# Carving Ruby arrays

I ran across an interesting method in a colleague’s commit recently, which monkey patched Ruby’s Array class. To me, it nearly epitomizes Ruby’s elegance by packing a number of powerful techniques into a single short line of code, without being unreadably complicated. No line noise here. While I’m not sold one way or the other on monkey patching, I was intrigued enough by the following carve! method to dig a bit deeper.

Note: this replicates functionality provided by Enumerable.partition, so there’s probably no reason to use this in production. On the other hand, as noted by Benjamin Tan in The Ruby Closures Book, implementing a cheap counterfeit is usually enlightening.

Here’s the snippet under consideration:

class Array
def carve!
dup.tap { delete_if &Proc.new } - self
end
end

It does exactly what the name carve! implies, separating one Array into two Arrays based on the condition passed in as a block.

How it works:

• dup makes a copy of the values in the array and returns those values in a new array. That’s pretty straightforward.
• tap yields self to the block then returns itself. This is the first key part of the technique.
• delete_if takes a block, and deletes every element matching the condition given by the block. Note that the deletion occurs on every match, not after the iteration is complete. Note that delete_if operates on the original array, not on the duplicated array.
• &Proc.new is an odd construction, even for Ruby, which can appear to have a number of odd constructions. We’ll examine this in more detail shortly. This is the third key part.

Then the original array (self) is subtracted from the duplicated and modified array. That looks simple enough.

The slightly mind-bending aspect of carve! is how it processes the block passed in as the condition. It was not at all obvious to me, so let’s take a deeper look at that.

## dup and tap

The dup.tap calls warrant a closer look. Especially tap which to me is one of those super simple constructions which I don’t use very often, hence haven’t developed an intuitive feel for it. Using tap saves two lines of code for the same functionality:

def carve2!
other = dup
delete_if &Proc.new
other - self
end

A case could be made for carve2 being easier to understand. A (strong) case could also be made for mastering Ruby, tap and all. I choose mastery.

Ruby 1.9 introduced the tap method, which passes self to a block and returns self after processing the block. Normally, the result of the block would be returned. In combinatory logic, this behavior is defined by a function called kestrel, or $K$ Combinator:

The theoretical importance of $K$ cannot be overstated. Combined with the $S$ combinator, the $SK$ calculus is equivalent to an untyped lambda calculus, i.e., is Turing complete.

But that takes us too far afield for today.

The important thing when using tap is understanding that the block has no implicit return, it is only used as a generator of side effects.

## delete_if and passing procs

Let’s take a closer look at delete_if. This is one of those Ruby methods which “just works” usually without having to think much about it. For our investigation, we need to get drill a bit deeper. We’ll start with some REPL code:

\$ irb
2.2.3 :001 > [1, 2].delete_if { |e| e < 2 }
=> [2] # as expected

2.2.3 :002 > deleter = ->(e) { e < 2 } # set us up the lambda
=> #<Proc:0x007f8f0981bcb8@(irb):3 (lambda)>

2.2.3 :003 > [1, 2].delete_if(deleter) # we have to try...
ArgumentError: wrong number of arguments (1 for 0) # as expected...
from (irb):4:in 'delete_if'
from (irb):4
from /Users/doolin/.rvm/rubies/ruby-2.2.3/bin/irb:15:in '<main>'

2.2.3 :004 > [1, 2].delete_if(&deleter)
=> [2] # success

2.2.3 :005 >

As can be seen, delete_if doesn’t take any arguments, unless it takes a single argument prefixed with an ampersand &. In which case, that argument is received as Proc object.

## &Proc.new

In Ruby, Proc is an object created from a block.

When the last argument of a method is prefixed with an ampersand &, that argument will receive the block as a Proc object. From ruby-doc, Proc.new:

Creates a new Proc object, bound to the current context. Proc::new may be called without a block only within a method with an attached block, in which case that block is converted to the Proc object.

What I need to do here is create my own method which yields, and see how &Proc works with that. Using map and delete_if doesn’t give me enough insight into how &Proc really works. The dup.tap gets in the way. How about a custom map method:

class Array
def newmap
map &Proc.new
end
end

nm = [1, 2].newmap { |element| element + 1 }
puts nm # 2, 3

And that’s the long and short of carve!.

## Carving arrays

Without making any judgement call on monkey patching, carve! feels like an elegant addition to Ruby’s Array class. The implementation given here is not likely performant due to delete_if’s behavior of deleting by each item. On the other hand, implementing something more computationally efficient becomes very difficult very quickly, as the deletion criteria can be arbitrary.

The coolest thing about carve! for me as a Ruby programmer is running across code which stretches my comprehension of the language, something which is increasingly rare as my Ruby ability grows.