This removes support for configurable repetition.
What? Why?
As it turns out, the complexity that repetition adds is quite significant
and is not worth the effort. The truth is that NIR is going to have to
allow zero-or-more matches on virtually everything _anyway_ because template
application is allowed virtually anywhere---it is not possible to fully
statically analyze TAME's sources because templates can expand into just
about anything. Given that, AIR (or something down the line) is going to
have to supply the necessary invariants instead.
It does suck, though, that this removes a lot of code that I fairly recently
wrote, and spent a decent amount of time on. But it's important to know
when to cut your losses.
Perhaps I could have planned better, but deriving this whole system as been
quite the experiment.
DEV-7145
If attributes fail to parse (e.g. missing required attribute) and parsing
reaches a dead state, this will recover by ignoring the entire element. It
previously panicked with a TODO.
DEV-7145
These were initially used to prevent conflicts with generated variants, but
we are no longer generating such variants since they're being jumped to via
the trampoline.
DEV-7145
I'm starting to clean up some TODOs, and this was a glaring one causing
panics when encountered. The recovery for this is simple, because we have
no choice: just stop parsing; leave it to the next lowering operation(s) to
complain that we didn't provide what was necessary. They'll have to,
anyway, since templates mean that NIR cannot ever have enough information to
guarantee that a document is well-formed, relative to what would expand from
the template.
DEV-7145
This allows for a construction like this:
```
ele_parse! {
[...]
StmtX := QN_X {
[...]
};
StmtY := QN_Y {
[...]
};
ExprA := QN_A {
[...]
};
ExprB := QN_B {
[...]
};
Expr := (A | B);
Stmt := (StmtX | StmtY);
// This previously was not allowed:
StmtOrExpr := (Stmt | Expr);
}
```
There were initially two barriers to doing so:
1. Efficiently matching; and
2. Outputting diagnostic information about the union of all expected
elements.
The first was previously resolved with the introduction of `NT::matches`,
which is macro-expanded in a way that Rust will be able to optimize a
bit. Worst case, it's effectively a linear search, but our Sum NTs are not
that deep in practice, so I don't expect that to be a concern.
The concern that I was trying to avoid was heap-allocated `NodeMatcher`s to
avoid recursive data structures, since that would have put heap access in a
very hot code path, which is not an option.
That left problem #2, which ended up being the harder problem. The solution
was detailed in the previous commit, so you should look there, but it
amounts to being able to format individual entries as if they were a part
of a list by making them a function of not just the matcher itself, but also
the number of items in (recursively) the sum type and the position of the
matcher relative to that list. The list length is easily
computed (recursively) at compile-time using `const`
functions (`NT::matches_n`).
And with that, NIR can be abstracted in sane ways using Sum NTs without a
bunch of duplication that would have been a maintenance burden and an
inevitable source of bugs (from having to duplicate NT references).
DEV-7145
This allows using a `[attr]` special form to stream attributes as they are
encountered rather than aggregating a static attribute list. This is
necessary in particular for short-hand template application and short-hand
function application, since the attribute names are derived from template
and function parameter lists, which are runtime values.
The syntax for this is a bit odd since there's a semi-useless and confusing
`@ {} => obj` still, but this is only going to be used by a couple of NTs
and it's not worth the time to clean this up, given the rather significant
macro complexity already.
DEV-7145
This uses the same mechanism that was introduced for handling `Text` nodes
in mixed content, allowing for arbitrary element `Open` matches for
preemption by the superstate.
This will be used to allow for template expansion virtually
anywhere. Unlike the existing TAME, it'll even allow for it at the root,
though whether that's ultimately permitted is really depending on how I
approach template expansion; it may fail during a later lowering operation.
This is interesting because this approach is only possible because of the
CPS-style trampoline implementation. Previously, with the composition-based
approach, each and every parser would have to perform this check, like we
had to previously with `Text` nodes.
