4.6.4.3 Distinctions Among the “Write” Predicates

• For write and writeq, the term is written with respect to current operator declarations (See ref-syn-ops for a discussion of operators).

  write_canonical(Term) writes Term to the current or specified output stream in standard syntax (see ref-syn on Prolog syntax), and quotes atoms and functors to make them acceptable as input to read/[1,2]. That is, operator declarations are not used and compound terms are always written in the form:

  name(arg1, …, argn)
  

  and the special list syntax, e.g. [a,b,c], or braces syntax, e.g. {a,b,c} are not used.

  Calling write_canonical/1 is a good way of finding out how Prolog parses a term with several operators.

• Atoms, and other terms, output by write/[1,2] cannot in general be read back using read/[1,2]. For example,

  | ?- write('a b').
  a b
  

  For this reason write/[1,2] is only useful as a way to treat atoms as strings of characters. It is rarely, if ever, useful to use write/[1,2] with other kinds of terms, i.e. variables, numbers or compound terms.

  If you want to be sure that the atom can be read back by read/[1,2], then you should use writeq/[1,2], or write_canonical/[1,2], which put quotes around atoms when necessary, or use write_term/[2,3] with the quoted option set to true. Note also that the printing of quoted atoms is sensitive to character escaping (see ref-syn-ces).

• write/[1,2] and writeq/[1,2] use the write option numbervars(true), so treat terms of the form '$VAR'(N) specially: they write ‘A’ if N=0, ‘B’ if N=1, …‘Z’ if N=25, ‘A1’ if N=26, etc. Terms of this form are generated by numbervars/3 (see ref-lte-anv).

  | ?- writeq(a('$VAR'(0),'$VAR'(1))).
  a(A,B)
  

  write_canonical/1 does not treat terms of the form '$VAR'(N) specially.