(apx:a.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. |