Commits on Jan 16, 2018

1. parseExprFromFile -> evalFile …

   parseExprFromFile() should be avoided since it doesn't cache anything.

   

   edolstra committed Jan 16, 2018
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2. Add pure evaluation mode …

   In this mode, the following restrictions apply:
   
   * The builtins currentTime, currentSystem and storePath throw an
     error.
   
   * $NIX_PATH and -I are ignored.
   
   * fetchGit and fetchMercurial require a revision hash.
   
   * fetchurl and fetchTarball require a sha256 attribute.
   
   * No file system access is allowed outside of the paths returned by
     fetch{Git,Mercurial,url,Tarball}. Thus 'nix build -f ./foo.nix' is
     not allowed.
   
   Thus, the evaluation result is completely reproducible from the
   command line arguments. E.g.
   
     nix build --pure-eval '(
       let
         nix = fetchGit { url = https://github.com/NixOS/nixpkgs.git; rev = "9c927de4b179a6dd210dd88d34bda8af4b575680"; };
         nixpkgs = fetchGit { url = https://github.com/NixOS/nixpkgs.git; ref = "release-17.09"; rev = "66b4de79e3841530e6d9c6baf98702aa1f7124e4"; };
       in (import (nix + "/release.nix") { inherit nix nixpkgs; }).build.x86_64-linux
     )'
   
   The goal is to enable completely reproducible and traceable
   evaluation. For example, a NixOS configuration could be fully
   described by a single Git commit hash. 'nixos-rebuild' would do
   something like
   
     nix build --pure-eval '(
       (import (fetchGit { url = file:///my-nixos-config; rev = "..."; })).system
     ')
   
   where the Git repository /my-nixos-config would use further fetchGit
   calls or Git externals to fetch Nixpkgs and whatever other
   dependencies it has. Either way, the commit hash would uniquely
   identify the NixOS configuration and allow it to reproduced.

   

   edolstra committed Jan 16, 2018
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3. Make show-trace a config setting

   

   edolstra committed Jan 16, 2018
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4. <nix/fetchurl.nix>: Don't access builtins.currentSystem …

   This doesn't work in pure evaluation mode.

   

   edolstra committed Jan 16, 2018
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