This section describes the basic term reading and writing predicates. The predicates format/[1,2] and writef/2 provide formatted output. Writing to Prolog data structures such as atoms or code-lists is supported by with_output_to/2 and format/3.
Reading is sensitive to the Prolog flag character_escapes,
which controls the interpretation of the \
ignore (ignore the attribute), dots
(write the attributes as {...}), write (simply
hand the attributes recursively to write_term/2)
and portray (hand the attributes to attr_portray_hook/2).string causes string
objects to be printed between back quotes and symbol_char
causes the backquote to be printed unquoted. In all other cases the
backquote is printed as a quoted atom.true (default), write {}(X) as {X}.
See also
dotlists and ignore_ops.portray,
portray/1
is called for each blob encountered. See section
12.4.10.true and quoted(true) is active, special
characters in quoted atoms and strings are emitted as ISO escape
sequences. Default is taken from the reference module (see below).true and character_escapes(true) and
quoted(true) are active escapted characters are written
using
\uXXXX or \UXXXXXXXX syntax. The default
depends on the Prolog flag character_escapes_unicodetrue (default), cyclic terms are written as
@(Template, Substitutions), where Substitutions
is a list
Var = Value. If cycles is false,
max_depth is not given, and Term is cyclic, write_term/2
raises a domain_error.106The
cycles option and the cyclic term representation using the @-term are
copied from SICStus Prolog. However, the default in SICStus is set to false
and SICStus writes an infinite term if not protected by, e.g., the depth_limit
option. See also the cycles option in
read_term/2.true (default false), write lists using the
dotted term notation rather than the list notation.107Copied
from ECLiPSe. Note that as of version 7, the list
constructor is
'[|]'. Using dotlists(true), write_term/2
writes a list using‘.' as constructor. This is intended for
communication with programs such as other Prolog systems, that rely on
this notation. See also the option no_lists(true) to use
the actual SWI-Prolog list functor.true (default false), add a fullstop token
to the output. The dot is preceded by a space if needed and followed by
a space (default) or newline if the nl(true) option is also
given.108Compatible with ECLiPSetrue, the generic term representation (<functor>(<args>
... )) will be used for all terms. Otherwise (default), operators will
be used where appropriate.109In
traditional systems this flag also stops the syntactic sugar notation
for lists and brace terms. In SWI-Prolog, these are controlled by the
separate options dotlists and brace_terms.
?- write_term(a(s(s(s(s(0)))), [a,b,c,d,e,f]),
[max_depth(3)]).
a(s(s(...)), [a, b|...])
true.
Used by the top level and debugger to limit screen output. See also the Prolog flags answer_write_options and debugger_write_options.
user). This defines
the default value for the character_escapes
option as well as the operator definitions to use. If Module
does not exist it is
not created and the user module is used. See also op/3
and read_term/2,
providing the same option.fullstop option.dotlists(true),
but uses the SWI-Prolog list functor, which is by default '[|]'
instead of the ISO Prolog '.'. Used by display/1.true, terms of the format $VAR(N), where N
is an integer that fits in 64-bit,110Larger
integers are ignored. As no term that fits into memory can have that
many variables, this is not a restriction. will be written
as a variable name. For N in 0..25 it emits A..Z. For higher
numbers it emits An..Zn, where
n is N//26. For negative numbers it emits S_N,
which is used for representing shared sub-terms and cyclic terms.
If N is an atom it is written without quotes. This
extension allows for writing variables with user-provided names. The
default is
false. See also numbervars/3
and the option
variable_names.
true (default false), do not reset the
logic that inserts extra spaces that separate tokens where needed. This
is intended to solve the problems with the code below. Calling write_value(.)
writes .., which cannot be read. By adding partial(true)
to the option list, it correctly emits . .. Similar
problems appear when emitting operators using multiple calls to write_term/3.
write_value(Value) :-
write_term(Value, [partial(true)]),
write('.'), nl.
