[ag] fmt::formatter view composition

[GuillaumeDua]

Part of #134

Motivation

Combine several modifiers to print csl::ag::concepts::aggregate in a specific ways

• indexed: incremental indexes... of each element
• typenamed: prefix each value with element csl::typeinfo::typename...
• indented_view ? (depth-based indentation)
• TBD.

fmt::println("{}", csl::ag::io::join<indexed, typenamed>(value)); // aggregate_join_view<indexed_t, typenamed_t>

Requires proper dispatching to underlying formatters.

POCs/Design

• minimalistic/naive typenamed
• WIP - composition with view_interface
  • 💡✅ flatten composition: compound_view<views...>

Question: infos-dedicated type.s dispatch ?

template <typename T>
struct meta_infos : std::type_identity<T> {};
template <typename T>
struct meta_infos_range : std::type_identity<T> {
    std::size_t index = 0;
};
template <typename T, std::size_t index>
struct meta_infos_tuple : std::type_identity<T> {
   constexpr inline static std::size_t index = index;
};

Question: should style information be part of the parse instead ?

• pros: indented<style>
• cons: inconsistent with fmt::formatters of range and tuple_like

Details/examples

• {:n}: none
• {:c}: compact (default)
• {:i}: indented -> requires constexpr string - constexpr std::basic_string(count, char)

so for instance:

struct A{ int i = 42; char c = 'c'; };
struct B{ bool b = true; A a; };

fmt::println("{:n}", B{}); // { true42c }
fmt::println("{:c}", B{}); // { true, { 42, c } }
fmt::println("{:i}", B{});
// {
//   true,
//   {
//     42,
//     c
//   }
// }