What is "FilterResult" type?

[Meta via Digitalmars-d-learn]

On Tuesday, 8 September 2015 at 11:08:59 UTC, Bahman Movaqar 
wrote:
> However, I have made this a strict practice of mine to specify 
> the full signature of my public API.  I suppose, if I want to 
> be pedantic, I have to realise the lazy value first and pass 
> the resulting array out.  Is this correct?

If you _really_ want to specify the return type, you can import 
std.range.interfaces[1] and use the appropriate return type 
(e.g., InputRange!int, RandomAccessFinite!char, etc.) and then 
append a `.inputRangeObject` to the end of your range chain.

Ex:

import std.algorithm;
import std.range;

InputRange!int doRangeyThings(R)(R r)
if (isInputRange!R)
{
	return r.filter!(x => !(x & 1))
			.map!(x => x * 2)
			.inputRangeObject;
}

void main()
{
	auto n = [1, 2, 3, 4];
	auto m = n.doRangeyThings();
	assert(is(typeof(m) == InputRange!int));
	assert(m.equal([4, 8]));
}

You should use this sparingly, however. The types in 
std.range.interfaces are interfaces that allow you to combine 
ranges with runtime polymorphism, and you have to pay the 
corresponding price for that, namely, you'll be hitting the GC 
and the virtual functions entail a speed hit.




1. http://dlang.org/phobos/std_range_interfaces.html