Question

I need to obtain the number of parameters a given block takes. For example:

foobar(1,2,3) { |a, b, c|
}

def foobar(x, y, z, &block)
  # need to obtain number of arguments in block
  # which would be 3 in this example
end

This is possible in the 1.9 trunk, but not in any official release. I was hoping if there’s any way to do this without having to download a separate gem/extension module.

Answer 1

When you materialize a block with &, it becomes a Proc object, which has an arity method. Just be careful – it returns the one’s complement if the proc takes a *splat arg.

def foobar(x, y, z, &block)
  p block.arity
end

(Answer via “The Ruby Programming Language” book.)