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stevegeek/typed_operation: Command pattern, which is callable, and can be partially applied, curried and has typed parameters. Authorization to execute via action_policy if desired.

An implementation of a Command pattern, which is callable, and can be partially applied.

Inputs to the operation are specified as typed attributes (uses literal).

Type of result of the operation is up to you, eg you could use Dry::Monads.

• Operations can be partially applied or curried
• Operations are callable
• Operations can be pattern matched on
• Parameters:
  • specified with type constraints (uses literal gem)
  • can be positional or named
  • can be optional, or have default values
  • can be coerced by providing a block

class ShelveBookOperation < ::TypedOperation::Base
  # Parameters can be specified with `positional_param`/`named_param` or directly with the
  # underlying `param` method.
  
  # Note that you may also like to simply alias the param methods to your own preferred names:
  # `positional`/`named` or `arg`/`key` for example.
  
  # A positional parameter (positional argument passed to the operation when creating it).
  positional_param :title, String
  # Or if you prefer:
  #   `param :title, String, positional: true`
  
  # A named parameter (keyword argument passed to the operation when creating it).
  named_param :description, String
  # Or if you prefer:
  #   `param :description, String`

  # `param` creates named parameters by default
  param :author_id, Integer, &:to_i
  param :isbn, String
  
  # Optional parameters are specified by wrapping the type constraint in the `optional` method, or using the `optional:` option
  param :shelf_code, optional(Integer)
  # Or if you prefer:
  #   `named_param :shelf_code, Integer, optional: true`

  param :category, String, default: "unknown".freeze

  # optional hook called when the operation is initialized, and after the parameters have been set
  def prepare
    raise ArgumentError, "ISBN is invalid" unless valid_isbn?
  end

  # optionally hook in before execution ... and call super to allow subclasses to hook in too
  def before_execute_operation
    # ...
    super
  end

  # The 'work' of the operation, this is the main body of the operation and must be implemented
  def perform
    "Put away '#{title}' by author ID #{author_id}#{shelf_code ? " on shelf #{shelf_code}" : "" }"
  end

  # optionally hook in after execution ... and call super to allow subclasses to hook in too
  def after_execute_operation(result)
    # ...
    super
  end
  
  private

  def valid_isbn?
    # ...
    true
  end
end

shelve = ShelveBookOperation.new("The Hobbit", description: "A book about a hobbit", author_id: "1", isbn: "978-0261103283")
# => #<ShelveBookOperation:0x0000000108b3e490 @attributes={:title=>"The Hobbit", :description=>"A book about a hobbit", :author_id=>1, :isbn=>"978-0261103283", :shelf_code=>nil, :category=>"unknown"}, ...

shelve.call
# => "Put away 'The Hobbit' by author ID 1"

shelve = ShelveBookOperation.with("The Silmarillion", description: "A book about the history of Middle-earth", shelf_code: 1)
# => #<TypedOperation::PartiallyApplied:0x0000000103e6f560 ...

shelve.call(author_id: "1", isbn: "978-0261102736")
# => "Put away 'The Silmarillion' by author ID 1 on shelf 1"

curried = shelve.curry
# => #<TypedOperation::Curried:0x0000000108d98a10 ...

curried.(1).("978-0261102736")
# => "Put away 'The Silmarillion' by author ID 1 on shelf 1"

shelve.call(author_id: "1", isbn: false)
# => Raises an error because isbn is invalid
# :in `initialize': Expected `false` to be of type: `String`. (Literal::TypeError)

Operations can also be partially applied and curried:

class TestOperation < ::TypedOperation::Base
  param :foo, String, positional: true
  param :bar, String
  param :baz, String, &:to_s

  def perform = "It worked! (#{foo}, #{bar}, #{baz})"
end

# Invoking the operation directly
TestOperation.("1", bar: "2", baz: 3)
# => "It worked! (1, 2, 3)"

# Partial application of parameters
partially_applied = TestOperation.with("1").with(bar: "2")
# => #<TypedOperation::PartiallyApplied:0x0000000110270248 @keyword_args={:bar=>"2"}, @operation_class=TestOperation, @positional_args=["1"]>

