From: "Eric I." <rubytraining@...>
Date: 2008-05-23T11:15:01+09:00
Subject: Re: Lambda Block vs. Method

On May 22, 9:18 pm, "APNelson.L" <APNelso...@gmail.com> wrote:
> I was just wondering what the benefit would be of creating a block
> that can be called rather than defining a method. Here is an example:
>
> def puts_time(x)
> puts x
> end
>
> instead of...
>
> puts_time = lambda { |x| puts x }
>
> personally I see no reason to create a block like this instead of a
> method.

Well lambda returns a Proc instance, and you can store a Proc in a
variable for later use, pass one or more Procs into a method as
parameters, and a method can return a Proc.  This allows code to be
treated as data and this gives you a higher level of abstraction or a
greater degree of flexibility.

Here's an example.  Suppose you have tons of data and would like to
allow the user to filter it in various ways.  Perhaps you'll even
provide the set of available filters in a drop-down menu or such.  Now
you could write some code where in some case statement you determine
which filter was selected and then call a method of a pre-determined
name that implements that filter.

Or you could do something along these lines:

require 'date'

date_count = 25

# create an array of some random dates in the last ~two
years
# somewhat biased to be more
recent
some_dates =
  (1..date_count).map { Date.today - rand(rand(2 * 365)) }

# set up some useful date
filters
date_filters = {
  "previous year" =>
    lambda { |d| d.year == Date.today.year - 1 },
  "this year" =>
    lambda { |d| d.year == Date.today.year },
  "last 12 months" =>
    lambda { |d| (0..364) === Date.today - d ||
      # add special leap year
check
      (Date.today - d == 365 && Date.today.mday != d.mday) },
  "last 7 days" =>
    lambda { |d| (0..6) === Date.today - d }
}

# try each filter out against the random
dates
date_filters.each do |label, filter|
  puts "The following dates are all in the #{label}:"
  puts some_dates.select { |d| filter.call(d) }.join("\n")
end

So now you have the filters stored as data in a data structure, and
you can use this relatively easily from multiple places throughout
your program.

What do you think?  Can you imagine this being useful?

Eric

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