[m-dev.] proposal: require_switch_arms_{det,semi,...}

[Zoltan Somogyi]

A while ago I added require_complete_switch to the language.
This requires the switch arms to cover all possible cases, but
allows them to have any determinism. I think it would be
a good idea to add another construct that is the opposite:
it would allow some function symbols in the type of the
switched-on variable not to be covered, but would require
each switch arm to have at least the specified determinism
(so you could allow a det arm in a require_switch_arms_semi
scope, but not a semidet arm in a require_switch_arms_det
scope).

The motivation is some code I will soon post for review,
which does parsing of string.format specifier strings. In the
presence of unicode, you cannot have a complete switch
on characters, but for each character that *is* a specifier,
you want to handle it with det code, since you want to guarantee
that you get back either a description of the specifier or an
error message.

You can get this effect now by wrapping each arm in a
require_det scope, but (a) this is ugly, and (b) it allows
bugs in which some switch arms are not so wrapped to go
undetected.

Any opinions?

Zoltan.