portray hookprint/1 is called from within the system in two places:
By default, the effect of print/1 is the same as that of
write/1, but you can change its effect
by providing clauses for the hook predicate portray/1.
If X is a variable, then it is printed using write(X).
Otherwise the user-definable procedure portray(X) is called.
If this succeeds, then it is assumed
that X 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 X is a compound term, then
write/1 is used to write the principal functor of X and
print/1 is called recursively on its arguments. If X
is atomic, it is written using write/1.
When print/1 has to print a list, say [X1,X2,...,Xn], 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 [X1,X2,...,Xn] is simply a different way of
writing .(X1,[X2,...,Xn]), one might expect print/1 to be called
recursively on the two arguments X1 and [X2,...,Xn], giving portray/1
a second
chance at [X2,...,Xn]. This does not happen; lists are a
special case in which print/1 is called separately for each of
X1,X2,...Xn.
If you would like lists of character codes printed
by print/1 using double-quote notation, you should
include library(printchars) (described in lib)
as part of your version
of portray/1.
Often it is desirable to define clauses for portray/1 in different files.
This can be achieved either by declaring it multifile in each of the files, or
by using library(addportray).