Add Fetch Warns About Superseding Block Argument as a Ruby TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1288 TILs and counting..._
+_1289 TILs and counting..._
 
 ---
 
@@ -1043,6 +1043,7 @@ _1288 TILs and counting..._
 - [Extract A Column Of Data From A CSV File](ruby/extract-a-column-of-data-from-a-csv-file.md)
 - [FactoryGirl Sequences](ruby/factory-girl-sequences.md)
 - [Fail](ruby/fail.md)
+- [Fetch Warns About Superseding Block Argument](ruby/fetch-warns-about-superseding-block-argument.md)
 - [Find The Min And Max With A Single Call](ruby/find-the-min-and-max-with-a-single-call.md)
 - [Finding The Source of Ruby Methods](ruby/finding-the-source-of-ruby-methods.md)
 - [Generate A Signed JWT Token](ruby/generate-a-signed-jwt-token.md)

ruby/fetch-warns-about-superseding-block-argument.md

@@ -0,0 +1,29 @@
+# Fetch Warns About Superseding Block Argument
+
+[Ruby's `#fetch`](https://ruby-doc.org/core-2.5.1/Hash.html#method-i-fetch) can
+be used in a couple ways beyond just grabbing the value out of a hash.
+
+If you include a second argument in the `#fetch` call, that will be treated as
+a default value to fallback to when the first argument key doesn't appear in
+the hash.
+
+If you instead specify a block argument, that block will be executed when the
+key is missing.
+
+What happens when you specify both a second argument and a block argument?
+
+```ruby
+data = { taco: 'bell' }
+
+data.fetch(:burrito, 'house') do
+  puts 'the block gets executed'
+  'shack'
+end
+
+warning: block supersedes default value argument
+the block gets executed
+=> 'shack'
+```
+
+The block argument wins. The second argument is ignored. And Ruby warns you
+that, "block supersedes default value argument".