Bug 13881 – Optional length template argument for std.range.takeExactly

In some cases it can be useful to specify the length as template argument to takeExactly, allowing the range to have a length known at compile-time: void main() pure @safe nothrow @nogc { import std.range: takeExactly; int[10] arr; auto r = arr.takeExactly!5; enum len = r.length; } If the first argument of takeExactly is a fixed-size array of sufficient length, or a range that has length known at compile-time (like another takeExactly!N or an infinite range), then takeExactly becomes nothrow @nogc. Now r.length is usable in various compile-time contexts: int[r.length] arr2 = void; copy(r, arr2[]); If you can't add such optional template argument, then you can add a similar function named std.range.takeExactlyCt. See also Issue 13880

Range length can't be a compile-time constant, or it won't be able to support popFront and isn't a range.

As Daniel said, a range with an immutable length might as well itself be immutable (not to be confused with a range of immutable elements). As such, it would be of very limited use. I don't see a point in pursuing this.