This chapter goes into the details of an artefact central to the Phobos standard library with a little core language support: ranges. Ranges are user-defined objects used to build iterators over a collection of items. The collection may be pre-existing, such as an array, or it may be generated on the fly by the range object.

Ranges are defined in a way that they can be plugged together like building blocks with generic algorithms and other transformations. Command-line pipelines and range code can be very similar.

The Unix command-line command cat file.txt | sort | uniq can be expressed similarly in D, using ranges from std.stdio and std.algorithm and a helper function from std.range, as shown in the following code:

foreach(line; File("file.txt").byLine.map!(a=>a.idup).array.sort.uniq)
    writeln("Unique line: ", line);

Each range feeds into the next, building a system of generic building blocks that can be combined to perform a variety of tasks. In this chapter, we'll look at how to use ranges and how to create our own.