On Fri, 2008-09-19 at 23:24 +0100, Magnus Therning wrote:
Hi all,
I'm looking for some inspiration for an elegant solution to a silly little problem I have. This might have a general well-known solution, or maybe there's something particularly elegant possible in Haskell. I just thought I'd ask.
When writing a command line tool I want to use a configuration file and then have the ability to override the configuration using command line arguments. When I've worked with command line arguments before I've used the trick of folding (>>=) over a list of functions that modify the "members" of a type, using the default values as the starting point. I like that, it's cute.
First I thought I'd treat the configuration in a similar way, but then I noticed a slight ordering problem. The command line arguments should take priority over the contents of the configuration file, but the location of the configuration can be given as an argument. I could read the arguments twice, first to get the correct location of the config file, then load the config, and then read the arguments again to make sure they take priority. But that feels a little silly. Are there any more elegant solutions people are using?
You could build a monoid, data Option a = Unspecified | Default a | Config a | CommandLine a With Unspecified being the identity and the multiplication being Default a * Config b = Config b Default a * CommandLine b = CommandLine b Config a * CommandLine b = CommandLine b and symmetrically and break ties to the right e.g. CommandLine a * CommandLine b = CommandLine b