CoCoA » CoCoALib

• if the fn is the main public fn for that operation (and documented) then it should produce errors when bad args are passed
• otherwise check using assertions.

With this approach, errors always have a run-time cost, while assertions cost nothing unless debugging was activated (and everything recompiled).
I can see an argument for using assertions instead of errors for short, fast, low-level functions because the overhead of always checking may be annoyingly high.

This last comment does suggest that indexing functions could/should perhaps use assertions instead of errors. This could be a problem for a careless CoCoA-5 user, because a bad index might cause a crash (or worse). Maybe it would be better to offer two such functions: one which does check its args, and one which does not -- and the latter should have a name indicating that checking has been disabled (but it probably should still use CoCoA_ASSERT).