[nailer]

> I.e. if you pass maybe-scores to something that computes the mean, you'll get a compiler error. The writer of the mean function doesn't need to care you've overloaded an error value onto your numbers.

If I pass a null into something that calculates an average, taking numbers, the TS compiler will complain now. The writer of mean() (assuming mean() is typed) doesn’t have to know anything about my code.

[groby_b]

This only works for non-sentinels. I.e. if "-1" happens to be the value indicating NoScore, I don't think the TS compiler can catch that?

[nailer]

Nobody is discussing using a magic number for null, nor is that an accepted practice in TS.