Add Define A Custom RSpec Matcher as a ruby til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_804 TILs and counting..._
+_805 TILs and counting..._
 
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@@ -615,6 +615,7 @@ _804 TILs and counting..._
 - [Create Listing Of All Middleman Pages](ruby/create-listing-of-all-middleman-pages.md)
 - [Create Thumbnail Image For A PDF](ruby/create-thumbnail-image-for-a-pdf.md)
 - [Defaulting To Frozen String Literals](ruby/defaulting-to-frozen-string-literals.md)
+- [Define A Custom RSpec Matcher](ruby/define-a-custom-rspec-matcher.md)
 - [Destructuring Arrays In Blocks](ruby/destructuring-arrays-in-blocks.md)
 - [Disassemble Some Codes](ruby/disassemble-some-codes.md)
 - [Double Splat To Merge Hashes](ruby/double-splat-to-merge-hashes.md)

ruby/define-a-custom-rspec-matcher.md

@@ -0,0 +1,32 @@
+# Define A Custom RSpec Matcher
+
+You can define your own custom RSpec matchers. This is a great way to keep
+your spec tight and readable. A custom, domain-specific matcher can convey
+more intent in a single line than several lines of built-in matchers.
+
+Here is a matcher to check if something is in a range:
+
+```ruby
+require 'rspec/expectations'
+
+RSpec::Matchers.define :be_betwen do |lower, upper|
+  match do |operand|
+    expect(operand).to be >= lower
+    expect(operand).to be <= upper
+  end
+end
+
+describe MyThing do
+  it "has a value between 0 and 10" do
+    thing = MyThing.new
+
+    expect(thing.value).to be_between(0, 10)
+  end
+end
+```
+
+By requiring `rspec/expectations` we are able to define a matcher that takes
+0 or more arguments (in our case, `lower` and `upper`) and then make
+assertions with them and the `expect` value.
+
+[source](https://relishapp.com/rspec/rspec-expectations/v/2-4/docs/custom-matchers/define-matcher)