Reader.md

Reader

The Reader data structure allows modelling values which depend on an environment. Reader turns out very useful for implementing computations which depend on some external configuration. It could even be used as a system for dependency injection.

Constructors

The Reader data structure has a single constructor, which requires a function with the environment as a parameter.

/**
 * @template E
 * @template A
 */
final class Reader
{
    /**
     * @template F
     * @template B
     * @param callable(F): B $f
     * @return Reader<F, B>
     */
    public static function reader(callable $f)
}

Its simplified type is

reader :: (F -> B) -> Reader<F, B>

Evaluators

The only evaluator for Reader requires an instance of the environment and computes the action for that particular environment.

    /**
     * @param E $environment
     * @return A
     */
    public function runReader($environment)

Its simplified type is

runReader :: (Reader<E, A>, E) -> A

Interpretation as an effect

The Reader datatype allows modelling values and computation which depend on an environment.

Typeclass instances

• ReaderApplicative
• ReaderApply
• ReaderFunctor
• ReaderMonad