This optional builder method creates higher-order propositions. Higher-order propositions make statements about set of models. In contrast, first-order propositions make statements about individual models.

You can create higher-order propositions without using the As() method by defining the model as a collection. However, at some point, you will likely want to surface the underlying assertions that contributed to determining whether the higher-order proposition was satisfied or not. The As() method (and its extensions) makes it possible to subsequently access these assertions.

Custom higher-order propositions

As(Func<IEnumerable<BooleanResult<TModel, TUnderlyingMetadata>>, bool> higherOrderPredicate)

The proposition is satisfied when the custom higher order predicate is satisfied, otherwise it is not satisfied.

Spec.Build((int n) => n % 2 == 0)
    .As(results => results.CountTrue() == results.CountFalse())
    .Create("has equal amounts of odd and even")

Custom higher-order propositions with causal result selection

As(
    Func<IEnumerable<BooleanResult<TModel, TUnderlyingMetadata>>, bool> higherOrderPredicate,  
    Func<bool, IEnumerable<BooleanResult<TModel, TUnderlyingMetadata>>, IEnumerable<BooleanResult<TModel, TUnderlyingMetadata>>> causeSelector)

The proposition is satisfied when the custom higher order predicate is satisfied, otherwise it is not satisfied.

Spec.Build((int n) => n % 2 == 0)
    .As(result => result.CountTrue() == result.CountFalse(),
        (_, results) => results)
    .Create("has equal amounts of odd and even")