I wanna write a parser that can read the input text from an arbitrary source. That's going to be files for most of the time, but I don't want to unnecessarily limit it. It's a line based format, so I supposed InputRange!string would do in this case?
I would recommend writing a range using a struct instead of inheriting from InputRange. See here for an introduction to ranges: http://ddili.org/ders/d.en/ranges.html
