How to get type returned by e.g. std.algorithm.iteration.filter

[Christian Köstlin via Digitalmars-d-learn]

I would like to join several sorted files into one big sorted file.
For that I came up with this snippet:

#!/usr/bin/env rdmd
import std.stdio;
import std.algorithm;
import std.typecons;
import std.array;
import std.range;

auto byMinimum(Ranges)(Ranges ranges)
{
    auto getNonEmpty()
    {
        return ranges.filter!("!a.empty");
    }

    auto nonEmpty = getNonEmpty;
    struct ByMinimum(Ranges)
    {
        bool empty()
        {
            return nonEmpty.empty;
        }

        auto front()
        {
            auto minRangeAndLine = nonEmpty.map!(range => tuple!("range",
                    "line")(range, range.front)).minElement!("a.line");
            return minRangeAndLine;
        }

        void popFront()
        {
            auto minRangeAndLine = nonEmpty.map!(range => tuple!("range",
                    "line")(range, range.front)).minElement!("a.line");
            minRangeAndLine.range.popFront;
            nonEmpty = getNonEmpty;
        }
    }

    return ByMinimum!(Ranges)();
}

void main(string[] files)
{
    foreach (n; files[1 .. $].map!(name => 
File(name).byLine(No.keepTerminator)).array.byMinimum)
    {
        writeln(n.line);
    }
}


I would like to get rid of the duplication that searches for the range 
with the next minimum in terms of runtime but also in terms of code 
written out. Unfortunately I have no idea how to even store the result 
of this search in an attribute of ByMinimum, as I cannot writeout its type.

Kind regards,
Christian