The issue you face here is one of program inputs. Any time you are passing functions as values, you can't guarantee how exactly the body of a function will exec

The issue you face here is one of program inputs. Any time you are passing functions as values, you can't guarantee how exactly the body of a function will execute. In a simple example: (lambda (a) (a (lambda () (b)) (lambda () (c))) You can't tell how the calls to b and c will be executed without also knowing something about the value of a. The function a could be a branching construct, or it could just evaluate it's inputs directly. You could assume the worst case and assign a high complexity to this code, but all lambda calculus terms are delayed in this way, so every function call would be assigned the worst case complexity for control flow. More generally, imagine an interpreter that accepts a string as input and that string contains a program. You can make the program contained in the string as complex as you like without affecting the complexity of the interpreter, and effectively cheat the metric.