We used to build our systems in the monolithic style. Now we use microservices. A microservice is definitely decomposable into smaller functions. Obviously, a function is the next step in architectural evolution. 

Monolithic is a software architecture that contains distinguishable software concerns. Every service is built into a single deployment module.

The microservice architecture, in contrast, separates coherent services inside a single monolithic module to be externally, loosely coupled services.

Function as a Service or FaaS is another level of separation. In this architecture, a microservice is split into more fine-grained units, functions: