An either can hold one of two possible things - a typical example would be either the successful result of a computation or an error explaining what went wrong.
By convention, a Left holds an error, while a Right holds the result of a successful computation.
Here's a simple example:
def divide x y
y == 0 ? Left("Cannot divide by zero") : Right(x / y)
end
divide(4,2).match do |m|
m.right { |res| puts "Division success: #{res}" }
m.left { |err| puts "Couldn't do this: #{err}" }
end
# The above will print "Division success: 2"
divide(4,0).match do |m|
m.right { |res| puts "Division success: #{res}" }
m.left { |err| puts "Couldn't do this: #{err}" }
end
# The above will print "Couldn't do this: Cannot divide by zero"