Extends [[Alt]] with an empty value:

```haskell
class Alt f <= Plus f where
  empty :: forall a. f a
```

> [!info]
> Conceptually, this lives next to [[Applicative|pure]] in my mind; in [[Maybe]] for example `pure` and `empty` are aliases for `Just` and `Nothing`.

In [[Array]], `empty` is empty array `[]`. In [[Maybe]], `empty` is `Nothing`.

The most common place you'll see `empty` is when using [[MaybeT]] as an early return:

```haskell
logDebug :: String -> Effect Unit
logDebug msg = void $ runMaybeT do
  env <- MaybeT $ Process.lookupEnv "NODE_ENV"
  when (env /= "DEVELOPMENT") empty
  lift $ Console.log msg
```

this is equivalent to:

```haskell
logDebug :: String -> Effect Unit
logDebug msg = do
  menv <- Process.lookupEnv "NODE_ENV"
  case menv of
    Just env ->
      if env == "DEVELOPMENT" then
        Console.log msg
      else
        pure unit
    Nothing -> pure unit
```

> [!info]
> Note that [guard](https://pursuit.purescript.org/packages/purescript-control/docs/Control.Alternative#v:guard) is a shorthand available for `when (..) empty`