gotchas/to_s_vs_to_str.rb

# coding: utf-8
# Concept: (The blockquote style does not look so well so I just pasted directly,
# but these are all quoted from the links in the bottom of this page). You should
# not implement `to_str` unless your object acts like a string, rather than just
# having a string *representation*. The only core class that implements `to_str`
# is String itself. [to_i and to_s] are not particularly strict: if an object has
# some kind of decent representation as a string, for example, it will probably
# have a to_s method… [to_int and to_str] are strict conversion functions: you
# implement them only if you object can naturally be used every place a string or
# an integer could be used. `to_str` is used by methods such as `String#concat`
# to convert their arguments to a string. Unlike to_s, which is supported by
# almost all classes, `to_str` is normally implemented only by those classes that
# act like strings. Of the built-in classes, only `Exception` and `String`
# implement `to_str`, `print`, `puts` and string interpolation(`#{}` syntax) use
# `to_s`. Mostly everything else (`glob`, `split`, `match`, string concatenation)
# uses `to_str`.

# Simple Summary: `to_s` is defined on every object and will always return
# something. `to_str` is only defined on objects that are string-like. For
# example, `Symbol` has `to_str` but `Array` does not. Thus, you can use
# `obj.respond_to?(:to_str)` instead of something like `obj.is_a?(String)` if you
# want to take advantage of duck typing without worrying about whether the class
# you're working with is a subclass of `String` or not. The documentation for
# `Exception#to_str` reads: "By supplying a `to_str` method, exceptions are
# agreeing to be used where `Strings` are expected." So it's all about
# expectations. `to_s`: Give me a String no matter what! `to_str`: I am pretty
# sure you are String-like. So sure, that I'd prefer to get a NoMethodException
# in the case where you're not. Use case?
#
# Borrowed from Confident Ruby.

class ArticleTitle
  def initialize(text)
    @text = text
  end

  def slug
    @text.strip.tr_s('^A-Za-z0-9', '-').downcase
  end

  def to_str
    puts 'to_str called'
    @text
  end
end

title = ArticleTitle.new('A Modest Proposal')

# By defining #to_str, ArticleTitle can be used in places Ruby expects a String:

puts 'Today’s Feature: ' + title
# => "to_str called"
# => "Today’s Feature: A Modest Proposal"