In addition, if the priority is not 1200 or 999 this assumes we are printing an operant of an operator. If Term is an atom that is also an operator it will always be embraced.111If the priority is 1200 it is assumed to be a toplevel term and if the priority is 999 it is assumed to be a list element or argument of a compound term.
portrayed(Bool). Deprecated.portray(true), but calls Goal rather
than the predefined hook portray/1. Goal
is called through call/3,
where the first argument is Goal, the second is the term to
be printed and the 3rd argument is the current write option list. The
write option list is copied from the write_term call, but the list is
guaranteed to hold an option priority that reflects the
current priority.true, the hook portray/1
is called before printing a term that is not a variable. If portray/1
succeeds, the term is considered printed. See also print/1.
The default is false. This option is an extension to the
ISO write_term options.
format('~w = ', [VarName]),
write_term(Value, [quoted(true), priority(699)])
true, atoms and strings that need quotes will be quoted.
The default is false. If character_escapes
is
true (default) characters in the quoted atom or string are
escaped using backslash (\\u0020)
are escaped. For those characters for which an ISO Prolog single
character escape, e.g.,
\t is defined, this is used. Otherwise the output depends
on the option character_escapes_unicode. If this flag
applies(default) the widely accepted \uXXXX or \UXXXXXXXX
is used. Otherwise the ISO Prolog \x<hex>\ syntax is
used.standard, adding only space
where needed for proper tokenization by read_term/3.
Currently, the only other value is next_argument, adding a
space after a comma used to separate arguments in a term or list.
The implementation binds the variables from List to a term
'$VAR'(Name). Like write_canonical/1,
terms that where already bound to '$VAR'(X)
before write_term/2
are printed normally, unless the option numbervars(true) is
also provided. If the option numbervars(true) is used, the
user is responsible for avoiding collisions between assigned names and
numbered names. See also the variable_names option of
read_term/2.
Possible variable attributes (see section 8.1) are ignored. In most cases one should use copy_term/3 to obtain a copy that is free of attributed variables and handle the associated constraints as appropriate for the use-case.
write_term(Term, Options). In addition to valid options for
write_term/2,
it processes the option:
ignore_ops,
quoted, quote_non_ascii, brace_terms(false)
and
numbervars after numbervars/4
using the singletons option.
Note that due to the use of numbervars/4,
non-ground terms must be written using a single write_canonical/1
call. This used to be the case anyhow, as garbage collection between
multiple calls to one of the write predicates can change the _<NNN>
identity of the variables.
write_term(Term, [quoted(true), numbervars(true)]).write(Term), nl.. The output stream is
locked, which implies no output from other threads can appear between
the term and newline.write(Stream, Term), nl(Stream).. The output
stream is locked, which implies no output from other threads can appear
between the term and newline.
print(Term) :-
current_prolog_flag(print_write_options, Options), !,
write_term(Term, Options).
print(Term) :-
write_term(Term, [ portray(true),
numbervars(true),
quoted(true)
]).
The print/1
predicate is used primarily through the ~p escape sequence
of format/2,
which is commonly used in the recipes used by
print_message/2
to emit messages.
The classical definition of this predicate is equivalent to the ISO
predicate write_term/2
using the options portray(true) and
numbervars(true). The portray(true) option
allows the user to implement application-specific printing of terms
printed during debugging to facilitate easy understanding of the output.
See also
portray/1
and library(portray_text). SWI-Prolog adds quoted(true)
to (1) facilitate the copying/pasting of terms that are not affected by
portray/1
and to (2) allow numbers, atoms and strings to be more easily
distinguished, e.g., 42, '42' and "42".
end_of_file. This is the same as read_term/2
using an empty option list.
[NOTE] You might have found this while looking for a predicate
to read input from a file or the user. Quite likely this is not what you
need in this case. This predicate is for reading a Prolog term
which may span multiple lines and must end in a full stop (dot
character followed by a layout character). The predicates for reading
and writing Prolog terms are particularly useful for storing Prolog data
in a file or transferring them over a network communication channel
(socket) to another Prolog process. The libraries provide a wealth of
predicates to read data in other formats. See e.g., library(readutil),
library(pure_input) or libraries from the extension
packages to read XML, JSON, YAML, etc.
dec10 (report and restart).true (default), call
prolog:comment_hook(Comments, TermPos, Term) if this
multifile hook is defined (see prolog:comment_hook/3).
