docs(tvix/eval): notes on the implementation of catchable errors r/7586

Change-Id: I3b2f039d2c2d40ee5d13d071915ced6f3deb18f2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11000
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

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+# (Possible) Implementation(s) of Catchable Errors for `builtins.tryEval`
+
+## Terminology
+
+Talking about “catchable errors” in Nix in general is a bit precarious since
+there is no properly established terminology. Also, the existing terms are less
+than apt. The reason for this lies in the fact that catchable errors (or
+whatever you want to call them) don't properly _exist_ in the language: While
+Nix's `builtins.tryEval` is (originally) based on the C++ exception system,
+it specifically lacks the ability of such systems to have an exception _value_
+whilst handling it. Consequently, these errors don't have an obvious name
+as they never appear _in_ the Nix language. They just have to be named in the
+respective Nix implementation:
+
+- In C++ Nix the only term for such errors is `AssertionError` which is the
+  name of the (C++) exception used in the implementation internally. This
+  term isn't great, though, as `AssertionError`s can not only be generated
+  using `assert`, but also using `throw` and failed `NIX_PATH` resolutions.
+  Were this terminology to be used in documentation addressing Nix language
+  users, it would probably only serve confusion.
+
+- Tvix currently (as of r/7573) uses the term catchable errors. This term
+  relates to nothing in the language as such: Errors are not caught, we rather
+  try to evaluate an expression. Catching also sort of implies that a value
+  representation of the error is attainable (like in an exception system) which
+  is untrue.
+
+In light of this I (sterni) would like to suggest “tryable errors” as an
+alternative term going forward which isn't inaccurate and relates to terms
+already established by language internal naming.
+
+However, this document will continue using the term catchable error until the
+naming is adjusted in Tvix itself.
+
+## Implementation
+
+Below we discuss different implementation approaches in Tvix in order to arrive
+at a proposal for the new one. The historical discussion is intended as a basis
+for discussing the proposal: Are we committing to an old or current mistake? Are
+we solving all problems that cropped up or were solved at any given point in
+time?
+
+### Original
+
+The original implementation of `tryEval` in cl/6924 was quite straightforward:
+It would simply interrupt the propagation of a potential catchable error to the
+top level (which usually happened using the `?` operator) in the builtin and
+construct the appropriate representation of an unsuccessful evaluation if the
+error was deemed catchable. It had, however, multiple problems:
+
+- The VM was originally written without `tryEval` in mind, i.e. it largely
+  assumed that an error would always cause execution to be terminated. This
+  problem was later solved (cl/6940).
+- Thunks could not be `tryEval`-ed multiple times (b/281). This was another
+  consequence of VM architecture at the time: Thunks would be blackholed
+  before evaluation was started and the error could occur. Due to the
+  interaction of the generator-based VM code and `Value::force` the part
+  of the code altering the thunk state would never be informed about the
+  evaluation result in case of a failure, so the thunk would remain
+  blackholed leading to a crash if the same thunk was `tryEval`-ed or
+  forced again. To solve this issue, amjoseph completely overhauled
+  the implementation.
+
+One key point about this implementation is that it is based on the assumption
+that catchable errors can only be generated in thunks, i.e. expressions causing
+them are never evaluated strictly. This can be illustrated using C++ Nix:
+
+```console
+> nix-instantiate --eval -E '[ (assert false; true) (builtins.throw "") <nixpkgs> ]'
+[ <CODE> <CODE> <CODE> ]
+```
+
+If this wasn't the case, the VM could encounter the error in a situation where
+the error would not have needed to pass through the `tryEval` builtin, causing
+evaluation to abort.
+
+### Present
+
+The current system (mostly implemented in cl/9289) uses a very different
+approach: Instead of relying on the thunk boundary, catchable errors are no
+longer errors, but special values. They are created at the relevant points (e.g.
+`builtins.throw`) and propagated whenever they are encountered by VM ops or
+builtins. Finally, they either encounter `builtins.tryEval` (and are converted to
+an ordinary value again) or the top level where they become a normal error again.
+
+The problems with this mostly stem from the confusion between values and errors
+that it necessitates:
+
+- In most circumstances, catchable errors end up being errors again, as `tryEval`
+  is not used a lot. So `throw`s usually end up causing evaluation to abort.
+  Consequently, not only `Value::Catchable` is necessary, but also a corresponding
+  error variant that is _only_ created if a catchable value remains at the end of
+  evaluation. A requirement that was missed until cl/10991 (!) which illustrate
+  how strange that architecture is. A consequence of this is that catchable
+  errors have no location information at all.
+- `Value::Catchable` is similar to other internal values in Tvix, but is much
+  more problematic. Aside from thunks, internal values only exist for a brief
+  amount of time on the stack and it is very clear what parts of the VM or
+  builtins need to handle them. This means that the rest of the implementation
+  need to consider them, keeping the complexity caused by the internal value
+  low. `Value::Catchable`, on the other hand, may exist anywhere and be passed
+  to any VM op or builtin, so it needs to be correctly propagated _everywhere_.
+  This causes a lot of noise in the code as well as a big potential for bugs.
+  Essentially, catchable errors require as much attention by the Tvix developer
+  as laziness. This doesn't really correlate to the importance of the two
+  features to the Nix language.
+
+### Future?
+
+The core assumption of the original solution does offer a path forward: After
+cl/9289 we should be in a better position to introspect an error occurring from
+within the VM code, but we need a better way of storing such an error to prevent
+another b/281. If catchable errors can only be generated in thunks, we can just
+use the thunk representation for this. This would mean that `Thunk::force_`
+would need to check if evaluation was successful and (in case of failure)
+change the thunk representation
+
+- either to the original `ThunkRepr::Suspended` which would be simple, but of
+  course mean duplicated evaluation work in some expressions. In fact, this
+  would probably leave a lot of easy performance on the table for use cases we
+  would like to support, e.g. tree walkers for nixpkgs.
+- or to a new `ThunkRepr` variant that stores the kind of the error and all
+  necessary location info so stack traces can work properly. This of course
+  reintroduces some of the difficulty of having two kinds of errors, but it is
+  hopefully less problematic, as the thunk boundary (i.e. `Thunk::force`) is
+  where errors would usually occur.
+
+Besides the question whether this proposal can actually be implemented, another
+consideration is whether the underlying assumption will hold in the future, i.e.
+can we implement optimizations for thunk elimination in a way that thunks that
+generate catchable errors are never eliminated?