Simple Slack Slash Command (https://api.slack.com/slash-commands) example.

app_missing_available_options.rb

######################################
# Ruby doc: http://ruby-doc.org/core #
######################################

#########
# Setup #
#########
#
# Install the gems:
# =================
# gem install sinatra pry shotgun --no-ri --no-rdoc
# 
# Run the server:
# ===============
# shotgun app.rb
#
# Examples using HTTPie: https://github.com/jkbrzt/httpie
# =======================================================
#
# http -F http -f POST localhost:9393 command="/calc" text="add 1 1"
# http -F http -f POST localhost:9393 command="/calc" text="sum 1 2"
# http -F http -f POST localhost:9393 command="/calc" text="sub 2 1"
# http -F http -f POST localhost:9393 command="/calc" text="minus 3 1"
#
# Tip:
# ====
# Use the ngrok (https://ngrok.com/) to expose the server to the internet.
#
# Plus:
# ====
# https://pt.wikipedia.org/wiki/Filosofia_Unix
# http://www.sinatrarb.com/
# http://pryrepl.org/

require "sinatra"
require "pry"

class CommandData
  attr_reader :name

  def initialize(params)
    @data = params["text"].split
    @name = @data[0]
  end

  def num_1
    num(1)
  end

  def num_2
    num(2)
  end

  private

  def num(index)
    Integer(@data[index])
  end
end

class CommandBase
  attr_reader :data

  def initialize(params)
    @data = CommandData.new(params)
  end

  def operation
    fail NotImplementedError
  end

  def invoke?
    @data.name =~ pattern
  end

  def result
    String(operation)
  end

  def pattern
    self.class.const_get(:PATTERN)
  end
end

class CommandSum < CommandBase
  PATTERN = /sum|add/i

  def operation
    data.num_1 + data.num_2
  end
end

class CommandSub < CommandBase
  PATTERN = /sub(tract)?|minus/i

  def operation
    data.num_1 - data.num_2
  end
end

COMMANDS = [CommandSum, CommandSub]

post "/" do
  commands = COMMANDS.map {|command| command.new(params) }
  command = commands.detect(&:invoke?)
  command.result
end

app_with_available_options.rb

require "sinatra"
require "pry"

# Standard library
require "forwardable"

class CommandData
  attr_reader :command_name

  def initialize(params)
    @data = params["text"].split
    @command_name = @data[0]
  end

  def num_1
    num(1)
  end

  def num_2
    num(2)
  end

  private

  def num(index)
    Integer(@data[index])
  end
end

class CommandBase
  extend Forwardable

  def_delegators :@data, :command_name, :num_1, :num_2

  def initialize(data)
    @data = data
  end

  def operation
    fail NotImplementedError
  end

  def result
    String(operation)
  end

  def invoke?
    command_name =~ pattern
  end

  def names
    self.class.const_get(:NAMES)
  end

  private

  def pattern
    Regexp.new(names.join("|"), Regexp::IGNORECASE)
  end
end

class CommandSum < CommandBase
  NAMES = ["sum", "add"]

  def operation
    num_1 + num_2
  end
end

class CommandSub < CommandBase
  NAMES = ["sub", "minus"]

  def operation
    num_1 - num_2
  end
end

class Commands
  LIST = [CommandSum, CommandSub]
  AVAILABLE_OPTIONS = "Available options: %{options}"

  def initialize(params)
    @data = CommandData.new(params)
  end

  def execute
    command = detect

    if command
      command.result
    else
      available_options
    end
  end

  private

  def available_options
    options = all.map(&:names).flatten.join(", ")

    AVAILABLE_OPTIONS % {options: options}
  end

  def detect
    all.detect(&:invoke?)
  end

  def all
    LIST.map {|command| command.new(@data) }
  end
end

get "/" do
  execute_command
end

post "/" do
  execute_command
end

private

def execute_command
  Commands.new(params).execute
end