This introduces XIR Flat (XIRF), which is conceptually between XIR and
XIRT. This provides a more appropriate level of abstraction for further
lowering operations to parse against, and removes the need for other parsers
to perform their own validations (inappropriately) to ensure well-formed
XML.
There is still some cleanup worth doing, including moving some of the
parsing responsibility up a level back into the XIR parser.
DEV-10863
It's a bit odd that I've done next to nothing with TAMER for the past week
or so, and decided to do this one small thing before I go on break for the
holidays, but I felt compelled to do _something_. Besides, this gets me in
a better spot for the inevitable mental planning and writing I'll be doing
over the holidays.
This move was natural, given what this has evolved into---it has nothing to
do with the concept of a "tree", and the modules imports emphasized that
fact given the level of inappropriate nesting.
Now that the parser has been simplified by removing attributes, we can
further simplify the state transitions to make it more clear what further
refactoring can be done.
DEV-11339
More information can be found in the prior commit message, but I'll
summarize here.
This token was introduced to create a LL(0) parser---no tokens of
lookahead. This allowed the underlying TokenStream to be freely passed to
the next system that needed it.
Since then, Parser and ParseState were introduced, along with
ParseStatus::Dead, which introduces the concept of lookahead for a single
token---an LL(1) grammar.
I had always suspected that this would happen, given the awkwardness of
AttrEnd; it was just a matter of time before the right abstraction
manifested itself to handle lookahead.
DEV-11339
Note that AttrParse{r=>}State needs renaming, and Stack will get a better
name down the line too. This commit message is accurate, but confusing.
This performs the long-awaited task of trying to observe, concretely, how to
combine two automata. This has the effect of stitching together the state
machines, such that the union of the two is equivalent to the original
monolith.
The next step will be to abstract this away.
There are some important things to note here. First, this introduces a new
"dead" state concept, where here a dead state is defined as an _accepting_
state that has no state transitions for the given input token. This is more
strict than a dead state as defined in, for example, the Dragon Book, where
backtracking may occur.
The reason I chose for a Dead state to be accepting is simple: it represents
a lookahead situation. It says, "I don't know what this token is, but I've
done my job, so it may be useful in a parent context". The "I've done my
job" part is only applicable in an accepting state.
If the parser is _not_ in an accepting state, then an unknown token is
simply an error; we should _not_ try to backtrack or anything of the sort,
because we want only a single token of lookahead.
The reason this was done is because it's otherwise difficult to compose the
two parsers without requiring that AttrEnd exist in every XIR stream; this
has always been an awkward delimiter that was introduced to make the parser
LL(0), but I tried to compromise by saying that it was optional. Of course,
I knew that decision caused awkward inconsistencies, I had just hoped that
those inconsistencies wouldn't manifest in practical issues.
Well, now it did, and the benefits of AttrEnd that we had in the previous
construction do not exist in this one. Consequently, it makes more sense to
simply go from LL(0) to LL(1), which makes AttrEnd unnecessary, and a future
commit will remove it entirely.
All of this information will be documented, but I want to get further in
the implementation first to make sure I don't change course again and
therefore waste my time on docs.
DEV-11268
These were missed from a couple of commits ago, after I recalled that I
could now simplify the Stack variants; they were made more complicated due
to isolated attribute parsing.
These progressive refactorings do a good job illustrating why composing
parsers is better than a monolith---the complexity of the parsers is
significantly reduced, and the number of combinations of states are also
greatly reduced, which allows us to reason about them in isolation.
DEV-11268
This was added only for isolated attribute parsing. Of course, this does
mean that a new union type will be needed when combining the two parsers,
depending on the desired resolution, but that'll come at a later time and
possibly in a more general way.
DEV-11268
This nearly completely integrates the new Parser with xir::tree, but does
not yet compose AttrParseState. I also need to determine what to do with
`parse()` and, further, make `parser_from` generic as part of mod parse.
If we take a moment to reflect on all of the changes, this struggle has been
a roundabout way of converting tree's parser into parse::Parser; providing
a trait for Stack (as ParseState); beginning parser decomposition; and
moving some common logic into Parser. The composition of parsers is the
final piece to be realized.
This could have been a lot less work if I really understood exactly what I
wanted to do up front, but as was mentioned in previous commits, I was
really confusing myself trying to maintain API BC in ways that I should not
have for XmloReader. More on that will be coming soon as well.
DEV-11268
This isn't currently used by anything, and this is collecting, which does
not fit well with the streaming model. AttrList was originally written for
Element parsing, and the isolated attr parser was written for test cases,
before it was fully decided how this system ought to work.
Instead, if AttrList is in fact needed, we can either collect (ideally not)
or implement Extend for AttrList. (Or create TryExtend.)
