Fun with Procs in Ruby 1.9

Ruby 1.9 adds a lot of features to Proc objects.

Currying is the ability to take a function that accepts n parameters and fffgenerate from it one of more functions with some parameter values already filled in. In Ruby 1.9, you create a curry-able proc by cthe curry method on it. If you subsequently call this curried proc with fewer parameters than it expects, it will not execute. Instead, it returns a new proc with those parameters already bound.

Let’s look at a trivial example. Here’s a proc that simply adds two values:

plus = lambda {|a,b| a + b}
puts plus[1,2]

I’m using the [ ] syntax to invoke the proc with arguments, in this case 1 and 2. The code will print 3.

Now let’s have some fun.

curried_plus = plus.curry n
# create two procs based on plus, but with the first parameter 
# already set to a value
plus_two = curried_plus[2]
plus_ten = curried_plus[10]

puts plus_two[3]
puts plus_ten[3]

On line 1, I create a curried version of the plus proc. I then call it twice, but both times I only pass it one parameter. This means it cannot execute the body. Instead, each time it returns a new proc which is like the original, but which has the first parameter preset to either 2 or 10. In the last two lines, I call these two new procs, supplying the missing parameter. This means they can execute normally, and the code outputs 5 and 13.

You can have a lot of fun with currying, but that’s not why we’re here today.

Over the weekend, Matz added a new method to the Proc class. You can now use Proc#=== as an alias for Proc.call. So, why on earth would you want to do that? Well, remember that === is used to match terms in a case statement. Over of the AimRed blog, they noted that this feature could be used to make the matching in case statements actually execute code. In their example, they manually added the ===method to class Proc

class Proc
  def ===( *parameters )
    self.call( *parameters )
  end
end

Then you can write something like

sunday = lambda{ |time| time.wday == 0 }
monday = lambda{ |time| time.wday == 1 }
# and so on...

case Time.now
when< sunday
<  puts "Day of rest"
when monday
  puts "work"
# ...
end

See how that works? As Ruby executes the case statement, it looks at each of the parameters of thewhen clauses in turn. For each, it invokes its === method, passing that method the original case discriminator (Time.now in this example). But with the new === method in class Proc, this will now execute the proc, passing it Time.now as a parameter.

While updating the PickAxe, I noticed that Matz liked this so much that it is now part of 1.9. And it means we can combine this trick with currying to write some fun code:

is_weekday = lambda {|day_of_week, time| time.wday == day_of_week}.curry

sunday    = is_weekday[0]
monday    = is_weekday[1]
tuesday   = is_weekday[2]
wednesday = is_weekday[3]
thursday  = is_weekday[4]
saturday  = is_weekday[6]

case Time.now
when sunday
  puts "Day of rest"
when monday, tuesday, wednesday, thursday, friday
  puts "Work"
when saturday 
  puts "chores"
end

Is this incredibly efficient? Not really :) But it opens up quite an interesting set of possibilities.