I recently had a use-case for:
parallel 'cmd {}' ::: {1..Inf}
Well, I actually just ended up hardcoding Inf as a very large number.
The use-case is that I was using parallel to request N number of pages
from a paged API in parallel, but I didn't know what N would be in
advance.
An exit code and --halt soon,fail=1 ensured that I could get by N
number of pages from the API and halt as soon as I reached the last
one.
It would be nice if there was some syntax to do this without the hack
of supplying a huge list via the shell to parallel, but as far as I
can tell there isn't. If there's not consider this a feature request.