As usual, this is still adding to the mess a bit, and it'll need some future
cleanup.
DEV-7145
This introduces the concept of superstate node preemption generally, which I
hope to use for template application as well, since templates can appear in
essentially any (syntatically valid, for XML) position.
This implements mixed content handling by defining the mapping on the
superstate itself, which really simplifies the problem but foregoes
fine-grained text handling. I had hoped to avoid that, but oh well.
This pushes the responsibility of whether text is semantically valid at that
position to NIR->AIR lowering (which we're not transition to yet), which is
really the better place for it anyway, since this is just the grammar. The
lowering to AIR will need to validate anyway given that template expansion
happens after NIR.
Moving on!
DEV-7145
This was accepting an early EOF when the active child `ParseState` was in an
accepting state, because it was not ensuring that anything on the stack was
also accepting.
Ideally, there should be nothing on the stack, and hopefully in the future
that's what happens. But with how things are today, it's important that, if
anything is on the stack, it is accepting.
Since `is_accepting` on the superstate is only called during finalization,
and because the check terminates early, and because the stack practically
speaking will only have a couple things on it max (unless we're in tail
position in a deeply nested tree, without TCO [yet]), this shouldn't be an
expensive check.
Implementing this did require that we expose `Context` to `is_accepting`,
which I had hoped to avoid having to do, but here we are.
DEV-7145
Along with this change we also had to change how we handle dead states in
the superstate. So there were two problems here:
1. Sum states were not yielding a dead state after recovery, which meant
that parsing was unable to continue (we still have a `todo!`); and
2. The superstate considered it an error when there was nothing left on
the stack, because I assumed that ought not happen.
Regarding #2---it _shouldn't_ happen, _unless_ we have extra input after we
have completed parsing. Which happens to be the case for this test case,
but more importantly, we shouldn't be panicing with errors about TAMER bugs
if somebody puts extra input after a closing root tag in a source file.
DEV-7145
This does two things:
1. Places the expected list on a separate help line as a footnote where
it'll be a bit more tolerable when it inevitably overflows the terminal
width in certain contexts (we may wrap in the future); and
2. Removes angled brackets from the element names so that they (a) better
correspond with the span which highlights only the element name and (b)
do not imply that the elements take no attributes.
DEV-7145
When we match a QName against a namespace, we ought to store the matching
QName to use (a) in error messages and (b) to make available as a
binding. The former is necessary for sensible errors (rather than saying
that it's e.g. expecting a closing `t:*`) and the latter is necessary for
e.g. getting the template name out of `t:foo`.
DEV-7145
This allows matching on a namespace prefix by providing a `Prefix` instead
of a `QName`. This works, but is missing a couple notable things (and
possibly more):
1. Tracking the QName that is _actually_ matched so that it can be used in
messages stating what the expected closing tag is; and
2. Making that QName available via a binding.
This will be used to match on `t:*` in NIR. If you're wondering how
attribute parsing is supposed to work with that (of course you're wondering
that, random person reading this)---that'll have to work differently for
those matches, since template shorthand application contains argument names
as attributes.
DEV-7145
This introduces `NodeMatcher`, with the intent of introducing wildcard QName
matches for e.g. `t:*` nodes. It's not yet clear if I'll expand this to
support text nodes yet, or if I'll convert text nodes into elements to
re-use the existing system (which I had initially planned on doing, but
didn't because of the work and expense (token expansion) involved in the
conversion).
DEV-7145
I need to move on, and there are (a) a couple different ways to proceed that
I want to mull over and (b) upcoming changes that may influence my decision
one way or another.
DEV-7145
This will utilize the superstate's error object in place of nested errors,
which was the result of the previous composition-based delegation.
As you can see, all we had to do was remove the special handling of these
errors; the existing delegation setup continues to handle the types properly
with no change. The composition continues to work for `*Attr_`.
The alternative was to box inner errors, since they're far from the hot code
path, but that's clearly unnecessary.
