By default, the effect of print/[1,2] is the same as that of
write/[1,2], but you can change its effect
by providing clauses for the hook predicate user:portray/1.
If X is a variable, it is printed using write(X).
Otherwise the user-definable procedure user:portray(X) is called.
If this succeeds, it is assumed
that X has been printed and print/[1,2] exits (succeeds).
If the call to user:portray/1 fails, and if X is a compound term,
write/[1,2] is used to write the principal functor of X and
print/[1,2] is called recursively on its arguments. If X
is atomic, it is written using write/[1,2].
When print/[1,2] has to print a list, say [X1,X2,...,Xn], it passes
the whole list to user:portray/1. As usual, if user:portray/1 succeeds,
it is assumed to have printed the entire list, and print/[1,2] does
nothing further with this term. Otherwise print/[1,2] writes the
list using bracket notation, calling print/[1,2] 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/[1,2] to be called
recursively on the two arguments X1 and [X2,...,Xn], giving user:portray/1
a second
chance at [X2,...,Xn]. This does not happen; lists are a
special case in which print/[1,2] is called separately for each of
X1,X2,...Xn.