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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 :HelloWorld
  # => "helloworld"

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

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


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

For an up-to-date version, try

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

Backwards compatibility breakages

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.

See also


A ruby library for prettier-still anonymous blocks.




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