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 who created an earlier version.

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, as everything in ruby is a method, 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"]

As an added bonus, the effect is transitive — you can chain method calls:

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

There are two 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]

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

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

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