Symbol to string issue

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" + oTypeError: 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=> nilirb(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_sputs result