The previous commit had the ASG implicitly constructed and then
discarded. This will keep it around, which will be necessary not only for
imports, but for passing the ASG off to the next phases of lowering.
DEV-13429
This does not yet yield the produces ASG, but does set up the lowering
pipeline to prepare to produce it. It's also currently a no-op, with
`NirToAsg` just yielding `Incomplete`.
The goal is to begin to move toward vertical slices for TAMER as I start to
return to the previous approach of a handoff with the old compiler. Now
that I've gained clarity from my previous failed approach (which I
documented in previous commits), I feel that this is the best way forward
that will allow me to incrementally introduce more fine-grained performance
improvements, at the cost of some throwaway work as this progresses. But
the cost of delay with these build times is far greater.
DEV-13429
This finalizes the implementation for interpolation. There is some more
cleanup that can be done, but it is now functioning as intended and
providing errors.
Finally. How deeply exhausting all of this has been.
DEV-13156
This just cleans up these tests a bit before I add to them. What we're left
with follows the structure of most other parser tests and is atm a good
balance between boilerplate and clarity in isolation (a fair level of
abstraction).
Could possibly do better by putting the inner objects in a callback so that
the `Close` can be asserted on commonly as well, but that's a bit awkward
with how the assertion is based on the collection; we'd have to keep the
last item from being collected from the iterator. I'd rather not deal with
such restructuring right now and figuring out a decent pattern. Perhaps in
the future.
DEV-13156
This is the culmination of all the recent work---the third attempt at trying
to integrate this. It ended up much cleaner than what was originally going
to be done, but only after gutting portions of the system and changing my
approach to how NIR is parsed (WRT attributes). See prior commits for more
information.
The final step is to fill the error branches with actual errors rather than
`todo!`s.
What a relief.
DEV-13156
This begins to introduce the new, simplified NIR by creating tokens that
serve as the expansion for interpolation. Admittedly, `Text` may change, as
it doesn't really represent `<text>foo</text>`, and I'd rather that node
change as well, though I'll probably want to maintain some sort of BC.
DEV-13156
This removes quite a bit of work, and work that was difficult to reason
about. While I'm disappointed that that hard work is lost (aside from
digging it up in the commit history), I am happy that it was able to be
removed, because the extra complexity and cognitive burden was significant.
This removes more `memcpy`s than the sum state could have hoped to, since
aggregation is no longer necessary. Given that, there is a slight
performacne improvement. The re-introduction of required and duplicate
checks later on should be more efficient than this was, and so this should
be a net win overall in the end.
DEV-13346
This cleans up the old implementation now that it's no longer used (as of
the previous commit) by `ele_parse!`. It also removes the two error
variants that no longer apply: required attributes and duplicate
attributes.
DEV-13346
This handles the bulk of the integration of the new `attr_parse_stream!` as
a replacement for `attr_parse!`, which moves from aggregate attribute
objects to a stream of attribute-derived tokens. Rationale for this change
is in the preceding commit messages.
The first striking change here is how it affects the test cases: nearly all
`Incomplete`s are removed. Note that the parser has an existing
optimization whereby `Incomplete` with lookahead causes immediate recursion
within `Parser`, since those situations are used only for control flow and
to keep recursion out of `ParseState`s.
Next: this removes types from `nir::parse`'s grammar for attributes. The
types will instead be derived from NIR tokens later in the lowering
pipeline. This simplifies NIR considerably, since adding types into the mix
at this point was taking an already really complex lowering phase and making
it ever more difficult to reason about and get everything working together
the way that I needed.
Because of `attr_parse_stream!`, there are no more required attribute
checks. Those will be handled later in the lowering pipeline, if they're
actually needed in context, with possibly one exception: namespace
declarations. Those are really part of the document and they ought to be
handled _earlier_ in the pipeline; I'll do that at some point. It's not
required for compilation; it's just required to maintain compliance with the
XML spec.
We also lose checks for duplicate attributes. This is also something that
ought to be handled at the document level, and so earlier in the pipeline,
since XML cares, not us---if we get a duplicate attribute that results in an
extra NIR token, then the next parser will error out, since it has to check
for those things anyway.
A bunch of cleanup and simplification is still needed; I want to get the
initial integration committed first. It's a shame I'm getting rid of so
much work, but this is the right approach, and results in a much simpler
system.
