Are there "rules" for Ruby syntactic sugar?

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

Hash["a", "b", "c", "d"] is equivalent to Hash.[]("a", "b", "c", "d"). Almost everything in Ruby is a method call. 5 + 5 is equivalent to 5.+(5).

Given a = ["a", "b", "c", "d"] Hash[*a] is equivalent to Hash["a", "b", "c", "d"], which in turn is equivalent to Hash.[]("a", "b", "c", "d"). Similarly, foo(*a) is equivalent to foo("a", "b", "c", "d") This is called the explode operator, and allows sending an array to a method and have each item in the array count as one argument to the method, instead of sending the array to the method as the first argument.

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To specifically answer your update, there's nothing special that lets you put *a inside the brackets. The brackets is just sugar for a normal method call. The only "special" thing here is that you can send *a to any method.

Answer 2

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

Answer 3

I think that Ruby's syntax is defined in parse.y for YARV Ruby.