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Function std.range.choose

Choose one of two ranges at runtime depending on a Boolean condition.

auto choose(R1, R2) (
  bool condition,
  scope return R1 r1,
  scope return R2 r2
if (isInputRange!(Unqual!R1) && isInputRange!(Unqual!R2) && !is(CommonType!(ElementType!(Unqual!R1), ElementType!(Unqual!R2)) == void));

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).


condition which range to choose: r1 if true, r2 otherwise
r1 the "true" range
r2 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)));


Andrei Alexandrescu, David Simcha, Jonathan M Davis, and Jack Stouffer. Credit for some of the ideas in building this module goes to Leonardo Maffi.


Boost License 1.0.