According to Command-Query Separation principle, as well as Thinking in Data and DDD with Clojure presentations one should separate side effects (modifying the world) from computations and decisions, so that it would be easier to understand and test both parts.

This leaves an unanswered question: where relatively to the boundary should we put "asking the world"? On the one hand, requesting data from external systems (like database, extental services' APIs etc) is not referentially transparent and thus should not sit together with pure computational and decision making code. On the other hand, it's problematic, or maybe impossible to tease them apart from computational part and pass it as an argument as because we may not know in advance which data we may need to request.