From: David MacMahon <davidm@...>
Date: 2013-02-07T09:54:53+09:00
Subject: [ruby-core:51956] Re: [ruby-trunk - Feature #7792] Make symbols and strings the same thing

Hi, Rodrigo,

FWIW, I sympathize with your symbols-vs-strings as keys frustration, but I think it's not so trivial to have the best of both worlds (flexibility and performance).  Here is a simplification of your example:

require 'redis'
cache = Redis.new
value = :value
key = 'key'

v1 = cache[key] || (cache[key] = value)
v2 = cache[key] || (cache[key] = value)

p v1                   # :value or "value"
p v2                   # always "value"
p cache[key] == value  # always false

IMHO, the crux of the problem here is that Redis converts all Symbols to Strings, but assigning "v1 = cache[key] = value" does not store a String in v1 if value is a Symbol.  This could be addressed by doing:

v1 = cache[key] || (cache[key] = value; cache[key])

which will store a String in v1 if value is a Symbol even if key does not yet exist in cache.  Yes, it is an extra cache fetch.

The same kind of thing happens if value is a Fixnum.  Surely you wouldn't want Fixnums and Strings to be the same thing! :-)

Thanks for starting this fascinating thread,
Dave

On Feb 6, 2013, at 3:45 PM, Rodrigo Rosenfeld Rosas wrote:

> Ok, you missed the point. Let me show you a complete example as I think it will help you understanding my concerns (I thought I made myself clear in the ticket description, but I hope this code example will help you understand what I meant):
> 
> require 'redis'
> require 'json'
> require 'sequel'
> 
> cache = Redis.new
> users = cache['users'] || begin
>  db = Sequel.connect('postgres://user:password@localhost/mydb')
>  cache['users'] = db[:users].select(:id, :name).map{|r| id: r[:id], name: r[:name]} # or just select(:id, :name).all
> end
> 
> p users.first[:id]
> 
> 
> What will be the output of the code above?
> 
> Exactly! It depends! If the results were cached it will print "nil", otherwise it will print "1" (or whatever the first id is).
> 
> This is the problem that symbols cause because they don't behave like strings.
> 
> Best,
> Rodrigo.