Add Double Splat To Merge Hashes as a ruby til.

ruby/double-splat-to-merge-hashes.md

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+# Double Splat To Merge Hashes
+
+One way of merging two hashes is with `#merge`:
+
+```ruby
+> h1 = {a: 1, b: 2}
+=> {:a=>1, :b=>2}
+> h2 = {c: 3, d: 4}
+=> {:c=>3, :d=>4}
+> h1.merge(h2)
+=> {:a=>1, :b=>2, :c=>3, :d=>4}
+```
+
+You can also use double splats for a slightly more concise approach:
+
+```ruby
+> h1 = {a: 1, b: 2}
+=> {:a=>1, :b=>2}
+> h2 = {c: 3, d: 4}
+=> {:c=>3, :d=>4}
+> {**h1, **h2}
+=> {:a=>1, :b=>2, :c=>3, :d=>4}
+```
+
+This works particularly well when you want to expand an existing hash into a
+hash you are creating on the fly:
+
+```ruby
+> h1 = {a: 1, b: 2}
+=> {:a=>1, :b=>2}
+> {c: 3, d: 4, **h1}
+=> {:c=>3, :d=>4, :a=>1, :b=>2}
+```