Table of Contents
Item 4: Erlang Best Practices - Enforce noninstantiability with a private constructor
Introduction to Enforcing Noninstantiability in [[Erlang]]
In Erlang, a functional programming language known for its concurrency and fault-tolerant design, the concept of enforcing noninstantiability is not directly applicable as it would be in object-oriented languages. However, Erlang modules often involve creating records (similar to structs) or using processes where controlling the instantiation and access to these structures can be important. Although Erlang does not have constructors in the traditional sense, you can enforce noninstantiability by restricting access to certain parts of your code, especially when dealing with records and process creation.
Advantages of Enforcing Noninstantiability in [[Erlang]]
Enforcing noninstantiability in Erlang offers several advantages: 1. **Prevents Misuse**: By controlling the instantiation and access to records or processes, you prevent unintended behaviors or misuse in your code. 2. **Encapsulates Implementation Details**: Restricting access to the creation functions allows you to encapsulate the internal logic and representation of a record or process, ensuring that only the intended API is exposed. 3. **Encourages Proper Design**: This approach encourages developers to use the module's public API functions for creating and interacting with instances, leading to more maintainable and consistent code.
Example 1: Enforcing Noninstantiability by Hiding Record Definitions
In Erlang, you can enforce noninstantiability by keeping the record definition private within a module and only providing controlled functions to create and interact with instances:
```erlang -module(my_module). -export([create_instance/1, get_value/1]).
% Define the record, but do not export it -record(my_record, {value}).
% Private function to create an instance create_instance(Value) when is_integer(Value) →
#my_record{value = Value}.
% Public function to access the value get_value(#my_record{value = Value}) →
Value.```
In this example, the `my_record` record is defined within the `my_module` module but is not exported. The `create_instance/1` function is the only way to create an instance of the record, enforcing noninstantiability.
Example 2: Using a Private Function for Process Creation
When working with processes, you can enforce noninstantiability by keeping the function that spawns a process private and exposing only controlled ways to interact with the process:
```erlang -module(singleton_process). -export([start/0, do_something/0]).
% Private function to start the process start_process() →
spawn(fun loop/0).
% Public function to start the process if it hasn't been started start() →
case whereis(singleton) of
undefined ->
Pid = start_process(),
register(singleton, Pid),
ok;
_Pid ->
{error, already_started}
end.
% Public function to interact with the process do_something() →
singleton ! {do_something, self()},
receive
ok -> ok
end.
% Private loop function loop() →
receive
{do_something, From} ->
%% Do something
From ! ok,
loop()
end.
```
In this example, `start_process/0` is a private function that spawns a process. The public `start/0` function ensures that the process is only started once, enforcing a form of noninstantiability by limiting process creation.
Example 3: Enforcing Noninstantiability with Opaque Data Structures
You can use opaque data structures in Erlang by keeping the details of the data structure private and only exposing an API for interacting with it:
```erlang -module(my_opaque_module). -export([new/1, get_value/1]).
% Define the opaque record -record(opaque_record, {data}).
% Public function to create a new instance new(Data) when is_binary(Data) →
#opaque_record{data = Data}.
% Public function to access the data get_value(#opaque_record{data = Data}) →
Data.```
In this example, `opaque_record` is a record that is only manipulated through the functions provided by the module. By keeping the record definition and instantiation logic hidden, you prevent misuse and ensure that the data structure is used as intended.
Example 4: Controlling Access Through Module Encapsulation
Another way to enforce noninstantiability is to use module encapsulation, where you control access to the creation of data types or processes by encapsulating the logic within the module:
```erlang -module(my_encapsulated_module). -export([public_function/1]).
% Define a private function that handles creation new(Value) when is_integer(Value) →
{ok, #state{value = Value}}.
% Public function that interacts with the encapsulated logic public_function(Value) →
case new(Value) of
{ok, State} -> do_something(State);
{error, _} -> {error, invalid_value}
end.
% Private function that operates on the state do_something(State) →
io:format("Doing something with value: ~p~n", [State#state.value]).
```
In this example, the `new/1` function is private and controls how instances of the state are created and managed. The `public_function/1` is the only way for external code to interact with the encapsulated logic.
When to Enforce Noninstantiability in [[Erlang]]
Enforcing noninstantiability in Erlang is useful in the following scenarios: - **Encapsulation**: When you want to hide the internal structure or logic of a module and expose only a controlled API. - **Process Management**: When you need to control the creation and management of processes, such as implementing singleton processes. - **Library Design**: When developing a library or module where certain data structures or processes should not be directly instantiated by users, enforcing noninstantiability can prevent misuse and maintain the integrity of the API. - **Opaque Data Structures**: When you want to hide the internal details of a data structure, ensuring that it is only manipulated through the provided API.
Conclusion
In Erlang, while the language does not have traditional constructors, enforcing noninstantiability can still be achieved through private functions, module encapsulation, and controlling access to records and processes. These techniques allow you to create more maintainable, secure, and reliable code by ensuring that data structures and processes are used only as intended.