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         


        

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

The *<Array> method tells ruby not to take the array as array, but to use the 'expanded' array.

Example:

def test(*argv)
  p argv
end

a = [1,2,3]
test(a) #-> [[1, 2, 3]]
test(*a) #-> [1, 2, 3]    

With test(a) the array a is the one and only parameter.

With test(*a) a is used as a list of parameters.

---

In your case

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

is similar to

Hash["foo", 1, "bar", 2]

Hash[*a] would be

Hash[["foo", 1, "bar", 2]]

Answer 4

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