Difference between prepend and append of colon ( :item vs item:)

Question

What is the difference between appending and prepending a colon in ruby?

Example:

#In rails you often have things like this:
has_many :models, dependent: :destroy

Why does dependent: have an appended colon, but :models and :destroy have a prepended colon? What is the difference?

Answer 1

This is a new syntax in Ruby 1.9 for defining symbols that are the keys in a hash.

Both prepended and appended :'s define a symbol, but the latter is only valid during the initialization of a hash.

You can think of a symbol as a lightweight string constant.

It is equivalent to

:dependent => :destroy

Prior to 1.9, hashes were defined with a syntax that is slightly more verbose and awkward to type:

hash = {
   :key => "value",
   :another_key => 4
}

They simplified it in 1.9:

hash = {
   key: "value",
   another_key: 4
}

If you are ever writing a module you want to use on Ruby prior to 1.9, make sure you use the older syntax.

Answer 2

Since Ruby allows you to omit parenthesis ()and in some cases curly braces {} it might not be very obvious but the above code is actually looking like this:

has_many(:models, { dependent: :destroy } )

Now, it means that has_many takes two arguments, one being a symbol :, an immutable string if you will, and also a hash where the dependent is the key and destroy is the value; also maybe seen as :dependent => destroy.

Answer 3

In both cases the colon indicates a symbol, but appending it is shorthand for when the symbol is a key in a hash.

dependent: :destroy

is the same as

:dependent => :destroy

Answer 4

The "appended" colon is simply the new common way of displaying hashes in 1.9.

dependent: :destroy is the same thing as :dependent => :destroy

On the other hand, a "prepended" colon indicates a symbol data type in Ruby.