Template Method Pattern using Callbacks and Behaviours in Elixir

7.5. Template Method Pattern using Callbacks and Behaviours

The Template Method Pattern is a behavioral design pattern that defines the skeleton of an algorithm in a method, deferring some steps to subclasses. In Elixir, we can leverage callbacks and behaviours to implement this pattern, allowing us to define algorithm skeletons while deferring the implementation of specific steps to other modules. This approach is particularly useful in functional programming, where we aim to create reusable and composable code.

Defining Algorithm Skeletons

In the Template Method Pattern, the algorithm’s structure is defined in a high-level function, while the specific steps are implemented in separate functions or modules. This separation of concerns allows for flexibility and reuse, as the high-level function can remain unchanged while the specific steps can be modified or extended.

Key Concepts

• Algorithm Skeleton: The high-level structure of the algorithm, defined in a function.
• Deferred Steps: Specific steps of the algorithm that are implemented in separate functions or modules.
• Callbacks: Functions that are called at specific points in the algorithm, allowing for customization.
• Behaviours: A way to define a set of functions that must be implemented by a module, ensuring consistency and structure.

Implementing the Template Method Pattern

In Elixir, we can use behaviours to define the required callbacks for our algorithm. A behaviour is essentially a contract that specifies which functions a module must implement. By defining a behaviour, we can ensure that all modules implementing the behaviour provide the necessary functionality.

Step-by-Step Implementation

1. Define the Behaviour: Create a behaviour module that specifies the required callbacks.
2. Implement the Template Method: Define the high-level function that outlines the algorithm’s skeleton.
3. Create Modules Implementing the Behaviour: Implement the required callbacks in separate modules, providing the specific functionality for each step.
4. Use the Template Method: Call the high-level function, passing in the modules that implement the behaviour.

Example: Processing a List of Items

Let’s consider an example where we want to process a list of items. The algorithm involves iterating over the list, performing an operation on each item, and then aggregating the results.

Step 1: Define the Behaviour

1defmodule ItemProcessor do
2  @callback process_item(item :: any()) :: any()
3  @callback aggregate_results(results :: list(any())) :: any()
4end

Step 2: Implement the Template Method

1defmodule ListProcessor do
2  @behaviour ItemProcessor
3
4  def process_list(items, processor_module) do
5    items
6    |> Enum.map(&processor_module.process_item/1)
7    |> processor_module.aggregate_results()
8  end
9end

Step 3: Create Modules Implementing the Behaviour

 1defmodule SumProcessor do
 2  @behaviour ItemProcessor
 3
 4  def process_item(item) do
 5    item
 6  end
 7
 8  def aggregate_results(results) do
 9    Enum.sum(results)
10  end
11end
12
13defmodule ProductProcessor do
14  @behaviour ItemProcessor
15
16  def process_item(item) do
17    item
18  end
19
20  def aggregate_results(results) do
21    Enum.reduce(results, 1, &*/2)
22  end
23end

Step 4: Use the Template Method

1items = [1, 2, 3, 4, 5]
2
3sum_result = ListProcessor.process_list(items, SumProcessor)
4IO.puts("Sum: #{sum_result}")
5
6product_result = ListProcessor.process_list(items, ProductProcessor)
7IO.puts("Product: #{product_result}")

Visualizing the Template Method Pattern

To better understand the Template Method Pattern, let’s visualize the flow of the algorithm using a sequence diagram.

    sequenceDiagram
	    participant Client
	    participant ListProcessor
	    participant SumProcessor
	    participant ProductProcessor
	
	    Client->>ListProcessor: process_list(items, SumProcessor)
	    ListProcessor->>SumProcessor: process_item(item)
	    SumProcessor-->>ListProcessor: processed_item
	    ListProcessor->>SumProcessor: aggregate_results(results)
	    SumProcessor-->>ListProcessor: aggregated_result
	    ListProcessor-->>Client: result
	
	    Client->>ListProcessor: process_list(items, ProductProcessor)
	    ListProcessor->>ProductProcessor: process_item(item)
	    ProductProcessor-->>ListProcessor: processed_item
	    ListProcessor->>ProductProcessor: aggregate_results(results)
	    ProductProcessor-->>ListProcessor: aggregated_result
	    ListProcessor-->>Client: result

Use Cases

The Template Method Pattern is widely used in Elixir, especially in scenarios where a common algorithm needs to be customized. Here are some common use cases:

• GenServer Callbacks: Define the structure of a server process, deferring specific operations to callbacks.
• Plug Pipelines in Phoenix: Define a sequence of operations for processing HTTP requests, allowing customization through plugs.

Design Considerations

When implementing the Template Method Pattern in Elixir, consider the following:

• Flexibility: Ensure that the behaviour allows for sufficient customization without compromising the algorithm’s integrity.
• Reusability: Design the algorithm skeleton to be reusable across different contexts and applications.
• Consistency: Use behaviours to enforce consistency in the implementation of callbacks.

Elixir Unique Features

Elixir’s support for behaviours and pattern matching makes it particularly well-suited for implementing the Template Method Pattern. The ability to define behaviours ensures that all modules implementing the pattern adhere to a consistent interface, while pattern matching allows for concise and expressive code.

Differences and Similarities

The Template Method Pattern is often compared to the Strategy Pattern, as both involve defining a high-level algorithm and deferring specific steps. However, the key difference is that the Template Method Pattern defines the algorithm’s structure in a single function, while the Strategy Pattern involves selecting a specific strategy at runtime.

Try It Yourself

Experiment with the code examples provided by modifying the process_item and aggregate_results functions to implement different operations. For example, try creating a module that calculates the average of the items or filters out certain values before aggregation.

Knowledge Check

• What is the purpose of the Template Method Pattern?
• How do behaviours facilitate the implementation of the Template Method Pattern in Elixir?
• What are some common use cases for the Template Method Pattern in Elixir?

Embrace the Journey

Remember, mastering design patterns is a journey. As you continue to explore and experiment with the Template Method Pattern, you’ll gain a deeper understanding of how to create flexible and reusable code. Keep experimenting, stay curious, and enjoy the journey!

Quiz: Template Method Pattern using Callbacks and Behaviours