maybeEmptyRange

maybe_empty.d

import std.stdio;

/*
 * Copy-paste from std.range.
 * Comment this and uncomment `import std.range`, and the static assert will fail.
 */
template isInputRange(R)
{
    enum bool isInputRange = is(typeof(
    (inout int = 0)
    {
        R r = R.init;     // can define a range object
        if (r.empty) {}   // can test for empty
        r.popFront();     // can invoke popFront()
        auto h = r.front; // can get the front of the range
    }));
}

//import std.range;

struct MyRange {
  void popFront() { }
  @property int front() { return 0; }
  @property bool empty() { return false; }
}

struct MaybeEmpty(R) if (isInputRange!R) {
  private bool _isEmpty;
  private R    _input;

  alias _input this;

  this(bool isEmpty, R input) {
    _input = input;
    _isEmpty = isEmpty;
  }

  @property bool empty() { 
    return _isEmpty || _input.empty;
  }
}

auto maybeEmpty(R)(bool empty, R input = R.init) if (isInputRange!R) {
  return MaybeEmpty!R(empty, input);
}

auto foo(bool b) {
  return b ? maybeEmpty!MyRange(true) : maybeEmpty!MyRange(false, MyRange());
}

unittest {
  auto a = maybeEmpty!MyRange(true);
  alias R = typeof(a);

  static assert(isInputRange!MyRange);
  static assert(isInputRange!R);
}