syntax - What do the different brackets in Ruby mean?

Question

In Ruby, what's the difference between {} and []?

{} seems to be used for both code blocks and hashes.

Are [] only for arrays?

The documention isn't very clear.

Answer 1

It depends on the context:

1. When on their own, or assigning to a variable, [] creates arrays, and {} creates hashes. e.g.

   a = [1,2,3] # an array
   b = {1 => 2} # a hash
   

2. [] can be overridden as a custom method, and is generally used to fetch things from hashes (the standard library sets up [] as a method on hashes which is the same as fetch)
   There is also a convention that it is used as a class method in the same way you might use a static Create method in C# or Java. e.g.

   a = {1 => 2} # create a hash for example
   puts a[1] # same as a.fetch(1), will print 2
   
   Hash[1,2,3,4] # this is a custom class method which creates a new hash
   

   See the Ruby Hash docs for that last example.

3. This is probably the most tricky one - {} is also syntax for blocks, but only when passed to a method OUTSIDE the arguments parens.

   When you invoke methods without parens, Ruby looks at where you put the commas to figure out where the arguments end (where the parens would have been, had you typed them)

   1.upto(2) { puts 'hello' } # it's a block
   1.upto 2 { puts 'hello' } # syntax error, ruby can't figure out where the function args end
   1.upto 2, { puts 'hello' } # the comma means "argument", so ruby sees it as a hash - this won't work because puts 'hello' isn't a valid hash