What does the "=>" in "rescue Exception => e" do?

Given the example:

def method_of_doom
my_string = "I sense impending doom."
my_string.ah_ha_i_called_a_nonexistent_method
rescue NoMethodError => e:
puts "PROBLEM: " + e.to_s
rescue Exception:
puts "Uhh...there's a problem with that there method."
end

On the line where it says:

rescue NoMethodError => e:

What is the '=>' doing?

How is it different than this usage:

module FighterValues
BAMBOO_HEAD = { 'life' => 120, 'hit' => 9 }
DEATH = { 'life' => 90, 'hit' => 13 }
KOALA = { 'life' => 100, 'hit' => 10 }
CHUCK_NORRIS = { 'life' => 60000, 'hit' => 99999999 }

def chuck_fact
puts "Chuck Norris' tears can cure cancer..."
puts "Too bad he never cries."
end
end

module ConstantValues
DEATH = -5 # Pandas can live PAST DEATH.
EASY_HANDICAP = 10
MEDIUM_HANDICAP = 25
HARD_HANDICAP = 50
end

puts FighterValues::DEATH
→ {'life'=>90,'hit'=>13}

puts ConstantValues::DEATH
→ -5

Solution

The Hash Rocket is a Syntactic Token

The hash rocket is actually a syntactic token. You can find the token in the grammar defined by ext/ripper/ripper.y:

%token tASSOC           /* => */

In other words, Ripper uses the hash rocket to associate things.

How tASSOC is Used

In general, this token is used in hash literals to associate a key with a value. For example:

{ :e => 'foo' }

associates the string literal foo with the symbol :e. This common usage is why people tend to think of the hash rocket as solely a hash-related construct.

On the other hand, the following associates a variable with an exception:

rescue => e

In this case, rather than associating a key with a value, Ripper is associating the variable e with the implied StandardError exception, and uses the variable to store the value of Exception#message.

What does the "=>" in "rescue Exception => e" do?

The Hash Rocket is a Syntactic Token

How tASSOC is Used

Further Reading