PartitionIds.md

<CppML/Algorithm/PartitionIds.hpp>

PartitionIds<Predicate, Pipe>

PartitionIds<Predicate, Pipe> is a metafunction that passes to Pipe two ml::ListT<ml::Int<Is>...>, where the first list consists of ml::Int<Is>..., indexes of types which evaluate to ml::Bool<true> under Predicate, and the second indexes of those ml::Int<Is>... that evaluate to ml::Bool<false>. Formally speaking, PartitionIds<Predicate, Pipe> forms (indexes of ) equivalence classes of Ts... under Predicate. Pipe defaults to ml::ToList.

f:: Ts... -> (ml::ListT<ml::Int<I0s>...>, ml::ListT<ml::Int<I1s>...>) >-> Pipe

Predicate

Predicate can be any metafunction returning a single type.

f:: T -> U

Example

We partition indexes a parameter pack, to those whose alignment is greater than 4, and those whose it is not.

using T = ml::f<
              ml::PartitionIds<
                    ml::AlignOf<
                        ml::PartialR<
                            ml::Greater<>,
                            ml::Int<4>>>>,
              int, char, long, bool, double>;
static_assert(
              std::is_same_v<
                  T,
                  ml::ListT<
                      ml::ListT<ml::Int<2>, ml::Int<4>>,
                      ml::ListT<ml::Int<0>, ml::Int<1>, ml::Int<3>>);

NOTE that

ml::AlignOf<          // take aligment of the input and pipe into
      ml::PartialRR<       // a partial evaluation of the metafunction
          ml::Greater<>, // greater
          ml::Int<4>>>   // on ml::Int<4>

is a metafunction of the type

f:: T -> AlignOf(T) >-> Curry<Greater>(4)

and conforms to the predicate constraints.