Multiple blog comment validations

comment_policy.rb

require 'pp'
require 'rumonade'

User = Struct.new(:user_id, :name)
Post = Struct.new(:post_id, :text, :comments)
Comment = Struct.new(:user, :post, :text)

Users = [User.new(101, "Dave")]
Posts = [Post.new(201, "Welcome to my blog", [])]

def validate_not_nil(arg, nil_error_msg)
  if arg.nil? then Left(nil_error_msg) else Right(arg) end
end

def validate_user(user_id)
  validate_not_nil(user_id, "Missing user id").right.flat_map do |id|
    validate_not_nil(Users.find {|u| u.user_id == id}, "Unknown user id")
  end
end

def validate_post(post_id)
  validate_not_nil(post_id, "Missing post id").right.flat_map do |id|
    validate_not_nil(Posts.find {|u| u.post_id == id}, "Unknown post id")
  end
end

Policy = { /\bdisagree\b/i => "disagreement", /\bjerk\b/i => "name calling" }

def validate_text(text)
  validate_not_nil(text, "Missing comment text").lift_to_a.right.flat_map do |txt, d|
    Policy.map { |rex, msg| if rex =~ txt then Left("No #{msg} allowed!") else Right(txt) end.lift_to_a }.
      inject(:+).right.map { |*txts| txts[0, 1] }
  end
end

def add_comment(user_id, post_id, text)
  [validate_user(user_id).lift_to_a, validate_post(post_id).lift_to_a, validate_text(text)].
    inject(:+).right.map { |u, p, t| p.comments << Comment.new(u, p, t.first); p }
end

pp add_comment(nil, nil, nil)
# Left(["Missing user id", "Missing post id", "Missing comment text"])

pp add_comment(42, 16, "I totally disagree with this jerk...")
# Left(["Unknown user id", "Unknown post id", "No disagreement allowed!", "No name calling allowed!"])

pp add_comment(101, 201, "I agree!")
# Right(#<struct Post post_id=201, text="Welcome to my blog", comments=[#<struct Comment user=#<struct User user_id=101, name="Dave">, post=#<struct Post:...>, text="I agree!">]>)