List Function Names from Packages

let
  pkgs = import <nixpkgs> { };
 
  tryEval =
    expr:
    let
      result = builtins.tryEval expr;
    in
    if result.success then result.value else null;
 
  safeGet = name: tryEval (builtins.getAttr name pkgs);
  isFunction = x: builtins.isFunction x;
  isTopLevelFunction = name: name != "lib" && isFunction (safeGet name);
  functionNames = builtins.filter isTopLevelFunction (builtins.attrNames pkgs);
in
builtins.trace (builtins.concatStringsSep "\n" functionNames) { }

$ nix eval -f <filename>.nix
trace:
...