simple-expr | Simple expression language for mapbox-gl-js expressions | Runtime Evironment library

You cannot, the configure method is for configuring the route(s). Its only invoked once during startup.

As you mention you can use an inlined processor to invoke the function. The simple language can also invoke bean/methods and use bean parameter binding so it can bind data from the exchange to the bean method signature. Then you can use the simple language directly in the Log EIP.

You can use nix-shell for this. It drops you into a shell configured to the given nix expression. Initially that expression could simply be along the lines of buildInputs = [ pkgs.ruby ]; and you can develop it from that. There are a number of helpful articles online written by nix users that give more examples of using nix-shell, like this one from garbas.si

You may also find it useful to get a better idea of how nix packages work. There's a separate nixpkgs manual that covers in greater detail using nix to create package expressions. A quick skim of the 3rd section should be useful to give a bit more understanding. There's also a chapter on using nix with ruby bundler that might be useful for you. Again there are articles that give more examples of its use, such as one from stesie.github.io.

If you need postgresql actually running in your environment nix won't manage that for you; its function is solely the building and management of packages, not their activation. You could simply activate postgres manually, use the nix-shell hook, or create some other integration with nix, but I think the most robust option is to make use of the Linux distribution that's built on top of Nix - NixOS. NixOS integrates with nix packages and manages the services provided by the packages. You could create a NixOS configuration with postgres active and your development environment present. This utility from github.com/chrisfarms may also be of interest.

To have pyparsing construct your classes for you, attach them as parse actions to the expressions. Here's a small example of how to do this: