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Logic Programming Introduction

Online Version of Learn Prolog Now: http://www.learnprolognow.org/

Hello World

  • Hypthoses are the "knowledge base"
    • Rules m(X):- h(X).<->
      • "implied by" - it's reversed!
    • Facts: h(s).
  • Query: ?- m(s).
  • is implied with uppercase variables

Knowledge Base

  • Hypotheses, called "Knowledge Base"
  • Every statement is a "clause" and ends with a .
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woman(mia).
woman(jody).
woman(yolanda).
playsAirGuitar(jody).
party.
  • If a query cannot be proven true, it is false
  • Unknown predicates return either false or throw an error (depending on the compiler)

Rules

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happy(yolanda). // fact
listens2music(mia). // fact
listens2music(yolanda):- happy(yolanda). // rule
playsAirGuitar(mia):- listens2music(mia).
playsAirGuitar(yolanda):- listens2music(yolanda).
  • Read: happy(yolanda) => listens2music(yolanda) (Reversed)

Conjuction

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playsAirGuitar(vincent):-
    listens2music(vincent),
    happy(vincent).
  • Read: "vincent plays air guitar if he listens to music and he's happy"

Disjuction

  • ; instead of ,

  • Same as defining to rules with the same goal

  • There's no negation in Prolog. An absence of rule is basically a negation

    • Prolog does no proofs of contradiction, but "negation as failure" (to construct a proof tree)

Asking for alternatives

  • ?- woman(X) gives every X that satisfies
  • (Uppercase) variables in the Knowledge Base take into account every occurence ( universially quantified)

Syntax

  • Atoms are strings starting with a lowercase letter or arbitrary strings in ''
  • Numbers are integers or floats (no type-checking)
  • Variables are Strings starting with an Uppercase Letter or with a _
    • Variables are always "bound" in a or
    • The variable _ is a "wildcard"
  • Complex Terms are e.g. playsAirGuitar(mia)
    • The functor must be an atom
  • "Arity" is the number of arguments in a complex term
    • Two predicates with different arity are treated as sepparate