let
  field = x:
    if (x == null || x == [ ] || x == "") then "-"
    else if (builtins.isInt x) then (builtins.toString x)
    else if (builtins.isList x) then (builtins.concatStringsSep "," x)
    else x;

  mkBuildMachine =
    { uri ? null
    , systems ? null
    , sshKey ? null
    , maxJobs ? null
    , speedFactor ? null
    , supportedFeatures ? null
    , mandatoryFeatures ? null
    , publicHostKey ? null
    }: ''
      ${field uri} ${field systems} ${field sshKey} ${field maxJobs} ${field speedFactor} ${field supportedFeatures} ${field mandatoryFeatures} ${field publicHostKey}
    '';

  mkBuildMachinesFile = x: builtins.toFile "machines" (
    builtins.concatStringsSep "\n" (
      map (mkBuildMachine) x
    )
  );

in
mkBuildMachinesFile