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CSE3302 Programming Languages (notes?)

CSE3302 Programming Languages (notes?). Dr. Carter Tiernan. Logic Programming. Nonprocedural programming Higher-level language allows one to express the same program with less detail Language does more automatically Programmer focuses more on what to do not on how to do it.

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CSE3302 Programming Languages (notes?)

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  1. CSE3302Programming Languages(notes?) Dr. Carter Tiernan Programming Languages

  2. Logic Programming • Nonprocedural programming • Higher-level language allows one to express the same program with less detail • Language does more automatically • Programmer focuses more on what to do not on how to do it Programming Languages

  3. Automatic deduction • Automatic theorem proving • The development of programs that can construct formal proofs of propositions stated in symbolic language • Side effect of proof is construction of a result which demonstrates the proof • Programs expressed in the form of propositions that assert the existence of a desired result Programming Languages

  4. Prolog • Programs are structured like theorems • Clauses that define the problem domain • Facts (database of relationships among atomic individuals) • Goals • Is a fact provable? • Is there an individual satisfying the goal? • Is there a solution satisfying the goal? Programming Languages

  5. Prolog Clauses • Clauses define relationships or “predicates” • Facts or hypotheses parent (charles, harry). • Goals :- grandparent (elizabeth, X). • Conditions or rules grandparent(Y, Z) :- parent (Y,W), parent(W,Z). Programming Languages

  6. Predicates • <head> :- <body> • Predicates • Relationship applied to terms • Relationships • Properties of • Relations among • Terms • Atoms • Variables • Compound terms • Horn clause form Programming Languages

  7. Goals • Executing a goal • Match clauses in predicate by finding an assignment of values to the variables that makes the goal identical to the head of one of the clauses (unification) • Variables are bound (instantiated) to create subgoal • Recursive application and pattern matching Programming Languages

  8. Compound terms • Compound term allows us to describe individuals without naming them • Functor with atoms or variables d(X, plus(U,V), plus(DU,DV)) or d(X, U + V, DU + DV) • Similar to LISP list structure • Acts somewhat like a function but is not a function call Programming Languages

  9. Data structures • No constructors • Data structures are implicitly defined by their properties • Few primitives • Compound terms can provide logical description of structure • Some Prologs allow infix notation for functors Programming Languages

  10. Complex structures • Compound terms represent themselves • Symbolic notations can be defined directly • Good for mathematical relationships • Predicates can define structures • Predicates can replace compound terms • Expressions matched to clauses must exist within the Prolog “closed world” • Good for object-oriented relationships Programming Languages

  11. More data structures • Abstract data types - so abstract they’re only described • Infinite terms - “occurs check” • Representation of ‘infinite’ list is finite • Circular structure Programming Languages

  12. Control Structures • Separation of logic and control • Independent analysis • Order of clauses has no effect on meaning or logic • Control affects generation and unification of subgoals • Efficiency is an issue but not meaning Programming Languages

  13. Subgoal Generation • Top-down • Start from goal; try to reach hypotheses • Recursive approach • Bottom-up • Start with hypotheses; try to reach goal • Iterative approach Programming Languages

  14. Backtracking • Multiple matching clauses may be available • If a failure occurs after a choice point, execution backtracks to the last choice point • Another match is made and execution continues. • Implementation of efficient backtracking is crucial in logic programming Programming Languages

  15. Input / Output Parameters • Goals attempt to satisfy subgoals with whatever value is unified • Parameters are neither inherently input nor output. Which ever is supplied is used as input. • When no parameter values are given, the systems attempts to search for any solution that satisfies the pattern Programming Languages

  16. Searching in Prolog • Depth-first search isspecified • Not pure logic programming • Attempts to satisfy goals in the order written • Will try matching clauses in the order in which they were entered into the DB Programming Languages

  17. Prolog vs. Logic Programming • Interpretation of arithmetic jumps beyond the bounds of strict logic programming unless handled as succ() • Efficiency requires the use underlying hardware support • ‘is’ gives an assignment • Binding forces ordering Programming Languages

  18. Search rationale • Breadth-first • searches paths in parallel • needs exponentially more space than depth-first • Depth-first • Can get caught in infinitely deep search • Programmer is required to order clauses to prevent endless search Programming Languages

  19. Nonmonotonic reasoning • Updateable database • Assert • Retract • Does not match logic • Does match world state changes over time Programming Languages

  20. Cuts • “You have found all the solutions there are; do not bother trying to find any others” • Predicate that always succeeds, but past which you can never backtrack • Used with repeat to provide looping Programming Languages

  21. Higher-order rules • Parameters must be terms not predicates • Logic programming is generally restricted to first-order logic • Resolution algorithm is complete only for first-order logic Programming Languages

  22. Negation • Unsatisfiability - cannot be proved true • Absence of data • Closed world assumption • Conclusions can be drawn about relationships that DO hold • NO conclusions can be made about relationships that do NOT hold • not( -- ) predicate succeeds if -- fails Programming Languages

  23. Equivalence • Term equality • Other types of equivalence cannot be defined • In terms of logical properties • Term inequality requires complete binding for correct interpretation Programming Languages

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