print/1 hookableprint(+Term)
print(+Stream, +Term)
Writes Term to the current output stream, or Stream.
Can be redefined with the hook portray/1.
By default, the effect of this predicate is the same as that of
write/1, but you can change its effect by providing clauses for
the predicate portray/1.
If Term is a variable, then it is printed using
write(Term).
Otherwise the user-definable procedure
portray(Term) is called. If this succeeds, then it is assumed
that Term has been printed and print/1 exits (succeeds).
Note that print/1 always calls portray/1 in module user.
Therefore, to be visible to print/1, portray/1 must either be
defined in or imported into module user.
If the call to portray/1 fails, and if Term is a compound term, then
write/1 is used to write the principal functor of Term and
print/1 is called recursively on its arguments. If Term
is atomic, it is written using write/1.
When print/1 has to print a list, say [Term1,Term2,...,TermN], it passes
the whole list to portray/1. As usual, if portray/1 succeeds,
it is assumed to have printed the entire list, and print/1 does
nothing further with this term. Otherwise print/1 writes the
list using bracket notation, calling print/1 on each element of the
list in turn.
Since [Term1,Term2,...,TermN] is simply a different way of
writing .(Term1,[Term2,...,TermN]), one might expect print/1 to be called
recursively on the two arguments Term1 and [Term2,...,TermN], giving portray/1
a second
chance at [Term2,...,TermN]. This does not happen; lists are a
special case in which print/1 is called separately for each of
Term1,Term2,...TermN.
If you would like lists of character codes printed
by print/1 using double-quote notation, you should
include library(printchars) (see lib-abs)
as part of your version of portray/1.
Succeeds without error, except for any errors
that may occur in the execution of portray/1.
portray/1, library(printchars)