Function main

example/optional/applicative.complex.cpp:40–50

BOOST_HANA_CONSTEXPR_LAMBDA auto evaluate = hana::ap(function<op>, digit<x>, digit<y>);

int main() {
    BOOST_HANA_CONSTEXPR_CHECK(evaluate<'1', '+', '2'> == hana::just(1 + 2));
    BOOST_HANA_CONSTEXPR_CHECK(evaluate<'4', '-', '2'> == hana::just(4 - 2));

    BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '+', '2'> == hana::nothing);
    BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '?', '2'> == hana::nothing);
    BOOST_HANA_CONSTANT_CHECK(evaluate<'1', '+', '?'> == hana::nothing);
    BOOST_HANA_CONSTANT_CHECK(evaluate<'?', '?', '?'> == hana::nothing);

    static_assert(hana::lift<hana::optional_tag>(123) == hana::just(123), "");
}