Add Fragment Node

[stephencelis]

This tweet points out a problem with the current API:

https://twitter.com/hoagy_ytfc/status/1079550683253219328

Without being able to represent a list of nodes as a node itself, we can't currently use Vapor's protocol-oriented ResponseEncodable API to return a doctype alongside more root nodes:

[
  doctype,
  html([
    // …
    ])
]

One solution is to add a fragment node, which represents a document or fragment of HTML. The DOM spec has included a DocumentFragment node type for a very long time and React recently added its own Fragment API, so the precedent is there. We've also struggled with our View construct in the past as to whether we should be composing (A) -> Node or (A) -> [Node], so this could help solve that problem.

The change in the PR results in:

Node.fragment([
  doctype,
  html([
    // …
    ])
  ])

A few unanswered questions:

1. Node.fragment is pretty verbose. Even when there's inference, .fragment([…]) might not read well. Do we wanna think of alternative names? Do we want to add a document function as an alias?

   document([
     doctype,
     html([
       // …
       ])
     ])

2. Do we remove the render([Node]) overload or keep it around as convenience?

3. Do we add ExpressibleByArrayLiteral or are we worried about Swift diagnostics?

[stephencelis]

One other consideration: if we start composing HTML from a bunch of fragments will it affect performance?

[mbrandonw]

Not sure I understand why this filter is needed. What's up with it?

[stephencelis]

Was just trying to get rid of the nesting and slipped it in here. Probably not necessary or probably better as an explicit traversal?

[mbrandonw]

you can drop the children.isEmpty since allSatisfy returns true for empty collections.

[mbrandonw]

ha oops! wonder how that slipped in