Sadly, Peter Landin died last year after a short illness. Peter was my PhD supervisor and was a huge influence on myself and many others as recalled here. Peter was responsible for a number of very important discoveries relating to Programming and Programming Languages. Although many of these were in the 1960s and 1970s, he was active throughout the time I knew him in the 90s and early 00s when he was developing the foundation of a topic he called Calculations:

calculations seek to characterize the concept of what a program does without being prescriptive about how the desired behaviour is achieved. By providing a way of abstractly capturing program behaviour, calculations offer: an approach to program analysis, an approach to designing programs based on the desired behaviour, and a new approach to teaching about programming, [...] calculations are a missing concept in the study and teaching of Computing.

A calculation is a historical record of what went on when a program executed with respect to some specific data items. An example calculation showing mutually dependent processing of integer streams is shown on the right. Peter's notes on calculations have been typed up and recently published in the Journal of Higher-Order and Symbolic Computation.