DO :

1 == my_var
"my string" == my_string

DO NOT :

my_var == 1
my_string == 'my_string'

I guess you see where I'm going here : syntax error detected is much better than execution error. zero is great. What about the rest of integers ? What about strings ?

The = 0 instead of == 0 argument does not make much sense anymore, does it ?

My two cents.
I did not "invent" it, I was instructed to do it decades ago.

It took me some time to understand that sugar syntax.
First of : it's & :to_s obviously.
Operator is &.

What does it do ?

it turns

mythings.map(& :upcase)
# is the same as 
mythings.map(&:upcase)

into

mythings.map { |mything| mything.upcase }

Remark : sugar syntax can be a pain in the butt. I hope you do see it.

The map method applies the "block" { |mything| mything.upcase } on each and every elements of mythings and returns an enumerable of class mythings.class containing each result.

The operator is & and the argument in my example is :upcase. :upcase is a Symbol.
Symbols have a smart to_proc method.

Another example :

require 'date'
[1,DateTime.now,3].map(&:to_s)
# => ["1", "2021-09-05T12:06:54+02:00", "3"]

So how does it work ?

• &̀ when a parameter of a method is prepended with & then ruby knows this parameter to be a Proc

• :upcase is a symbol.

So the &:myproc is a smart shortcut implemented to turn the symbol :myproc into a proc.

Let's try it :

def call_my_block ( my_parameter, &myblock)
  myblock.call.myblock
end

The parameter &myblock is known to be a block.

When I use method call_my_block with the argument &:my_symbol :

• The second parameter is known to be a block.
• The argument will be translated into :my_symbol.to_proc
  If the first argument is of class KlassandKlassactually has a my_symbol method, then it will call it.

Example :

call_my_block DateTime.now , &:to_s translates into :

:to_s.to_proc.call DateTime.now 

note that :

a = :to_s.to_proc
#=> #<Proc:0x000055e77c191c98(&:to_s)>
a.call(13)
#=> "13"
a.class
#=> Proc

Hence :

call_my_block DateTime.now , &:to_s
# => "2021-09-05T12:06:00+02:00" 
[1,DateTime.now,3].map { |val| call_my_block(val,&:to_s) }
#=> ["1", "2021-09-05T12:21:54+02:00", "3"]
[1,DateTime.now,3].map(&:to_s)
["1", "2021-09-05T12:23:22+02:00", "3"]

Note ::

• A function/methods has parameters.
• When called the function is given arguments

In other terms, an argument is the value of a parameter.