Rational Developer for i (RDi), the modern editor for IBM i RPG, COBOL and other languages, runs on Windows and Linux, but not Mac.
IBM has shown some willingness to port the product to Mac. They have posted a poll to allow users to vote for this enhancement or to comment on the proposal: https://www.ibm.com/developerworks/rfe/execute?use_case=viewRfe&CR_ID=13454
Those wishing to vote will find a “vote” link near the bottom of the poll page.