Add Construct A Constant From A String as a ruby til.

ruby/construct-a-constant-from-a-string.md

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+# Construct A Constant From A String
+
+Ruby's
+[`Module.const_get`](http://ruby-doc.org/core-2.1.0/Module.html#method-i-const_get)
+can be used to look for and retrieve the constant for the given name.
+
+This can be used to construct a class name
+
+```ruby
+> Object.const_get("Math")
+#=> Math
+> Object.const_get("Math")::PI
+#=> 3.141592653589793
+```
+
+It can also be used to reference a constant
+
+```ruby
+> Object.const_get("Math::PI")
+#=> 3.141592653589793
+```
+
+You can even be more specific if you'd like
+
+```ruby
+> Math.const_get("PI")
+#=> 3.141592653589793
+```