back in its place and then I don't know whether or not that should override the config file (supposing the user has verbose=True in the file). Very clever though.