Ask a method, lambda or proc for the range of arguments it can be called with from minimum to maximum.

arity_range.rb

module ArityRange
  def arity_range
    args = parameters.map &:first

    req = args.count :req
    opt = args.include?(:rest) ? Float::INFINITY : args.count(:opt)

    keyreq = args.count :keyreq
    keyopt = args.include?(:keyrest) ? Float::INFINITY : args.count(:key)

    {arguments: req..req + opt, keywords: keyreq..keyreq + keyopt}
  end
end

class Proc
  include ArityRange
end

class Method
  include ArityRange
end

class UnboundMethod
  include ArityRange
end

arity_range_example.rb

require_relative 'arity_range'

# For example, #puts can take zero or more args.
method(:puts).arity_range
#=> {:arguments=>0..Infinity, :keywords=>0..0}

method(:==).arity_range
#=> {:arguments=>1..1, :keywords=>0..0}

->{ }.arity_range
#=> {:arguments=>0..0, :keywords=>0..0}

lambda { |x, y| }.arity_range
#=> {:arguments=>2..2, :keywords=>0..0}

proc { |x, y| }.arity_range
#=> {:arguments=>0..2, :keywords=>0..0}

def req_keyword_example(required:); end
method(:req_keyword_example).arity_range
#=> {:arguments=>0..0, :keywords=>1..1}

def keyword_example(required:, optional: true); end
method(:keyword_example).arity_range
#=> {:arguments=>0..0, :keywords=>1..2}

def opt_keyword_example(optional: true); end
method(:opt_keyword_example).arity_range
#=> {:arguments=>0..0, :keywords=>0..1}

def opt_keywords_example(optional: true, another: true); end
method(:opt_keywords_example).arity_range
#=> {:arguments=>0..0, :keywords=>0..2}

def double_splat_keyword_example(optional: true, **hash); end
method(:double_splat_keyword_example).arity_range
#=> {:arguments=>0..0, :keywords=>0..Infinity}

->(a, b = true, c:, d: true) { }.arity_range
#=> {:arguments=>1..2, :keywords=>1..2}