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:
type::type/1
type::check/2
arbitrary::arbitrary/1
arbitrary::arbitrary/2
arbitrary::shrinker/1
arbitrary::shrink/3
arbitrary::edge_case/2
Uses:
optional
optional(Optional)
random
type
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)

## Public predicates

### `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`

### `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`

### `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)

See also

optional, optional(Optional), type, arbitrary