The Ampex (&X) library provides a Metavariable X that can be used in conjunction with the unary ampersand to create anonymous blocks in a slightly more readable way than the default. It was inspired by the clever Symbol#to_proc method which handles the most common case very elegantly, and discussion with Sam Stokes about creating lazy enumerators in ruby.

At its simplest, &X can be used as a drop-in replacement for Symbol#to_proc:

[1,2,3].map &X.to_s
  # => ["1", "2", "3"]

However the real strength in the library comes from allowing you to call methods with arguments:

[1,"2",3].select &X.is_a?(String)
  # => ["2"]

And to chain method calls:

[1, 2, 3].map &X.to_f.to_s
  # => ["1.0", "2.0", "3.0"]

As everything in Ruby is a method call, you can create readable expressions without the noise of a one-argument block:

[{1 => 2}, {1 => 3}].map &X[1]
  # => [2, 3]

[1,2,3].map &-X
  # => [-1, -2, -3]

["a", "b", "c"].map &(X * 2)
  # => ["aa", "bb", "cc"]

You can use this in any place a block is expected, for example to create a lambda:

normalizer = lambda &X.to_s.downcase
normalizer.call :HelloWorld
  # => "helloworld"

Last but definitly not least you can use ampex in a case clause:

case something
when X.respond_to? :foo
  something.foo
when X.respond_to? :[]
  something['foo']
end

There are a few things to watch out for:

Firstly, &X can only appear on the left:

[1, 2, 3].map &(X + 1)
  # => [2, 3, 4]

[1, 2, 3].map &(1 + X) # WRONG
  # => TypeError, "coerce must return [x, y]"

[[1],[2]].map &X.concat([2])
  # => [[1, 2], [2, 2]]

[[1],[2]].map &[2].concat(X) # WRONG
  # => TypeError, "Metavariable#to_ary should return Array"

Secondly, other arguments or operands will only be evaluated once, and not every time:

i = 0
[1, 2].map &(X + (i += 1)) # WRONG
  # => [2, 3]

i = 0
[1, 2].map{ |x| x + (i += 1) }
  # => [2, 4]

&X has been tested on MRI ruby 1.8.6, 1.8.7, 1.9.2, 1.9.3, jruby, and rubinius.

For bug-fixes or enhancements, please contact the author: Conrad Irwin conrad.irwin@gmail.com

For an up-to-date version, try https://github.com/rapportive-oss/ampex

This library is copyrighted under the MIT license, see LICENSE.MIT for details.

Between version 1.2.1 and version 2.0.0, the support for assignment operations was removed from ampex. These had a very non-obvious implementation, and it was impossible to support assigning of falsey values; and did not work on rubinius.

Between version 2.0.0 and 3.0.0 Metavariable#=== was implemented to provide case statement support. This will break code that was expecting &(X === foo) to work.

• http://cirw.in/blog/ampex — a blog post that describes the ideas.
• https://github.com/raganwald/homoiconic/blob/master/2012/05/anaphora.md#anaphora-in-ruby-2012-edition — a detailed overview of all similar libraries available for Ruby.
• https://github.com/danielribeiro/RubyUnderscore — which uses an underscore in place of &X and works by rewriting the syntax tree.
• https://gist.github.com/1224361 — a patch for Rubinius that enables the underscore in a similar way.
• http://blog.railsware.com/2012/03/13/ruby-2-0-enumerablelazy/ — The upcoming lazy enumerable support for Ruby 2.0.