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# write_term(+Stream, ?Term, ++Options)

The term Term is written to the output stream Stream in a format specified by Options

Stream
  Integer (stream number) or Atom (reserved or user-defined symbolic stream name)
Term
  An arbitrary term
Options
  List of option terms

## Description

This is a generalisation of the predicates write/2, writeq/2, print/2, display/2, write_canonical/2. It is used to write an arbitrary term Term onto the current output stream according to the given options. Options is a (possibly empty) list of the following options:

as(term) -- default

  do not assume any particular meaning of the printed term.

as(clause)

  print the term as a clause, i.e. clause macros will be taken into account.

as(goal)

  print the term as a goal, i.e. goal write transformations will be taken into account.

attributes(none) -- default

  do not print any variable attributes, i.e. print attributed variables like plain variables.

attributes(pretty)

  variable attributes are printed using the corresponding print handlers. See meta_attribute/2.

attributes(full)

  print the full contents of all variable attributes. This is necessary if the term is to be written out and read back in.

compact(false) -- default

  print extra blank space (around operators, after commas, etc.) for better readability.

compact(true)

  don't print blank space unless necessary.

depth(0) -- default

  print the term only up to a maximum nesting depth determined by the (stream-specific or global) flag 'print_depth'. See get_stream_info/3 and get_flag/2.

depth(MaxDepth)

  print the term only up to a maximum nesting depth of MaxDepth. MaxDepth is a positive integer.

depth(full)

  do not observe any depth limit and print the whole term. Note that this will cause looping when the term is cyclic.

dotlists(false) -- default

  write lists in the common square bracket notation, e.g. [1, 2].

dotlists(true)

  write lists in the dot functor notation rather than using the square bracket notation, e.g. .(1, .(2, [])) rather than [1, 2].

newlines(false) -- default

  print newline (NL) characters as escape sequence, when they occur in quoted atoms or strings.

newlines(true)

  print newline (NL) characters as newlines rather than as an escape sequence, even when they occur in quoted atoms or strings. This only makes sense together with the quoted(true) option.

numbervars(false) -- default

  do not treat '$VAR'/1 terms specially. ISO-Prolog compatible.

numbervars(true)

  any term of the form '$VAR'(N), where N is a non-negative integer, is printed as a variable name consisting of a capital letter followed by a number. The capital letter is the ((N mod 26)+1)st letter of the alphabet, and the integer is N//26. If N is an atom, this atom gets printed instead of the term. ISO-Prolog compatible.

operators(true) -- default

  obey operator declarations. All infix, prefix and postfix operators are printed in infix, prefix or postfix form, respectively.

operators(false)

  ignore operator declarations. All terms are written in the canonical notation, with a functor followed by the arguments in parentheses.

portrayed(false) -- default

  do not use portray/1,2.

portrayed(true)

  call the user-defined predicate portray/1,2 in the way print/1,2 does.

quoted(false) -- default

  do not print quotes around strings or atoms. ISO-Prolog compatible.

quoted(true)

  quote atoms and strings if necessary. ISO-Prolog compatible.

transform(true) -- default

  apply portray (write) transformations before printing.

transform(false)

  do not apply any portray (write) transformations.

variables(default) -- default

  print variables using their source name, if available. Otherwise print a system-generated name, which consists of an underscore and a number, e.g. `_123`. Note that this format cannot be reliably read back, because different variables may have the same source name.

variables(raw)

  print all variables using a system-generated name, which consists of an underscore and a number, e.g. `_123`. This format is suitable when the term needs to be read back later. It makes sure that multiple occurrences of the same variable have the same name, and different variables have different names.

variables(full)

  print variables using their source name, if available, followed by a unique number, e.g. Alpha_132. Variables without source name are printed in the raw format. Since variables with identical source names are named apart, this format is suitable when the term needs to be read back later.

variables(anonymous)

  print every variable as a simple underscore. Any information about multiple occurrences of a variable is lost with this format. It is mainly useful to produce output that can be compared easily with the output of a different Eclipse session.

When an option is omitted altogether, then the corresponding option settings for the output stream will come into effect (see set_stream_property/3, get_stream_info/3, open/4).

The following additional options are supported for compatibility with other Prolog systems:

ignore_ops(true)

  the same as [operators(false),dotlists(true),transform(false)]. ISO-Prolog compatibility.

ignore_ops(false)

  the same as [operators(true),dotlists(false),transform(true)]. ISO-Prolog compatibility.

max_depth(0)

  the same as depth(full). SICStus-Prolog compatibility.

max_depth(N)

  the same as depth(N). SICStus-Prolog compatibility.

The correspondence between write_term/2,3 and the other output predicates is as follows:

write(T)

  write_term(T, [])

writeq(T)

  write_term(T, [variables(raw),attributes(full),transform(false), quoted(true),depth(full)])

write_canonical(T)

  write_term(T, [variables(raw),attributes(full),transform(false), quoted(true),depth(full),dotlist(true),operators(false)])

print(T)

  write_term(T, [portrayed(true)])

display(T)

  write_term(T, [dotlist(true),operators(false)])

Note that as usual, the output is buffered, so it may need to be flushed either by closing the stream, by writing again or by using flush/1.

### Modes and Determinism

- write_term(+, ?, ++) is det

### Modules

This predicate is sensitive to its module context (tool predicate, see @/2).

### Exceptions

(4) instantiation fault
  Stream is not instantiated.
(5) type error
  Stream is not an atom or an integer.
(5) type error
  Options is not a list of compound terms.
(6) out of range
  Options list contains a unrecognised option.
(192) illegal stream mode
  Stream is not an output stream.
(193) illegal stream specification
  Stream is an illegal stream specification.

## Examples

    	?- write_term(*(^(1,2),+(3,4)), []).
    	1 ^ 2 * (3 + 4)

    	?- write_term(*(^(1,2),+(3,4)), [operators(false)]).
    	*(^(1, 2), +(3, 4))

    	?- write_term(['a-b',"cd"], []).
    	[a-b, cd]

    	?- write_term(['a-b',"cd"], [quoted(true)]).
    	['a-b', "cd"]

    	?- write_term(['a-b',"cd"], [quoted(true),dotlists(true)]).
    	.('a-b', .("cd", []))

## See Also

write_term / 2, display / 2, print / 2, printf / 3, write / 2, writeq / 2, write_canonical / 2, get_stream_info / 3, get_flag / 2