To get the static record of names you need to make good error messages for the missing things, I think it should also be possible to write a GHC.Generics function with an end result like: gNames :: (GNames r) => r (Const String) by digging through the metadata for each field in the record type r, extracting the name, using Const . fromString, and packaging it up in the record? But exactly how the Generic1 machinery for this works is escaping me at this hour. Perhaps someone can help fill in the details? On Thu, May 24, 2018 at 6:45 PM, lennart spitzner wrote:

Thanks matt for this pointer. I have also used the higher-kinded approach in the past, but using a slightly different abstraction. This is introduced in a very recent post [1]. I think this gives you the custom `gtraverse` you describe essentially for free (I assume you'd still need to write that, given that it has a custom type, right?). However it does not use the type family trick to avoid the `Identity` wrappers.

The examples in my post still mostly assume that there is a default config, but I think you could work around this. This would involve the `CZipWithM` class instead of just `CZipWith`: The simple path would be

cTraverse (fmap Identity) :: MyConfig Option -> Option (MyConfig Identity)

which roughly corresponds to `gvalidate` from the "Higher-Kinded Data" post. The downside is that a Nothing result would not tell you which field(s) were missing. To fix that, you could define a static value of type `MyConfig (Const String)` that adds a value-level name to each field, and use `cZipWithM` to produce a `Either String (MyConfig Identity)` or perhaps even `Either [String] (MyConfig Identity)` by using the right traversal monad.

Hope this helps.

-- lennart

[1] http://hexagoxel.de/postsforpublish/posts/2018-05- 24-program-configuration.html

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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

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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 wrote:

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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

On 25/05/18 00:03, Matt wrote: the 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.

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