DEV-13346
This really does need documentation.
With that said, this changes things up a bit: the value is now derived from
an `SPair` rather than an `Attr`, given that the name is redundant. We do
not need the attribute name span, since the philosophy is that we're
stripping the document and it should no longer be important beyond the
current context.
It does call into question errors, but my intent in the future is to be able
to have the lowering pipline augment errors with its current state---since
we're streaming, then an error that is encountered during lowering of an
element will still have the element parser in the state representing the
parsing of that element; so that information does not need to be propagated
down the pipeline, but can be augmented as it bubbles back up.
More on that at some point in the future; not right now.
DEV-13346
As I talked about in the previous commit, this is going to be the
replacement for the aggreagte `attr_parse!`; the next commit will integrate
it into `ele_parse!` so that I can begin to remove the old one.
It is disappointing, since I did put a bit of work into this and I think the
end result was pretty neat, even if was never fully utilized. But, this
simplifies things significantly; no use in maintaining features that serve
no purpose but to confound people.
DEV-13346
Alright, this has been a rather tortured experience. The previous commit
began to state what is going on.
This is reversing a lot of prior work, with the benefit of
hindsight. Little bit of history, for the people who will probably never
read this, but who knows:
As noted at the top of NIR, I've long wanted a very simple set of general
primitives where all desugaring is done by the template system---TAME is a
metalanguage after all. Therefore, I never intended on having any explicit
desugaring operations.
But I didn't have time to augment the template system to support parsing on
attribute strings (nor am I sure if I want to do such a thing), so it became
clear that interpolation would be a pass in the compiler. Which led me to
the idea of a desugaring pass.
That in turn spiraled into representing the status of whether NIR was
desugared, and separating primitives, etc, which lead to a lot of additional
complexity. The idea was to have a Sugared and Plan NIR, and further within
them have symbols that have latent types---if they require interpolation,
then those types would be deferred until after template expansion.
The obvious problem there is that now:
1. NIR has the complexity of various types; and
2. Types were tightly coupled with NIR and how it was defined in terms of
XML destructuring.
The first attempt at this didn't go well: it was clear that the symbol types
would make mapping from Sugared to Plain NIR very complicated. Further,
since NIR had any number of symbols per Sugared NIR token, interpolation was
a pain in the ass.
So that lead to the idea of interpolating at the _attribute_ level. That
seemed to be going well at first, until I realized that the token stream of
the attribute parser does not match that of the element parser, and so that
general solution fell apart. It wouldn't have been great anyway, since then
interpolation was _also_ coupled to the destructuring of the document.
Another goal of mine has been to decouple TAME from XML. Not because I want
to move away from XML (if I did, I'd want S-expressions, not YAML, but I
don't think the team would go for that). This decoupling would allow the
use of a subset of the syntax of TAME in other places, like CSVMs and YAML
test cases, for example, if appropriate.
This approach makes sense: the grammar of TAME isn't XML, it's _embedded
within_ XML. The XML layer has to be stripped to expose it.
And so that's what NIR is now evolving into---the stripped, bare
repsentation of TAME's language. That also has other benefits too down the
line, like a REPL where you can use any number of syntaxes. I intend for
NIR to be stack-based, which I'd find to be intuitive for manipulating and
querying packages, but it could have any number of grammars, including
Prolog-like for expressing Horn clauses and querying with a
Prolog/Datalog-like syntax. But that's for the future...
The next issue is that of attribute types. If we have a better language for
NIR, then the types can be associated with the NIR tokens, rather than
having to associate each symbol with raw type data, which doesn't make a
whole lot of sense. That also allows for AIR to better infer types and
determine what they ought to be, and further makes checking types after
template application natural, since it's not part of NIR at all. It also
means the template system can naturally apply to any sources.
Now, if we take that final step further, and make attributes streaming
instead of aggregating, we're back to a streaming pipeline where all
aggregation takes place on the ASG (which also resolves the memcpy concerns
worked around previously, also further simplifying `ele_parse` again, though
it sucks that I wasted that time). And, without the symbol types getting
in the way, since now NIR has types more fundamentally associated with
tokens, we're able to interpolate on a token stream using simple SPairs,
like I always hoped (and reverted back to in the previous commit).
