ex.md

File.open("foo.csv", "w") { |fh| fh.puts "Hello world" }

File.open takes 3 arguments.

1. A string for the filename: "foo.csv"
2. A string for the mode: "w"
3. A function taking one argument, named "fh"

This syntax would have been more honest looking like this:

File.open("foo.csv", "w", -> { |fh| fh.puts "Hello world" })

Or using separate variables:

handler = -> { |fh| fh.puts "Hello world" }
filename = "foo.csv"
mode = "w"

File.open(filename, mode, handler)

Ruby does let you do this kind of, if you use an annoying little & in there.

handler = -> { |fh| fh.puts "Hello world" }
filename = "foo.csv"
mode = "w"

File.open(filename, mode, &handler)