if row has property X, find all rows that has property X

For example I have table addresses and usually I want 2 things:
1. Find id of bad addresses.
2. Check if this address is good or bad.

For this I write two plpgsql functions
1. find_all_bad_addresses
2. is_bad_address(id)

These functions will duplicate logic of each other. How to not repeat myself?
Thanks