Why is it important to create a method as a symbol?

Question

I'm trying to understand the extent of what symbols do in Ruby. I understand that it is much more faster and efficient to use symbols as keys as opposed to strings, but how is it faster?

And from my understanding, when referencing methods it has to be represented as a symbol, :to_i as opposed to to_i. What is the purpose of this?

`when referencing methods it has to be represented as a symbol. (i.e :to_i as opposed to .to_i) What is the purpose of this?` - do you mean something like array.map(&:to_i)? — idej, Apr 07 '17 at 18:19
Welcome to Stack Overflow. Do you want to know what symbols do and how they work, or do you want to know why we use symbols when passing method names? The questions are not related and result in a broad question. Please read "[ask]" and the linked pages. — the Tin Man, Apr 07 '17 at 19:04
it's a duplicate of others questions about symbols. Just try to use search — idej, Apr 07 '17 at 19:09

score 8 · Accepted Answer · answered Apr 07 '17 at 18:48

In Ruby, a symbol is just an immutable string:

"hello " + "world" #=> "hello world"
:hello_ + :world #=> NoMethodError: undefined method `+' for :hello:Symbol

Being immutable makes symbols a safe and reliable reference, for example:

 Object.methods => [:new, :allocate, :superclass, #etc..]

If Ruby were to use strings here, users would be able to modify the strings, thus ruining future calls of Object.methods. This could be fixed by making copies of the strings each time the method is called, but that would be a huge memory footprint.

In fact, since Ruby knows symbols are never going to be modified, it saves each symbol only once, no matter how many times you declare it:

"hello".object_id #=> 9504940
"hello".object_id #=> 9565300

:hello.object_id #=> 1167708
:hello.object_id #=> 1167708

This takes the memory-saving potential of symbols even further, allowing you to use symbol literals in your code anywhere and everywhere with little memory overhead.

So, the round-about answer to your question: symbols can't be modified, but they're safer and more memory efficient; therefore, you should use them whenever you have a string that you know shouldn't be modified.

Symbols are used as the keys to hashes because:

You should never modify the key of a hash while it's in the hash.
Hashes require literal referencing a lot, ie my_hash[:test], so it's more memory-efficient to use symbols.

As for method references: you can't reference a method directly, ie send(my_method()) because can't tell the difference between passing the method in and executing it. Strings could have been used here, but since a method's name never changes once defined, it makes more sense to represent the name as a symbol.

Why is it important to create a method as a symbol?

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