For a "fully general" approach, the problem is well expressed by the
"higher kinded data" pattern:
http://reasonablypolymorphic.com/blog/higher-kinded-data/
A `Config f = Config { configFoo :: f Foo, ... }` type would use either the
First or Last monoids, depending on if you want earlier updates to take
precedence over later ones. Then, you would get a `Config First` from your
CLI parser, a `Config First` from your environment variable parser, and a
`Config First` from your config file parser. After `mappend`ing them all
together, you'd use a `gtraverse` function with a signature like: `Config
First -> Either [Text] (Config Identity)` -- you'd either have a list of
all fields that were missing, or a complete Config.
Matt Parsons
On Thu, May 24, 2018 at 3:00 PM, Olaf Klinke
Dear cafe,
a recent post here [1] mentioned that configurations, such as the ones read from a config file, can be given Monoid instances, where mempty is the empty or default configuration and mappend merges two partial configurations, producing a more complete one. The vgrep package explicitly does this, for instance. Although the ConfigParser type from the ConfigFile package has a binary 'merge' operation, it does define neither a Monoid not a Semigroup instance.
I'm struggling to make the concept of monoidal configuration work when there is no sensible default configuration. Suppose my configuration type is
data Config = Config {foo :: Bool, bar :: Int}
with no reasonable default, e.g.
emptyConfig = Config { foo = error "you did not specify option foo", bar = error "you did not specify option bar" }
Some configuration monoids seem to have the second operand override the first, or the other way around. However, I wish that when cfg1 = emptyConfig {foo = True} cfg2 = emptyConfig {bar = 4} then cfg1 <> cfg2 == Config {foo = True, bar = 4}.
So it seems that for mappend to work as intended one needs a terminating function that tells me if a record field is already defined, e.g. when all fields are Maybes. Vgrep.Environment.Config.Monoid does it this way. My solution so far was to resort to the monoid of endofunctions (as the getflag package does), that is, define
cfg1, cfg2 :: Config -> Config cfg1 = \cfg -> cfg {foo = True} cfg2 = \cfg -> cfg {bar = 4}
And then build (cfg1.cfg2) emptyConfig. (Alternatively, one might structure these as lenses instead of endofunctions, see e.g. Data.Monoid.Endo.Fold in the endo package.) Thus I arrived at
class Config cfg where emptyConfig :: cfg -- may contain some defaults configOptions :: [Parser (cfg -> cfg)]
Do you think every other concept of configuration parsing can be cast into this typeclass? -- Olaf
[1] https://mail.haskell.org/pipermail/haskell-cafe/2018-May/129063.html _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.