How does this code with send :[] work?

Question

The following code generates an output of 9. I understand send is simply calling the method :[], but am confused how the parameters work.

 x = [1,2,3]
 x.send :[]=,0,4                             #why is x now [4,2,3]
 x[0] + x.[](1) + x.send(:[],2)              # 4 + 2 + 3

How do line 2 and line 3 work?

Answer 1

Object#send provides another way to invoke a method.

x.send :[]=,0,4  

is saying, invoke []= method on x, and pass the argument 0, 4, it's equivalent to:

x[0] = 4

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The name send is because in Ruby, methods are invoked by sending a message to an object. The message contains the method's name, along with parameters the method may need. This idea comes from SmallTalk.

Answer 2

x.send(:[]=, 0, 4)

is the same* as

x.[]=(0, 4) 

which has the syntactic sugar of

x[0] = 4

which should be familiar from other languages. Parentheses are, of course, optional.

The .send or .public_send variety has the advantage that the method to invoke doesn't have to be hardcoded—it can come from a variable.

All this is congruent with ruby's paradigm of object orientedness: objects communicate with each other by sending messages, and the messages trigger code execution.

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*almost, #send will invoke private methods too