The current version of SWI-Prolog provides two formatted write predicates. The first is writef/[1,2], which is compatible with Edinburgh C-Prolog. The second is format/[1,2], which is compatible with Quintus Prolog. We hope the Prolog community will once define a standard formatted write predicate. If you want performance use format/[1,2] as this predicate is defined in C. Otherwise compatibility reasons might tell you which predicate to use.
write(Term), nl.
writef(Atom, []).
Escape sequences to generate a single special character:
| \n | Output a newline character (see also nl/[0,1]) | 
| \l | Output a line separator 
(same as \n) | 
| \r | Output a carriage-return character (ASCII 13) | 
| \t | Output the ASCII character TAB (9) | 
| \\ | The character is output | 
| \% | The character is output | 
| \nnn | where <nnn> is an integer (1-3 digits) the character with character code <nnn> is output (NB : <nnn> is read as decimal) | 
Note that \l, \nnn and \\ are 
interpreted differently when character-escapes are in effect. See
section 2.15.1.2.
Escape sequences to include arguments from Arguments. Each time a % escape sequence is found in Format the next argument from Arguments is formatted according to the specification.
| %t | print/1 the next item (mnemonic: term) | 
| %w | write/1 the next item | 
| %q | writeq/1 the next item | 
| %d | Write the term, ignoring operators. See also write_term/2. Mnemonic: old Edinburgh display/1. | 
| %p | print/1 
the next item (identical to %t) | 
| %n | Put the next item as a character (i.e., it is a character code) | 
| %r | Write the next item N times where N is the second item (an integer) | 
| %s | Write the next item as a String (so it must be a list of characters) | 
| %f | Perform a ttyflush/0 (no items used) | 
| %Nc | Write the next item Centered in N columns. | 
| %Nl | Write the next item Left justified in N columns. | 
| %Nr | Write the next item Right justified in N columns. N is a decimal number with at least one digit. The item must be an atom, integer, float or string. | 
?- swritef(S, '%15L%w', ['Hello', 'World']). S = "Hello World"
swritef(String, Format, []).
format(Format) :- format(Format, []).'Special sequences start with the tilde (~`<character>, 
representing the character code value of the character (only useful for ~t) 
or a asterisk (*
Numeric conversion (d, D, e, E, f, g 
and G) accept an arithmetic expression as argument. This is 
introduced to handle rational numbers transparently (see
section 4.26.2.2. The floating 
point conversions allow for unlimited precision for printing rational 
numbers in decimal form.
~
a
c
d
D
e
E
fgGiknNpqr~16r prints its argument hexadecimal. The argument should 
be in the range [2, ... , 36]. Lower case letters are used 
for digits above 9.
R
s
@current_output stream is inserted at this place. Goal 
is called in the module calling format/3. 
This option is not present in the original definition by Quintus, but 
supported by some other Prolog systems.
t~t statements between the tab stops. This space is padded 
with spaces by default. If an argument is supplied this is taken to be 
the character code of the character used for padding. This can be used 
to do left or right alignment, centering, distributing, etc. See also ~| 
and ~+ to set tab stops. A tab stop is assumed at the start 
of each line.
|~t's to be distributed between the previous and this tab 
stop.
+~|. 
w
Wformat(' W', [Term, [numbervars(true)]]). This 
option is SWI-Prolog specific.
Example:
simple_statistics :-
    <obtain statistics>         % left to the user
    format('~tStatistics~t~72|~n~n'),
    format('Runtime: ~`.t ~2f~34|  Inferences: ~`.t ~D~72|~n',
                                            [RunT, Inf]),
    ....
Will output
                             Statistics
Runtime: .................. 3.45  Inferences: .......... 60,345
library(backcomp). For example:
?- format(atom(A), '~D', [1000000]). A = '1,000,000'
~c (tilde, followed by some character) is 
found, the format derivatives will first check whether the user has 
defined a predicate to handle the format. If not, the built in 
formatting rules described above are used. Char is either an ASCII 
value, or a one character atom, specifying the letter to be (re)defined. Head 
is a term, whose name and arity are used to determine the predicate to 
call for the redefined formatting character. The first argument to the 
predicate is the numeric argument of the format command, or the atom default 
if no argument is specified. The remaining arguments are filled from the 
argument list. The example below redefines ~n to produce Arg 
times return followed by linefeed (so a (Grr.) DOS machine is happy with 
the output).
:- format_predicate(n, dos_newline(_Arg)).
dos_newline(default) :- !,
        dos_newline(1).
dos_newline(N) :-
        (   N > 0
        ->  write('\r\n'),
            N2 is N - 1,
            dos_newline(N2)
        ;   true
        ).
user, Head is 
qualified with the definition module of the predicate.