Default values in Ruby blocks

The ruby parser does not allow default values to be set in blocks, unlike in method signatures. This is of course valid ruby:

def method(a, b=0)
#...
end

but this isn't:

Proc.new {|a, b=0| } ### syntax error!

See e.g. this discussion in Ruby forum.

However it is possible to simulate the behaviour, creating a de facto signature with default values. For example lets say I want a proc accepting the same parameters as a method defined as def a(b, c=1):

lambda do |*args| # simulated signature: |b, c=1|
 b, c = args[0], args[1] || 1
end

This has the disadvantage that is not validating the number of arguments, so let's add some validation code to the block:

lambda do |*args| # simulated signature: |b, c=1|
 b, c = args[0], args[1] || 1
  # validate number of arguments:
 err = "wrong number of arguments"
  if args.size > 2
   raise ArgumentError, "#{err} (#{args.size} for 2)"
  elsif args.size == 0
   raise ArgumentError, "#{err} (0 for 1)"
  end
end

Now the block behaves like if it had the signature |b, c=1|.