Install SWI-Prolog interpreter with sudo snap install swi-prolog. Start it with swi-prolog.swipl.

Basic Syntax

A clause is ended with full stop (period).

Text between /* and */ is comment and is ignored.

Notes of LPP

Notes of book "Logic Programming with Prolog", 2nd edition by Max Bramer.

1.1

BIP: built-in predicates, like write, nl, halt and statistics.

1.2 - 1.3

Demo program:

dog(fido).
cat(felix).
animal(X):- dog(X).

Key concepts:

• predicate: dog(fido), in section 2.2, predicate is defined as: true or false, a relation between its arguments.

• function: a expression in other language, for example: 6 + 3, first three letter of 'hello world'. Anything can be evaluated to number or string. Only be used in arithmetic expression

• atom: a constant which is not a number, for example fido. Beginning with a lower case letter. Can be one of the three forms:

  • literal constant, e.g.: john
  • string: sequence of characters enclosed in single quotes, e.g.: 'This is a tring'
  • special character sequence, e.g.: +++, >=, +-

• variable: X, beginning with a capital letter or a underscore

• query: animal(fido).

• backtrack: make Prolog provide another solution with semicolon ; after a solution

• term: numbers, atoms, variable, compound term

• compound terms: like record in other programming language. General form: f(arg1, arg2, ...), where f is a functor, arg1, arg2, etc is *argument`

• functor: first part of a compound term, like record name, must be atom

• arity: number of arguments in a compound term

• argument: terms in parenthsis, like record fields, must be a (compound) term

• list: [arg1, arg2, ...], arguments in a list may be any kind

• call terms: atoms and compound terms

2.1

• Clause: facts, or rules

• Form of facts: head.

• head must be a call term, represent the conclusion

• Form of rules: head:- t_1,t_2,...,t_k

• :-: neck of a clause, read as "if"

• t1,t2,... as a whole is called the body of the clause, specifies the conditions that must be met for the conclusion represented by the head

• Each component seperated by comma in the body is called a goal. Each goal must be a call term

• Commas in the body are read as and

A rule is read as "head is true if t_1, t_2, ..., t_k are all true". Or "to achieve goal head, it's necessary to achieve subgoals t_1, t_2, ..., t_k in turn.