[#currying]
[section Currying]

A [link metafunction template metafunction] supports ['currying] when it accepts
less arguments than it normally expects. When less arguments are provided, then
it returns a [link metafunction_class template metafunction class] expecting
the remaining arguments. That template metafunction class is also expected to
support currying.

For example assuming the following metafunction is given:

  template <class A, class B>
  struct plus;

It takes two values, adds them and returns their result. For example:

  static_assert(
    plus<
      std::integral_constant<int, 11>,
      std::integral_constant<int, 2>
    >::type::value == 13,
    "This should work"
  );

If it supports currying, then the following should also work:

  using inc = plus<std::integral_constant<int, 1>>;
  
  static_assert(
    inc::apply<std::integral_constant<int, 12>>::type::value == 13,
    "This should work"
  );

The above example defines the `inc` template metafunction class by calling
`plus` with just one argument: the [link boxed_value boxed] `1` value.

[endsect]