Equivalent Transformation by Safe Extension of Data Structures

Equivalent transformation has been proposed as a method ology for providing programs with appropriate data structures For in stance logic programs which use lists are transformed into equivalent programs that use di erence lists However lists and di erence lists are both usual terms and in this sense no new data structures are introduced in the transformation Since logic programming has xed data structure called terms no one can develop theoretical foundations for introducing new data structures into programs as far as only logic programs are dis cussed In this paper we develop a theoretical foundation of equivalent transformation that introduces new data structures We introduce a pa rameter for data structures by which many languages with di erent data structures are characterized By changing this parameter say from to we can discuss data structure change for programs We de ne a concept of safe extension of data structures and prove that the meaning of a program on a data structure is preserved by safe extension of the data structure