Wadler's deforestation algorithm eliminates intermediate data structures from functional programs. To be suitable for inclusion in a compiler, it must terminate on all programs. Several techniques to ensure termination of deforestation on all first-order programs are known, but a technique for higher-order programs was only recently introduced by Hamilton, and elaborated and implemented in the Glasgow Haskell compiler by Marlow. We introduce a new technique for ensuring termination of deforestation on all higher-order programs that allows useful transformation steps prohibited in Hamilton's and Marlowe's techniques. 1 Introduction Lazy, higher-order, functional programming languages lend themselves to a certain style of programming which uses intermediate data structures [28]. Example 1 Consider the following program. letrec a = x; y:case x of [] ! y (h : t) ! h : a t y in u; v; w: a (a u v) w The term u; v; w:a (a u v) w appends the three lists u, v, and w. Appending u and v ...

. Deforestation is a transformation of functional programs to remove intermediate data structures. It is based on outermost unfolding of function calls where folding occurs when unfolding takes place within the same nested function call. Since unrestricted unfolding may encounter arbitrarily many terms, a termination analysis has to determine those subterms where unfolding is possibly dangerous. We show that such an analysis can be obtained from a control flow analysis by an extension with integer constraints -- essentially at no loss in efficiency. 1 Introduction The key idea of flow analysis for functional languages is to define an abstract meaning in terms of program points , i.e., subexpressions of the program possibly evaluated during program execution [Pa95]. Such analysises have been invented for tasks like type recovery [Sh91], binding time analysis [Co93], or safety analysis [PS95]. Conceptually, these are closely related to A. Deutsch's store--based alias analysis [D...