# You can partially apply more than one parameter at a time, and chain calls to `.with`.
# With all the required parameters set, the operation is 'prepared' and can be instantiated and called
prepared = TestOperation.with("1", bar: "2").with(baz: 3)
# => #<TypedOperation::Prepared:0x0000000110a9df38 @keyword_args={:bar=>"2", :baz=>3}, @operation_class=TestOperation, @positional_args=["1"]>

# A 'prepared' operation can instantiated & called
prepared.call
# => "It worked! (1, 2, 3)"

# You can provide additional parameters when calling call on a partially applied operation
partially_applied.call(baz: 3)
# => "It worked! (1, 2, 3)"

# Partial application can be done using `.with or `.[]`
TestOperation.with("1")[bar: "2", baz: 3].call
# => "It worked! (1, 2, 3)"

# Currying an operation, note that *all required* parameters must be provided an argument in order  
TestOperation.curry.("1").("2").(3)
# => "It worked! (1, 2, 3)"

# You can also curry from an already partially applied operation, so you can set optional named parameters first.
# Note currying won't let you set optional positional parameters.
partially_applied = TestOperation.with("1")
partially_applied.curry.("2").(3)
# => "It worked! (1, 2, 3)"

# > TestOperation.with("1").with(bar: "2").call
# => Raises an error because it is PartiallyApplied and so can't be called (it is missing required args)
#      "Cannot call PartiallyApplied operation TestOperation (key: test_operation), are you expecting it to be Prepared? (TypedOperation::MissingParameterError)"

TestOperation.with("1").with(bar: "2").with(baz: 3).operation
# same as > TestOperation.new("1", bar: "2", baz: 3)
# => <TestOperation:0x000000014a0048a8 ...>

# > TestOperation.with(foo: "1").with(bar: "2").operation
# => Raises an error because it is PartiallyApplied so operation can't be instantiated
#      "Cannot instantiate Operation TestOperation (key: test_operation), as it is only partially applied. (TypedOperation::MissingParameterError)"

Create an operation by subclassing TypedOperation::Base or TypedOperation::ImmutableBase and specifying the parameters the operation requires.

• TypedOperation::Base (uses Literal::Struct) is the parent class for an operation where the arguments are potentially mutable (ie not frozen). No attribute writer methods are defined, so the arguments can not be changed after initialization, but the values passed in are not guaranteed to be frozen.
• TypedOperation::ImmutableBase (uses Literal::Data) is the parent class for an operation where the operation instance is frozen on initialization, thus giving a somewhat stronger immutability guarantee.

Note: you cannot include TypedOperation::ActionPolicyAuth into a TypedOperation::ImmutableBase.

The subclass must implement the #perform method which is where the operations main work is done.

The operation can also implement:

• #prepare - called when the operation is initialized, and after the parameters have been set
• #before_execute_operation - optionally hook in before execution ... and call super to allow subclasses to hook in too
• #after_execute_operation - optionally hook in after execution ... and call super to allow subclasses to hook in too

# optionally hook in before execution...
def before_execute_operation
  # Remember to call super
  super
end

def perform
  # ... implement me!
end

# optionally hook in after execution...
def after_execute_operation(result)
  # Remember to call super, note the result is passed in and the return value of this method is the result of the operation
  # thus allowing you to modify the result if you wish
  super
end

Parameters are specified using the provided class methods (.positional_param and .named_param), or using the underlying .param method.

Types are specified using the literal gem. In many cases this simply means providing the class of the expected type, but there are also some other useful types provided by literal (eg Union).

