From: Jarmo Pertman <jarmo.p@...>
Date: 2011-02-26T09:55:31+09:00
Subject: Re: Symbol#to_proc helping out with #select to beat Scala-s solution

I tried to modify the methods directly in Enumerable and succeeded
only 50%.

Consider the code below:
module Enumerable
  def select
    puts 1
  end
end

class Array
  include Enumerable
end

[2,3].select # doesn't print 1

If i use find_all instead then it works:
module Enumerable
  def find_all
    puts 1
  end
end

class Array
  include Enumerable
end

[2,3].find_all # prints 1

Now i'm confused as to why isn't #select working as i was hoping for?

Jarmo


On Feb 26, 1:43 am, Sean O'Halpin <sean.ohal...@gmail.com> wrote:
> [snip]
>
> > I'm thinking that one possible way to shorten my calls would be to use
> > instance-eval in #select, #map and #sort_by so i could do it like
> > this:
> > sales.select{value > 50000}.map{buyer}.sort_by{age}
>
> Here's a trivial implementation of that idea:
>
> module Relational
>   def project(&block)
>     map { |x| x.instance_eval(&block) }
>   end
>
>   def where(&block)
>     select { |x| x.instance_eval(&block) }
>   end
>
>   def order_by(&block)
>     sort_by { |x| x.instance_eval(&block) }
>   end
> end
>
> class Buyer < Struct.new(:name, :age)
> end
>
> class Sale < Struct.new(:value, :buyer)
> end
>
> alice = Buyer["Alice", 30]
> bob = Buyer["Bob", 40]
> charlie = Buyer["Charlie", 50]
>
> sales = [
>          Sale[80000, alice],
>          Sale[40000, bob],
>          Sale[60000, charlie],
>         ]
>
> class Array
>   include Relational
> end
>
> p sales.where{ value > 50000 }.project{ buyer }.order_by{ age }
> # => [#<struct Buyer name="Alice", age=30>, #<struct Buyer
> name="Charlie", age=50>]
>
> # But it becomes uglier when you want to do this:
> p sales.where{ value > 50000 }.project{ [value, buyer] }.order_by{ self[1].age }
> # => [[60000, #<struct Buyer name="Alice", age=30>], [80000, #<struct
> Buyer name="Charlie", age=50>]]
>
> # Better to do it this way round:
> p sales.where{ value > 50000 }.order_by{ buyer.age }.project{ [value, buyer] }
> # => [[60000, #<struct Buyer name="Alice", age=30>], [80000, #<struct
> Buyer name="Charlie", age=50>]]
>
> However, in practice, I don't use it. Ruby's block syntax is perfectly
> elegant as it is. Why not be content with it?
>
> Regards,
> Sean