Add If You Detect None as a ruby til

ruby/if-you-detect-none.md

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+# 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
+```