[Haskell-cafe] Config Files and monoids

Fri May 25 02:52:42 UTC 2018

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 <hexagoxel at hexagoxel.de>
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
>
>
> On 25/05/18 00:03, Matt wrote:
> > 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 <olf at aatal-apotheke.de>
> wrote:
> >
> >> 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
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> >
> >
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-- 
J. Douglas McClean

(781) 561-5540 (cell)
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