.. index:: optional .. _optional/0:

.. rst-class:: right

object

optional

Constructors for optional terms. An optional term is either empty or holds a value. Optional terms should be regarded as opaque terms and always used with the optional/1 object by passing the optional term as a parameter.

| Availability: | logtalk_load(optionals(loader))

| Author: Paulo Moura | Version: 2:1:0 | Date: 2021-01-03

| Compilation flags: | static, context_switching_calls

| Provides: | :ref:`type::type/1 <type/0::type/1>` | :ref:`type::check/2 <type/0::check/2>`

| Remarks:

• Type-checking support: This object also defines a type optional for use with the type library object.

| Inherited public predicates: | (none)

.. contents:: :local: :backlinks: top

Public predicates

.. index:: empty/1 .. _optional/0::empty/1:

empty/1 ^^^^^^^^^^^

Constructs an empty optional term.

| Compilation flags: | static

| Template: | empty(Optional) | Mode and number of proofs: | empty(--nonvar) - one

---

.. index:: of/2 .. _optional/0::of/2:

of/2 ^^^^^^^^

Constructs an optional term holding the given value.

| Compilation flags: | static

| Template: | of(Value,Optional) | Mode and number of proofs: | of(@term,--nonvar) - one

---

.. index:: from_goal/3 .. _optional/0::from_goal/3:

from_goal/3 ^^^^^^^^^^^^^^^

Constructs an optional term holding a value bound by calling the given goal. Returns an empty optional term if the goal fails or throws an error.

| Compilation flags: | static

| Template: | from_goal(Goal,Value,Optional) | Meta-predicate template: | from_goal(0,*,*) | Mode and number of proofs: | from_goal(+callable,--term,--nonvar) - one

---

.. index:: from_goal/2 .. _optional/0::from_goal/2:

from_goal/2 ^^^^^^^^^^^^^^^

Constructs an optional term holding a value bound by calling the given closure. Returns an empty optional term if the closure fails or throws an error.

| Compilation flags: | static

| Template: | from_goal(Closure,Optional) | Meta-predicate template: | from_goal(1,*) | Mode and number of proofs: | from_goal(+callable,--nonvar) - one

---

.. index:: from_generator/3 .. _optional/0::from_generator/3:

from_generator/3 ^^^^^^^^^^^^^^^^^^^^

Constructs optional terms with the values generated by calling the given goal. On goal error or failure, returns an empty optional.

| Compilation flags: | static

| Template: | from_generator(Goal,Value,Optional) | Meta-predicate template: | from_generator(0,*,*) | Mode and number of proofs: | from_generator(+callable,--term,--nonvar) - one_or_more

---

.. index:: from_generator/2 .. _optional/0::from_generator/2:

from_generator/2 ^^^^^^^^^^^^^^^^^^^^

Constructs optional terms with the values generated by calling the given closure. On closure error or failure, returns an empty optional.

| Compilation flags: | static

| Template: | from_generator(Closure,Optional) | Meta-predicate template: | from_generator(1,*) | Mode and number of proofs: | from_generator(+from_generator,--nonvar) - one_or_more

---

Protected predicates

(no local declarations; see entity ancestors if any)

Private predicates

(no local declarations; see entity ancestors if any)

Operators

(none)

.. seealso::

:ref:`optional(Optional) <optional/1>`, :ref:`type <type/0>`