Expose parse to the public

[clebert]

I am currently working on making the prettier-plugin-svelte compatible with Svelte 3. I noticed that compile has to be called to get the AST. This leads to errors in some tests, because they cannot be compiled.

For example, the following test: https://github.com/UnwrittenFun/prettier-plugin-svelte/blob/master/test/printer/samples/binding-shorthand.html
Error: value is not declared

But regardless of my use case, I think it makes sense to get the AST without a compilation step.

[Conduitry]

If svelte.compile fails, then it's an invalid component (or there's a bug). Do other Prettier plugins support formatting malformed or invalid files?

This isn't to take away from the idea of exposing svelte.parse, but imo that test will need to be updated for v3 because it's no longer a valid component.

[halfnelson]

As a workaround, I have extracted the parser from svelte as a module here
https://github.com/halfnelson/svelte-component-parser

[clebert]

@Conduitry I think only the syntactic correctness of a program is important for a formatter. A program does not have to be semantically correct. Of course it is a question of definition, if an undeclared variable is bound, whether it is a semantic or syntactic error? I plead on the former. I can also call a misspelled function in the JavaScript part, and it will still be formatted.

I might create an issue for further discussion. I will temporarily use svelte-component-parser from @halfnelson.

[clebert]

I didn't find svelte-component-parser on npm. I have therefore published svelte-parser, and will maintain it until this PR is merged or the parser is made accessible in another way.

[Conduitry]

I've come around to thinking that it would be nice to expose just the parser, without any of the additional validation that compile performs. The cost seems pretty low, as I don't think this ties us down any more than having ast in the compiler response already did. I'll defer to whatever @Rich-Harris thinks though.

[Rich-Harris]

Thanks, look forward to checking out prettier-plugin-svelte 😍