Oh, and what about that desugaring pass? There's the issue of how to
represent such a thing in the type system---ideally we'd know statically
that desugaring always lowers into a more primitive NIR that reduces the
mapping that needs to be done to AIR. But that adds complexity, as
mentioned above. The alternative is to just use the templat system, as I
originally wanted to, and resolve shortcomings by augmenting the template
system to be able to handle it. That not only keeps NIR and the compiler
much simpler, but exposes more powerful tools to developers via TAME's
metalanguage, if such a thing is appropriate.
Anyway, this creates a system that's far more intuitive, and far
simpler. It does kick the can to AIR, but that's okay, since it's also
better positioned to deal with it.
Everything I wrote above is a thought dump and has not been proof-read, so
good luck! And lets hope this finally works out...it's actually feeling
good this time. The journey was necessary to discover and justify what came
out of it---everything I'm stripping away was like a cocoon, and within it
is a more beautiful and more elegant TAME.
DEV-13346
Also: Revert "tamer: nir::desugar::interp: Token {SPair=>Attr}"
This reverts commit 7fd60d6cdafaedc19642a3f10dfddfa7c7ae8f53.
This reverts commit 12a008c66414c3d628097e503a98c80687e3c088.
This has been quite a tortured experience, trying to figure out how to best
fit desugaring into the existing system. The truth is that it ultimately
failed because I was not sticking with my intuition---I was trying to get
things out quickly by compromising on the design, and in the end, it saved
me nothing.
But I wouldn't say that it was a waste of time---the path was a dead end,
but it was full of experiences.
More to come, but interpolation is back to operating on NIR directly, and I
chose to treat it as a source-to-source mapping and not represent it using
the type system---interpolation can be an optional feature when writing TAME
frontends (the principal one being the XML-based one), and it's up to later
checks to assert that identifiers match a given domain.
I am disappointed by the additional context we lose here, but that can
always be introduced in the future differently, e.g. by maintaining a
dictionary of additional context for spans that can be later referenced for
diagnostic purposes. But let's worry about that in the future; it doesn't
make sense to further complicate IRs for such a thing.
DEV-13346
Converts to use TAME's diagnostic panics, same as previous commits. Also
introduces impl for `Result`, which I apparently hadn't needed yet.
In the future, I hope trait impl specializations will be available to
automatically derive and expose span information in these diagnostic
messages for certain types.
DEV-13156
This changes the input token from a more generic `SPair` to `Attr`, which
reflects the new target integration point in the `attr_parse!`
parser-generator.
This is a compromise---I'd like for it to remain generic and have stitching
deal with all integration concerns, but I have spent far too much time on
this and need to keep moving.
With that said, we do benefit from knowing where this must fit in---it's
easier to reason about in a more concrete way, and we can take advantage of
the extra information rather than being burdened by its presence and
ignoring it. We need to be able to convert back into `XirfToken` (see a
recent commit that discusses that) for `StitchExpansion`, which is why
`Attr` is here. And since it is, we can use it to explain to the user not
just the interpolation specification used to derive params, but also the
attribute it is associated with. This is what TAME (in XSLT) does today,
IIRC (I wrote it, I just forget exactly). It also means that I can name the
parameters after the attribute.
So, that'll be in a following commit; I was disappointed when my prior
approach with `SPair` didn't give me enough information to be able to do
that, since I think it's important that the system be as descriptive as
possible in how it derives information. Of course, traces would reveal how
the parser came about the derivation, but that requires recompilation in a
special tracing mode.
DEV-13156
Of course I would run into integration issues. My foresight is lacking.
The purpose of this is to allow for type narrowing before passing data to a
more specialized ParseState, so that the other ParseState doesn't need to
concern itself with the entire domain of inputs that it doesn't need, and
repeat unnecessary narrowing.
For example, consider XIRF: it has an `Attr` variant, which holds an `Attr`
object. We'll want to desugar that object. It does not make sense to
require that the desugaring process accept `XirfToken` when we've already
narrowed it to an `Attr`---we should accept an Attr.
However, we run into a problem immediately: what happens with tokens that
bubble back up due to lookahead or errors? Those tokens need to be
converted _back_ (widened). Fortunately, widening is a much easier process
than narrowing---we can simply use `From`, as we do today so many other
places.
So, this still keeps the onus of narrowing on the caller, but for now that
seems most appropriate. I suspect Rust would optimize away duplicate
checks, but that still leaves the maintenance concern---the two narrowings
could get out of sync, and that's not acceptable.
