A combinator for making monadic controls operator stack-safe (using `MonadRec`

).

For example, `purescript-lists`

defines `replicateM_`

, which is not stack-safe. It gives a stack overflow when applied to large inputs using, for example, the `Eff`

monad.

```
> replicateM_ 100000 (log "Testing...")
RangeError: Maximum call stack size exceeded
```

However, we can make it stack-safe by using the `safely`

combinator.

For any `MonadRec`

, we can form the `Monad`

`FreeT Identity m`

in which
arbitrarily-associated binds are stack-safe.

The `safely`

function performs the wrapping and unwrapping for us, and provides a
pair of natural transformations to lift and lower actions to and from the
safe monad.

This means that we can write stack-safe monadic code for any `MonadRec`

by
simply writing the naive implementation, and wrapping it in a call to
`safely`

:

```
replicateS_ :: forall m a. MonadRec m => Int -> m a -> m Unit
replicateS_ n x = safely \lift _ ->
let go i | i <= 0 = pure unit
| otherwise = do lift x
go (i - 1)
in go n
```