A Graphical Foundation for Schedules

In 2007, Harmer, Hyland and Melliès gave a formal mathematical foundation for game semantics using a notion they called a schedule. Their definition was combinatorial in nature, but researchers often draw pictures when describing schedules in practice. Moreover, a proof that the composition of schedules is associative involves cumbersome combinatorial detail, whereas in terms of pictures the proof is straightforward, reflecting the geometry of the plane. Here, we give a geometric formulation of schedule, prove that it is equivalent to Harmer et al.ʼs definition, and illustrate its value by giving a proof of associativity of composition.