ConfigBuilder (was: Add Config::with_sources())

[matthiasbeyer]

This patch adds Config::with_sources() for builder-pattern multi-merge
without calling Config::refresh().
The documentation added in the patch serves as rationale.

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I'd love to see discussion on this.

[szarykott]

As discussed in #148 this kind of code could have the desired effect. It would be easy to extend with asynchronous configuration sources if they are ever implemented just by adding merge_async or similar methods.

I am thinking, however, that it might be cleaner to implement a separate struct called, for instance, ConfigBuilder that would produce a frozen config at the end that would be an entry point by itself, without a need to go through Config. Having such a separate struct would indicate clearly that workflow is different. On the other hand discoverability of such a feature would be smaller than in the case of a single entry point as it is a case with a single Config struct with methods on it.

Just a thought of mine on this matter.

[matthiasbeyer]

So what'd you think? Merge? 😄

[matthiasbeyer]

I will merge after Fri 2nd April 2021 12:00:00 UTC if there are no complains.

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Ah, I will implement the suggested changes first, tho.

[matthiasbeyer]

So this is a bit off-topic for this branch, but anyways: The latest commit (which really should be squashed if we merge this) implements a builder-pattern entry point as you @szarykott suggested.

I kept the entry point for the builder on the Config type with Config::builder(), because that keeps the visibility of the whole thing. I did not (yet) remove the functions on the Config type itself, as I think we should rather deprecate them in one release and remove them in the following one. Changing all interface in one release seems to be a bit too big of a change for our users, IMO.

What do you think?

[szarykott]

I'd consider moving ConfigBuilder to a separate file as it seems a functionality of its own.

As you might have seen in #196 I'd like to separate Config and ConfigBuilder, which basically means having two structs - one with cache, read-only, and the other one having all mutable stuff inside. Right now they are kinda mixed as Builder just spawns a mutable Config underneath and uses its mutable methods.

Besides it might be better to merge #190 before considering merging this (due to ConfigKind::Frozen being removed).