Add 55: "Struggling Four Equality"

source/categories/index.html.erb

@@ -80,6 +80,7 @@ title: Idiosyncratic Ruby - Browse by Category
         <li><a href="/50-naming-too-good.html">Identifiers to Avoid</a>/<li>
         <li><a href="/52-constant-visibility.html">Private & Derecated Constants</a></li>
         <li><a href="/54-try-converting.html">Class <code>.try_convert</code></a></li>
+        <li><a href="/55-struggling-four-equality.html"><code>.equal?</code>, <code>eql?</code>, <code>==</code>, <code>===</code></a></li>
       </ul>
 
       <h2>Miscellaneous</h2>

source/posts/55-struggling-four-equality.html.md

@@ -0,0 +1,69 @@
+---
+title: Struggling Four Equality
+date: 2016-05-24
+tags: core, equal, types
+---
+
+Another of Ruby's idiosycrasies is equalness. It's not too complicated, but naming is an issue here.
+
+ARTICLE
+
+## Four Concepts of Equalness
+
+### `equal?` Object Identity Comparison
+
+This one is easy. Two objects should be considered identical. Think: `x.object_id == y.object_id`
+
+### `==` Equality Equality
+
+This is the usual method to care about. Two objects should be treated the same.
+
+### `eql?` Hash Key Equality
+
+Normally, this is the same as `==`: ["…`eql?` is usually aliased to the overridden `==` method"](http://ruby-doc.org/core-2.3.1/Hash.html#class-Hash-label-Hash+Keys)
+
+The most important effect of the result of `eql?` is to distinguish between hash keys: ["Two objects refer to the same hash key when their `hash` value is identical and the two objects are `eql?` to each other"](http://ruby-doc.org/core-2.3.1/Hash.html#class-Hash-label-Hash+Keys). A real life example:
+
+    1 == 1.0 # => true
+    1.eql?(1.0) # => false
+
+    # this means that the following will be treated as two different keys
+    {1: "Idiosyncratic", 1.0 "Hash"}
+
+So `eql?` is a little stricter than `==`, because it will return `false` if two objects are not instances of the same class. A typical implementation looks like this:
+
+    def eql?(other)
+      instance_of?(other.class) && self == other
+    end
+
+### `===` Fancy Equality
+
+Implicitely used for `case` statements. Usually like `==`, but can also mean that something has some kind of relationship, like being [some kind of a class](https://github.com/janlelis/sig/blob/v1.0.1/lib/sig.rb#L108-L129).
+
+## Equality Implementations for Core Classes
+
+Class             | `eql?` | `==` | `===`
+------------------|--------|------|------
+ Object           | Identity (like `equal?`) | Same as `eql?` | Same as `==`
+ Symbol           | -      | -    | -
+ Numeric          | Same type, same value | Same value, according to [spaceship returning `0`](http://ruby-doc.org/core-2.3.1/Numeric.html#method-i-3C-3D-3E) | -
+ String           | Same length, same contents | If other is a String: `eql?`, else: `other.to_str === self` | -
+ Regexp           | If other is a Regexp: Same pattern, same options, same encoding | Same as `eql?` | If other is a String: Match against self
+ Array            | Same length, every element `.eql?` corresponding other element | Same length, every element `==` corresponding other element. Will [implicetly convert](/54-try-converting.html) other object via `.to_ary` | -
+ Hash             | Same length, every element `==` corresponding other element (order not relevant) | Same as `eql?` | -
+ Module           | -      | -    | `other.is_a?(self)`
+ Class            | -      | -    | `other.is_a?(self)`
+{:.table-13-29-29-X}
+
+Meaning of `-`: Not defined / Use `Object`'s implementation
+
+## Best Practices for Sub Classes
+
+- Define a meaningful `==` which returns `true` if two objects should be considered the same
+- Make `eql?` return the same value `==`, but also limit it to return only `true` if both object are instances of the same class
+- Don't redefine `equal?`
+- Be creative with `===` (On a reasonable level. Other people using your code expect it to be some kind of useful relationship check)
+
+## Also See
+
+- [equalizer: Automatically define `==` and `eql?` based on the object's instance variables](https://github.com/dkubb/equalizer)

source/stylesheets/table-width.scss

@@ -119,3 +119,9 @@
   th:nth-child(3) { width: 15%; }
   th:nth-child(4) { width: 25%; }
 }
+
+.table-13-29-29-X {
+  th:nth-child(1) { width: 13%; }
+  th:nth-child(2) { width: 29%; }
+  th:nth-child(3) { width: 29%; }
+}