Symbol to string issue

Following code fails

world = :world
result = 'hello' + world
puts result #=> can't co开发者_如何学编程nvert Symbol into String

Following code works

world = :world
result = "hello #{world}"
puts result #=> hello world

Why?

Using ruby 1.8.7

String interpolation is an implicit to_s call. So, something like this:

result = "hello #{expr}"

is more or less equivalent to this:

result = "hello " + expr.to_s

As karim79 said, a symbol is not a string but symbols do have to_s methods so your interpolation works; your attempt at using + for concatenation doesn't work because there is no implementation of + available that understand a string on the left side and a symbol on the right.

The same behaviour would occur if world were a number.

"hello" + 1 # Doesn't work in Ruby
"hello #{1}" # Works in Ruby

If you want to add a string to something, implement to_str on it:

irb(main):001:0> o = Object.new
=> #<Object:0x134bae0>
irb(main):002:0> "hello" + o
TypeError: can't convert Object into String
        from (irb):2:in `+'
        from (irb):2
        from C:/Ruby19/bin/irb:12:in `<main>'
irb(main):003:0> def o.to_str() "object" end
=> nil
irb(main):004:0> "hello" + o
=> "helloobject"

to_s means "You can turn me into a string", while to_str means "For all intents and purposes, I am a string".

A symbol is not a string, and as such it cannot be concatenated to one without explicit conversion. Try this:

result = 'hello ' + world.to_s
puts result

As a side note, you can always define the method yourself :)

ruby-1.9.2-p0 > class Symbol
ruby-1.9.2-p0 ?>  def +(arg)
ruby-1.9.2-p0 ?>    [to_s, arg].join(" ")
ruby-1.9.2-p0 ?>    end
ruby-1.9.2-p0 ?>  end
 => nil 
ruby-1.9.2-p0 > :hello + "world"
 => "hello world"