Oh what a tortured journey. I had originally tried to avoid formalizing
lookahead for all parsers by pretending that it was only needed for dead
state transitions (that is---states that have no transitions for a given
input token), but then I needed to yield information for aggregation. So I
added the ability to override the token for `Dead` to yield that, in
addition to the token. But then I also needed to yield lookahead for error
conditions. It was a mess that didn't make sense.
This eliminates `ParseStatus::Dead` entirely and fully integrates the
lookahead token in `Parser` that was previously implemented.
Notably, the lookahead token is encapsulated in `TransitionResult` and
unavailable to `ParseState` implementations, forcing them to rely on
`Parser` for recursion. This not only prevents `ParseState` from recursing,
but also simplifies delegation by removing the need to manually handle
tokens of lookahead.
The awkward case here is XIRT, which does not follow the streaming parsing
convention, because it was conceived before the parsing framework. It needs
to go away, but doing so right now would be a lot of work, so it has to
stick around for a little bit longer until the new parser generators can be
used instead. It is a persistent thorn in my side, going against the grain.
`Parser` will immediately recurse if it sees a token of lookahead with an
incomplete parse. This is because stitched parsers will frequently yield a
dead state indication when they're done parsing, and there's no use in
propagating an `Incomplete` status down the entire lowering pipeline. But,
that does mean that the toplevel is not the only thing recursing. _But_,
the behavior doesn't really change, in the sense that it would infinitely
recurse down the entire lowering stack (though there'd be an opportunity to
detect that). This should never happen with a correct parser, but it's not
worth the effort right now to try to force such a thing with Rust's type
system. Something like TLA+ is better suited here as an aid, but it
shouldn't be necessary with clear implementations and proper test
cases. Parser generators will also ensure such a thing cannot occur.
I had hoped to remove ParseStatus entirely in favor of Parsed, but there's a
lot of type inference that happens based on the fact that `ParseStatus` has
a `ParseState` type parameter; `Parsed` has only `Object`. It is desirable
for a public-facing `Parsed` to not be tied to `ParseState`, since consumers
need not be concerned with such a heavy type; however, we _do_ want that
heavy type internally, as it carries a lot of useful information that allows
for significant and powerful type inference, which in turn creates
expressive and convenient APIs.
DEV-7145
This isn't conceptally all that significant of a change, but there was a lot
of modify to get it working. I would generally separate this into a commit
for the implementation and another commit for the integration, but I decided
to keep things together.
This serves a role similar to AttrSpan---this allows deriving a span
representing the element name from a span representing the entire XIR
token. This will provide more useful context for errors---including the tag
delimiter(s) means that we care about the fact that an element is in that
position (as opposed to some other type of node) within the context of an
error. However, if we are expecting an element but take issue with the
element name itself, we want to place emphasis on that instead.
This also starts to consider the issue of span contexts---a blob of detached
data that is `Span` is useful for error context, but it's not useful for
manipulation or deriving additional information. For that, we need to
encode additional context, and this is an attempt at that.
I am interested in the concept of providing Spans that are guaranteed to
actually make sense---that are instantiated and manipulated with APIs that
ensure consistency. But such a thing buys us very little, practically
speaking, over what I have now for TAMER, and so I don't expect to actually
implement that for this project; I'll leave that for a personal
project. TAMER's already take a lot of my personal interests and it can
cause me a lot of grief sometimes (with regards to letting my aspirations
cause me more work).
DEV-7145
This is the first parser generator for the parsing framework. I've been
waiting quite a while to do this because I wanted to be sure that I
understood how I intended to write the attribute parsers manually. Now that
I'm about to start parsing source XML files, it is necessary to have a
parser generator.
Typically one thinks of a parser generator as a separate program that
generates code for some language, but that is not always the case---that
represents a lack of expressiveness in the language itself (e.g. C). Here,
I simply use Rust's macro system, which should be a concept familiar to
someone coming from a language like Lisp.
This also resolves where I stand on parser combinators with respect to this
abstraction: they both accomplish the exact same thing (composition of
smaller parsers), but this abstraction doesn't do so in the typical
functional way. But the end result is the same.
The parser generated by this abstraction will be optimized an inlined in the
same manner as the hand-written parsers. Since they'll be tightly coupled
with an element parser (which too will have a parser generator), I expect
that most attribute parsers will simply be inlined; they exist as separate
parsers conceptually, for the same reason that you'd use parser combinators.
It's worth mentioning that this awkward reliance on dead state for a
lookahead token to determine when aggregation is complete rubs me the wrong
way, but resolving it would involve reintroducing the XIR AttrEnd that I had
previously removed. I'll keep fighting with myself on this, but I want to
get a bit further before I determine if it's worth the tradeoff of
reintroducing (more complex IR but simplified parsing).
DEV-7145
Mike Gerwitz