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.
Although this end symbol appears as just a close-brace in source, it's
worth differentiating it because the scanner must differentiate it anyway
(to recognize moving back into template-scanning mode) and it avoids the
parser from having to similarly re-recognize the difference.
On reflection, it seems easier to maintain the necessary state we need
by doing all of the scanning in a single pass, since we can then just
use local variables within the scanner function.
Using Ragel here because the scanner is going to be somewhat complex due
to the need to switch back and forth between normal and template states,
etc. This should be easier to maintain than a hand-written scanner, while
ragel gives us the extra features we need to implement things that would
normally be too complex for a "regular" scanner generator.
This is the first non-trivial expression Value implementation. Lots of
code here, so hopefully while implementing other expressions some
opportunities emerge to factor out some of these details.
The implementation of Variables will be identical for every Expression
implementation since we just wrap our AST-walk-based "Variables" function
to do the work.
Rather than manually copy-pasting the declaration for each expression
type, instead we'll generate this programmatically using "go generate".
This will need to be re-run each time a new expression node type is
added, in order to make it actually implement the Expression interface.
This function is effectively the implementation of Variables for all
expressions, but unfortunately we still need to declare a wrapper around
it as a method on every single expression type.
This package will grow to contain all of the gory details of the native
zcl syntax, including it AST, parser, etc. Most callers should access
this via the simpler API in the top-level package, which then gives
automatic support for other syntaxes too.
