The idea is to make the functions pure and return the state ‘a(s_1) -> s_2, value’. Makes so much easier to test and write concurrent programs. Though it’s not

The idea is to make the functions pure and return the state ‘a(s_1) -> s_2, value’. Makes so much easier to test and write concurrent programs. Though it’s not always practical, like when needing to push an element to an array for making updated state. It is nevertheless possible to shave of pure parts considerably making mutability easier to maintain.