feat(config): support TOML configuration files

[jorgegonzalez]

Closes #4845

In order to support .toml configuration files, cosmiconfig must first be updated to v5.0. This update seems to break a bunch of tests. The following errors are thrown all over the place:

Update 29/7/2018: This PR adds support for .prettierrc.toml config files.

[jorgegonzalez]

Also, I recognize adding another dependency is not ideal for what is essentially a minor feature, but toml is quite light.

[foray1010]

@jorgegonzalez how about @iarna/toml? it is actively maintained, and it is the only js package support toml 0.5.0 at the time
https://github.com/iarna/iarna-toml

[j-f1]

It’s also much smaller slightly bigger than toml when bundled.

(packagephobia tells you how big a package is when installed on your computer, while bundlephobia tells you how big it is when bundled+minified(+gzipped).)

[jorgegonzalez]

@foray1010 I'll use @iarna/toml if @j-f1 will allow the size increase 🙏

[jorgegonzalez]

Thanks @ikatyang for upgrading cosmiconfig!

[j-f1]

Should be fine 👍

[jorgegonzalez]

Not sure why it's failing on node4 🤔

[jorgegonzalez]

Ah I see.

[j-f1]

I think you could use a snapshot in that test, which would save having to create an object to match against.

[ikatyang]

The currentFile looks weird to me, it confused me why there's a TOML parsing error for a js file. We should rename it to something ends with .toml.

[ikatyang]

We could use @iarna/toml/parse-string to further reduce the bundle size.

[ikatyang]

I'll let other maintainers to decide if we should merge it as I hesitate about adding another config format. If someone wants to add support for other format later, what should we do?

I'm thinking of if we should make loaders be part of plugins so that people can use whatever format they want.

[jorgegonzalez]

I understand the hesitance to add another format, and this is indeed an opinionated PR. Personally, I would love to see the JavaScript community embrace TOML configuration files more, as it is subjectively superior to both YAML and JSON configurations, and Prettier has the opportunity to lead the community in this. I suspect more users would also lean toward this format if they were given the chance to try it.

As linked in #4845:

Some reasons why TOML is better than YAML for configuration files: https://arp242.net/weblog/yaml_probably_not_so_great_after_all.html

Some arguments against JSON as a configuration language: https://www.lucidchart.com/techblog/2018/07/16/why-json-isnt-a-good-configuration-language/

[j-f1]

I think this is a great idea! It seems fairly lightweight and it shouldn’t take too much maintenance, especially if it’s integrated into cosmiconfig directly. All that’s missing now is support for printing it 😉

[suchipi]

There's not a huge value-add imo since prettier config files are very small and you're unlikely to need comments in them, but since the work is already done let's merge it 👍

[j-f1]

Thank you for contributing to Prettier!