Added test for subexpressions

[Nthalk]

#123

[machty]

Wanna take a stab at the implementation? :) :) :) :)

[Nthalk]

Machty, I've loved reading your blog about writing parsers to just to learn about it, however, I haven't the first idea of how to start. Is this low hanging fruit that I can attempt?

Given that this is your playground, could you give me a basic idea about what needs to be done or where I could start?

[Nthalk]

I imagine I would want to detect ('s and match them with )'s and leave everything there for the Handlebars compiler?

[machty]

@Nthalk you would need to start here

https://github.com/machty/emblem.js/blob/master/src/grammar.pegjs#L322

which defines the grammar for the inside of a mustache, and, therefore, the inside of a sexpr. You can trace that rule down to its component rules, such as what a param or hash param can be, and add a subexpressions rule which is essentially an inMustache wrapped in parentheses, and in such cases, you'd need to return an AST.SexprNode, which is the new node I added in my handlebars work.

Honestly, this is a perfect and satisfying PR for you to work on and wrap your brain around PEGjs; I think the bulk of the challenge is just learning PEG and not so much anything specifically nitpicking or annoying about this particular change, so I definitely encourage you to go for it.