Add If You Detect None as a ruby til

Author: jbranchaud

README.md

@@ -230,6 +230,7 @@ _323 TILs and counting..._
 - [FactoryGirl Sequences](ruby/factory-girl-sequences.md)
 - [Finding The Source of Ruby Methods](ruby/finding-the-source-of-ruby-methods.md)
 - [Identify Outdated Gems](ruby/identify-outdated-gems.md)
+- [If You Detect None](ruby/if-you-detect-none.md)
 - [Invoking Rake Tasks Multiple Times](ruby/invoking-rake-tasks-multiple-times.md)
 - [Last Raised Exception In The Call Stack](ruby/last-raised-exception-in-the-call-stack.md)
 - [Limit Split](ruby/limit-split.md)

ruby/if-you-detect-none.md

@@ -0,0 +1,33 @@
+# If You Detect None
+
+The
+[`Enumerable#detect`](http://ruby-doc.org/core-2.2.3/Enumerable.html#method-i-detect)
+method, which is synonymous with `#find`, can be given an optional argument,
+`ifnone`, that is called when nothing in the array meets the conditional in
+the block. Though I am not sure how this is practically useful and cannot
+find an example of it in use, this contrived example illustrates how it
+works.
+
+```ruby
+# without the fallback behavior
+> [2,4,6,8].detect { |x| x.odd? }
+=> nil
+
+# with a proc as an argument
+> [2,4,6,8].detect(->{0}) { |x| x.odd? }
+=> 0
+```
+
+The last example can also be written as:
+
+```ruby
+> [2,4,6,8].detect(->{0}, &:odd?)
+=> 0
+```
+
+And if you want to be really explicit:
+
+```ruby
+> [2,4,6,8].detect(ifnone=->{0}, &:odd?)
+=> 0
+```