These can be either accessed via the Literal module, eg Literal::Types::BooleanType:

class MyOperation < ::TypedOperation::Base
  param :name, String
  param :age, Integer, optional: true
  param :choices, Literal::Types::ArrayType.new(String)
  param :chose, Literal::Types::BooleanType
end

MyOperation.new(name: "bob", choices: ["st"], chose: true)

or by including the Literal::Types module into your operation class, and using the aliases provided:

class MyOperation < ::TypedOperation::Base
  include Literal::Types
  
  param :name, String
  param :age, _Nilable(Integer) # optional can also be specifed using `.optional`
  param :choices, _Array(String)
  param :chose, _Boolean
end

Type constraints can be modified to make the parameter optional using .optional.

Note that you may also like to alias the param methods to your own preferred names in a common base operation class.

Some possible aliases are:

• positional/named
• arg/key

For example:

class ApplicationOperation < ::TypedOperation::Base
  class << self
    alias_method :arg, :positional_param
    alias_method :key, :named_param
  end
end

class MyOperation < ApplicationOperation
  arg :name, String
  key :age, Integer
end

MyOperation.new("Steve", age: 20)

Defines a positional parameter (positional argument passed to the operation when creating it).

The following are equivalent:

• param <param_name>, <type>, positional: true, <**options>
• positional_param <param_name>, <type>, <**options>

The <para_name> is a symbolic name, used to create the accessor method, and when deconstructing to a hash.

The <type> constraint provides the expected type of the parameter (the type is a type signature compatible with literal).

The <options> are:

• default: - a default value for the parameter (can be a proc or a frozen value)
• optional: - a boolean indicating whether the parameter is optional (default: false). Note you may prefer to use the .optional method instead of this option.

Note when positional arguments are provided to the operation, they are matched in order of definition or positional params. Also note that you cannot define required positional parameters after optional ones.

Eg

class MyOperation < ::TypedOperation::Base
  positional_param :name, String, positional: true
  # Or alternatively => `param :name, String, positional: true` 
  positional_param :age, Integer, default: -> { 0 }
  
  def perform
    puts "Hello #{name} (#{age})"
  end
end

MyOperation.new("Steve").call 
# => "Hello Steve (0)"

MyOperation.with("Steve").call(20)
# => "Hello Steve (20)"

Defines a named parameter (keyword argument passed to the operation when creating it).

The following are equivalent:

• param <param_name>, <type>, <**options>
• named_param <param_name>, <type>, <**options>

The <para_name> is a symbol, used as parameter name for the keyword arguments in the operation constructor, to create the accessor method and when deconstructing to a hash.

The type constraint and options are the same as for positional parameters.

class MyOperation < ::TypedOperation::Base
  named_param :name, String
  # Or alternatively => `param :name, String` 
  named_param :age, Integer, default: -> { 0 }
  
  def perform
    puts "Hello #{name} (#{age})"
  end
end

MyOperation.new(name: "Steve").call 
# => "Hello Steve (0)"

MyOperation.with(name: "Steve").call(age: 20)
# => "Hello Steve (20)"

You can use both positional and named parameters in the same operation.

class MyOperation < ::TypedOperation::Base
  positional_param :name, String
  named_param :age, Integer, default: -> { 0 }
  
  def perform
    puts "Hello #{name} (#{age})"
  end
end

MyOperation.new("Steve").call 
# => "Hello Steve (0)"

MyOperation.new("Steve", age: 20).call
# => "Hello Steve (20)"

MyOperation.with("Steve").call(age: 20)
# => "Hello Steve (20)"

Optional parameters are ones that do not need to be specified for the operation to be instantiated.

An optional parameter can be specified by:

• using the optional: option
• using the .optional method around the type constraint

class MyOperation < ::TypedOperation::Base
  param :name, String
  param :age, Integer, optional: true
  param :nickname, optional(String)
  # ...
end

MyOperation.new(name: "Steve")
MyOperation.new(name: "Steve", age: 20)
MyOperation.new(name: "Steve", nickname: "Steve-o")

This .optional class method effectively makes the type signature a union of the provided type and NilClass.

You can specify a block after a parameter definition to coerce the argument value.

param :name, String, &:to_s
param :choice, Literal::Types::BooleanType do |v|
  v == "y"
end

You can specify a default value for a parameter using the default: option.

