Ruby Cookbook, 2nd Edition

Chapter 8. Code Blocks and Iteration

In Ruby, a code block (or just “block”) is an object that contains some Ruby code, and the context necessary to execute it. Code blocks are the most visually distinctive aspect of Ruby, and also one of the most confusing to newcomers from other languages. Essentially, a Ruby code block is a method that has no name.

Most other languages have something like a Ruby code block: C’s function pointers, C++’s function objects, Python’s lambdas and list comprehensions, Perl’s anonymous functions, Java’s anonymous inner classes. These features live mostly in the corners of those languages, shunned by novice programmers. Ruby can’t be written without code blocks. Of the major languages, only Lisp is more block-oriented.

Unlike most other languages, Ruby makes code blocks easy to create and imposes few restrictions on them. In every other chapter of this book, you’ll see blocks passed into methods like it’s no big deal (which it isn’t):

[1,2,3].each { |i| puts i }
# 1
# 2
# 3

In this chapter, we’ll show you how to write that kind of method, the kinds of methods that are useful to write that way, and when and how to treat blocks as first-class objects.

Ruby provides two syntaxes for creating code blocks. When the entire block will fit on one line, it’s most readable when enclosed in curly braces:

[1,2,3].each { |i| puts i }
# 1
# 2
# 3

When the block is longer than one line, it’s more readable to begin it with the do keyword and end it with the end keyword: ...