From function level semantics to program transformation and optimization
The software crisis results from our disorderly concepts of "program". These make programming an art, rather than an engineering discipline. Such a discipline would at least require that we have stocks of useful off-the-shelf programs and collections of standard theorems that can be applied repeatedly. We have neither.
Mathematical systems are often distinguished by a set of operations that (a) map a set of entities into itself, (b) have simply understood results, and (c) obey a set of strong algebraic laws. Neither conventional programs nor "object level" functional programs are entities belonging to such a system. The standard operations on conventional programs violate (b) and (c); object level functional programs normally employ lambda abstraction as their program building operation and it violates (a) and (c). Other problems of these program concepts are reviewed.
Function level programs are the entities of just such a mathematical system: programs are built by program-forming operations having good algebraic properties. Hence they are the subject of a large number of general theorems, theorems that are applicable in practice. We give examples. Function level programs also have the possibility of providing solutions to many of the other problems reviewed.
The paper reviews the function level FP system of programs, sketches a function level semantics for it, and from the equations of that semantics develops some moderately general results concerning linear, recursively defined functions, one concerning recursion removal. It then discusses other general, directly applicable results in the literature and shows that they are essentially function level results and are best presented and recognized in that form.
The final section is about optimization; it shows how some FP programs can be transformed into others that run as fast as Fortran programs. It introduces "Fortran constructs" into FP, pure functions that have an obvious corresponding Fortran-like program. It exhibits a number of function level identities for these constructs and shows how these can be used to convert inefficient FP programs into efficient Fortran-like ones.
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