Basic DSL to define operations that can fail

[waiting-for-dev]

Overview

We introduce a thin DSL on top of dry-monads' result type to define operations that can fail.

Dry::Operation#steps accepts a block where individual operations can be called with #step. When they return a Success, the inner value is automatically unwrapped, ready to be consumed by subsequen steps. When a Failure is returned along the way, the remaining steps are skipped and the failure is returned.

Example:

require "dry/operation"

class MyOperation < Dry::Operation
  def call(input)
    steps do
      attrs = step validate(input)
      user = step persist(attrs)
      step notify(user)
      user
    end
  end

  def validate(input)
   # Dry::Monads::Result::Success or Dry::Monads::Result::Failure
  end

  def persist(attrs)
   # Dry::Monads::Result::Success or Dry::Monads::Result::Failure
  end

  def notify(user)
   # Dry::Monads::Result::Success or Dry::Monads::Result::Failure
  end
end

include Dry::Monads[:result]

case MyOperation.new.call(input)
in Success(user)
  puts "User #{user.name} created"
in Failure[:invalid_input, validation_errors]
  puts "Invalid input: #{validation_errors}"
in Failure(:database_error)
  puts "Database error"
in Failure(:email_error)
  puts "Email error"
end

The approach is similar to the so-called "do notation" in Haskell, but done in an idiomatic Ruby way. There's no magic happening between every line within the block (i.e., "programmable semicolons"). Besides not being something possible in Ruby, it'd be very confusing for people to require all the lines to return a Result type (e.g., we want to allow debugging). Instead, it's required to unwrap intermediate results through the step method. Notice that not having logic to magically unwrap results is also intentional to allow flexibility to transform results in between steps (e.g., validate(input).value_or({}))

Please, check also the first two commits that perform some extra setup for development.

[timriley]

Looking great, thanks @waiting-for-dev! I've one little request about flatting the file structure for our unit specs, but otherwise, let's get this in :)