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writeclause(+Stream, ?Clause)

The clause Clause is pretty printed on the output stream Stream .
Stream
Integer (stream number) or Atom (reserved or user-defined symbolic stream name).
Clause
A Prolog term.

Description

Used to pretty print the clause Clause on the output stream Stream according to the current operator declarations.

When reading Prolog clauses from one file, and then writing to another, the latter part can be done using writeclause/2. This is because the clauses are terminated by a period and a newline, which are not retained by prolog. writeclause/2 replaces these, and flushes the output.

writeclause/1,2 knows about the special meaning of ,/2, ;/2, ->/2, fg -->/2 and :-/2 and prints the clause with the appropriate indentation of subgoals and some (redundant) parantheses to show the clause structure. Everything else is written as with writeq/1,2, so output of writeclause/1,2 is readable for read/1,2.

Modes and Determinism

Modules

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

Exceptions

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

Examples

Success:
      ?- writeclause(output, f(1,2,3)), writeclause(output, h(2,3)).
      f(1, 2, 3) .
      h(2, 3) .
      yes.

      ?- writeclause(output, X + 2).
      _56 + 2.
      yes.

      ?- writeclause(output, a(k):-write(k)).
      a(k) :-
              write(k) .
      yes.

      ?- writeclause(output, (a:-write(k),date(K))).
      a :-
              write(k),
              date(_68) .
      yes.

      ?- open(file1,update,s), writeclause(s, X + 2), close(s).
      X = _72
      yes.
      [eclipse]: sh('cat file1').
      _72 + 2.
      yes.

      ?- set_stream(a,output), writeclause(a, (:- dynamic f/1)).
      :- dynamic f / 1 .
      yes.

      ?- writeclause(output, (head:-a1,a2;a3,a4->a5;a6)).
      head :-
                (
                    a1,
                    a2
                ;
                    (
                        a3,
                        a4
                    ->
                        a5
                    ;
                        a6
                    )
                ).
      yes.

Error:
      writeclause(S, a(b,c)).         (Error 4).
      writeclause("string" a(b,c)).   (Error 5).

See Also

writeq / 2, writeclause / 1