Why can I not concatenate two strings and assign them to a symbol?

Question

. . . as in this example:

helloworld.rb:1: syntax error, unexpected '=', expecting $end
:helloworld = "hello ".concat("world")

I thought if I use concat I'm modifying the string "hello " and adding "world" to it and then ultimately assigning the resulting string - "hello world" - to the :helloworld symbol on the left side of the equals sign. I thought that would be legal, just as if I had written:

:helloworld = "hello world"

oh, wait, that doesn't work either. (scratches head).

Answer 1

Try this:

:"hello world"

Answer 2

Ruby symbols cannot have values assigned to them since they represent a canonical value. If you want to build a symbol from a string, you can use "hello ".concat("world").to_sym.

Answer 3

symbols are confusing when coming from languages that don't have anything like them. You can't assign to symbols, that's not what they're for.

Here are a couple of examples that may help explain it.

5.times do |i|
  # a new string is created in each iteration of the loop
  puts "my string".object_id

  # there is only ever one symbol for each possible value
  # so a new object is not created during each iteration
  puts :my_symbol.object_id

  # this often makes a difference when you're using them for things like hash keys
  #some_method(:name => 'bob')
  #some_method('name' => 'bob')
end

The other big difference is that a symbol comparison is simply a pointer comparison.

Answer 4

Perhaps what you're actually looking for is a constant? They can be dynamically generated, referred to later, and you get a warning if you try to reassign one.

>> HELLO_WORLD = "hello ".concat("world")
=> "hello world"
>> HELLO_WORLD = "foo"
(irb):3: warning: already initialized constant HELLO_WORLD
=> "foo"

Answer 5

You cannot assign to a symbol but you can return a symbol which is a concatenation of other symbols, although you have to pass through strings as far as I know (I'd like to know if there is a better way).

Suppose you want to obtain the symbol :myway starting from the symbols :my and :way ;

(:my.to_s + :way.to_s).to_sym # return :myway

Answer 6

Well, yeah. You can't assign to symbols. Do you mean something like:

foo = { :helloworld => "hello ".concat("world") }

?

Answer 7

class Symbol
  def + (to)
    return "#{self}#{to}".to_sym
  end
end

Answer 8

This worked for me

@foo = 'world'
"hello #{@foo}".to_s.to_sym

output=> :"hello world"