// lesson 3: variables
//
// a name is a list of characters. a variable holds a type, not a value, and the
// only type that carries a value is ::decltype(...). the ::conditional_t around
// it is not required. it is just what everyone does

::declare<
    ::name<'x'>,
    ::conditional_t<
        ::true_type::value,
        ::decltype(::difference<'F', 'A'>),
        ::void
    >
>::commit;

// reading it back is ::value_of<NAME>, which is a value, so ::decltype it again

::emit<::decltype(::value_of<::name<'x'>>)>::commit;

::declare<
    ::name<'y'>,
    ::decltype(
        ::sum<
            ::value_of<::name<'x'>>,
            ::difference<'B', 'A'>
        >
    )
>::commit;

::emit<::decltype(::value_of<::name<'y'>>)>::commit;

// there is no assignment. to change x, undeclare it and declare it again

::undeclare<::name<'x'>>::commit;

::declare<
    ::name<'x'>,
    ::decltype(
        ::product<
            ::value_of<::name<'y'>>,
            ::difference<'C', 'A'>
        >
    )
>::commit;

::emit<::decltype(::value_of<::name<'x'>>)>::commit;