This is used to drive PlDoc.process_comment(false).
The singletons option of read_term/3
is initialised from the active style-checking mode. The module
option is initialised to the current compilation module (see prolog_load_context/2).
true, read `...` to a string
object (see
section 5.2). The default
depends on the Prolog flag
back_quotes.\ escape sequences in quoted atoms. See
the Prolog flag character_escapes
in current_prolog_flag/2.
(SWI-Prolog).true (default false), re-instantiate
templates as produced by the corresponding write_term/2
option. Note that the default is false to avoid
misinterpretation of @(Template, Substitutions), while the
default of write_term/2
is true because emitting cyclic terms without using the
template construct produces an infinitely large term (read: it will
generate an error after producing a huge amount of output).true (default false), read .(a,[])
as a list, even if lists are internally constructed a different functor
([|](Head,Tail)). This is primarily intended to read the
output from write_canonical/1
from other Prolog systems. See
section 5.1.user is used because new modules by
default inherit from userquasi_quotation(+Syntax,
+Quotation, +VarDict, -Result), where Syntax is the
term in
{|Syntax||..|}, Quotation is a list of character
codes that represent the quotation, VarDict is a list of
Name=Variable and Result is a variable
that shares with the place where the quotation must be inserted. This
option is intended to support tools that manipulate Prolog source text.variable_names, but only reports the variables occurring
only once in the Term read (ISO). If Vars is the
constant
warning, singleton variables are reported using print_message/2.
The variables appear in the order they have been read. The latter option
provides backward compatibility and is used to read terms from source
files. Not all singleton variables are reported as a warning. See
section 2.16.1.10
for the rules that apply for warning about a singleton variable.112As
of version 7.7.17, all variables starting with an underscore
except for the truly anonymous variable are returned in Vars.
Older versions only reported those that would have been reported if warning
is used.error (default), throw an exception on a syntax error.
Other values are fail, which causes a message to be printed
using
print_message/2,
after which the predicate fails, quiet which causes the
predicate to fail silently, and dec10 which causes syntax
errors to be printed, after which read_term/[2,3]
continues reading the next term. Using dec10, read_term/[2,3]
never fails. (Quintus, SICStus).").{...}, as used in DCG rules. Arg
describes the argument.|none.key_value_position/7 terms. The
key_value_position/7 terms appear in the order of the
input. Because maps do not preserve ordering, the key is provided in the
position description.term_position/5 that would
be created, except that the key and value positions do not need an
intermediate list and the key is provided in Key to enable
synchronisation of the file position data with the data structure.{|Syntax||Content|},
SyntaxTerm is the parsed term representation from Syntax,
e.g., {|string(X)||Hello {{X}}|} produces Syntax
string(X) and SyntaxPos describes the layout of
this term. ContentPos is always a term From-To
describing the character range of Content.113The
layout of the term produced by the quasi quotation parser is not
available. Future versions may provide an interface that allows
contributing a layout term.true, demand variables to start with an underscore. See
section 2.16.1.8.~! ?-. A sequence
~! is replaced by the current history event number.!history.!help.user.user_input.
Most applications will use the read_term/2
option variable_names to get access to the names of the
variables in Term. SWI-Prolog calls read_term_with_history/2
as follows:
read_term_with_history(
Goal,
[ show(h),
help('!h'),
no_save([trace, end_of_file]),
prompt('~! ?-'),
variable_names(Bindings)
]).
user_input
stream.
Old is first unified with the current prompt. On success the
prompt will be set to New (an atom). A prompt is printed if
data is read from user_input, the cursor is at the left
margin and the user_input is considered to be connected to
a terminal. See the tty(Bool) property of stream_property/2
and set_stream/2.
The default prompt is '|: '. Note that the toplevel loop
(see
prolog/0)
sets the prompt for the first prompt (see prompt1/1)
to
'?- ', possibly decorated by the history event number,
break level and debug mode. If the first line does not complete
the term, subsequent lines are prompted for using the prompt as defined
by prompt/2.