They vary based on how they deal with

• arity
  • do they care how many parameters they're passed?
• return semantics
  • does return return from the closure or from the function?
• scope
  • does it inherit scope or define a new one?

Rule #1: ignore lambdas

• move along; nothing to see here
• if you need to declare a closure, use proc
• procs are easier to use
  • due to their answers to the arity and return questions

Rule #2: blocks are procs

• or at least they act like they are
• there are some minor performance differences in some Ruby VMs

Rule #3: def breaks scope; do doesn't

x = 7

proc { puts x }.call #=> 7

def foo
  puts x
end
foo #=> NameError: undefined local variable or method `x'

Rule #4: don't worry about it

• just muddle along and things will probably work ok

Block Parameters

• Can use splats and defaults and tuples
• There's also a lot of weird options, see http://www.paperplanes.de/2012/2/16/fun-with-ruby-block-parameters.html