To be clear: this is necessary to allow for recursive grammars in
`ele_parse` without creating recursive data structures in Rust.
DEV-7145
Comments ought not have any more semantic meaning than whitespace. Other
languages may have conventions that allow for various types of things in
comments, like annotations, but those are symptoms of language
limitations---we control the source language here.
DEV-7145
This properly integrates the trampoline into `ele_parse!`. The
implementation leaves some TODOs, most notably broken mixed text handling
since we can no longer intercept those tokens before passing to the
child. That is temporarily marked as incomplete; see a future commit.
The introduced test `ParseState`s were to help me reason about the system
intuitively as I struggled to track down some type errors in the monstrosity
that is `ele_parse!`. It will fail to compile if those invariants are
violated. (In the end, the problems were pretty simple to resolve, and the
struggle was the type system doing its job in telling me that I needed to
step back and try to reason about the problem again until it was intuitive.)
This keeps around the NT states for now, which are quickly used to
transition to the next NT state, like a couple of bounces on a trampoline:
NT -> Dead -> Parent -> Next NT
This could be optimized in the future, if it's worth doing.
This also makes no attempt to implement tail calls; that would have to come
after fixing mixed content and really isn't worth the added complexity
now. I (desperately) need to move on, and still have a bunch of cleanup to
do.
I had hoped for a smaller commit, but that was too difficult to do with all
the types involved.
DEV-7145
I had previously used `Context` to hold the parser configuration for
repetition, since that was the easier option. But I now want to utilize the
`Context` for a stack for the superstate trampoline, and I don't want to
have to deal with the awkwardness of the repetition in doing so, since it
requires that the configuration be created during delegation, rather than
just being passed through to all child parsers.
This adds to a mess that needs cleaning up, but I'll do that after
everything is working.
DEV-7145
And here's the thing that I've been dreading, partly because of the
`macro_rules` issues involved. But, it's not too terrible.
This module was already large and complex, and this just adds to it---it's
in need of refactoring, but I want to be sure it's fully working and capable
of handling NIR before I go spending time refactoring only to undo it.
_This does not yet use trampolining in place of the call stack._ That'll
come next; I just wanted to get the macro updated, the superstate generated,
and tests passing. This does convert into the
superstate (`ParseState::Super`), but then converts back to the original
`ParseState` for BC with the existing composition-based delegation. That
will go away and will then use the equivalent of CPS, using the
superstate+`Parser` as a trampoline. This will require an explicit stack
via `Context`, like XIRF. And it will allow for tail calls, with respect to
parser delegation, if I decide it's worth doing.
The root problem is that source XML requires recursive parsing (for
expressions and statements like `<section>`), which results in recursive
data structures (`ParseState` enum variants). Resolving this with boxing is
not appropriate, because that puts heap indirection in an extremely hot code
path, and may also inhibit the aggressive optimizations that I need Rust to
perform to optimize away the majority of the lowering pipeline.
Once this is sorted out, this should be the last big thing for the
parser. This unfortunately has been a nagging and looming issue for months,
that I was hoping to avoid, and in retrospect that was naive.
DEV-7145
I'm disappointed that I keep having to implement features that I had hoped
to avoid implementing.
This introduces a "superstate" feature, which is intended really just to be
a sum type that is able to delegate to stitched `ParseState`s. This then
allows a `ParseState` to transition directly to another `ParseState` and
have the parent `ParseState` handle the delegation---a trampoline.
This issue naturally arises out of the recursive nature of parsing a TAME
XML document, where certain statements can be nested (like `<section>`), and
where expressions can be nested. I had gotten away with composition-based
delegation for now because `xmlo` headers do not have such nesting.
The composition-based approach falls flat for recursive structures. The
typical naive solution is boxing, which I cannot do, because not only is
this on an extremely hot code path, but I require that Rust be able to
deeply introspect and optimize away the lowering pipeline as much as
possible.
