Re: lambda, ->, haskell, and so on

On Thu, May 22, 2008 at 5:37 PM, Berger, Daniel <[e-mail]> wrote:
> Could someone show me the use case for lambdas with default arguments?
> Why would we need this in practice?
>
> I also don't understand the use case for blocks with default values.

It's the same case as default values for regular method (functions).
Lambdas, blocks are like anonymous functions (and closures).
For example in ActiveRecord there is recently added new feature named_scope:

class Post
  named_scope :recent, lambda { |*args| {:conditions => ["released_at
> ?", (args.first || 2.weeks.ago)]} }
end

usage:

Post.recent # all posts from latest 2 weeks
Post.recent(3.weeks.ago) # all posts from latest 3 weeks

As you see there is very ugly trick with *args (because you can't just
pust |time=2.weeks.ago| and for |time| there is warning when you don't
put parameter).

This example comes from Ryan Bates railscasts
(http://railscasts.com/episodes/108).
In Ruby 1.9 it's easier with ->

named_scope :recent, ->(time = 2.weeks.ago) { {:conditions =>
["released_at > ?", time]} }


-- 
Rados梶w Buウat

http://radarek.jogger.pl - m blog