Unfortunately, this is just one of the problems with integration...
DEV-13156
My initial plan with expansion was to wrap a `PasteState` in another that
unwraps `Expansion` and converts into a `Dead` state, so that existing
`TransitionResult` stitching methods (`delegate`, specifically) could be
used.
But the desire to use that existing method was primarily because stitching
was a complex operation that was abstracted away _as part of the `delegate`
method_, which made writing new ones verbose and difficult. Thus began the
previous commits to begin to move that responsibility elsewhere so that it
could be more composable.
This continues with that, introducing a new trait that will culminate in the
removal of a wrapping `ParseState` in favor of a stitching method. The old
`StitchableExpansionState` is still used for tests, which demonstrates that
the boilerplate problem still exists despite improvements made here These
will become more generalized in the future as I have time (and the
functional aspects of the code more formalized too, now that they're taking
shape).
The benefit of this is that we avoid having to warp our abstractions in ways
that don't make sense (use of a dead state transition) just to satisfy
existing APIs. It also means that we do not need the boilerplate of a
`ParseState` any time we want to introduce this type of
stitching/delegation. It also means that those methods can eventually be
extracted into more general traits in the future as well.
Ultimately, though, the two would have accomplished the same thing. But the
difference is most emphasized in the _parent_---the actual stitching still
has to take place for desugaring in the attribute parser, and I'd like for
that abstraction to still be in terms of expansion. But if I utilized
`StitchableExpansionState`, which converted into a dead state, I'd have to
either forego the expansion abstraction---which would make the parser even
more confusing---or I'd have to create _another_ abstraction around the dead
state, which would mean that I stripped one abstraction just to introduce
another one that's essentially the same thing. It didn't feel right, but it
would have worked.
The use of `PhantomData` in `StitchableExpansionState` was also a sign that
something wasn't quite right, in terms of how the abstractions were
integrating with one-another.
And so here we are, as I struggle to wade my way through all of the yak
shavings and make any meaningful progress on this project, while others
continue to suffer due to slow build times.
I'm sorry. Even if the system is improving.
DEV-13156
This is just intended to simplify the job of panicing when something is
expected to be `None`. In my case, `Lookahead`; see upcoming commits.
This is intended to be generalized to more than just `Option`, but I have no
use for it elsewhere yet; I primarily just needed to implement a method on
`Option` so that I could have the ergonomics of the dot notation.
DEV-13156
There's no use in duplicating this in util::expand.
Lookahead tokens are one of the few invariants that I haven't taken the time
of enforcing using the type system, because it'd be quite a bit of work that
I do not have time for, and may not be worth it with changes that may make
the system less ergonomic. Nonetheless, I do hope to address it at some
point in the (possibly-far) future.
If ever you encounter this diagnostic message, ask yourself how stable TAMER
otherwise is and how many other issues like this have been entirely
prevented through compile-time proofs using the type system.
DEV-13156
As in previous commits, this continues to replace panics with
`diagnostic_panic!`, which provides much more useful information both for
debugging and to help the user possibly work around the problem. And lets
the user know that it's not their fault, and it's a TAMER bug that should be
reported.
...am I going to rationalize it in each commit message?
DEV-13156
This moves enough of the handling of complex type conversions into the
various components of `TransitionResult` (and itself), which simplifies
delegation and opens up the possibility of having specialized
delegation/stitching methods implemented atop of `TransitionResult`.
DEV-13156
These delegation methods have been a pain in my ass for quite some time, and
their lack of generalization makes the introduction of new delegation
methods (in the general sense, not necessarily trait methods) very tedious
and prone to inconsistencies.
I'm going to progressively refactor them in separate commits so it's clear
what I'm doing, primarily for future me to reference if need be.
DEV-13156
This beings to introduce more primitive operations to `TransitionResult` and
its components so that I can actually work with them without having to write
a bunch of concrete, boilerplate implementations. This is demonstrated in
part by `EchoState` (which is nearly all boilerplate, but whose correctness
should be verifiable at a glance), which will be used going forward as a
basis for default implementations for parsers (e.g. expansion delegation).
DEV-13156
This has evolved into a more robust and independent concept, but it is still
a utility in the sense that it's utilizing existing parsing framework
features and making them more convenient.
