Question

I’m learning the basics of Ruby (just starting out), and I came across the Hash.[] method. It was introduced with

a = ["foo", 1, "bar", 2]
=> ["foo", 1, "bar", 2]
Hash[*a]
=> {"foo"=>1, "bar"=>2}

With a little thought, I figured out that Hash[*a] is equivalent to Hash.[](*a) or Hash.[] *a. My question is why that’s the case. What is it that lets you put the *a inside the square brackets, and is there some kind of rule for where and when else “it” can be used?

Edit: My wording seems to be causing some confusion. I’m not asking about the array expansion. I get that. My question is basically: if [] is a method name, why is it okay to put arguments inside the brackets? It seems almost–but not quite–like saying that if you have a method Foo.dood, and you wanted to pass the string "hey" to it, then you could write Foo.do"hey"od.

Answer 1

There are a couple methods that ruby lets you call in a special way. These are the [] as you mentioned, the +, -, == and the like as someone else mentioned. Another important example are methods of the form something=(value) which can be called with object.something = value and allow you to create accessors.

Edit:

Fun fact 1: if you define a + method you get += for free.

Fun fact 2: if you define a <=> you get all comparison methods, courtesy of Comparable