Using closures, continuations in real life

rubyonrails-talk
1

Hi all !! I'm fairly new to Rails programming and finding somewhat different n the approach than .NET programming I did previosuly. I found this blog post : http://deadprogrammersociety.blogspot.com/2007/02/ruby-blocks-closure…

It's about closures and contiinuations, I get the basic idea about closures for example class GreetingGenerator def initialize(greeting) @block = lambda {|name| puts "#{greeting}, #{name}"} end def greet(name) @block.call name end end

frenchGreeting = GreetingGenerator.new "Bonjour" englishGreeting = GreetingGenerator.new "Hello" southernGreeting = GreetingGenerator.new "Howdy" japaneseGreeting = GreetingGenerator.new "Ohayou" germanGreeting = GreetingGenerator.new "Guten Tag"

frenchGreeting.greet "John" englishGreeting.greet "John" southernGreeting.greet "John" japaneseGreeting.greet "John" germanGreeting.greet "John"

puts greeting # just checking that the `greeting` variable doesn't exist in the main scope In this case, each block carries a little information with it ("greeting").

This gives me the idea that clousures might be used in practice as a functional replacement for a class behaving as an abstract factory (the pattern). In this example the function makes a closure over the type of language (i.e. english, german) and then we pass to each function the parameters for the fabric to produce the output. Could anyone post some more comments and tell me if this idea is OK? I would love to know about continuations also. !

Thanks & regards Federico

Espero sus opiniones y comentarios tambien sobre por ejemplo continuations.

2

What you're doing seems fine to me, although I'm no expert. I sometimes see something like class_eval used to write the entire method instead. Most of what little I know of the topic comes from the Little Schemer; here's a pretty long way to make a factorial function in Ruby using closures:

Y = lambda { |le| lambda { |f| f[f] }[ lambda { |f| le[ lambda { |x| f[f] }]}]}

lil_fib = lambda { |f|              lambda { |n|                if n.zero?                  1               else                  n * f[n-1]               end             }}

Y[lil_fib][10] => 3628800

Continuations still screw me up big time. Chad Fowler and Jim Weirich wrote this on it: Chicago Area Ruby Group - Continuations Demystified - Cover I still don't really grok the concept. I've heard that Seaside (a web framework using Smalltalk) makes heavy use of continuations. Maybe playing with that would help make the concept more concrete?

Jim