为什么 `sudo nixos-rebuild` 不显示跟踪输出？

Question

我期待这个here

let config_ = lib.debug.showVal (config); in
....
systemd     = import ./systemd     { inherit pkgs; config = config_; };

显示配置内容，为什么我看不到？

$ sudo nixos-rebuild dry-build --show-trace
building the system configuration...
these derivations will be built:
  /nix/store/g24yj8lzz2zg921daibfbj2yz5933fwn-hubstaff-1.3.0-9b2ba62.drv
  /nix/store/hps81xprfk0b4lhq8z2vycn1jq4ds841-system-path.drv
  /nix/store/1s689dqbl45g094mnd5sjzdh44wrd6g5-dbus-1.drv
  /nix/store/wqhr5z2f7l0a49fxb4arkwagb1iwmkx4-unit-dbus.service.drv
  .....
  /nix/store/8r03578gxmk2plvxn4p0jbj8aal63vc6-lightdm.conf.drv
  /nix/store/i2ikmkxhgyns0ylj17cw2yv0v82m0lfh-etc.drv
  /nix/store/kwqbq4mmim8ph4i3zjbsi5hhwjr6qkg7-nixos-system-machine-18.03.131954.2569e482904.drv

Answer 1

That version of the systemd/default.nix function doesn't evaluate the config attribute of its argument, so it will not evaluate your debugging function either. The Nix language evaluates by need.

要打印 config，请确保对其进行评估。一个可能对你有帮助的函数是 builtins.seq 它计算它的第一个参数，但 returns 第二个。在文件顶部试试这个：

{ config, pkgs, ... }:

with pkgs;

builtins.seq (lib.debug.showVal config) {

  imports = [
    /* etcetera */

为什么 `sudo nixos-rebuild` 不显示跟踪输出？

Why `sudo nixos-rebuild` doesnt show output of trace?

nix