Ruby Symbols are not Lisp Symbols

Saturday November 29, 2014

I heard about How Emacs influenced Ruby and the main thing I took from it was the possible connection between (Emacs) Lisp symbols and Ruby symbols, as suggested by Xah. But Ruby symbols and Lisp symbols are really quite different.

In Ruby you write a symbol starting with a colon. This looks similar to writing an apostrophe before a symbol in Lisp to quote it.

# Ruby
:aaron         # This is a symbol.
               # It is written and displayed with a leading colon.
# => :aaron

; Lisp
'aaron         ; This is shorthand for the next line.
(quote aaron)  ; These both evaluate to the symbol itself, as shown.
; => aaron

The big difference is that in Lisp, symbols evaluate as variables and functions. That's why you have to quote a symbol in Lisp to work with the symbol itself. In Ruby, on the other hand, symbols never evaluate to anything else. Variable and method names in Ruby are not symbols; they're off in their own world. Note the differing error messages here:

# Ruby
aaron        # This is not a symbol but evaluates as a variable or method.
# => NameError: undefined local variable or method `aaron' for main:Object

; Lisp
aaron        ; This is a symbol which will (try to) evaluate as a variable.
; => *** Eval error ***  Symbol's value as variable is void: aaron

You can't set (or bind) a Ruby symbol to a value at all—it's a syntax error. But that's exactly what you do in Lisp.

# Ruby
:aaron = 12       # This attempts to set a symbol to 12, which Ruby hates.
# => SyntaxError: (irb):10: syntax error, unexpected '=', expecting end-of-input

; Lisp
(set 'aaron 12)   ; This sets a symbol's value as a variable to 12.
; => 12

Conversely, what works naturally in Ruby is nonsense in Lisp.

# Ruby
aaron = 27        # This sets a variable to 27.
# => 12

; Lisp
(set aaron 27)    ; This attempts to set 12 to 27.
; => *** Eval error ***  Wrong type argument: symbolp, 12

Deeper differences between Lisp and Ruby are evident, for example, in the behavior of eval. In Ruby, eval takes a string, while in Lisp, it takes a Lisp object. So while it's true that in Ruby eval(:aaron.to_s) will give the variable aaron's value, it's fairly unrelated to the symbol :aaron, and quite different from (eval aaron) in Lisp.

Some uses of Ruby and Lisp symbols are similar, but by and large trying to think of them as related was doing me more harm than good.

I found that Christian Neukirchen has a 2005 post with the same name as this one, which points out even more differences between Lisp symbols and Ruby symbols. His link to the Common Lisp HyperSpec "K" glossary page (for "keyword") is broken; here's a working one. I'm playing with Emacs Lisp here, not Common Lisp. There may be more to be said.