print/1 hookable



print(+Stream, +Term)

Writes Term to the current output stream, or Stream. Can be redefined with the hook portray/1.


Stream stream_object

Term term


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.

See Also

portray/1, library(printchars)