Monadic vs Imperative code in ruby

README.md

Monadic vs Imperative

Here is a comparison of two pieces of code that do basically the same thing.

So what we have here is a chain of functions depending on each other, each consecutive takes the result of the previous one for further processing. If either of them fails, the rest shall not be executed.

1. validate parameters received from the outside world in a Hash
2. convert one of the parameters into an object
3. do a search in the database

You could express the whole operation like this:

find_in_db(convert_to_bson_id(get_id( params )))

The important thing is, I assume using a debugger (also sometime puts-driven-development) an expensive operation on the human side, and hence should be avoided. The implication is, that each operation needs to report errors in a clean and predictive way.

You can find more examples and a description of the monadic stuff (Try, Either) used here https://github.com/pzol/monadic

imperative.rb

module Transaction
  # @return [Hash, nil]
  def self.fetch_imp(params)
    raise ArgumentError, 'params must be hash and contain id' unless params.is_a? Hash && params.has_key?('id')
    id = params['id']

    begin
      bson_id = BSONTestConverter.from_string(id)
      DatabaseReader.find(v)
    rescue InvalidBSONString
      raise ArgumentError, "'#{id}' is not a valid BSON id"
    rescue SomeDatabaseError => dberror # just for the record
      raise SomeDatabaseError, "#{dberror.message}: '#{id}' not found"
    end
  end
end

monadic.rb

module Transaction
  # @return [Success, Failure]
  def self.fetch(params)
    id = Maybe(params)['id']

    Either(id).else('id is missing').
      >= {|id| Try { BSONTestConverter.from_string(id) }.or("'#{id}' is not a valid BSON id") }.
      >= {|id| Try { DatabaseReader.find(id)           }.or("'#{id}' not found")              }
  end
end

monadic_refactored.rb

module Transaction
  def self.fetch2(params)
    id = Maybe(params)['id']

    Either(id).else('id is missing').
      bind {|id| id_to_bson(id) }.
      bind {|id| find_in_db(id) }
  end

  def self.id_to_bson(id)
    Try { BSONTestConverter.from_string(id) }.or("'#{id}' is not a valid BSON id")
  end

  def self.find_in_db(bson_id)
    Try { DatabaseReader.find(bson_id) }.or("'#{bson_id}' not found")
  end
end