write_term/2

Availability:built-in

[ISO]write_term(+Term, +Options)

The predicate write_term/2 is the generic form of all Prolog term-write predicates. Valid options are:

attributes(Atom)

Define how attributed variables (see section 7.1) are written. The default is determined by the Prolog flag write_attributes. Defined values are 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).

back_quotes(Atom)

Fulfills the same role as the back_quotes prolog flag. Notably, the value 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.

brace_terms(Bool)

If true (default), write {}(X) as {X}. See also dotlists and ignore_ops.

blobs(Atom)

Define how non-text blobs are handled. By default, this is left to the write handler specified with the blob type. Using portray, portray/1 is called for each blob encountered. See section 11.4.7.

character_escapes(Bool)

If 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).

cycles(Bool)

If true (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.^{91The
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.

dotlists(Bool)

If true (default false), write lists using the dotted term notation rather than the list notation.^{92Copied
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.

fullstop(Bool)

If true (default false), add a fullstop token to the output. The dot is preceeded by a space if needed and followed by a space (default) or newline if the nl(true) option is also given.^{93Compatible with ECLiPSe}

ignore_ops(Bool)

If true, the generic term representation (<functor>(<args> ... )) will be used for all terms. Otherwise (default), operators will be used where appropriate.^{94In
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}.

max_depth(Integer)

If the term is nested deeper than Integer, print the remainder as ellipses ( ... ). A 0 (zero) value (default) imposes no depth limit. This option also delimits the number of printed items in a list. Example:

?- 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.

module(Module)

Define the reference module (default user). This defines the default value for the character_escapes option as well as the operator definitions to use. See also op/3.

nl(Bool)

Add a newline to the output. See also the fullstop option.

numbervars(Bool)

If true, terms of the format $VAR(N), where N is a non-negative integer, will be written as a variable name. 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.

partial(Bool)

If 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.

portray(Bool)

Same as portrayed(Bool). Deprecated.

portray_goal(:Goal)

Implies 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.

portrayed(Bool)

If 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.

priority(Integer)

An integer between 0 and 1200 representing the `context priority'. Default is 1200. Can be used to write partial terms appearing as the argument to an operator. For example:

        format('~w = ', [VarName]),
        write_term(Value, [quoted(true), priority(699)])

quoted(Bool)

If true, atoms and functors that need quotes will be quoted. The default is false.

spacing(+Spacing)

Determines whether and where extra white space is added to enhance readability. The default is 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.

variable_names(+List)

Assign names to variables in Term. List is a list of terms Name = Var, where Name is an atom that represents a valid Prolog variable name. Terms where Var is bound or is a variable that does not appear in Term are ignored. Raises an error if List is not a list, one of the members is not a term Name = Var, Name is not an atom or Name does not represent a valid Prolog variable name.

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 7.1) 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.