[ Term I/O | Reference Manual | Alphabetic Index ]

# print(?Term)

The term Term is written on the output stream according to the current operator declarations, using the predicate portray/2 or portray/1 if it exists.

Term
  Prolog term.

## Description

Used to print the term Term on the current output according to the current operator declarations, i.e. the same as write/1, however the user has the possibility to influence the way the term is printed. If the predicate portray/2 is visible in the module where print/1 was called from, it is used by print/1 in the following way:

* If Term is a variable, it is printed using write/1.

* If Term is a nonvariable or an attributed variable, then portray(output, Term) is called. If it succeeds, so does print/1. Otherwise, if Term is atomic, it is written using write/1 and the predicate succeeds. If Term is a compound term, its main functor is printed using write/1 and print/1 is called recursively on its arguments.

If portray/2 is not visible but portray/1 is, it is called instead of portray/2.

Note that when this predicate is used to print a list, only the elements of the list, i.e. the heads, are passed to the recursive calls of print/2, but not the list tails. Thus e.g. a list [1,2,3] will be passed once to portray/2 as a whole and then the elements 1, 2, 3, but not [2,3], [3] and [].

portray/1, 2 is used by the system when printing the answer bindings in the top-level loop, and by the debugger to print trace lines.

print(Term) is equivalent to write_term(Term, [portrayed(true), numbervars(true)]).

As usual, the output is buffered, so it may need to be flushed (e.g. explicitly using flush/1).

Note The output of print/1 is not necessarily in a form acceptable to read/1,2.

### Modes and Determinism

- print(?) is det

### Modules

This predicate is sensitive to its module context (tool predicate, see @/2).

## Examples

    Success:
        ?- [user].
         portray(S, a) :- write(S, b).
         user   compiled 100 bytes in 0.02 seconds
        yes.
        ?- print([a, b, c, d]).
        [b, b, c, d]
        yes.

        ?- [user].
         portray(S, '$VAR'(X)) :- write(S, 'X_'), write(S, X).
         user   compiled 180 bytes in 0.00 seconds
        yes.
        ?- lib(numbervars).
        yes.
        ?- F=f(_,_,_,_), numbervars(F, 0, _), write(F).
        f(A, B, C, D)                % default printing of '$VAR'/1
        F = f(X_0, X_1, X_2, X_3)    % toplevel uses portray
        yes.

## See Also

display / 1, display / 2, print / 2, write / 1, write / 2, writeq / 1, writeq / 2