contramap.rb

class Proc
  def contramap(&fn) self << fn end
end

adds = -> a, b { a + b }.curry

[1, 2, 3].map(&adds[2])
# => [3, 4, 5]

# What if the type on the left doesn't work with `adds`?
%w(1 2 3).map(&adds[2])
# TypeError: String can't be coerced into Integer

# ...well we can always transform them first:
%w(1 2 3).map(&:to_i).map(&adds[2])
# => [3, 4, 5]

# ...or apply a transformation to the input before it
# gets to the function with contramap:
%w(1 2 3).map(&adds[2].contramap(&:to_i))
# => [3, 4, 5]