DEV-11268
This removes the layer of encapsulation that was hiding Stack, which is the
actual parser. The new layer of encapsulation is parse::Parser, which will
be introduced here soon. Baby steps, so it's clear how this evolves.
DEV-11268
The old Parsed was renamed to ParseStatus to be used by Parser, and Parser
converts it into Parsed, which has the same variants as it did before and
has all but the Done variant, since it's not possible for Parser to yield
it.
DEV-11268
This removes Option from ParseState, as mentioned in previous commits.
This is ideal because it not only removes a layer of abstraction, but also
makes the intent very clear; the use of None was too tied to the concept of
an Iterator, which is the concern of Parser, _not_ ParseState.
This is now similar to tree::Parsed, which will help with that refactoring
shortly.
The Done variant is not accessible outside of Parser, since it always
coverts it to None (to halt iteration); given that, we should have another
public-facing type, as was also mentioned in a previous commit.
DEV-11268
This also renames related types.
See previous commits for more in formation. In essence, this trait
represents the reification of all parser state. The omission of "r" in the
name ParseState is intentional, since it indicates the state of a current
parse. We'll see whether that naming ends up being too confusing; it's easy
enough to change.
DEV-11268
This just leaves Parser, which is what I started with, but I wasn't sure how
far I was going to take this. I went against my usual judgment in creating
a trait that I may not need, in an attempt to try to reason about the API
that I wanted, because it wasn't yet clear at the time whether the Parser
ought to be generic.
Since then (as detailed in the last commit), this has become more of a
coordinator/mediator, and the real parser is actually TokenStreamState,
which will be renamed shortly.
DEV-11268
This begins to integrate the isolated AttrParser. The next step will be
integrating it into the larger XIRT parser.
There's been considerable delay in getting this committed, because I went
through quite the struggle with myself trying to determine what balance I
want to strike between Rust's type system; convenience with parser
combinators; iterators; and various other abstractions. I ended up being
confounded by trying to maintain the current XmloReader abstraction, which
is fundamentally incompatible with the way the new parsing system
works (streaming iterators that do not collect or perform heap
allocations).
There'll be more information on this to come, but there are certain things
that will be changing.
There are a couple problems highlighted by this commit (not in code, but
conceptually):
1. Introducing Option here for the TokenParserState doesn't feel right, in
the sense that the abstraction is inappropriate. We should perhaps
introduce a new variant Parsed::Done or something to indicate intent,
rather than leaving the reader to have to read about what None actually
means.
2. This turns Parsed into more of a statement influencing control
flow/logic, and so should be encapsulated, with an external equivalent
of Parsed that omits variants that ought to remain encapsulated.
3. TokenStreamState is true, but these really are the actual parsers;
TokenStreamParser is more of a coordinator, and helps to abstract away
some of the common logic so lower-level parsers do not have to worry
about it. But calling it TokenStreamState is both a bit
confusing and is an understatement---it _does_ hold the state, but it
also holds the current parsing stack in its variants.
Another thing that is not yet entirely clear is whether this AttrParser
ought to care about detection of duplicate attributes, or if that should be
done in a separate parser, perhaps even at the XIR level. The same can be
said for checking for balanced tags. By pushing it to TokenStream in XIR,
we would get a guaranteed check regardless of what parsers are used, which
is attractive because it reduces the (almost certain-to-otherwise-occur)
risk that individual parsers will not sufficiently check for semantically
valid XML. But it does _potentially_ match error recovery more
complicated. But at the same time, perhaps more specific parsers ought not
care about recovery at that level.
Anyway, point being, more to come, but I am disappointed how much time I'm
spending considering parsing, given that there are so many things I need to
move onto. I just want this done right and in a way that feels like it's
working well with Rust while it's all in working memory, otherwise it's
going to be a significant effort to get back into.
DEV-11268
This permits retrieving a Span from any Token variant. To support this,
rather than having this return an Option, Token::AttrEnd was augmented with
a Span; this results in a much simpler and friendlier API.
DEV-11268
This removes XIRT support for attribute fragments. The reason is that
because this is a write-only operation---fragments are used to concatenate
SymbolIds without reallocation, which can only happen if we are generating
XIR internally.
Given that this cannot happen during read, it was a mistake to complicate
the parsers. But it makes sense why I did originally, given that the XIRT
parser was written for simplifying test cases. But now that we want parsers
for real, and are writing production-quality parsers, this extra complexity
is very undesirable.
As a bonus, we also avoid any potential for heap allocations related to
attributes. Granted, they didn't _really_ exist to begin with, but it was
part of XIRT, and was ugly.
DEV-11268
The XIRT parser was initially written for test cases, so that unit tests
should assert more easily on generated token streams (XIR). While it was
planned, it wasn't clear what the eventual needs would be, which were
expected to differ. Indeed, loading everything into a generic tree
representation in memory is not appropriate---we should prefer streaming and
avoiding heap allocations when they’re not necessary, and we should parse
into an IR rather than a generic format, which ensures that the data follow
a proper grammar and are semantically valid.
