Indeed, any useful program must have state and therefore cannot be entirely functional. Only components or individual functions can be functional in style, the

Indeed, any useful program must have state and therefore cannot be entirely functional. Only components or individual functions can be functional in style, the goal being to have as many of those as practical so that you don't touch state often and hopefully, only in a few places and using methods which set the state (rather than letting individual components "reach in" and set it themselves).