Since their introduction in 1996, Dyalog's direct-definition functions (D-fns) have grown from an experimental toy to a notation used to implement large pieces of commercial software. D-fns are not only useful for expressing idioms, but also as a tool of thought for any problem that can benefit from a functional approach (and some would say that covers almost everything). Owing to their functional nature, D-fns also have greater potential for internal optimization, including compilation — and have been selected as the foundation for the new function syntax in the APL# dialect