The problem is that the most obvious way of doing it - File.readlines('foo.txt').map { ... }.select { ... } etc. - is not stream-oriented.

arguably, it's trivial to make that stream oriented

    open('tmp.rb').each_line.lazy.map {...}.select {...}

the problem with processing big files with ruby (in my humble experience) is usually that it's still slow enough that "preprocessing with grep&uniq" is worthwhile.

    > open('tmp.rb').each_line.lazy
    NoMethodError: undefined method `lazy' for #<Enumerator: #<File:Procfile>:each_line>

Not everybody is using Ruby 2.0.

> if you're reading the whole file into memory theres a good chance you're doing it wrong

GP: "working with file lines _naively_ in Ruby"

ah, my bad, read that as natively and chalked it up to odd wording.