# `optionals`

This library provides an implementation of optional terms with an API modeled after the Java 8 `Optional` class (originally due to requests by users working in Logtalk/Java hybrid applications). An optional term is an opaque compound term that may or may not hold a value. Optional terms avoid forcing the user to define a representation for the absence of a value by providing an API with predicates that depend on the presence or absence of a value. Optional terms also allow separating the code that constructs optional terms from the code that processes them, which is then free to deal if necessary and at its convenience with any case where the values held by optional terms are not present.

## API documentation

Open the ../../apis/library_index.html#optionals link in a web browser.

## Loading

To load all entities in this library, load the `loader.lgt` file:

    | ?- logtalk_load(optionals(loader)).

## Testing

To test this library predicates, load the `tester.lgt` file:

    | ?- logtalk_load(optionals(tester)).

## Usage

The `optional` object provides constructors for optional terms. For example:

    | ?- optional::of(1, Optional).
    ...

The created optional terms can then be passed as parameters to the `optional/1` parametric object. For example:

    | ?- optional::of(1, Optional), optional(Optional)::or_else(Term, 0).
    Optional = optional(1),
    Term = 1
    yes

    | ?- optional::empty(Optional), optional(Optional)::or_else(Term, 0).
    Optional = empty,
    Term = 0
    yes

The `maybe` object provides types and predicates for type-checking of the term held by optional terms. It also provides some predicates for handling lists of optional terms, including `sequence/2` and `traverse/3`.

The `optional/1` parametric object also provides `map_or_else/3` for applying a closure to the value held by the optional term if not empty, returning a default value otherwise:

    | ?- optional::of(a, Optional),
         optional(Optional)::map_or_else(char_code, 0, Value).
    Value = 97
    yes

    | ?- optional::empty(Optional),
         optional(Optional)::map_or_else(char_code, 0, Value).
    Value = 0
    yes

The `zip/3` predicate combines two optional terms using a closure when both are not empty:

    | ?- optional::of(1, O1), optional::of(3, O2),
         optional(O1)::zip([X,Y,Z]>>(Z is X+Y), O2, NewOptional).
    NewOptional = optional(4)
    yes

    | ?- optional::of(1, O1), optional::empty(O2),
         optional(O1)::zip([X,Y,Z]>>(Z is X+Y), O2, NewOptional).
    NewOptional = empty
    yes

The `flatten/1` predicate unwraps a nested optional term:

    | ?- optional::of(1, Inner), optional::of(Inner, Outer),
         optional(Outer)::flatten(NewOptional).
    NewOptional = optional(1)
    yes

    | ?- optional::empty(Inner), optional::of(Inner, Outer),
         optional(Outer)::flatten(NewOptional).
    NewOptional = empty
    yes

The `to_expected/2` predicate converts an optional to an expected term:

    | ?- optional::of(1, Optional),
         optional(Optional)::to_expected(missing, Expected).
    Expected = expected(1)
    yes

    | ?- optional::empty(Optional),
         optional(Optional)::to_expected(missing, Expected).
    Expected = unexpected(missing)
    yes

The `from_goal/3` and `from_goal/2` constructors silently convert goal exceptions to empty optional terms. Use `from_goal_or_throw/3` or `from_goal_or_throw/2` if exceptions should be propagated instead:

    | ?- optional::from_goal_or_throw(Y is 1+2, Y, Optional).
    Optional = optional(3)
    yes

    | ?- optional::from_goal_or_throw(2 is 3, _, Optional).
    Optional = empty
    yes

    | ?- catch(
           optional::from_goal_or_throw(Y is _, Y, _),
           Error,
           true
         ).
    Error = error(instantiation_error, ...)
    yes

Examples:

    | ?- optional::of(1, O1), optional::of(2, O2),
         maybe::sequence([O1, O2], Optional).
    Optional = optional([1,2])
    yes

    | ?- maybe::traverse({optional}/[X,O]>>optional::of(X, O), [1,2], Optional).
    Optional = optional([1,2])
    yes

    | ?- maybe::traverse({optional}/[X,O]>>(
           integer(X) -> optional::of(X, O)
         ; optional::empty(O)
         ), [1,a,2], Optional).
    Optional = empty
    yes

    | ?- optional::of(1, O1), optional::empty(O2),
         maybe::sequence([O1, O2], Optional).
    Optional = empty
    yes

## See also

The `expecteds` and `validations` libraries.