which range to choose: r1 if true, r2 otherwise
the "true" range
the "false" range
A range type dependent on R1 and R2.
import std.algorithm.comparison : equal; import std.algorithm.iteration : filter, map; auto data1 = only(1, 2, 3, 4).filter!(a => a != 3); auto data2 = only(5, 6, 7, 8).map!(a => a + 1); // choose() is primarily useful when you need to select one of two ranges // with different types at runtime. static assert(!is(typeof(data1) == typeof(data2))); auto chooseRange(bool pickFirst) { // The returned range is a common wrapper type that can be used for // returning or storing either range without running into a type error. return choose(pickFirst, data1, data2); // Simply returning the chosen range without using choose() does not // work, because map() and filter() return different types. //return pickFirst ? data1 : data2; // does not compile } auto result = chooseRange(true); assert(result.equal(only(1, 2, 4))); result = chooseRange(false); assert(result.equal(only(6, 7, 8, 9)));
Choose one of two ranges at runtime depending on a Boolean condition.
The ranges may be different, but they must have compatible element types (i.e. CommonType must exist for the two element types). The result is a range that offers the weakest capabilities of the two (e.g. ForwardRange if R1 is a random-access range and R2 is a forward range).