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 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"]
[{}].each &X[1] = 2
# => [{1 => 2}]
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"
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]
In normal usage there are no known bugs. That said, if you accidentally miss the & from in front of the X, in an expression that ends in an assignment (e.g. X.formatter = :inspect); then the #to_proc method of the object assigned will respond with the expression generated by that X the next time you call it from anywhere else in the same thread.
&X has been tested on MRI ruby 1.8.6, 1.8.7 and 1.9.2 and jruby 1.5.3. It is thread-safe.
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.
- https://github.com/danielribeiro/RubyUnderscore — which uses an underscore in place of
&Xand works by rewriting the syntax tree.