Many months ago, I figured that such a solution would require a trampoline,
as it typically does in stack-based languages, but I was hoping to avoid
it. Well, no longer; let's just get on with it.
This intends to implement trampolining in a `ParseState` that serves as that
sum type, rather than introducing it as yet another feature to `Parser`; the
latter would provide a more convenient API, but it would continue to bloat
`Parser` itself. Right now, only the element parser generator will require
use of this, so if it's needed beyond that, then I'll debate whether it's
worth providing a better abstraction. For now, the intent will be to use
the `Context` to store a stack that it can pop off of to restore the
previous `ParseState` before delegation.
DEV-7145
"Mixed content" is the XML term representing element nodes mixed with text
nodes. For example, `foo <strong>bar</strong> baz` is mixed.
TAME supports text nodes as documentation, intended to be in a literate
style but never fully realized. In any case, we need to permit them, and I
wanted to do more than just ignore the nodes.
This takes a different approach than typical parser delegation---it has the
parent parser _preempt_ the child by intercepting text before delegation
takes place, rather than having the child reject the token (or possibly
interpret it itself!) and have to handle an error or dead state.
And while this makes it more confusing in terms of state machine stitching,
it does make sense, in the sense that the parent parser is really what
"owns" the text node---the parser is delegating _element_ parsing only, take
asserts authority when necessary to take back control where it shouldn't be
delegated.
DEV-7145
Previously a `Depth` was provided only for `Open` and `Close`. This depth
information, for example, will be used by NIR to quickly determine whether a
given parser ought to assert ownership of a text/comment token rather than
delegating it.
This involved modifying a number of test cases, but it's worth repeating in
these commits that this is intentional---I've been bit in the past using
`..` in contexts where I really do want to know if variant fields change so
that I can consider whether and how that change may affect the code
utilizing that variant.
DEV-7145
Recent changes regarding whitespace were all to support this change (though
it was also needed for XIRF, pre- and post-root).
Now I'll have to conted with how I want to handle text nodes in various
circumstances, in terms of `ele_parse!`.
DEV-7145
Various DUMMY_SPAN-derived spans are used by many test cases, so this
finally extracts them---something I've been meaning to do for some time.
This also places DUMMY_SPAN behind a `cfg(test)` directive to ensure that it
is _only_ used in tests; UNKNOWN_SPAN should be used when a span is actually
unknown, which may also be the case during development.
DEV-7145
Whether or not quoting is appropriate depends on context, and that parent
context is already performing the quoting. For example:
error: expected `</rater>`, but found `<import>`
--> /home/[...]/foo.xml:2:1
|
2 | <rater xmlns="http://www.lovullo.com/rater"
| ------ note: element starts here
--> /home/[...]/foo.xml:7:3
|
7 | <import package="/rater/core/base" />
| ^^^^^^^ error: expected `</rater>`
In these cases (obviously I'm still working on the parser, since this is
nonsense), the parser is responsible for quoting the token "<import>".
DEV-7145
There were two problem errors: one showing "element element" and one showing
the value along with the name of the attribute.
The change for `<Attr as Display>::fmt` is debatable. I'm going to do this
for now (only show `@name`) and adjust later if necessary.
I'll need to go use `crate::fmt` consistently in previously-existing format
strings at some point, too.
DEV-7145
This teaches XIRF to optionally refine Text into RefinedText, which
determines whether the given SymbolId represents entirely whitespace.
This is something I've been putting off for some time, but now that I'm
parsing source language for NIR, it is necessary, in that we can only permit
whitespace Text nodes in certain contexts.
The idea is to capture the most common whitespace as preinterned
symbols. Note that this heuristic ought to be determined from scanning a
codebase, which I haven't done yet; this is just an initial list.
The fallback is to look up the string associated with the SymbolId and
perform a linear scan, aborting on the first non-whitespace character. This
combination of checks should be sufficiently performant for now considering
that this is only being run on source files, which really are not all that
large. (They become large when template-expanded.) I'll optimize further
if I notice it show up during profiling.
