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Programming declaratively in C++ using the Logic paradigm Roshan Naik Hewlett Packard Co. 16 th April 2008 Northwest C++ Users Group Redmond, WA

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Agenda Introduction to Logic Programming (LP) concepts. Introduction to facilities available for LP in C++. Will use the open source library : Castor (www.mpprogramming.com). Live examples. From trivial to simple to more complex ones.

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Underlying Theme LP as a general purpose programming. Demonstrate Multiparadigm Programming by mixing LP with the Imperative, Object-oriented, Functional and Generic paradigms.

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Logic Paradigm (LP) –Computational model: Predicate calculus (Turing complete). –Declarative : Focuses on what to compute not how. –Holy Grail of programming: “The user states the problem, the computer solves it”. LP is one approach in Computer Science. Basic mechanics of LP –Programmer provides sufficient information to the computer. By using relations –Computer employs a general purpose problem solving technique. By using backtracking and unification.

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Declarative vs. Imperative Why are most programming languages imperative ? What is a key weakness of declarative programming ? What is a key weakness of imperative programming ?

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“relation” A set is a collection of (unique) objects. A simple way of thinking about relations is as an association/mapping between elements in two or more sets. People Frank Sam Mary Denise Genders Male Female GenderOf (Frank, Male) (Sam, Male) (Mary, Female) (Denise, Female)

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“relation” A relation is essentially a set. Thinking of relations as mappings between sets really helps when designing relations in LP. A relation is to the logic paradigm what a function is to the imperative paradigm.

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Functions vs. Relations As functions // check if (p,g) bool checkGender(string p, string g) {... } // get gender of p string genderOf(string p) {... } // get all people having gender g list havingGender(string g) {... } // list all (p,g) list > getItems() {... } GenderOf (Frank, Male) (Sam, Male) (Mary, Female) (Denise, Female)

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Functions vs. Relations As a relation // declarative reading : p’s gender is g relation gender(lref p, lref g){ return eq(p,”Frank”) && eq(g,”male”) || eq(p,”Sam”) && eq(g,”male”) || eq(p,”Mary”) && eq(g,”female”) || eq(p,”Denise”)&& eq(g,”female”); } –Specification is declarative. –One relation subsumes functionality of all four functions. GenderOf (Frank, Male) (Sam, Male) (Mary, Female) (Denise, Female)

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Demo

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lref : logic reference Unlike C++ references, it does not have to be initialized. Use method defined() to check. Operator * used to access underlying value. cout << *lx; Internally stores a copy of the value assigned to it. lref li=2; Copy constructing an lref binds it with source lref. i.e. both refer to the same underlying object. lref lj (li); Assigning one lref to another simply copies the underlying value over. lref lk; lk=lj;

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lref : Logic reference Initialization lref li1=1, li2; // li2.defined()==false li2=10; // li2.defined()==true Copy construction lref li3=li1; // co-references li3=5; cout << *li1; // prints 5 Assignment lref li4 = 4; li4=li1; // copy value from li1 to li4 li1=9; cout << *li1 << ", " << *li4; // prints 9, 1

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eq : The unification relation Attempts to make both arguments equal. Semantically a combination of == and = if (both args are initialized) return 1stArg == 2ndArg; else uninitializedArg = value of the other; return true; At least one of the two arguments must be initialized!

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The magic type : relation Represents any function object that produces a bool and takes no arguments. struct FuncObj { bool operator() (void) {...} }; FuncObj f; relation r = f; r(); // execute FuncObj::operator() Type erasure at work –Explicit conversion or assignment operators not needed to assign FuncObj to relation. –FuncObj does not have to inherit from any type or be templated. –Key to smooth integration of paradigms and simple syntax. –Fairly similar to boost::function

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Disjunction: Operator || relation operator || (relation lhs, relation rhs) Example: foo(…) || bar(…) In plain English: –Generate all solutions from lhs (one per invocation), then generate all solutions from rhs (one per invocation). Coroutine style pseudo code (C# like syntax). while( lhs() ) yield return true; //‘yield’ borrowed from C# while( rhs() ) yield return true; return false;

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Conjunction: Operator && In plain English: –Produce all solutions (one per invocation) in the 2 nd relation for each solution in the 1 st relation. Coroutine style pseudo code (C# like syntax). relation tmp = rhs; //make copy of rhs while( lhs() ) { while( rhs() ) yield return true; //‘yield’ borrowed from C# rhs = tmp; //reset rhs } return false;

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Exclusive Disjunction: Operator ^ In plain English: –Generate solutions from 2 nd relation (one per invocation), only if 1 st relation did not produce any. Coroutine style pseudo code (C# like syntax). bool lhsSucceded = false; while( lhs() ) { lhsSucceded = true; yield return true; } while(!lhsSucceded && rhs()) yield return true; return false;

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Examples Demo

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Directed Acyclic Graphs // Edges in the graph relation edge(lref n1, lref n2) { return eq(n1,1) && eq(n2,2) || eq(n1,2) && eq(n2,3) || eq(n1,3) && eq(n2,4) || eq(n1,5) && eq(n2,4) || eq(n1,5) && eq(n2,2) ; } // Definition of path relation path(lref start, lref end) { lref nodeX; return edge(start, end) || edge(start, nodeX) && recurse(path,nodeX,end); }

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Disjunctions : Dynamic relations Think of it as a dynamic list of clauses separated by || operator. Disjunctions is itself a relation. relation edge(lref n1, lref n2) { Disjunctions result; result.push_back( eq(n1,1) && eq(n2,2) ); result.push_back( eq(n1,2) && eq(n2,3) );... return result; }

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Examples Demo

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Castor About: –Pure header library (i.e. nothing to link). –Download from –Open source. (MIT License). –Sourceforge project name: castor-logic Compilers supported: –aCC, A (Mar ) –Borland C++ Builder 2007 –GCC, v4.1.0 (and above) –Microsoft Visual C and 2008

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More information Introduction to Logic Programming with C++ –www.mpprogramming.com/resources/CastorTutorial.pdf Castor reference manual –www.mpprogramming.com/resources/CastorReference.pdf Blending the Logic Programming Paradigm into C++ (on design & implementation of Castor) –www.mpprogramming.com/resources/CastorDesign.pdfwww.mpprogramming.com/resources/CastorDesign.pdf RSS feed on mpprogramming.com for updates.

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Q / A

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