A system to understand incorrect programs

This paper presents a systems (PHENARETE) which understands and improves incompletely defined LISP programs, such as those written by students beginning to program in LISP. This system takes, as input, the program without any additional information. In order to understand the program, the system meta-evaluates it, using a library of "pragmatic rules", describing the construction and correction of general program constructs, and a set of "specialists", describing the syntax and semantics of the standard LISP functions. The system can use its understanding of the program to detect errors in it, to debug them and, eventually, to justify its proposed modification. This paper gives a brief survey of the working of the system, emphasizing on some commented examples.