This also frees XIR itself from being concerned by Whitespace. Initially I
had used quick-xml's whitespace trimming, but it messed up my span
calculations, and those were a pain in the ass to implement to begin with,
since I had to resort to pointer arithmetic. I'd rather avoid tweaking it.
tameld will not check for whitespace, since it's not important---xmlo files,
if malformed, are the fault of the compiler; we can ignore text nodes except
in the context of code fragments, where they are never whitespace (unless
that's also a compiler bug).
Onward and yonward.
DEV-7145
Because of recovery, the trace otherwise paints a really confusing-looking
picture when given unexpected input.
This is large enough now that it really ought to be extracted from
`feed_tok`, but I'll wait to see how this evolves further. I considered
adding color too, but it's not yet clear to me that the visual noise will be
all that helpful.
DEV-7145
impl for `&Token` instead of Token; the writer is just copying data into the
destination stream anyway.
This will allow us to continue writing the token while also using it for
further processing, like `tee`.
DEV-7145
We need to be able to export generated identifiers. Trying to figure out a
syntax for this was a bit tricky considering how much is generated, so I
just settled on something that's reasonably clear and easy to parse with
`macro_rules!`.
I had intended to just make everything public by default and encapsulate
using private modules, but that then required making everything else that it
uses public (e.g. error and token objects), which would have been a bizarre
thing to do in e.g. test cases.
DEV-7145
Values can be parsed using `TryFrom<Attr>`. Previously only `From<Attr>`
was supported, which could not fail.
This is critical for parsing values into types, which will wrap `SymbolId`
to provide data assurances.
DEV-7145
The tests had certain things in scope, but now that I'm trying to use it
outside of those modules, some fixes are needed.
This is admittedly a sloppy commit, with a number of miscellaneous fixes. I
didn't bother separating it more because most of them are type fixes, and
the `From<Attr>` stuff is going to have to change into, likely,
`TryFrom<Attr>` so that parse failures can occur when attributes do not
match certain patterns.
DEV-7145
The only additional information needed was opening spans so that we can
provide useful information regarding closing tags.
This uses a generic Span in place of {Open,Close}Span because the latter
wasn't necessary, but more descriptive types would be nice; it may be
beneficial later on to introduce newtypes for each of the span generated by
{Open,Close}Span.
DEV-7145
This was a TODO for the attribute parser generator. The first attribute
will be kept and later ones will be ignored, producing an error. Recovery
permits further attribute parsing having ignored the duplicate.
DEV-7145
This allows an element to be repeated by the parent NT. The easiest way I
saw to implement this for now was to abuse the Context to provide a runtime
configuration that would allow the state machine to reset after it has
completed parsing.
This also influences error recovery, in that if we're expecting zero or more
of something, we cannot provide an error for an unexpected name, and instead
must emit a dead state so that the caller can determine what to do.
DEV-7145
This produces useful parse traces that are output as part of a failing test
case. The parser generator macros can be a bit confusing to deal with when
things go wrong, so this helps to clarify matters.
This is _not_ intended to be machine-readable, but it does show that it
would be possible to generate machine-readable output to visualize the
entire lowering pipeline. Perhaps something for the future.
I left these inline in Parser::feed_tok because they help to elucidate what
is going on, just by reading what the trace would output---that is, it helps
to make the method more self-documenting, albeit a tad bit more
verbose. But with that said, it should probably be extracted at some point;
I don't want this to set a precedent where composition is feasible.
