Beware not to forget about .to_proc conversions.

Since Ruby 2.3.0 Hash class has a .to_proc conversion. It’s pretty useful with & operator.

h = { a: 1, b: 2, c: 3 }
[ :a, :b, :c, ].map(&h) # => [ 1, 2, 3 ]

Another case is using Hash with gsub

h = {  'a' => '1',  'b' => '2', 'c' => '3' }
"xxxabcxxx".gsub /[abc]/, h # => "xxx123xxx"

default

And very useful is Hash default for non entry values.

h = {  'a' => '1',  'b' => '2', 'c' => '3' }
h.default = 'Y'
"xxxabcxxx".gsub /[abcx]/, h # => "YYY123YYY"

You may use default_proc to create new key-value pairs.

h = {  'a' => '1',  'b' => '2', 'c' => '3' }
h.default_proc = proc { |h, c|  h[c] = c.upcase }
"xxxabcxxx".gsub /[abcx]/, h # => "XXX123XXX"

Interesting that we can do cache strategy as simple as follows

cache = Hash.new { |cache,url| cache[url] = open(url) }
puts cache['http://site.com/resource']

Array#to_proc

I have found interesting suggestion to add to_proc to array with similar functionality. https://thepugautomatic.com/2014/11/array-to-proc-for-hash-access/

If that fits your eyes

[ { name: "A" }, { name: "B" } ].map(&[:name])  # => [ "A", "B" ]

Here the way you can get it

class Array
  def to_proc
    ->(h) { length == 1 ? h[first] : h.values_at(*self) }
  end
end

And that will work with multiple keys too

[ { name: 'A', 'age' => 41 }, { name: 'B', 'age' => 42 } ].map(&[:name, 'age'])
# => [ [ "A", 41 ], [ "B", 42 ] ]