#I can't say much about the subtle differences. However, I can point out that Ruby #1.9 now allows optional parameters for lambdas and blocks.

#Here's the new syntax for the stabby lambdas under 1.9:

stabby = ->(msg='inside the stabby lambda') { puts msg }

#Ruby 1.8 didn't have that syntax. Neither did the conventional way of declaring #blocks/lambdas support optional args:

# under 1.8
l = lambda { |msg = 'inside the stabby lambda'|  puts msg }
#SyntaxError: compile error
#(irb):1: syntax error, unexpected '=', expecting tCOLON2 or '[' or '.'

#Ruby 1.9, however, supports optional arguments even with the old syntax:

l = lambda { |msg = 'inside the regular lambda'|  puts msg }
#=> #<Proc:0x0e5dbc@(irb):1 (lambda)>
l.call
#=> inside the regular lambda
l.call('jeez')
#=> jeez