[Nix-dev] removing null attrs in nix

[Matthew O'Gorman]

Hi I am working on a package and wanted to know the bet way to achieve
this. i have an env variable like this

  huginnEnv = {
    GEM_HOME = gemHome;
    BUNDLE_GEMFILE = "${cfg.packages.huginn}/share/huginn/Gemfile";
    UNICORN_PATH = "${cfg.statePath}/";
    HUGINN_PATH = "${cfg.packages.huginn}/share/huginn/";
    HUGINN_STATE_PATH = "${cfg.statePath}";
    HUGINN_UPLOADS_PATH = "${cfg.statePath}/uploads";
    HUGINN_LOG_PATH = "${cfg.statePath}/log/huginn.log";
    RAILS_ENV = "production";
    LD_LIBRARY_PATH = "${pkgs.curl.out}/lib:${pkgs.postgresql}/lib";
    DATABASE_ADAPTER = "postgresql";
    RAILS_SERVE_STATIC_FILES = (if serve_static_files then "true" else "");
    # I set this by hand because I wanted to maintain sessions between restarts.
    #I totally know what I am doing security wise.
    APP_SECRET_TOKEN=(if cfg.app_secret_token != "random" then cfg.app_secert_token else "");
    DOMAIN=(if !cfg.enableUnicorn then cfg.host else "");
    PORT=(if !cfg.enableUnicorn then toString cfg.port else "");
  };

I would like to take huginnEnv but prune out any of the Null results,
"", so that I only set real variables no blank ones.  is there a way to
filter on this? or mapttrs to list filter and then list to attrs is the
simplest way?

thanks

-- 
Matthew O'Gorman
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