Elm best practices - enforce the singleton property with a private constructor or an enum type Page

Item 3: Elm Best Practices - Enforce the singleton property with a private constructor or an enum type

Enforce the Singleton Property

In traditional object-oriented programming languages, the Singleton pattern is commonly used to ensure that a class has only one instance and provides a global point of access to that instance. This is typically enforced using private constructors or enum types. However, Elm is a functional programming language without classes, constructors, or mutable state. Therefore, enforcing the singleton property in Elm is approached differently, using the language's functional paradigms.

Understanding the Singleton Pattern in Elm

In Elm, enforcing the singleton property typically means ensuring that a specific value or configuration is shared across the entire application and that only one instance of it exists. Since Elm values are immutable and globally accessible when defined at the top level, the singleton pattern in Elm can be enforced by simply defining a value once and reusing it everywhere it's needed.

Using a Top-Level Constant to Enforce Singleton

The most straightforward way to enforce a singleton in Elm is to define a top-level constant that represents the single instance of a value or configuration. This value is then shared across the entire application.

### Example 1: Simple Singleton Using a Top-Level Constant

```elm
module Config exposing (config)

-- Singleton configuration
config : { apiUrl : String, apiKey : String }
config =
{ apiUrl = "https://api.example.com", apiKey = "123456" }

-- Usage in other modules
import Config exposing (config)

apiRequest : String -> String
apiRequest endpoint =
config.apiUrl ++ "/" ++ endpoint ++ "?apiKey=" ++ config.apiKey
```

In this example, the `config` value is defined at the top level of the `Config` module and represents a singleton configuration for the application. Since `config` is immutable and defined at the top level, it is effectively a singleton that can be shared across the application without risk of duplication or unintended modification.

Using a Custom Type for Singleton Values

Another way to enforce the singleton property in Elm is to use a custom type that ensures only a specific value is used throughout the application. This approach can be useful when you need to enforce a particular state or configuration that should remain consistent.

### Example 2: Singleton Using a Custom Type

```elm
module Mode exposing (Mode, production, development, currentMode)

-- Define a custom type with two possible values
type Mode
= Production
| Development

-- Define singleton instances of the custom type
production : Mode
production =
Production

development : Mode
development =
Development

-- Define the current mode as a singleton
currentMode : Mode
currentMode =
production -- or development, depending on your environment

-- Usage in other modules
import Mode exposing (Mode, currentMode)

logMessage : String -> String
logMessage message =
case currentMode of
Production ->
"Logging in production: " ++ message

Development ->
"Logging in development: " ++ message
```

In this example, the `Mode` type represents different application modes, such as `Production` and `Development`. The `currentMode` value is a singleton that holds the current mode of the application, ensuring that this mode is consistent across the entire application.

Enforcing Singleton with a Factory Function

For more complex scenarios, you can use a factory function to enforce that only one instance of a specific type is created. While this isn't a typical singleton pattern as seen in object-oriented languages, it ensures that certain values are only constructed once and reused.

### Example 3: Singleton Using a Factory Function

```elm
module Logger exposing (Logger, getLogger)

-- Define a type for the logger
type Logger
= LoggerInstance { logLevel : String }

-- Define a factory function to create a logger instance
getLogger : Logger
getLogger =
LoggerInstance { logLevel = "INFO" }

-- Usage in other modules
import Logger exposing (Logger, getLogger)

log : Logger -> String -> String
log (LoggerInstance loggerConfig) message =
"[" ++ loggerConfig.logLevel ++ "] " ++ message

main =
let
logger = getLogger
in
log logger "This is a log message."
```

In this example, the `getLogger` function returns a singleton instance of the `Logger`. This ensures that the same logger configuration is used throughout the application, enforcing a consistent logging behavior.

When to Enforce Singleton Property in Elm

Enforcing the singleton property is appropriate in Elm when:
- **Global Configuration**: You have a configuration or state that should be globally consistent across the application.
- **Single Instance Requirement**: You need to ensure that only one instance of a particular value or type is used.
- **Shared Resources**: You have resources or utilities (like a logger) that should be shared and consistent across the application.

Conclusion

In Elm, enforcing the singleton property is accomplished using functional programming techniques, such as defining top-level constants, using custom types, or employing factory functions. While Elm does not have classes or constructors, these patterns allow you to achieve similar goals by ensuring that specific values or configurations are consistent and shared across your application. By following these best practices, you can maintain a clean, efficient, and predictable codebase.