Braces: [Brackets], (Parentheses) & {Curlies} in Ruby & Rails

Question

So the loose tolerance of Ruby to use braces sometimes and not REQUIRE them has led to alot of confusion for me as I'm trying to learn Rails and when/where to use each and why?

Sometimes parameters or values are passed as (@user, @comment) and other times they seem to be [ :user => comment ] and still others it's just: :action => 'edit'

I'm talking about the us of [ ] vs ( ) vs { }

What ARE the rules? And are there any tricks you have to help you remember?

Answer 1

Parentheses () are for grouping logical or mathematical expressions and grouping arguments to a function call, e.g.:

a = 2 * (3 + 4)
b = (x==y) || (m==n)
Hash.new.send('[]=', :a, :b)

Curly Braces {} are used for hash literals and blocks, e.g.:

h = {1=>2, 2=>3}
h.each {|k,v| puts k+v}

Square Brackets [] are used for array literals, array indexing and slicing, and fetching from a hash, e.g.:

arr = [1, 2, 3]
two = arr[1]
three = h[2]

To confuse the matter, hash literals can also be used in-place as an argument to a method call without needing the curly braces or parentheses as long as it is the last argument (thanks samuil). Additionally, hash literals can be used in-place in square brackets to create a single-item array containing the hash:

puts 1=>2, 3=>4 #=> 1234
[5=>6, 7=>8]    #=> [{5=>6, 7=>8}]

When in doubt, always use parentheses to group items and wrap your hashes in curly braces.