Assumption that an expression is dynamic, so not all the required data is known in advance.

Example:
sometimes the expression is:
foo + bar
another time can be only:
bar

If fetch for foo needs web request , how to avoid web request for each time and only fetch and set value of foo to env when found in expression?

I think it's a very normal case in business projects.For example, in a marketing system, the combination of conditions can be very flexible, and the data sources for these conditions include basic account information, order information, login information, and so on.

Different combinations of conditions require fetching different combinations of entities(Account, Order , Login...). Pre-fetching all the data every time would result in wastage of resources. It is desired to dynamically retrieve the data sources when evaluating a specific condition and cache the data sources to the environment (env).