get from tuple by type

[ketmar via Digitalmars-d-learn]

On Sun, 15 Mar 2015 21:59:16 +0000, Charles Cooper wrote:

> C++14 has:
> template<class T, class... Types> constexpr T& get(tuple<Types...>& t);
> Which allows you to get a member of the tuple struct by type. Is there
> an idiomatic / library way to do this in D? Preferably by indexing.

why indexing? functional programming is fun!

  import std.typecons : tuple;

  template GetByType(Type, Tuple...) {
    import std.typecons : isTuple;
    static if (Tuple.length == 0)
      static assert(false, Type.stringof~" not found");
    else static if (Tuple.length == 1 && isTuple!(typeof(Tuple[0])))
      enum GetByType = GetByType!(Type, Tuple[0].expand);
    else static if (is(typeof(Tuple[0]) == Type))
      enum GetByType = Tuple[0];
    else
      enum GetByType = GetByType!(Type, Tuple[1..$]);
  }

  static assert(2.5 == GetByType!(double, 1, 2.5));
  static assert(2.5 == GetByType!(double, 1, 2.5, 3.1));
  //static assert(2.5 == GetByType!(char, 1, 2.5, 3.1));
  static assert(2.5 == GetByType!(double, std.typecons.tuple(1, 2.5)));
  static assert(2.5 == GetByType!(double, std.typecons.tuple(1,2.5,3.1)));
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