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print(+Stream, ?Term)

The term Term is written on the output stream Stream according to the current operator declarations, using the predicate portray/2 or portray/1 if it exists.
Stream handle or alias (atom)
Prolog term.


Used to print the term Term on the output stream Stream according to the current operator declarations, i.e. the same as write/2, 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/2 was called from, it is used by print/2 in the following way:

* If Term is a variable, it is printed using write/2.

* If Term is a nonvariable or an attributed variable, then portray(Stream, Term) is called. If it succeeds, so does print/2. Otherwise, if Term is atomic, it is written using write/2 and the predicate succeeds. If Term is a compound term, its main functor is printed using write/2 and print/2 is called recursively on its arguments.

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 [].

If portray/2 is not visible but portray/1 is, it is called instead of portray/2, with the 'output' stream temporarily redirected to Stream. Because of this side effect, defining portray/2 is preferrable.

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(S, Term) is equivalent to write_term(S, 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/2 is not necessarily in a form acceptable to read/1,2 and there is no 'printq' predicate.

Modes and Determinism


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


(4) instantiation fault
Stream is not instantiated.
(5) type error
Stream is not an atom or a stream handle.
(192) illegal stream mode
Stream is not an output steam.
(193) illegal stream specification
Stream is not a stream specification.


    ?- [user].
     portray(S, a) :- write(S, b).

     user   compiled 148 bytes in 0.00 seconds

    ?- write(write(a)), nl, print(output, print(a)).
    ?- trace.

    Debugger switched on - creep mode
    ?- p(a).
      (1) 0  CALL   p(a) (dbg)?- output: write        ('o' typed)
      (1) 0  CALL   p(a) (dbg)?- output: display
      (1) 0  CALL   p(a) (dbg)?- output: print/writeq
      (1) 0  CALL   p(b) (dbg)?- creep
      (1) 0  EXIT   p(b) (dbg)?- creep


     print(S, a(b,c)).         (Error 4).
     print("str", a(b,c)).     (Error 5).
     print(input, X).          (Error 192).
     print(nostr, X + 2).      (Error 193).

See Also

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