Previously it was !{, but in real examples this looked confusing since
an exclamation point after a word looks (to humans) like literal
punctuation rather than syntax.
% is not ideal either since it's also the marker traditionally used for
printf, but has the advantage that programmers are already primed for it
to be syntax.
Previously we were failing to return back to template-scanning mode due
to decrementing "braces" too early, causing the remainder of the template
to be scanned as if it were an expression.
The context where a string literal was found affects what sort of escaping
it can have, so we need to distinguish these cases so that we will only
look for and handle backslash escapes in quoted strings.
This alternative scanning mode makes the scanner start in template
context rather than normal context. This will be later used by the parser
to allow parsing of standalone templates that aren't embedded inside a
zcl configuration file.
This is important because our syntax for objects uses newlines as the
separator between items, so this is the only signal we'll get that a
given item has ended and another is beginning.
A scanner "mode" decides which state it starts in, allowing us to start
in template mode for parsing top-level templates. However, currently the
only mode implemented is "normal" mode, which is the behavior we had
before.
This requires some extra state-keeping because we allow templates to be
nested inside templates. This takes us outside of the world of regular
languages, but we accept that here because it makes things easier to
deal with down the line in the parser.
The methodology is to keep track of how many braces are open at a given
time and then, when a nested template interpolation begins, record the
current brace level. Then, when a closing brace is encountered, if its
nesting level is at the top of the stack then we pop off the stack and
return to "main" parsing mode.
Ragel's existing idea of calling and returning from machines is important
here too. As this currently stands this is not actually needed, but once
heredocs are in play we will have two possible places to return to at
the end of an interpolation sequence, so the state return stack maintained
by Ragel will determine whether to return to string mode or heredoc mode.
