Built-in predicates

10.1. Built-in predicates#

Term manipulation


`Term1 = Term2`	`Term1` and `Term2` are unified.
`Term1 \= Term2`	`Term1` and `Term2` cannot be unified.
`Term1 == Term2`	`Term1` and `Term2` are bound to the same term.
`Term1 \== Term2`	`Term1` and `Term2` are bound to different terms.
`var(V)`	`V` is an unbound variable.
`arg(N,T,A)`	`A` is the `N`-th argument of term `T`.
`functor(T,F,N)`	`T` is a term with functor `F` and arity `N`.
`Term =.. List`	`List` is a list starting with the functor of `Term`, followed by its arguments.
`varsin(Term,Vs)`	`Vs` is a list of the variables in `Term`.
`numbervars(T,N,M)`	the variables in term `T` are instantiated to terms of the form `'$VAR'(` n `)`, where n is an integer which has a different value for each distinct variable. The variables will be numbered starting from `N`, and `M` is the next unused number.

Database manipulation


`assert(Clause)`	`Clause` is added at the end of the database (can also be a single atom, without a body).
`asserta(Clause)`	`Clause` is added at the top of the database.
`clause(H,B)`	`H:-B` is a clause in the database (`H` must be instantiated).
`retract(Clause)`	the first clause unifying with `Clause` is removed from the database.
`retractall(Head)`	all clauses with head unifying with `Head` are removed from the database.
`kill(Predicate)`	all clauses defining `Predicate` are removed from the database.
`save(File)`	save the clauses in the database to `File`.
`consult(File)`	load the clauses in `File` into the database.
`op(P,Type,Name)`	declare an operator `Name` with priority `P` (a number between 0 and 1200, lower priority binds stronger); `Type` is `fx` or `fy` for prefix, `xfx`, `xfy` or `yfx` for infix, and `xf` or `yf` for postfix.

Control


`call(Goal)`	call `Goal` (must be instantiated to a goal).
`not(Goal)`	`Goal` is not provable (must be instantiated to a goal). `not/1` could be defined as `not(Goal):-call(Goal),!,fail.` `not(Goal).`
`fail`, `false`	forces failure.
`true`, `otherwise`	always succeeds. `true/0` could be defined as `true.`
`repeat`	succeeds indefinitely many times on backtracking. `repeat/0` could be defined as `repeat.` `repeat:-repeat.`
`findall(X,G,L)`	`L` is a list of `X`’s, one for each solution of `G` (succeeds with the empty list if no solutions are found).
`bagof(X,G,L)`	`L` is a list of `X`’s, one for each solution of `G`, which may be preceded by existential variables (fails if no solutions are found).
`setof(X,G,L)`	as `bagof/3`, but `L` is a sorted list without duplicates.
`forall(G,C)`	for all the solutions of `G`, `C` is true. `forall/2` could be defined as `forall(G,C):-not((G,not(C))).`

Interaction


`read(Term)`	`Term` is instantiated to the next line typed by the user (must be a Prolog term).
`write(Term)`	write `Term` to the screen.
`tab(N)`	write `N` spaces to the screen.
`nl`	write a newline to the screen.
`get(C)`	`C` is ASCII code of next character typed by the user.
`put(C)`	write character with ASCII code `C` to the screen.
`tell(File)`	redirect output to `File`.
`told`	stop redirecting output.
`see(File)`	redirect input from `File`.
`seen`.	stop redirecting input.