The default value can be a proc or a frozen value. If the value is specified as nil then the default value is literally nil and the parameter is optional.

param :name, String, default: "Steve".freeze
param :age, Integer, default: -> { rand(100) }

If using the directive # frozen_string_literal: true then you string values are frozen by default.

.with(...) creates a partially applied operation with the provided parameters.

It is aliased to .[] for an alternative syntax.

Note that .with can take both positional and keyword arguments, and can be chained.

An important caveat about partial application is that type checking is not done until the operation is instantiated

MyOperation.new(123)
# => Raises an error as the type of the first parameter is incorrect:
#    Expected `123` to be of type: `String`. (Literal::TypeError)

op = MyOperation.with(123)
# => #<TypedOperation::Prepared:0x000000010b1d3358 ...
#     Does **not raise** an error, as the type of the first parameter is not checked until the operation is instantiated

op.call # or op.operation
# => Now raises an error as the type of the first parameter is incorrect and operation is instantiated

An operation can be invoked by:

• instantiating it with at least required params and then calling the #call method on the instance
• once a partially applied operation has been prepared (all required parameters have been set), the call method on TypedOperation::Prepared can be used to instantiate and call the operation.
• once an operation is curried, the #call method on last TypedOperation::Curried in the chain will invoke the operation
• calling #call on a partially applied operation and passing in any remaining required parameters
• calling #execute_operation on an operation instance (this is the method that is called by #call)

See the many examples in this document.

TypedOperation::Base and TypedOperation::PartiallyApplied implement deconstruct and deconstruct_keys methods, so they can be pattern matched against.

case MyOperation.new("Steve", age: 20)
in MyOperation[name, age]
  puts "Hello #{name} (#{age})"
end

case MyOperation.new("Steve", age: 20)
in MyOperation[name:, age: 20]
  puts "Hello #{name} (#{age})"
end

Get a proc that calls .call(...)

Get a proc that calls the #call method on an operation instance

Check if an operation is prepared

Return an operation instance from a Prepared operation. Will raise if called on a PartiallyApplied operation

List of the names of the positional parameters, in order

List of the names of the keyword parameters

List of the names of the required positional parameters, in order

List of the names of the required keyword parameters

List of the names of the optional positional parameters, in order

List of the names of the optional keyword parameters

You can use the provided generator to create an ApplicationOperation class in your Rails project.

You can then extend this to add extra functionality to all your operations.

This is an example of a ApplicationOperation in a Rails app that uses Dry::Monads:

# frozen_string_literal: true

class ApplicationOperation < ::TypedOperation::Base
  # We choose to use dry-monads for our operations, so include the required modules
  include Dry::Monads[:result]
  include Dry::Monads::Do.for(:perform)
  
  class << self
    # Setup our own preferred names for the DSL methods
    alias_method :positional, :positional_param
    alias_method :named, :named_param
  end
  
  # Parameters common to all Operations in this application
  named :initiator, optional(::User)

  private

  # We setup some helper methods for our operations to use
  
  def succeeded(value)
    Success(value)
  end

  def failed_with_value(value, message: "Operation failed", error_code: nil)
    failed(error_code || operation_key, message, value)
  end

  def failed_with_message(message, error_code: nil)
    failed(error_code || operation_key, message)
  end

  def failed(error_code, message = "Operation failed", value = nil)
    Failure[error_code, message, value]
  end

  def failed_with_code_and_value(error_code, value, message: "Operation failed")
    failed(error_code, message, value)
  end

  def operation_key
    self.class.name
  end
end

Note, this optional feature requires the action_policy gem to be installed and does not yet work with ImmutableBase.

Add TypedOperation::ActionPolicyAuth to your ApplicationOperation (first require the module):

require "typed_operation/action_policy_auth"

class ApplicationOperation < ::TypedOperation::Base
  # ...
  include TypedOperation::ActionPolicyAuth

  # You can specify a parameter to take the authorization context object, eg a user (can also be optional if some
  # operations don't require authorization)
  param :initiator, ::User # or optional(::User)
end

Every operation must define what action_type it is, eg:

class MyUpdateOperation < ApplicationOperation
  action_type :update
end

Any symbol can be used as the action_type and this is by default used to determine which policy method to call.

