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diff --git a/tvix/docs/src/lang-version.md b/tvix/docs/src/lang-version.md new file mode 100644 index 000000000000..c288274c9105 --- /dev/null +++ b/tvix/docs/src/lang-version.md @@ -0,0 +1,62 @@ +# Nix language version history + +The Nix language (“Nix”) has its own versioning mechanism independent from its +most popular implementation (“C++ Nix”): `builtins.langVersion`. It has been +increased whenever the language has changed syntactically or semantically in a +way that would not be introspectable otherwise. In particular, this does not +include addition (or removal) of `builtins`, as this can be introspected using +standard attribute set operations. + +Changes to `builtins.langVersion` are best found by viewing the git history of +C++ Nix using `git log -G 'mkInt\\(v, [0-9]\\)'` for `builtins.langVersion` < 7. +After that point `git log -G 'v\\.mkInt\\([0-9]+\\)'` should work. To reduce the +amount of false positives, specify the version number you are interested in +explicitly. + +## 1 + +The first version of the Nix language is its state at the point when +`builtins.langVersion` was added in [8b8ee53] which was first released +as part of C++ Nix 1.2. + +## 2 + +Nix version 2 changed the behavior of `builtins.storePath`: It would now [try to +substitute the given path if missing][storePath-substitute], instead of creating +an evaluation failure. `builtins.langVersion` was increased in [e36229d]. + +## 3 + +Nix version 3 changed the behavior of the `==` behavior. Strings would now be +considered [equal even if they had differing string context][equal-no-ctx]. + +## 4 + +Nix version 4 [added the float type][float] to the language. + +## 5 + +The [increase of `builtins.langVersion` to 5][langVersion-5] did not signify a +language change, but added support for structured attributes to the Nix daemon. +Eelco Dolstra writes as to what changed: + +> The structured attributes support. Unfortunately that's not so much a language +> change as a build.cc (i.e. daemon) change, but we don't really have a way to +> express that... + +Maybe `builtins.nixVersion` (which was added in version 1) should have been +used instead. In any case, the [only `langVersion` check][nixpkgs-langVersion-5] +in nixpkgs verifies a lower bound of 5. + +## 6 + +Nix version 6 added support for [comparing two lists][list-comparison]. + +[8b8ee53]: https://github.com/nixos/nix/commit/8b8ee53bc73769bb25d967ba259dabc9b23e2e6f +[storePath-substitute]: https://github.com/nixos/nix/commit/22d665019a3770148929b7504c73bcdbe025ec12 +[e36229d]: https://github.com/nixos/nix/commit/e36229d27f9ab508e0abf1892f3e8c263d2f8c58 +[equal-no-ctx]: https://github.com/nixos/nix/commit/ee7fe64c0ac00f2be11604a2a6509eb86dc19f0a +[float]: https://github.com/nixos/nix/commit/14ebde52893263930cdcde1406cc91cc5c42556f +[langVersion-5]: https://github.com/nixos/nix/commit/8191992c83bf4387b03c5fdaba818dc2b520462d +[list-comparison]: https://github.com/nixos/nix/commit/09471d2680292af48b2788108de56a8da755d661 +[nixpkgs-langVersion-5]: https://github.com/NixOS/nixpkgs/blob/d7ac3423d321b8b145ccdd1aed9dfdb280f5e391/pkgs/build-support/closure-info.nix#L11 |