XIRF introduced the concept of `Transition` to help document code and
provide mental synchronization points that make it easier to reason about
the system. I decided to hoist this into XIR's parser itself, and have
`parse_token` accept an owned state and require a new state to be returned,
utilizing `Transition`.
Together with the convenience methods introduced on `Transition` itself,
this produces much clearer code, as is evidenced by tree::Stack (XIRT's
parser). Passing an owned state is something that I had wanted to do
originally, but I thought it'd lead to more concise code to use a mutable
reference. Unfortunately, that concision lead to code that was much more
difficult than necessary to understand, and ended up having a net negative
benefit by leading to some more boilerplate for the nested types (granted,
that could have been alleviated in other ways).
This also opens up the possibility to do something that I wasn't able to
before, which was continue to abstract away parser composition by stitching
their state machines together. I don't know if this'll be done immediately,
but because the actual parsing operations are now able to compose
functionally without mutability getting the way, the previous state coupling
issues with the parent parser go away.
DEV-10863
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
This behavior is unchanged, but it allows us to create more constant spans
for testing. For example:
const S = DUMMY_SPAN.offset_add(1).unwrap();
This, in turn, will allow for removing lazy_static! for tests that use it
for span generation.
DEV-10863
Petgraph was previously held back due to petgraph-graphml. I'd like to
transition away from that at some point, given that it's tied to petgraph
and also pulls in xmlns, on top of quick-xml and our XIR, but that can come
down the line.
The Options here are awkward and will be able to go away in the new reader
and in AsgBuilder once it has a proper state machine.
This gets rid of some of the initial migratory work for the new reader,
because PackageAttrs is gone. I'm going to wait to update this to the new
way until I get further into this.
DEV-11449
I'm finally back to TAMER development.
The original plan, some time ago, was to gate an entirely new XmloReader
behind a feature flag (wip-xmlo-xir-reader), and go from there, leaving the
existing implementation untouched. Unfortunately, it became too difficult
and confusing to marry the old aggregate API with the new streaming one.
AsgBuilder is the only system interacting with XmloReader, so I decided (see
previous commits) to just go the route of refactoring the existing
one. I'm not yet sure if I'll continue to progressively refactor this one
and eliminate the two separate implementations behind the flag, or if I'll
get this API similar and then keep the flag and reimplement it. But I'll
know soon.
DEV-11449
This is simply not worth it; the size is not going to be the bottleneck (at
least any time soon) and the generic not only pollutes all the things that
will use ASG in the near future, but is also incompatible with the SymbolId
default that is used everywhere; if we have to force it to 32 bits anyway,
then we may as well just default it right off the bat.
I thought that this seemed like a good idea at the time, and saving bits is
certainly tempting, but it was premature.
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 will allow Parser to operate on both owned and &mut values, and is the
same approach that Rust's built-in iterators take.
This is at first quite surprising, and I often forget that this is a
feature, and, as a bonus, an attractive way to avoid lifetimes in struct
definitions when generics are used for the type that may become a
reference.
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 stores the last seen Span and uses that when reporting EOF, so that the
user will be able to be notified of where exactly the problem occurred.
When I get into creating combinators, it'll be the responsibility of those
combinators to ensure that any None return value will be supplemented by its
own last span.
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.
There are a number of reasons for this, where the benefits do not make up
for the losses.
First: this is actually invoking cargo. Not only is this not necessary, but
it's not desirable: cargo by default hits the network and does all sorts of
other stuff, when all we want to do is invoke the executable. So the tests
aren't really testing the right thing in that sense. See the previous
commit for more information.
The way it invokes cargo is different than the way the Makefile invokes
cargo, so on my system, it's actually invoking a _different cargo_! This is
causing problems, in particular with lock files, which causes my tests to
fail.
Importantly, this also removes a _lot_ of dependencies, which removes a lot
of supplier chain risk and a lot of code to audit. This provides
significant security benefits, especially given that what was being tested
was rather small, and could be done in a shell script.
TAMER will receive significant system testing later on. But for now, none
of this was worth it.
Further audits of dependencies will come later on. I've always been fairly
insistent on keeping the dependency graph small and auditable, but recent
supply chain attacks have given me a better way to rationalize the security
risk. Further, I'm the only one on this project right now.
Cargo's default behavior is unfortunately to issue network calls each time
it is invoke in order to check for dependencies updates. This is not only
bad for reproducibility and privacy, but it's also a concern for supply
chain attacks, since most developers are unaware that this is occurring.
Instead, we pin to the lockfile. Installing dependencies can be done with
`cargo fetch` and updating dependencies must be explicitly done by the
developer, with the lockfile updated.
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.
To maintain a proper abstraction, this cannot be the responsibility of the
caller; most callers should not know that fragments exist, letalone how to
handle them.
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 adds a constant `ST_COUNT` representing the number of statically
allocated symbols, and uses that to estimate an initial capacity for the
`CachingEscaper`.
This is just a guess (and is certainly too low), but we can adjust later on
after profiling, if it ever comes up.
Mike Gerwitz