DEV-13156
These traits serve to abstract away some of the type-level details and
clearly state what the end result is (something stitchable with a parent).
I'm admittedly battling myself on this concept a bit. The proper layer of
abstraction is the concept of expansion, which is an abstraction that is
likely to be maintained all the way through, but we strip the abstraction
for the sake of delegation. Maybe the better option is to provide a
different method of delegation and avoid the stripping at all, and avoid the
awkward interaction with the dead state.
The awkwardness comes from the fact that delegating right now is so rigid
and defined in terms of a method on state rather than a mapping between
`TransitionResult`s. But I really need to move on... ;_;
The original design was trying to generalize this such that composition at
the attribute parser level (for NIR) would be able to just accept any
sitchable parser with the convention that the dead state is the replacement
token. But that is the wrong layer of abstraction, which not only makes it
confusing, but is asking for trouble when someone inevitably violates that
contract.
With all of that said, `StitchableExpansionState` _is_ a delegation. It
could just as easily be a function (`is_accepting` always delegates too), so
perhaps that should just be generalized as reifying delegation as a
`ParseState`.
DEV-13156
This parser really just allows me to continue developing the NIR
interpolation system using `Expansion` terminology, and avoid having to use
dead states in tests. This allows for the appropriate level of abstraction
to be used in isolation, and then only be stripped when stitching is
necessary.
Future commits will show how this is actually integrated and may introduce
additional abstraction to help.
DEV-13156
This is a shift in approach.
My original idea was to try to keep NIR parsing the way it was, since it's
already hard enough to reason about with the `ele_parse!` parser-generator
macro mess. The idea was to produce an IR that would explicitly be denoted
as "maybe sugared", and have a desugaring operation as part of the lowering
pipeline that would perform interpolation and lower the symbol into a plain
version.
The problem with that is:
1. The use of the type was going to introduce a lot of mapping for all the
NIR token variants there are going to be; and
2. _The types weren't even utilized for interpolation._
Instead, if we interpolated _as attributes are encountered_ while parsing
NIR, then we'd be able to expand directly into that NIR token stream and
handle _all_ symbols in a generic way, without any mapping beyond the
definition of NIR's grammar using `ele_parse!`.
This is a step in that direction---it removes `NirSymbolTy` and introduces a
generic abstraction for the concept of expansion, which will be utilized
soon by the attribute parser to allow replacing `TryFrom` with something
akin to `ParseFrom`, or something like that, which is able to produce a
token stream before finally yielding the value of the attribute (which will
be either the original symbol or the replacement metavariable, in the case
of interpolation).
(Note that interpolation isn't yet finished---errors still need to be
implemented. But I want a working vertical slice first.)
DEV-13156
This was a substantial change. Design and rationale are documented on
`AttrFieldSum` and related as part of this change, so please review the diff
for more information there.
If you're a Ryan employee, DEV-13209 gives plenty of profiling information,
including raw data and visualizations from kcachegrind. For everyone else:
you're able to easy produce your own from this commit and the previous and
comparing the `__memcpy_avk_unaligned_erms` calls. The reduction is
significant in this commit (~90%), and the number of Parsers invoking it has
been reduced. Rust has been able to optimize more aggressively, and
compound some of those optimizations, with the smaller `NirParseState`
width.
It also worth noting that `malloc` calls do not change at all between
these two changes, so when we refer to memory, we're referring to
pre-allocated memory on the stack, as TAMER was designed to utilize.
DEV-13209
This is a diagnostic replacement for `unreachable!`.
Eventually TAMER'll have build-time checks to enforce the use of these over
alternatives; I need to survey the old instances on a case-by-case basis to
see what diagnostic information can be reasonably presented in that context.
DEV-13209
The spans were previously not being calculated relative to the offset of the
original symbol span. Tests were passing because all of those spans began
at offset 0.
DEV-13156
This demonstrates how desugaring of interpolated strings will work, testing
one of the happy paths. The remaining work to be done is largely
refactoring; handling some other cases; and errors. Each of those items are
marked with `todo!`s.
I'm pleased with how this is turning out, and I'm excited to see diagnostic
reporting within the specification string using the derived spans once I get
a bit further along; this robust system is going to be much more helpful to
developers than the existing system in XSLT.
