Add Introspecting Block Parameters

SUMMARY.md

@@ -54,6 +54,7 @@
   * [Array Sample (enumerables)](idiomatic_ruby/array_sample.md)
   * [Use Fixnum#times](idiomatic_ruby/use_times.md)
   * [Implicit Return](idiomatic_ruby/implicit_return.md)
+  * [Introspecting Block Parameters](idiomatic_ruby/introspecting_block_parameters.md)
 * [Refactorings](refactorings.md)
   * [Conditionals when object is nil](refactorings/conditionals_when_object_is_nil.md)
   * [Switch case with Hashes](refactorings/case_with_hashes.md)

idiomatic_ruby.md

@@ -9,4 +9,4 @@
  * [Array Sample (enumerables)](idiomatic_ruby/array_sample.md)
  * [Use Fixnum#times](idiomatic_ruby/use_times.md)
  * [Implicit Return](idiomatic_ruby/implicit_return.md)
-
+ * [Introspecting Block Parameters](idiomatic_ruby/introspecting_block_parameters.md)

idiomatic_ruby/introspecting_block_parameters.md

@@ -0,0 +1,51 @@
+## Introspecting Block Parameters
+
+Suppose you would like to iterate over a hash, get its elements and use those
+in a block. One thing you can do is use `Proc#parameters` to help you.
+
+For example:
+
+```ruby
+hash = {
+  first_name: "John",
+  last_name:  "Smith",
+  age: 35,
+  # ...
+}
+
+hash.using do |first_name, last_name|
+  puts "Hello, #{first_name} #{last_name}."
+end
+
+# or even...
+
+circle = {
+  radius: 5,
+  color: "blue",
+  # ...
+}
+
+area = circle.using { |radius| Math::PI * radius**2 }
+```
+
+You can check how the implementation is really simple:
+
+```ruby
+class Hash
+  module Using
+    def using(&block)
+      values = block.parameters.map do |(type, name)|
+        self[name]
+      end
+
+      block.call(*values)
+    end
+  end
+
+  include Using
+end
+```
+
+From:
+
+http://weblog.jamisbuck.org/2015/12/12/little-things-proc-parameters.html