A Practical Study of Control in Objected-Oriented-Functional-Logic Programming with Paisley

ions, nondeterminism by backtracking, encapsulated search, and metaprogramming, without requiring changes to either the Java host language or the code of the data object models to be queried. In past papers, we have demonstrated how to use Paisley to sanitize legacy query interfaces [5], how to combine nondeterminism and search-plan metaprogramming to solve logical puzzles [7], and how to express complex relational queries in pattern (Kleene) algebra [8], respectively. In summary, Paisley turns Java into an effective functional–logic programming language, without constraining the use of features of the object-oriented host environment. However, the demonstrations have so far focused on encapsulated search, and omitted a different aspect that is ubiquitous in declarative pattern matching: the joint specification of data and control flow in case distinctions by pattern matching clauses. The present paper fills this gap. The Java language has adopted, in its recent version 8, mechanisms for local functional programming that are well-integrated with the object-oriented background. We demonstrate how to use these to encode the right hand sides of pattern matching clauses as continuations, such that the ordinary Java means for clause selection, namely conditional statements and short-circuiting Boolean operators, can be used freely and effectively. Unlike for the preceding papers [7, 8], we illustrate the real-world use of this novel expressivity not by constructing a new program for the purpose, but instead by reengineering parts of an existing third-party open-source project. We discuss and evaluate the impact of our modifications both qualitatively and quantitatively. 2 Paisley in a Nutshell As a running example, consider Figure 1 (left): a simple data model of LISP-style universal lists, implemented in Java in object-oriented textbook style. It is a simple matter to denote the construction of a list, for instance of three elements x, y and z. It is however a very different matter to use the public API to denote the inverse operation: checking whether a given list contains exactly three elements, and if so, extract them as x, y and z, or otherwise do something else. See Figure 2 (top and center). The style 152 Control for Paisley list1 = new Pair(x, new Pair(y, new Pair(z, empty) ) ); boolean success = false; // [S] if (list1 instanceof Pair) { // [T] Pair pair1 = (Pair)list1; // [C] Object x = pair1.getCar( ); // [B,P] Object list2 = pair1.getCdr( ); // [b,P] if (list2 instanceof Pair) { // [T] Pair pair2 = (Pair)list2; // [C] Object y = pair2.getCar( ); // [B,P] Object list3 = pair2.getCdr( ); // [b,P] if (list3 instanceof Pair) { // [T] Pair pair3 = (Pair)list3; // [C] Object z = pair3.getCar( ); // [B,P] Object list4 = pair3.getCdr( ); // [b,P] if (list4 == empty) { // [T] succeed(x, y, z); // [R] success = true; // [S]