Fix the evaluation order of arithmetic operands

[gasche]

Note: I think that a simpler translation of e1 + e2, which would avoid issues with evaluation order as well, would be

(if <mutate> then (-) else (+)) e1 e2

[jmid]

Thanks - and good points!

I agree that (if <mutate> then (-) else (+)) e1 e2 would be simpler. My immediate reaction was to think "but that will then allocate a closure" - which it probably won't precisely because of the "lets-do-right-to-left-eval-to-save-closure-alloc" ZINC-trick! 😄 I'm wondering whether there would be any performance difference between the two though 🤔

Your implicit suggestions for

• a dont_mutate attribute and
• a "remove let () = e in" mutation were also well received!