Box a final value for early termination

A value boxed with done() signals to its caller that it should stop iterating. Use it to shortcircuit a loop.

done(x)

is_done_box(x, empty = NULL)

Arguments

x	For `done()`, a value to box. For `is_done_box()`, a value to test.
empty	Whether the box is empty. If `NULL`, `is_done_box()` returns `TRUE` for all done boxes. If `TRUE`, it returns `TRUE` only for empty boxes. Otherwise it returns `TRUE` only for non-empty boxes.

A boxed value.

done(3)
#> <done>
#> [1] 3

x <- done(3)
is_done_box(x)
#> [1] TRUE