Abstract

ion. A common form of transformation, which is easily justified by appealing to reversibility, is abstraction. The abstraction transformation lifts some instances of subexpressions from the right-hand sides of a set of definitions and replaces them with function calls for some new functions. The abstraction process can be used in conjunction with a call-by-need implementation to avoid repeated evaluation of subexpressions. A well-known example is Hughes' supercombinator abstraction [Hughes 1982]. Another form of abstraction which is common in program transformation is syntactic generalization in which an expression e is replaced by a function call g e 1 : : : e n , where g is a new function defined by g x 1 : : : xn \Delta = e 0 , such that e j e 0 f e 1 : : : e n= x 1 : : : xn g. General statements about abstractions and their correctness are notationally rather complex. In practice we have found it is easier to appeal to a reversibility argument on a case-by-case basis than...

Citations