From 33c8838d569262b96e56012fd24087015951445a Mon Sep 17 00:00:00 2001
From: jbranchaud <jbranchaud@gmail.com>
Date: Thu, 9 Jul 2026 11:07:04 -0500
Subject: [PATCH] Add Avoid Dynamically Dispatching Private Methods as a Ruby
 TIL

---
 README.md                                     |  3 +-
 ...dynamically-dispatching-private-methods.md | 84 +++++++++++++++++++
 2 files changed, 86 insertions(+), 1 deletion(-)
 create mode 100644 ruby/avoid-dynamically-dispatching-private-methods.md

diff --git a/README.md b/README.md
index 6a448f1..c42fc09 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
 For a steady stream of TILs, [sign up for my newsletter](https://visualmode.kit.com/newsletter).
 
-_1814 TILs and counting..._
+_1815 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1427,6 +1427,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Assoc For Hashes](ruby/assoc-for-hashes.md)
 - [Audit Your Ruby Project For Any CVEs](ruby/audit-your-ruby-project-for-any-cves.md)
 - [Avoid Double Negation With Minitest Refute](ruby/avoid-double-negation-with-minitest-refute.md)
+- [Avoid Dynamically Dispatching Private Methods](ruby/avoid-dynamically-dispatching-private-methods.md)
 - [Block Comments](ruby/block-comments.md)
 - [Block Syntaxes Have Different Precedence](ruby/block-syntaxes-have-different-precedence.md)
 - [Build HTTP And HTTPS URLs](ruby/build-http-and-https-urls.md)
diff --git a/ruby/avoid-dynamically-dispatching-private-methods.md b/ruby/avoid-dynamically-dispatching-private-methods.md
new file mode 100644
index 0000000..2e706cc
--- /dev/null
+++ b/ruby/avoid-dynamically-dispatching-private-methods.md
@@ -0,0 +1,84 @@
+# Avoid Dynamically Dispatching Private Methods
+
+Ruby makes it easy to call any method on an object with the
+[`Object#send`](https://docs.ruby-lang.org/en/4.0/Object.html#method-i-send)
+method. It's powerful because I can dynamically call methods (e.g. based on a
+variable that is being passed in) and I can even dispatch private methods which
+is normally disallowed.
+
+```ruby
+obj.send(:initialize, new_args)
+```
+
+If I want the benefits of dynamic dispatch without allow private methods to be
+called, then I can instead reach for the
+[`Object#public_send`](https://docs.ruby-lang.org/en/4.0/Object.html#method-i-public_send)
+method.
+
+Here is a basic `Invoice` class to demonstrate:
+
+```ruby
+class Invoice
+  TAX_RATE = 0.08
+
+  def initialize(subtotal:)
+    @subtotal = subtotal
+  end
+
+  def total
+    (@subtotal + tax).round(2)
+  end
+
+  private
+
+  def tax
+    @subtotal * TAX_RATE
+  end
+end
+
+invoice1 = Invoice.new(subtotal: 250)
+
+puts "Total: #{invoice1.send(:total)}"
+puts "Tax: #{invoice1.send(:tax)}"
+
+# this will error
+invoice1.public_send(:tax)
+```
+
+When I run this file, the two `send` statements both work while the
+`public_send` on a private method raises:
+
+```bash
+❯ ruby object_send.rb
+Total: 270.0
+Tax: 20.0
+object_send.rb:25:in 'Kernel#public_send': private method 'tax' called for an instance of Invoice (NoMethodError)
+
+invoice1.public_send(:tax)
+        ^^^^^^^^^^^^
+Did you mean?  tap
+	from object_send.rb:25:in '<main>'
+```
+
+Where this behavior might be most useful is in a Rails _concern_ file where I am
+defining some shared behavior that needs to work dynamically across classes that
+are including it.
+
+```ruby
+module ValidatesAttachment
+  extend ActiveSupport::Concern
+
+  IMAGE_TYPES = %w[image/gif image/jpeg image/jpg image/png].freeze
+  MAX_SIZE = 5.megabytes
+
+  class_methods do
+    def validates_attachment(name, content_types: IMAGE_TYPES, max_size: MAX_SIZE, types_message: nil)
+      validate do
+        attachment = public_send(name)
+
+        # ... validation logic omitted
+      end
+    end
+  end
+end
+```