Description
In #450, there are two classes of expression attributes: built-in values such as @locale and implementation or private-use values (which might include values like @can-reorder or @_websiteId)
@eemeli proposed in the PR to use the presence of a hyphen in the name (@can-reorder) as the namespacing mechanism.
@aphillips proposed to use a starting character of underscore (@_myAttribute) as the namespacing mechanism
Both are consistent with name in that they don't require modification to the syntax to make them valid.
What we choose here should be consistent with other naming rules. We currently don't have similar distinctions elsewhere, but we are discussing using a "second character" or separate sigil for local vs. external variables in the discussion on mutability. I'm writing this issue to ensure that we making our naming schemes consistent so that we don't have different naming requirements in different parts. If would be weird to require (for example) a hyphen in expression attributes but an underscore for local vars:
let $_local = {:amendLocale nu=Latn @locale=th-TH @some-attr=foo}