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 therefore always written in the form:
predicate_name(arg1, ..., argn)
write/[1,2] cannot in general be read back using
read/[1,2]. For example,
| ?- write('a b').
a b
If you want to be sure that the atom can be read back by read/[1,2], 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 yes. Note also that the printing
of quoted atoms is sensitive to character escaping (see ref-syn-ces).
write/[1,2] and writeq/[1,2] treat terms of the form '$VAR'(N)
specially: they write `A' if N=0, `B' if N=1, ...`Z' if N=25,
`A1' if N=26, etc. Terms of this form are generated by
numbervars/3 (see ref-lte-anv). Terms of the form '$VAR'(X),
where X is not a number are written as unquoted terms. For example,
| ?- writeq(a('$VAR'(0),'$VAR'('Test'))).
a(A,Test)
write_canonical/1 does not treat terms of the form '$VAR'(N)
specially. It writes square bracket lists using ./2 and
[] (that is, [a,b] is written as `.(a,.(b,[]))').