Here's an example from test cases:
[Parser::feed_tok] (input IR: XIRF)
| ==> Parser before tok is parsing attributes for `package`.
| | Attrs_(SutAttrsState_ { ___ctx: (QName(None, LocalPart(NCName(SymbolId(46 "package")))), OpenSpan(Span { len: 0, offset: 0, ctx: Context(SymbolId(1 "#!DUMMY")) }, 10)), ___done: false })
|
| ==> XIRF tok: `<unexpected>`
| | Open(QName(None, LocalPart(NCName(SymbolId(82 "unexpected")))), OpenSpan(Span { len: 0, offset: 1, ctx: Context(SymbolId(1 "#!DUMMY")) }, 10), Depth(1))
|
| ==> Parser after tok is expecting opening tag `<classify>`.
| | ChildA(Expecting_)
| | Lookahead: Some(Lookahead(Open(QName(None, LocalPart(NCName(SymbolId(82 "unexpected")))), OpenSpan(Span { len: 0, offset: 1, ctx: Context(SymbolId(1 "#!DUMMY")) }, 10), Depth(1))))
= note: this trace was output as a debugging aid because `cfg(test)`.
[Parser::feed_tok] (input IR: XIRF)
| ==> Parser before tok is expecting opening tag `<classify>`.
| | ChildA(Expecting_)
|
| ==> XIRF tok: `<unexpected>`
| | Open(QName(None, LocalPart(NCName(SymbolId(82 "unexpected")))), OpenSpan(Span { len: 0, offset: 1, ctx: Context(SymbolId(1 "#!DUMMY")) }, 10), Depth(1))
|
| ==> Parser after tok is attempting to recover by ignoring element with unexpected name `unexpected` (expected `classify`).
| | ChildA(RecoverEleIgnore_(QName(None, LocalPart(NCName(SymbolId(82 "unexpected")))), OpenSpan(Span { len: 0, offset: 1, ctx: Context(SymbolId(1 "#!DUMMY")) }, 10), Depth(1)))
| | Lookahead: None
= note: this trace was output as a debugging aid because `cfg(test)`.
DEV-7145
This resolves a TODO by including the name of the element whose attributes
are currently being parsed.
This also frees a parent from having to provide additional context, allowing
Display to be fully delegated when stitching.
DEV-7145
This introduces `Nt := (A | ... | Z);`, where `Nt` is the name of the
nonterminal and `A ... Z` are the inner nonterminals---it produces a parser
that provides a choice between a set of nonterminals.
This is implemented efficiently by understanding the QName that is accepted
by each of the inner nonterminals and delegating that token immediately to
the appropriate parser. This is a benefit of using a parser generator macro
over parser combinators---we do not need to implement backtracking by
letting inner parsers fail, because we know ahead of time exactly what
parser we need.
This _does not_ verify that each of the inner parsers accept a unique QName;
maybe at a later time I can figure out something for that. However, because
this compiles into a `match`, there is no ambiguity---like a PEG parser,
there is precedence in the face of an ambiguous token, and the first one
wins. Consequently, tests would surely fail, since the latter wouldn't be
able to be parsed.
This also demonstrates how we can have good error suggestions for this
parsing framework: because the inner nonterminals and their QNames are known
at compile time, error messages simply generate a list of QNames that are
expected.
The error recovery strategy is the same as previously noted, and subject to
the same concerns, though it may be more appropriate here: it is desirable
for the inner parser to fail rather than retrying, so that the sum parser is
able to fail and, once the Kleene operator is introduced, retry on another
potential element. But again, that recovery strategy may happen to work in
some cases, but'll fail miserably in others (e.g. placing an unknown element
at the head of a block that expects a sequence of elements would potentially
fail the entire block rather than just the invalid one). But more to come
on that later; it's not critical at this point. I need to get parsing
completed for TAME's input language.
DEV-7145
This adds the ability to bind identifiers to represent `OpenSpan` and
`CloseSpan`, available to the `@` and `/` maps. Since identifiers in TAME
originate from attributes, this may not get a whole lot of use, but it's
important to be available.
There is some awkwardness in that the opening span appears to be scoped to
the entire nonterminal, but it's actually only available in the `@`
mapping. I'll change this if it's actually needed; this keeps things simple
for now.
DEV-7145
Mike Gerwitz