History

Aspen Smith 5e31096154 feat(tvix/eval): Store string context alongside data Previously, Nix strings were represented as a Box (within Value) pointing to a tuple of an optional context, and another Box pointing to the actual string allocation itself. This is pretty inefficient, both in terms of memory usage (we use 48 whole bytes for a None context!) and in terms of the extra indirection required to get at the actual data. It was necessary, however, because with native Rust DSTs if we had something like `struct NixString(Option<NixContext>, BStr)` we could only pass around fat pointers to that value (with the length in the pointer) and that'd make Value need to be bigger (which is a waste of both memory and cache space, since that memory would be unused for all other Values). Instead, this commit implements manual allocation of a packed string representation, with the length in the allocation as a field past the context. This requires a big old pile of unsafe Rust, but the payoff is clear: hello outpath time: [882.18 ms 897.16 ms 911.23 ms] change: [-15.143% -13.819% -12.500%] (p = 0.00 < 0.05) Performance has improved. Fortunately this change can be localized entirely within value/string.rs, since we were abstracting things out nicely. Change-Id: Ibf56dd16c9c503884f64facbb7f0ac596463efb6 Reviewed-on: https://cl.tvl.fyi/c/depot/+/10852 Tested-by: BuildkiteCI Reviewed-by: raitobezarius <tvl@lahfa.xyz> Autosubmit: aspen <root@gws.fyi>		2024-02-21 20:53:04 +00:00
..
benches	refactor(tvix/eval): move Evaluation::{default,new_pure}() again	2024-01-18 14:24:40 +00:00
builtin-macros	refactor(tvix/eval): Generalize propagation of catchable values	2024-02-08 19:59:21 +00:00
docs	docs(tvix/eval): notes on the implementation of catchable errors	2024-02-21 13:32:38 +00:00
proptest-regressions/value	refactor(tvix/eval): flatten call stack of VM using generators	2023-03-13 20:30:59 +00:00
src	feat(tvix/eval): Store string context alongside data	2024-02-21 20:53:04 +00:00
tests	refactor(tvix/eval): move Evaluation::{default,new_pure}() again	2024-01-18 14:24:40 +00:00
.skip-subtree	feat(tvix/tests): check in Nix' language test suite	2022-08-24 21:25:41 +00:00
build.rs	feat(tvix/eval): implement builtins.currentSystem	2022-10-24 12:20:01 +00:00
Cargo.toml	fix(tvix): Represent strings as byte arrays	2024-01-31 14:51:49 +00:00
default.nix	refactor(tvix): build Rust projects using crate2nix	2022-12-15 17:26:45 +00:00
README.md	docs(tvix/eval): update test suite documentation	2023-06-15 19:28:16 +00:00

README.md

Tvix Evaluator

This project implements an interpreter for the Nix programming language. You can experiment with an online version of the evaluator: tvixbolt.

The interpreter aims to be compatible with nixpkgs, on the foundation of Nix 2.3.

Important note: The evaluator is not yet feature-complete, and while the core mechanisms (compiler, runtime, ...) have stabilised somewhat, a lot of components are still changing rapidly.

Please contact TVL with any questions you might have.

Building tvix-eval

Please check the README.md one level up for instructions on how to build this.

The evaluator itself can also be built with standard Rust tooling (i.e. cargo build).

If you would like to clone only the evaluator and build it directly with Rust tooling, you can do:

git clone https://code.tvl.fyi/depot.git:/tvix/eval.git tvix-eval

cd tvix-eval && cargo build

Tests

Tvix currently has three language test suites for tvix-eval:

nix_tests and tvix_tests are based on the same mechanism borrowed from the C++ Nix implementation. They consist of Nix files as well as expected output (if applicable). The test cases are split into four categories: eval-okay (evaluates successfully with the expected output), eval-fail (fails to evaluate, no expected output), parse-okay (expression parses successfully, no expected output) and parse-fail (expression fails to parse, no expected output). Tvix currently ignores the last two types of test cases, since it doesn't implement its own parser.

Both test suites have a notyetpassing directory. All test cases in here test behavior that is not yet supported by Tvix. They are considered to be expected failures, so you can't forget to move them into the test suite proper when fixing the incompatibility.

Additionally, separate targets in the depot pipeline, under //tvix/verify-lang-tests, check both test suites (including notyetpassing directories) against C++ Nix 2.3 and the default C++ Nix version in nixpkgs. This way we can prevent accidentally introducing test cases for behavior that C++ Nix doesn't exhibit.
- nix_tests has the test cases from C++ Nix's language test suite and is sporadically updated by manually syncing the directories. The notyetpassing directory shows how far it is until we pass it completely.
- tvix_tests contains test cases written by the Tvix contributors. Some more or less duplicate test cases contained in nix_tests, but many cover relevant behavior that isn't by nix_tests. Consequently, it'd be nice to eventually merge the two test suites into a jointly maintained, common Nix language test suite.
  
  It also has a notyetpassing directory for missing behavior that is discovered while working on Tvix and isn't covered by the nix_tests suite.
nix_oracle can evaluate Nix expressions in Tvix and compare the result against C++ Nix (2.3) directly. Eventually it should gain the ability to property test generated Nix expressions. An additional feature is that it can evaluate expressions without --strict, so thunking behavior can be verified more easily.

rnix-parser

Tvix is written in memory of jD91mZM2, the author of rnix-parser who sadly passed away.

Tvix makes heavy use of rnix-parser in its bytecode compiler. The parser is now maintained by Nix community members.