String concatenation in Rails 3

Question

I'm wondering why this is so: Ruby concatenates two strings if there is a space between the plus and the next string. But if there is no space, does it apply some unary operator?

params['controller'].to_s + '/'
# => "posts/"

params['controller'].to_s +'/'
# => NoMethodError: undefined method `+@' for "/":String

Answer 1

The parser is interpreting +'/' as the first parameter to the to_s method call. It is treating these two statements as equivalent:

> params['controller'].to_s +'/'
# NoMethodError: undefined method `+@' for "/":String

> params['controller'].to_s(+'/')
# NoMethodError: undefined method `+@' for "/":String

If you explicitly include the parenthesis at the end of the to_s method call the problem goes away:

> params['controller'].to_s() +'/'
=> "posts/"

Answer 2

If you want to concatenate a string, the safest way is to write "#{params[:controller].to_s} /" ruby's string escaping is safer and better in many cases

Answer 3

Look closely the error:

p "hi".to_s +'/'
p "hi".to_s -'2'

#=> in `<main>': undefined method `+@' for "/":String (NoMethodError)

This is because unary operator +,- etc is defined only Numeric class objects. It will be clear if you look at the code below:

p "hi".to_s +2
#=>in `to_s': wrong number of arguments (1 for 0) (ArgumentError)

Now the above error is exactly right for to_s. As to_s doesn't take any argument when it is called.

Correct version is:

p "hi".to_s + '2' #=> "hi2"