A path cost function is a function that assigns a cost to a path. In all cases we will consider, the cost of a path is the sum of the costs of the individual actions along the path.
The path cost function is often denoted by g.
Together, the initial state, operator set, goal test, and path cost function define a problem.
Naturally, we can then define a datatype with which to represent problems:
datatype PROBLEM
components: INITIAL-STATE, OPERATORS, GOAL-TEST, PATH-COST-FUNCTION