.authorized_via is used to specify how to authorize the operation. You must specify the name of a parameter for the policy authorization context. You can also specify multiple parameters if you wish.

You can then either provide a block with the logic to perform the authorization check, or provide a policy class.

The record: option lets you provide the name of the parameter which will be passed as the policy 'record'.

For example:

class MyUpdateOperation < ApplicationOperation
  param :initiator, ::AdminUser
  param :order, ::Order
  
  action_type :update
  
  authorized_via :initiator, record: :order do
    # ... the permissions check, admin users can edit orders that are not finalized
    initiator.admin? && !record.finalized
  end
  
  def perform
    # ...
  end
end

You can instead provide a policy class implementation:

class MyUpdateOperation < ApplicationOperation
  action_type :update
  
  class MyPolicyClass < OperationPolicy
    # The action_type defined on the operation determines which method is called on the policy class
    def update?
      # ... the permissions check
      initiator.admin?
    end
    
    # def my_check?
    #   # ... the permissions check
    # end
  end
  
  authorized_via :initiator, with: MyPolicyClass
  
  # It is also possible to specify which policy method to call
  #      authorized_via :initiator, with: MyPolicyClass, to: :my_check?
end

with multiple parameters:

class MyUpdateOperation < ApplicationOperation
  # ...
  param :initiator, ::AdminUser
  param :user, ::User
  
  authorized_via :initiator, :user do
    initiator.active? && user.active?
  end
  
  # ...
end

To ensure that subclasses always implement authentication you can add a call to .verify_authorized! to your base operation class.

This will cause the execution of any subclasses to fail if no authorization is performed.

class MustAuthOperation < ApplicationOperation
  verify_authorized!
end

class MyUpdateOperation < MustAuthOperation
  def perform
    # ...
  end
end

MyUpdateOperation.call # => Raises an error that MyUpdateOperation does not perform any authorization

A hook is provided to allow you to do some work on an authorization failure.

Simply override the #on_authorization_failure(err) method in your operation.

class MyUpdateOperation < ApplicationOperation
  action_type :update
  
  authorized_via :initiator do
    # ... the permissions check
    initiator.admin?
  end
  
  def perform
    # ...
  end
  
  def on_authorization_failure(err)
    # ... do something with the error, eg logging
  end
end

Note you are provided the ActionPolicy error object, but you cannot stop the error from being re-raised.

As per the example in Dry::Monads documentation

class MyOperation < ::TypedOperation::Base
  include Dry::Monads[:result]
  include Dry::Monads::Do.for(:perform, :create_account)

  param :account_name, String
  param :owner, ::Owner
  
  def perform
    account = yield create_account(account_name)
    yield AnotherOperation.call(account, owner)

    Success(account)
  end

  private
  
  def create_account(account_name)
    # returns Success(account) or Failure(:cant_create)
  end
end

Add this line to your application's Gemfile:

And then execute:

Or install it yourself as:

$ gem install typed_operation

bin/rails g typed_operation:install

Use the --dry_monads switch to include Dry::Monads[:result] into your ApplicationOperation (don't forget to also add gem "dry-monads" to your Gemfile)

Use the --action_policy switch to add the TypedOperation::ActionPolicyAuth module to your ApplicationOperation (and you will also need to add gem "action_policy" to your Gemfile).

bin/rails g typed_operation:install --dry_monads --action_policy

bin/rails g typed_operation TestOperation

You can optionally specify the directory to generate the operation in:

bin/rails g typed_operation TestOperation --path=app/operations

The default path is app/operations.

The generator will also create a test file.

Bug reports and pull requests are welcome on GitHub at https://github.com/stevegeek/typed_operation. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

The gem is available as open source under the terms of the MIT License.

Everyone interacting in the TypedOperation project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.

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