First let’s define an Infinity constant (since Ruby does not come with one):

Inf = 1.0 / 0.0

Now let’s see if we can create a Range Object with it:

(1..Inf)

Okay, no errors. Now let’s try to use it:

(1..Inf).include?(10000000) #=> true (1..Inf).include?(-10) #=> false (-Inf..0).include?(-1000) #=> true (-Inf..Inf).include?(-1000.456) #=> true

Now let’s try using it as a ‘lazy’ list:

(1..Inf).each.take(5) #=> [1, 2, 3, 4, 5] (1..Inf).each.take(5).select { |v| v.even? } #=> [2, 4] (100..Inf).step(100).take(3) #=> [100, 200, 300]

One last thing:

(1..Inf).each { |v| puts v } output: 1 2 3 4 5 ...etc (quite alot of numbers...)

Applications

lazy evaluation?

Some of the applications of Lazy Enumerators (discussed here: http://www.michaelharrison.ws/weblog/?p=163) can be accomplished using Infinite Ranges instead.

In particular, the lazy_select, and lazy_map family of functions defined in that article will also work fine with Infinite Ranges:

module Enumerable def lazy_select Enumerator.new do |yielder| each do |obj| yielder.yield(obj) if yield(obj) end end end end # first 4 even numbers (1..Inf).lazy_select { |v| v.even? }.take(4) output: [2, 4, 6, 8]

Credits

This trick was discovered by Beoran in response to a question by FreakGuard in #ruby-lang on freenode.

lazy_select by gfarfl.

EDIT: Okay, this trick isn’t 100% new, see here: http://weblog.jamisbuck.org/2007/2/7/infinity

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