As a side note in his talk "Simple Made Easy" (https://www.infoq.com/presentations/Simple-Made-Easy, around minute 42)
Rich Hickey mentions, that conditional statements are complex, because they spread (business-)logic throughout the program.
As a simpler (in the Hickey-sense) alternative, he lists
rule systems and logic programming. For example, keeping parts of the business logic ("What do we consider an 'active' user?", "When do we notify a user?", etc...) as datalog expressions, maybe even storing them in a database, specifies them all in a single place. This helps to ensure consistency throughout the program. One could even give access to these specifications to a client, who can then customise the application directly in logic, instead of chasing throughout the whole code base.
Basically everyone involved agrees on a common language of predicates explicitly, instead of informally in database queries, UI, application code, etc...
But Hickey also notes that this thinking is pretty "cutting-edge" and probably not yet terribly practical.
It can work. My current company uses a rule system to represent most of our business logic since it is so dynamic. The downside is that we have to rebuild the entire graph into memory (times the number of threads, times the number of app servers) every time anything changes (which is constant).
Facebook wrote about rebuilding a similar system in Haskell that only changes memory incrementally, so it's definitely possible to do better.
As a simpler (in the Hickey-sense) alternative, he lists rule systems and logic programming. For example, keeping parts of the business logic ("What do we consider an 'active' user?", "When do we notify a user?", etc...) as datalog expressions, maybe even storing them in a database, specifies them all in a single place. This helps to ensure consistency throughout the program. One could even give access to these specifications to a client, who can then customise the application directly in logic, instead of chasing throughout the whole code base.
Basically everyone involved agrees on a common language of predicates explicitly, instead of informally in database queries, UI, application code, etc...
But Hickey also notes that this thinking is pretty "cutting-edge" and probably not yet terribly practical.