This also eliminates the ~50% performance degredation mentioned in a recent
commit by eliminating the SugaredNirSymbol enum and replacing it with a
newtype; this is a much better approach, though it doesn't change that I do
need to eventually address the excessive `memcpy`s on hot code paths.
DEV-13156
Not sure why I didn't add a prelude sooner, considering all the import
boilerplate. This will evolve as needed and I'll go back and replace other
imports when I'm not in the middle of something.
DEV-13156
Add initial descriptions and consolodate some of the types. There'll be
more to come; this is just to get `Display` derives working for types
that'll be using it. I'd like to see where this description manifests
itself before I decide how user-friendly I'd like it to be.
DEV-13156
This mirror is only a `Todo` variant at the moment, but my hope had been to
try to creatively nest or use generics to simplify the conversaion between
the two flavors without a lot of boilerplate. But it doesn't seem like I'm
going to be successful, and may have to resort to macros to remove
boilerplate.
But I need to stop fighting with myself and move on. Though I would still
like to keep the types purely compile-time via const generics if possible,
since they're not needed in memory (or disk) until we get to templates;
they're otherwise static relative to a NIR token variant.
DEV-13209
This simply detects whether a value will need to be further parsed for
interpolation; it does not yet perform the parsing itself, which will happen
during desugaring.
This introduces a performance regression, for an interesting reason. I
found that introducing a single new variant to `SugaredNir` (with a
`(SymbolId, Span)` pair), was causing the width of the `NirParseState` type
to increase just enough to cause Rust to be unable to optimize away a
significant number of memcpys related to `Parser` moves, and consequently
reducing performance by nearly 50% for `tamec`. Yikes.
I suspected this would be a problem, and indeed have tried in all other
cases to avoid aggregation until the ASG---the problem is that I had wanted
to aggregate attributes for NIR so that the IR could actually make some
progress toward simplifying the stream (and therefore working with the
data), and be able to validate against a grammar defined in a single
place. The problem is that the `NirParseState` type contains a sum type for
every attribute parser, and is therefore as wide as the largest one. That
is what Rust is having trouble optimizing memcpy away for.
Indeed, reducing the number of attributes improves the situation
drastically. However, it doesn't make it go away entirely.
If you look at a callgrind profile for `tameld` (or a dissassembly), you'll
notice that I put quite a bit of effort into ensuring that the hot code path
for the lowering pipeline contains _no_ memcpys for the parsers. But that
is not the case with `tamec`---I had to move on. But I do still have the
same escape hatch that I introduced for `tameld`, which is the mutable
`Context`.
It seems that may be the solution there too, but I want to get a bit further
along first to see how these data end up propagating before I go through
that somewhat significant effort.
DEV-13156
Various parts of the system have to be converted to use `diagnostic_panic!`,
which makes it very clear that this is a bug in TAMER that should be
reported. I just happened to see this one near code I was about to touch.
DEV-13156
This introduces the concept of sugared NIR and provides the boilerplate for
a desugaring pass. The earlier commits dealing with cleaning up the
lowering pipeline were to support this work, in particular to ensure that
reporting and recovery properly applied to this lowering operation without
adding a ton more boilerplate.
DEV-13158
I'm struggling to go much further yet without sorting out some other things
first with regards to mutable `Context` and, in particular, the ASG.
I'm going to pause on refactoring the lowering pipeline---it's been improved
significantly with the recent work---and I will continue in the next few
weeks.
DEV-13158
Lowering errors in tamec end up utilizing recovery and reporting, so there
is a distinction between recoverable and unrecoverable errors.
tameld aborts on the first error, since recovery is not currently
supported (we'll want to add it, since tameld should output e.g. lists of
unresolved externs).
Note that tamec does not yet handle `FinalizeError` like tameld because it
uses `Lower::lower`, which does not yet finalize (though it does in practice
when it reaches the end of the stream and auto-finalizes, but that is
widened into a `ParseError`).
DEV-13158
This helps to clarify the situations under which these errors can occur, and
the generality also helps to show why the inner types are as they
are (e.g. use of `String`).
But more importantly, this allows for an error type in `finalize` that is
detached from the `ParseState`, which will be able to be utilized in the
lowering pipeline as a more general error distinguishable from other
lowering errors. At the moment I'm maintaining BC, but a following commit
will demonstrate the use case to introduce recoverable vs. non-recoverable
errors.
DEV-13158
Mike Gerwitz