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Re: Guidelines for the "symbol" syntax class

From: Dmitry Gutov
Subject: Re: Guidelines for the "symbol" syntax class
Date: Mon, 4 Jan 2016 04:34:04 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:43.0) Gecko/20100101 Thunderbird/43.0

On 01/04/2016 04:21 AM, John Yates wrote:
On Sun, Jan 3, 2016 at 8:35 PM, Dmitry Gutov <address@hidden
<mailto:address@hidden>> wrote:
 > => [:<=>, :==, :===, :eql?, :hash, :casecmp, :+, :*, :%, :[], :[]=,
:insert, :length, :size, :bytesize, :empty?, ...]

It seems similar to C++'s operator keyword.  Am I getting closer?  Is
the colon required to abut the subsequent characters or can one write :
<=> (note intervening space)?

No space allowed. See this usage example:

irb(main):003:0> "abc" == "def"
=> false
irb(main):004:0> String.instance_method(:==)
=> #<UnboundMethod: String#==>
irb(main):005:0> "abc".size
=> 3
irb(main):006:0> String.instance_method(:size)
=> #<UnboundMethod: String#size>
irb(main):007:0> "abc".method(:size)
=> #<Method: String#size>
irb(main):008:0> "abc".method(:size).call
=> 3

You can have Symbols with any name, though. So they are not tied to methods, variables or anything.

Here's a good explanation: http://www.randomhacks.net/2007/01/20/13-ways-of-looking-at-a-ruby-symbol/, in particular, comparison #6 rings true: Ruby Symbols are similar to Lisp symbols. In Lisp, one can reference a dynamically bound variable, or call a function, using a symbol:

(let ((sym 'car)) (funcall sym nil))

or even

(let ((sym 'car)) (funcall (symbol-function sym) nil))

which is similar to "abc".method(:size).call I've tried above.

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