CSE 425: Logic Programming I Logic and Programs Most programs use Boolean expressions over data Logic statements can express program semantics –I.e., axiomatic semantics Logic also can be the subject of computation –E.g., automatic deduction systems, theorem-provers Can handle computation and proof interchangeably –E.g., for restricted forms of logic (Horn Clauses  FOL) Programming languages then can help automate this –E.g., logic programming in Prolog Mechanisms readily implemented in a functional style –E.g., in Scheme or other functional programming languages

CSE 425: Logic Programming I Logic Programs Logic programming languages (and programming frameworks in other languages like Scheme or C++) –Provide a means for encoding well formed statements –Provide algorithms for implementing inference rules Sometimes called “deductive databases” –Logic programming systems as logic + control where the logic is specified and the control is automated –To be feasible, however, some restrictions are needed Widely applicable beyond theorem proving –E.g., programming robots to adapt to situational factors

CSE 425: Logic Programming I Intro to Prolog The most widely used logic programming language –Though lots of logic programming is done in other languages Syntax is similar to first-order-logic (FOL) syntax –E.g., ancestor(x,y)^ancestor(y,z) → ancestor(x,z) in FOL is ancestor(X,Z) :- ancestor(X,Y), ancestor(Y,Z) in Prolog –Note: variables are capitalized (or begin with an underscore) –Note: comma indicates conjunction (and) –Also, Prolog uses =< instead of <= for less-than-or equal Lists are delimited by square brackets in Prolog –E.g., [alice, bob, X] Can do head|tail matching (as in ML, Haskell) –E.g., [H|T] = [alice, bob, X] gives [alice] and [bob, X]

CSE 425: Logic Programming I Prolog Interpreter Environment Maintains a database of statements and variables –E.g., loaded from a file, and/or entered by the user User can enter a predicate as a query –E.g., ancestor(bob,alice) –Interpreter matches query to statements in its database –If query is fully bound, will produce a yes or no answer –If query has a variable, will find a satisfactory binding User also can ask for the result of an expression –E.g., X is 3 + 5, write(X) Unification in Prolog –A constant only unifies with itself (exact match is required), while a variable may unify via renaming and/or binding –Predicates unify if same signature, and the arguments unify

CSE 425: Logic Programming I Today’s Studio Exercises We’ll code up initial ideas from Scott Chapter 11 –Looking at basic parsing and evaluation mechanisms for propositional and predicate logic, and proceeding from there Today’s exercises are again in C++ –Please take advantage of the text books and the on-line pages that are linked on the course web site –As always, please ask for help as needed When done, your answers to the course account, with subject line “Logic Programming Studio I”