Explain :colon and colon: in Ruby Like I'm Five

I am learning rails, finished ruby basics. Still confused with colon before and after in ruby.

 def create
    @user = User.new(user_params)

    respond_to do |format|
      if @user.save
        format.html { redirect_to @user, notice: 'User was successfully created.' }
        format.json { render :show, status: :created, location: @user }
      else
        format.html { render :new }
        format.json { render json: @user.errors, status: :unprocessable_entity }
      end
    end
  end

This is a create method automatically generated by rails scaffold command.

and please explain whats going on with the code.

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DISCUSSION (3)

:colon and colon: are both symbols in Ruby, a type of data (like strings, integers, etc).

As this Stack Exchange answer says, symbols are immutable. The link explains it better than I would so I'll leave that for them.

In the context of Ruby and Rails, you won't have to know too much besides some syntax rules:

# in a hash:
my_hash = {
  :cool => :symbol
}
# is the same as...
my_hash = {
  cool: :symbol
}

In previous versions of Ruby, the rocket or arrow syntax was the only way to have a symbol key point to a symbol value. Now, you can do a short-hand version using the after colon syntax colon:.

One thing to note is that if you ever want a hash with a key that is a string, you have to use the arrow/rocket syntax:

# this will work
my_hash = {
  "cool" => :thing
}
#=> {"cool" => :thing }

# this will not work
my_hash = {
  "cool": :thing
}
#=> { :cool => :thing }
# converts the string "cool" to a symbol

For more info about that code, I recommend reading the Rails guide section about scaffolding, which explains the code generated via scaffolding.

just doing a nitpick addition :-) :

actually, this works just fine: {"symbol-key-with-dash": :cool}. its that you can write a Symbol literal with quotes: :"looks-weird-still-is-symbol"

as a newcomer, my first "whoa" effect was when i realized that every function call like render :show, status: :created, location: @user is ultimately translated to: render(:show, {:status => :created, :location => @user});. it's just that in ruby you don't have to specify that curly braces to make the last argument a Hash. ruby is implicit. you can omit the brackets, the semicolon.

Oh interesting! Didn't know that. I personally think it's a bit confusing to have do "string": or :"string"if you wanted to have a key as a String instead of a Symbol. Good point to bring up though.

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