name: pure-eval internalName: pureEval type: bool default: false

Pure evaluation mode ensures that the result of Nix expressions is fully determined by explicitly declared inputs, and not influenced by external state:

• File system and network access is restricted to accesses to immutable data only:

  • Path literals relative to the home directory like ~/lix are rejected at parse time.

  • Access to absolute paths that did not result from Nix language evaluation is rejected when such paths are given as parameters to builtins like, for example, builtins.readFile.

    Access is nonetheless allowed to (absolute) paths in the Nix store that are returned by builtins like builtins.filterSource, builtins.fetchTarball and similar.

  • Impure fetches such as not specifying a commit ID for builtins.fetchGit or not specifying a hash for builtins.fetchTarball are rejected.

  • In flakes, access to relative paths outside of the root of the flake's source tree (often, a git repository) is rejected.

• The evaluator ignores NIX_PATH, -I and the nix-path setting. Thus, builtins.nixPath is an empty list.
• The builtins builtins.currentSystem and builtins.currentTime are absent from builtins.
• builtins.getEnv always returns empty string for any variable.
• builtins.storePath throws an error (Lix may change this, tracking issue: https://git.lix.systems/lix-project/lix/issues/402)