IO.md

IO

The IO<A> datatype describes a computation which will return a value of type A.

Constructors

The IO<A> data structure has only one constructor, called action, which requires a callable with no arguments and return type A.

/**
 * @template A
 */
class IO
{
    /**
     * @param callable(): A $action
     * @return IO<A>
     */
    public static function action(callable $action): self
}

Its simplified type is

action :: (() -> A) -> IO<A>

Eliminators

The only eliminator of IO is eval, which just evaluates the action represented by IO.

    /**
     * @return A
     */
    public function eval()

It simplified type is

eval :: IO<A> -> A

Interpretation as an effect

Representing a generic computation, the IO datatype given access to any possible effect.

The IO data structure should be used whenever there is the need to interact with the external world, being it a database, a user or a web api.

Typeclass instances

• IOApplicative
• IOApply
• IOFunctor
• IOMonad