Opaque types

Opaque types are types that cannot be created outside of a specific module. For example:

module Lib exposing (Config)

type Config = Config { size: Int, style: Style }

This module expose the Config type but not the constructor. An external module can't construct or modify a Config type.

Opaque types are useful for:

• Enforcing invariants: Only the relevant module can change the data and ensure it follows some invariants.
• Hiding the implementation to external modules. This is quite useful for building packages. Using opaque types makes it easier to change the implementation without breaking the code using it.

Anti-pattern

module Lib exposing (Config)

type alias Config = { size: Int, style: Style }

This module exposes Config transparently. Any changes we want to make will require changes in the module using this. In the case of a package, if we remove or change a type in Config we will have to publish a major version of this package.

Pattern

module Lib exposing (Config, newConfig, withSize)

type Config = Config { size: Int, style: Style }

newConfig : Config
newConfig =
	Config { size: 1, style : Big }

withSize : Int -> Config -> Config

This module allows an application to create a Config and update it. But if we decide to change how we store the Config we can do so without any breaking changes for the caller module using this.

The next page explains using opaque types for enforcing invariants.