# `expecteds`

This library provides an implementation of expected terms with an API that is inspired by the `optional` library and C++ standardization proposals for an `Expected<T>` type. An expected term is an opaque compound term that either contains an expected value or an error informing why the expected value is not present. Expected terms provide an alternative to generating an exception (or a failure) when something unexpected happens when asking for a value. This allows, e.g., separating the code that constructs expected terms from the code that processes them, which is then free to deal if necessary and at its convenience with any unexpected events.

## API documentation

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

## Loading

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

    | ?- logtalk_load(expecteds(loader)).

## Testing

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

    | ?- logtalk_load(expecteds(tester)).

## Usage

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

    | ?- expected::of_expected(1, Expected).
    ...

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

    | ?- expected::of_expected(1, Expected), expected(Expected)::or_else(Value, 0).
    Expected = expected(1),
    Value = 1
    yes

    | ?- expected::of_unexpected(-1, Expected), expected(Expected)::or_else(Value, 0).
    Expected = unexpected(-1),
    Value = 0
    yes

The `either` object provides types and predicates for extended type-checking and predicates for handling lists of expected terms, including `sequence/2` and `traverse/3`.

The `expected/1` parametric object provides `filter/3` for conditionally rejecting values, converting them to unexpected terms with a given error:

    | ?- expected::of_expected(1, Expected),
         expected(Expected)::filter(integer, not_integer, NewExpected).
    NewExpected = expected(1)
    yes

    | ?- expected::of_expected(a, Expected),
         expected(Expected)::filter(integer, not_integer, NewExpected).
    NewExpected = unexpected(not_integer)
    yes

The `map_or_else/3` predicate applies a closure to the value if present, returning a default value otherwise (symmetric with the optionals library):

    | ?- expected::of_expected(a, Expected),
         expected(Expected)::map_or_else(char_code, 0, Value).
    Value = 97
    yes

    | ?- expected::of_unexpected(-1, Expected),
         expected(Expected)::map_or_else(char_code, 0, Value).
    Value = 0
    yes

The `or/2` predicate chains expected terms, returning the current term if it holds a value, or calling a closure to produce an alternative otherwise:

    | ?- expected::of_expected(1, Expected),
         expected(Expected)::or(NewExpected, expected::of_expected(2)).
    NewExpected = expected(1)
    yes

    | ?- expected::of_unexpected(-1, Expected),
         expected(Expected)::or(NewExpected, expected::of_expected(2)).
    NewExpected = expected(2)
    yes

The `zip/3` predicate combines two expected terms using a closure when both hold values, returning the first error otherwise:

    | ?- expected::of_expected(1, E1), expected::of_expected(3, E2),
         expected(E1)::zip([X,Y,Z]>>(Z is X+Y), E2, NewExpected).
    NewExpected = expected(4)
    yes

    | ?- expected::of_unexpected(-1, E1), expected::of_expected(3, E2),
         expected(E1)::zip([X,Y,Z]>>(Z is X+Y), E2, NewExpected).
    NewExpected = unexpected(-1)
    yes

The `map_unexpected/2` predicate transforms the error held by an expected term:

    | ?- expected::of_unexpected(-1, Expected),
         expected(Expected)::map_unexpected([X,Y]>>(Y is abs(X)), NewExpected).
    NewExpected = unexpected(1)
    yes

The `map_catching/2` predicate applies a closure that may throw an error, catching it and wrapping it as an unexpected term:

    | ?- expected::of_expected(a, Expected),
         expected(Expected)::map_catching(char_code, NewExpected).
    NewExpected = expected(97)
    yes

    | ?- expected::of_expected(1, Expected),
         expected(Expected)::map_catching(char_code, NewExpected),
         expected(NewExpected)::is_unexpected.
    yes

The `map_both/3` predicate is a bifunctor map that transforms both the expected value and unexpected error using separate closures:

    | ?- expected::of_expected(1, Expected),
         expected(Expected)::map_both([X,Y]>>(Y is X+1), [X,Y]>>(Y is abs(X)), NewExpected).
    NewExpected = expected(2)
    yes

The `swap/1` predicate swaps expected and unexpected terms:

    | ?- expected::of_expected(1, Expected),
         expected(Expected)::swap(NewExpected).
    NewExpected = unexpected(1)
    yes

    | ?- expected::of_unexpected(error, Expected),
         expected(Expected)::swap(NewExpected).
    NewExpected = expected(error)
    yes

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

    | ?- expected::of_expected(1, Inner), expected::of_expected(Inner, Outer),
         expected(Outer)::flatten(NewExpected).
    NewExpected = expected(1)
    yes

    | ?- expected::of_unexpected(oops, Inner), expected::of_expected(Inner, Outer),
         expected(Outer)::flatten(NewExpected).
    NewExpected = unexpected(oops)
    yes

Conversion between expected and optional terms is provided by the `to_optional/1`, `from_optional/3`, and `optional/1::to_expected/2` predicates:

    | ?- expected::of_expected(1, Expected),
         expected(Expected)::to_optional(Optional).
    Optional = optional(1)
    yes

    | ?- expected::of_unexpected(error, Expected),
         expected(Expected)::to_optional(Optional).
    Optional = empty
    yes

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

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

Examples:

    | ?- expected::of_expected(1, E1), expected::of_expected(2, E2),
         either::sequence([E1, E2], Expected).
    Expected = expected([1,2])
    yes

    | ?- either::traverse({expected}/[X,E]>>expected::of_expected(X, E), [1,2], Expected).
    Expected = expected([1,2])
    yes

    | ?- either::traverse({expected}/[X,E]>>(
           integer(X) -> expected::of_expected(X, E)
         ; expected::of_unexpected(not_integer(X), E)
         ), [1,a,2], Expected).
    Expected = unexpected(not_integer(a))
    yes

    | ?- expected::of_expected(1, E1), expected::of_unexpected(e, E2),
         either::sequence([E1, E2], Expected).
    Expected = unexpected(e)
    yes

## See also

The `optionals` and `validations` libraries.