Term input/output

These predicates^8.4 enable a Prolog term to be input from a stream, or to be output to a stream. A term that is to be input must be followed by a period that is followed by whitespace.

• read_term(Stream,Term,Options): Inputs a term Term from the stream Stream, using options Options. After reaching the end of file, it unifies Term with end_of_file. The Options is a list of options that can include:
  • variables(V_list) - After reading a term, V_list will be unified with the list of variables that occur in the term.
  • variable_names(VN_list) - After reading a term, VN_list will be unified with a list of elements in the form of N = V, where V is a variable occurring in the term, and N is the name of V.
  • singletons(VS_list) - After reading a term, VS_list will be unified with a list of elements in the form N = V, where V is a singleton variable in Term, and N is its name.

• read_term(Term,Options): This is the same as read_term(Stream,Term,Options), except that the current input stream is used.

• read(Stream,Term): This is equivalent to read_term(Stream,Term),[]).

• read(Term): This is equivalent to read_term(Term,[]).

• write_term(Stream,Term,Options): Outputs a term Term into a stream Stream, using the option list Options. The list of options Options can include^8.5:
  • quoted(Bool) - When Bool is true, each atom and functor is quoted, such that the term can be read by read/1.

  • ignore_ops(Bool) - When Bool is true, each compound term is output in functional notation, i.e., in the form of f(A1,...,An), where f is the functor, and Ai (i=1,...,n) are arguments.

• write_term(Term,Options): This is the same as write_term(Stream,Term,Options), except that the current output stream is used.

• write(Stream,Term): This is equivalent to write_term(Stream,Term,[]).

• write(Term): This is equivalent to

        current_output(Stream),write(Stream,Term).
  

• write_canonical(Stream,Term): This is equivalent to

        write_term(Stream,Term,[quoted(true),ignore_ops(true)]).
  

• write_canonical(Term): This is equivalent to

        current_output(Stream),write_canonical(Stream,Term).
  

• writeq(Stream,Term): This is equivalent to

        write_term(Stream,Term,[quoted(true)]).
  

• writeq(Term): This is equivalent to

        current_output(Stream),writeq(Stream,Term).
  

• portray_clause(Clause):
• portray_clause(Stream,Clause): After the variables in Clause are numbered, writes Clause with the body indented, the same as in listing.

• op(Priority,Specifier,Name): Makes atom Name an operator of type Specifier and priority Priority^8.6. Specifier specifies the class (prefix, infix or postfix) and the associativity, which can be:
  • fx - prefix, non-associative.
  • fy - prefix, right-associative.
  • xfx - infix, non-associative.
  • xfy - infix, right-associative.
  • yfx - infix, left-associative.
  • xf - postfix, non-associative.
  • yf - postfix, left-associative.
  The priority of an operator is an integer greater than 0 and less than 1201. The lower the priority, the stronger the operator binds its operands.

• current_op(Priority,Specifier,Operator): This predicate is true if Operator is an operator with properties that are defined by a specifier Specifier and precedence Priority.