v5 proposal

[lesnitsky]

Hello @mockturtl 👋

This is a proposal PR which is by no means final, but a prototype for better understanding of the underlying problems and proposed solutions:

Motivation

dotenv is used in many backend dart projects, and we've had a few support tickets raised at globe.dev regarding env vars loading.

includeParentEnvironment

I'm not sure why merging .env and Platform.environment is not a default behavior. Typically .env is used to override production env vars locally for development purposes. .env files are rarely checked in into version control system and cloud providers/hosting environments have their own secure way to define env vars. Bundling .env is less secure, since it's just a plain text inside a file inside an archive with the code.

This PR implements merging Platform.environment with whatever is defined in .env as a default behaviour.

Initialization API

Compared to NodeJS's require('dotenv').config(), current initialization API is a bit more complicated, requires calling both constructor of DotEnv and load(), which has 2 parementers.

This PR implements singleton pattern on DotEnv class, similar to how it's done in firebase/flutterfire. Reading env var becomes as easy as:

final myVar = DotEnv.instance['MY_VAR'];

Although I'm not sure when this could be useful, there's still a way to have a custom parser:

final dotenv = DotEnv(MyDotEnvParser())..load();
final myVar = dotenv['MY_VAR'];

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Let me know what you think, if you're willing to accept these breaking changes, I'm happy to proceed with this PR, and update tests, docs, and readmes.