class HashMD {”a” => {”b” => {”c” => “xxx”}}}

http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/13408

Excellent writeup.

hash = Hash.new(&(p=lambda{|h,k| h[k] = Hash.new(&p)}))

First you have to understand how a Hash allows you to set the default value of an empty key. Normally, the default is simply “nil”, meaning that if you access any old key, such as my_hash[:duane], without having first set :duane to something, then my_hash will return nil.

This default can be customized, like this:
my_hash = Hash.new { |h,k| h[k] = 0 }

In this case, my_hash[:brother] will return zero (0).

We could have just as well done this with a Proc object (instead of a block, as above) by doing something like this:

p = Proc.new { |h,k| h[k] = 0 }
my_hash = Hash.new(&p)

In this case, we’re using the Ruby notation ‘&’ to denote that the proc we’re passing in is to take the place of the optional block.

“lambda” is a synonym for “Proc.new”. Thus, Kent is creating a new Proc object assigned to the variable “p”. Because blocks (and thus, Proc objects, aka lambdas) are aware of the variable scope outside of themselves, Kent is able to pass the “p” he just barely assigned right back in to the Proc itself–essentially creating a recursive lambda function.

Thus, in English, this single line:

hash = Hash.new(&(p=lambda{|h,k| h[k] = Hash.new(&p)}))

says, “Create a hash whose default value is a hash whose default value is a hash whose default value is a hash whose …” etc.

–mortal

Nice solution–I wish I’d thought of it!

hash = Hash.new(&(p=lambda{|h,k| h[k] = Hash.new(&p)}))