Questions and Answers
What happens when two free variables A and B are unified?
Which of the following pairings is not valid for unification in compound terms?
Why can't the variable A in f(A, b) = f(a, A) be unified with both a and b?
What is the correct structure of a rule in Prolog?
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Which of the following statements best describes facts in Prolog?
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In the expression parent(GP, GC) :- parent(GP, P), parent(P, GC), what does GP represent?
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What will the unification f(a) = f(b) return?
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If two compound terms have different names, what is the result of attempting to unify them?
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Which statement about compound term unification is false?
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In the context of Prolog, what is the main purpose of rules?
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Study Notes
SWI-Prolog Overview
- SWI-Prolog is a free implementation of the Prolog programming language, developed at the University of Amsterdam.
- Available for download at http://swi-prolog.org, with a web-based version, SWISH, accessible at http://swish.swi-prolog.org.
- Prolog files use the .pl extension.
Prolog Program Structure
- Programs consist of facts, predicates, relations defined as true, and rules that establish relationships between predicates.
- Distinction exists between functional programming (using functions) and logical programming (using predicates).
Querying in Prolog
- Prolog uses a querying system instead of traditional execution.
- Queries determine if a statement can be proven from existing facts and rules.
- The predicate
plus(Addend1, Addend2, Sum)
checks ifAddend1 + Addend2 = Sum
. - Examples of queries:
-
?- plus(1, 2, 3).
asks if 1 + 2 equals 3. -
?- plus(1, 2, Sum).
seeks the value of Sum that satisfies the equation.
-
Atoms in Prolog
- Atoms serve as tokens with meaning defined by the programmer (e.g.,
socrates
could represent a person or a pet). - Example:
GP is a grandparent of GC
if there exists a parent-child relationship.
Query Resolution Mechanism
- Prolog resolves queries by unifying them with facts or rules.
- A successful unification with a fact indicates truth; with a rule, the rule's body becomes the subgoal.
- Multiple potential unifying facts/rules create choice points in Prolog.
Handling Query Failure
- A query failure signifies that a statement cannot be proven, not necessarily that it is false.
Conjunction in Prolog
- Expressions can be evaluated for equality or inequality using
=:=
and=\=
respectively. - Example rules show how comparisons between expressions are made.
Arithmetic Operations
-
Term1 = Term2
denotes unification, whileExpr1 =:= Expr2
represents mathematical equality. - Example:
1 =:= 1.0
is true, but1 = 1.0
is false due to type differences.
Lists in Prolog
- Lists are structured as a head element(s) followed by a tail list, similar to structures in Scheme.
Variable Unification
- Unifying two free variables creates a single variable with multiple designations.
- Example:
A = B
links both names to the same unidentified object.
Unification of Compound Terms
- Compound terms unify only if they share the same name and arity, with corresponding arguments also unifying.
- Examples:
-
f(a) = f(a)
is valid. -
f(a) = g(a)
fails due to differing names.
-
Rules and Facts in Prolog
- Facts convey unconditional truths and are declared with syntax ending in a period, e.g.,
parent(marge, maggie).
- Rules derive new facts from known ones, formatted as
head :- term_1, term_2,..., term_n.
- Example:
grandparent(GP, GC) :- parent(GP, P), parent(P, GC).
demonstrates how to establish grandparent relationships through parenthood.
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Description
This quiz covers the fundamental principles of logic programming using SWI-Prolog, including installation and usage details. Explore the features of this powerful programming language and understand its application in various programming contexts.