When parsing attributes in an isolated context became necessary for the
aforementioned task, the state machine of the XIRT parser was modified to
accommodate. The opposite approach should have been taken---instead of
adding complexity and special cases to the parser, and from a complex parser
extracting a simple one (an attribute parser), we should be composing the
larger (full XIRT) parser from smaller ones (e.g. attribute, child
elements).
A combinator, when used in a functional sense, refers not to combinatory
logic but to the composition of more complex systems from smaller ones. The
changes made as part of this commit begin to work toward combinators, though
it's not necessarily evident yet (to you, the reader) how that'll work,
since the code for it hasn't yet been written; this is commit is simply
getting my work thusfar introduced so I can do some light refactoring before
continuing on it.
TAMER does not aim to introduce a parser combinator framework in its usual
sense---it favors, instead, striking a proper balance with Rust’s type
system that permits the convenience of combinators only in situations where
they are needed, to avoid having to write new parser
boilerplate. Specifically:
1. Rust’s type system should be used as combinators, so that parsers are
automatically constructed from the type definition.
2. Primitive parsers are written as explicit automata, not as primitive
combinators.
3. Parsing should directly produce IRs as a lowering operation below XIRT,
rather than producing XIRT itself. That is, target IRs should consume
XIRT and produce parse themselves immediately, during streaming.
In the future, if more combinators are needed, they will be added; maybe
this will eventually evolve into a more generic parser combinator framework
for TAME, but that is certainly a waste of time right now. And, to be
honest, I’m hoping that won’t be necessary.
Well, parse to the extent that it was being parsed before, anyway.
The core of this change demonstrates how well TAMER's abstractions work well
together. (As long as you have an e.g. LSP to help you make sense of all of
the inference, I suppose.)
Token::Open(QN_LV_PACKAGE | QN_PACKAGE, _) => {
return Ok(XmloEvent::Package(
attr_parser_from(&mut self.reader)
.try_collect_ok()??,
));
}
This finally makes use of `attr_parser_from` and `try_collect_ok`. All of
the types are inferred---from the iterator transformations, to the error
conversions, to the destination PackageAttrs type.
DEV-10863
This was forgotten when the attribute parser was introduced, and led to the
parser continuing to the token following AttrEnd, which properly caused a
failure given that the parser was in the Done state.
There is a future task I have in my backlog to properly address the Done
state, but this is sufficient for now.
Like previous commits, this replaces the explicit escaping context with the
convention that all values retrieved from `xir` are unescaped on read and
escaped on write.
Comments are a notable TODO, since we must escape only `--`.
CData is also an issue. I had _expected_ to use it as a means to avoid
unescaping fragments, but I had forgotten that quick_xml hard-codes escaping
on read, so that it can re-use BytesStart! That is terribly unfortunate,
and may result in us having to re-implement our own read method in the
future to avoid this nonsense. So I'm just leaving it as a TODO for now.
DEV-11081
This rewrites a good portion of the previous commit.
Rather than explicitly storing whether a given string has been escaped, we
can instead assume that all SymbolIds leaving or entering XIR are unescaped,
because there is no reason for any other part of the system to deal with
such details of XML documents.
Given that, we need only unescape on read and escape on write. This is
customary, so why didn't I do that to begin with?
The previous commit outlines the reason, mainly being an optimization for
the echo writer that is upcoming. However, this solution will end up being
better---it's not implemented yet, but we can have a caching layer, such
that the Escaper records a mapping between escaped and unescaped SymbolIds
to avoid work the next time around. If we share the Escaper between _all_
readers and the writer, the result is that
1. Duplicate strings between source files and object files (many of which
are read by both the linker and compiler) avoid re-unescaping; and
2. Writers can use this cache to avoid re-escaping when we've already seen
the escaped variant of the string during read.
The alternative would be a global cache, like the internment system, but I
did not find that to be appropriate here, since this is far less
fundamental and is much easier to compose.
DEV-11081
The previous implementation took ownership over the provided iterator, which
was an oversight, considering that this is intended to be used in contexts
where doing so is not possible. A good example where isolated test cases
aren't necessarily painting the correct picture.
`scan` takes owned values, so this instead uses the same parsing method as
`parse_attrs`, but using a `FromFn` iterator to avoid having to create a
whole new iterator type. This will work well so long as we don't need to
store the type returned by this (while also wanting to avoid boxing).
DEV-11062
See the previous commit. There is no sense in some common "IR" namespace,
since those IRs should live close to whatever system whose data they
represent.
In the case of these, they are general IRs that can apply to many different
parts of the system. If that proves to be a false statement, they'll be
moved.
DEV-10863
Mike Gerwitz