What is a elegant way in Ruby to tell if a variable is a Hash or an Array?

To check what @some_var is, I am doing a

if @some_var.class.to_s == 'Hash' 

I am sure there is a more elegant way to check if @some_var is a Hash or an Array.

You can just do:

@some_var.class == Hash

or also something like:

@some_var.is_a?(Hash)

It's worth noting that the "is_a?" method is true if the class is anywhere in the objects ancestry tree. for instance:

@some_var.is_a?(Object)  # => true

the above is true if @some_var is an instance of a hash or other class that stems from Object. So, if you want a strict match on the class type, using the == or instance_of? method is probably what you're looking for.

First of all, the best answer for the literal question is

Hash === @some_var

But the question really should have been answered by showing how to do duck-typing here. That depends a bit on what kind of duck you need.

@some_var.respond_to?(:each_pair)

or

@some_var.respond_to?(:has_key?)

or even

@some_var.respond_to?(:to_hash)

may be right depending on the application.

Usually in ruby when you are looking for "type" you are actually wanting the "duck-type" or "does is quack like a duck?". You would see if it responds to a certain method:

@some_var.respond_to?(:each)

You can iterate over @some_var because it responds to :each

If you really want to know the type and if it is Hash or Array then you can do:

["Hash", "Array"].include?(@some_var.class)  #=> check both through instance class
@some_var.kind_of?(Hash)    #=> to check each at once
@some_var.is_a?(Array)   #=> same as kind_of

Hash === @some_var #=> return Boolean

this can also be used with case statement

case @some_var
when Hash
   ...
when Array
   ...
end
 

I use this:

@var.respond_to?(:keys)

It works for Hash and ActiveSupport::HashWithIndifferentAccess.

In practice, you will often want to act differently depending on whether a variable is an Array or a Hash, not just mere tell. In this situation, an elegant idiom is the following:

case item
  when Array
   #do something
  when Hash
   #do something else
end

Note that you don't call the .class method on item.

You can use instance_of?

e.g

@some_var.instance_of?(Hash)

If you want to test if an object is strictly or extends a Hash, use:

value = {}
value.is_a?(Hash) || value.is_a?(Array) #=> true

But to make value of Ruby's duck typing, you could do something like:

value = {}
value.respond_to?(:[]) #=> true

It is useful when you only want to access some value using the value[:key] syntax.

> Please note that Array.new["key"] will raise a TypeError.

irb(main):005:0> {}.class
=> Hash
irb(main):006:0> [].class
=> Array