CSC 270 Survey of Programming Languages. Basic Constructs of Prolog

Transcription

1 CSC 270 Survey of Programming Languages Prolog Lecture 1 Facts, Rules, and Queries Basic Constructs of Prolog Prolog has only three basic constructs: Facts statements that are unconditionally true Rules statements that are conditionally true Queries questions pertaining to the information stored in a knowledge base A knowledge base is a collection of facts and rules Writing a program in Prolog is all about creating the knowledge base. 1

2 Knowledge Base #1 woman(mia). woman(jody). woman(yolanda). playsairguitar(yolanda). party. Querying Knowledge Base #1 1?- % c:/documents and settings/adelphi/my documents/prolog/kb1 compiled 0.00 sec, 6 clauses 1?- woman(mia). true. 2?- playsairguitar(jody). false. 3?- playsairguitar(mia). false. 4?- playsairguitar(vincent). false. 2

3 5?- tatooed(jody). ERROR: toplevel: Undefined procedure: tatooed/1 (DWIM could not correct goal) 6?- party. true. 7?- rockconcert. ERROR: toplevel: Undefined procedure: rockconcert/0 (DWIM could not correct goal) 8?- Infer (or Deduce) Prolog determines that Vincent doesn t play Air Guitar because there is no fact from which it can infer (or deduce). The error messages for tatooed and rockconcert are due to the fact that SWI Prolog did not find these terms when compiling the knowledge base. 3

4 Knowledge Base #2 happy(yolanda). listens2music(mia). listens2music(yolanda) :- happy(yolanda). playsairguitar(mia) :- listens2music(mia). playsairguitar(yolanda) :- listens2music(yolanda). Querying Knowledge Base #2 9?- playsairguitar(mia). true. 10?- playsairguitar(yolanda). true. 11?- 4

5 Knowledge Base #3 This knowledge base consists of 2 facts and 3 rules. happy(vincent). listens2music(butch). playsairguitar(vincent):- listens2music(vincent),happy(vincent). playsairguitar(butch):-happy(butch). playsairguitar(butch):-listens2music(butch). Running Knowledge Base #3 11?- "playsairguitar(vincent)"? false. 12?- This is because while KB3 contains happy(vincent), it does not explicitly contain the information listens2music(vincent), and this fact cannot be deduced either. 5

6 Running Knowledge Base #3 (continued) 12?- playsairguitar(butch). true. 13?- The knowledge base gives us two ways of deducing this through two different rules: playsairguitar(butch):-happy(butch). playsairguitar(butch):-listens2music(butch). This is effectively equivalent to an OR. Knowledge Base #4 woman(mia). woman(jody). woman(yolanda). loves(vincent,mia). loves(marsellus,mia). loves(pumpkin,honey_bunny). loves(honey_bunny,pumpkin). 6

7 Querying Knowledge Base #4 16?- woman(x). X = mia ; X = jody ; X = yolanda. 17?- loves(marsellus, X). X = mia. 18?- loves(marsellus, X), woman(x). X = mia. Knowledge Base #5 loves(vincent,mia). loves(marsellus,mia). loves(pumpkin,honey_bunny). loves(honey_bunny,pumpkin). jealous(x,y):- loves(x,z), loves(y,z). 7

8 Running Knowledge Base #5 20?- jealous(marsellus, W). W = vincent Prolog Syntax Facts, rules and queries are built from terms. There are four types of terms in Prolog: atoms numbers variables complex terms (or structures) Atoms and numbers are constants and constants and variables make up the simple terms of Prolog. 8

9 An atom is either: Atoms 1. A string of uppercase letters, lowercase letters, underscores and digits that begin with a lowercase letter. (E.g., butch, big_kahuna, etc.) 2. An arbitrary sequence of characters inside single quotes (E.g., Vincent, The Gimp, ((*&^*&^*&^ ) 3. A string of special characters ===>, ;, and :-) Numbers The typical Prolog application does not make heavy use of numbers. Most Prolog implmentations support floating point numbers and integers. 9

10 Variables A variable is a string of uppercase letters, lowercase letters, digits and underscores that starts with either an uppercase letter or an underscore. (E.g., X, Y, Variable, _tag, X_526, List, List24, etc.) _ is a special variable and it is called the anonymous variable. Complex Terms We can combine atoms, numbers and variables to form complex terms (also called structures). Complex terms are built out of a functor followed by a sequence of arguments. Examples playsairguitar(jody). loves(vincent, mia). hide(x, father(father(father(butch)))). 10

11 Recursive Structure Nesting complex terms inside complex terms indefinitely leads to recursive structures. Arity Arity refers to the number of arguments that a functor has. It becomes important when the same functor can have more than one different count of arguments (or different arities). 11