Proc#real_arity

real_arity.rb

# Currying a proc causes #arity to return -1
#
# This adds a #real_arity method to a Proc that always reflects the 
# number of *remaining* parameters to be accepted (minus any partially applied)
class Proc
  @r_arity = nil

  def real_arity
    @r_arity || arity
  end

  o_curry = instance_method(:curry)

  define_method(:curry) do |*i|
    ar = real_arity
    o_curry.bind(self).(*i).tap do |o|
      o.instance_eval { @r_arity = i.first || ar }
    end
  end

  o_call = instance_method(:call)

  define_method(:call) do |*args|
    ar = real_arity
    o_call.bind(self).(*args).tap do |r|
      n_ar = ar - args.size
      r.instance_eval { @r_arity = n_ar } if r.kind_of?(Proc)
    end
  end
end

add = ->(a, b) { a + b }.curry
add.arity # => -1
add.real_arity # => 2
add1 = add.(1)
add1.arity # => -1
add1.real_arity # => 1

Ruby procs return -1 as arity for any curried Proc. Knowing the arity of a curried proc turns out to be pretty useful if you are working with a number of them in various stages of partial application.

This monkey patches a #real_arity method on that always returns the correct value