[Haskell-cafe] Wrapping all fields of a data type in e.g. Maybe

John Lato jwlato at gmail.com
Wed Jul 17 02:41:11 CEST 2013

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

On Wed, Jul 17, 2013 at 4:57 AM, Michael Orlitzky <michael at orlitzky.com>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?
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