.. index:: single: maybe .. _maybe/0:

.. rst-class:: right

object

maybe

Types and predicates for type-checking and handling optional terms. Inspired by Haskell.

| Availability: | logtalk_load(optionals(loader))

| Author: Paulo Moura | Version: 0:9:0 | Date: 2025-06-19

| Compilation flags: | static, context_switching_calls

| Provides: | :ref:`type::type/1 <type/0::type/1>` | :ref:`type::check/2 <type/0::check/2>` | :ref:`arbitrary::arbitrary/1 <arbitrary/0::arbitrary/1>` | :ref:`arbitrary::arbitrary/2 <arbitrary/0::arbitrary/2>` | :ref:`arbitrary::shrinker/1 <arbitrary/0::shrinker/1>` | :ref:`arbitrary::shrink/3 <arbitrary/0::shrink/3>` | :ref:`arbitrary::edge_case/2 <arbitrary/0::edge_case/2>` | Uses: | :ref:`optional <optional/0>` | :ref:`optional(Optional) <optional/1>` | :ref:`random <random/0>` | :ref:`type <type/0>`

| Remarks:

• Type-checking support: Defines type maybe(Type) for checking optional terms where the value hold by the optional term must be of the given type.
• QuickCheck support: Defines clauses for the arbitrary::arbitrary/1-2, arbitrary::shrinker/1, arbitrary::shrink/3, and arbitrary::edge_case/2 predicates to allow generating random values for the maybe(Type) type.

| Inherited public predicates: | (none)

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

Public predicates

.. index:: cat/2 .. _maybe/0::cat/2:

cat/2 ^^^^^^^^^

Returns the values stored in the non-empty optional terms.

| Compilation flags: | static

| Template: | cat(Optionals,Values) | Mode and number of proofs: | cat(+list(optional),-list) - one

---

.. index:: sequence/2 .. _maybe/0::sequence/2:

sequence/2 ^^^^^^^^^^^^^^

Returns an optional term with a list of all values when all optional terms are not empty. Otherwise returns an empty optional term.

| Compilation flags: | static

| Template: | sequence(Optionals,Optional) | Mode and number of proofs: | sequence(+list(optional),--nonvar) - one

---

.. index:: traverse/3 .. _maybe/0::traverse/3:

traverse/3 ^^^^^^^^^^^^^^

Applies a closure to each list element to generate optional terms and then sequences them into a single optional term holding all values or an empty optional term.

| Compilation flags: | static

| Template: | traverse(Closure,Terms,Optional) | Meta-predicate template: | traverse(2,*,*) | Mode and number of proofs: | traverse(+callable,+list,--nonvar) - one

---

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