On 07/16/2013 09:57 PM, Michael Orlitzky wrote:
I have a common pattern in my command-line programs; I start out with a configuration data type, which over-simplified looks like:
data Cfg = Cfg { verbose :: Bool }
Now, there's usually a default configuration,
default :: Cfg default = Cfg False
The user can override the defaults one of two ways, either via a config file, or from the command-line. If both are specified, the command-line takes precedence. The way I do this is with,
data OptionalCfg = OptionalCfg { verbose :: Maybe Bool }
And then I define I Monoid instance for OptionalCfg which lets me merge two ofthem. Once the two OptionalCfgs are merged, I merge *that* with the default Cfg.
This all works great, except that when there's 20 or so options, I duplicate a ton of code in the definition of OptionalCfg. Is there some pre-existing solution that will let me take a Cfg and create a new type with Cfg's fields wrapped in Maybe?
One option is to combine OptionalCfg with Cfg, by parameterizing Cfg. If you make data Cfg a = Cfg { cfgVerbose :: a Bool } Then you can choose between Cfg Identity and Cfg Maybe. Furthermore, these are different types, so you can still have a monoid over Cfg Maybe. There might be some lens magic that makes working with this easier too. For example, verbose :: Lens' (Cfg a) Bool verbose = cfgVerbose.traverse Or something to that effect (that actually compiles). - ollie