Going Towards a More General Looping Mechanism - this should be a serious proposal.

[robertramey]

Going Towards a More General Looping Mechanism

Document number: DXXXXR0
Date: 20XX-MM-DD
Audience: EWG
Reply-to: Tony Van Eerd. tt at forecode.com

Summary

Current looping mechanisms (forloops, while and do...while loops)
are great - in their niche uses;
but they are often found wanting, resulting in awkward constructs.
In particular, in a few cases it is impossible to NOT repeat yourself (ie a violation of DRY) or to be concise, using those mechanisms.

Looking at how all these mechanisms work, we can refactor them into a single, more general, control flow mechanism.
In fact, it could replace all the other keywords with this single mechanism!

(Note, for the sake of backwards compatibility, we do not recommend deprecation at this time.
If the new style is widely adopted - as we expect it will - we can deprecate the old keywords
when common coding practices show they have fallen out of favour.)

Extra Checks	DRY and Concise
bool res = false; for(int retry = 10; !res && retry; retry--) { res = askHardware(question, &num); // don't know why it succeeds while failing, // but it does. :-( // Sadly, re-asking is the solution. if (res && num == 0) { sleepHack(200); continue; } } return res;	// pre-conditions bool res; // undetermined int retry = 10; // invariant // retry >= 0 // post-conditions auto post_condition = bool [res, num, retry](){ res && num == 0 || retry == 0 } do{ int num; sleepHack(200); res = askHardware(question, &num); --retry; // advances state - must terminate } while(! post_condition()); return res;

More Elucidating Examples

... show more problematic uses of current control flow, compared against new mechanism

Proof of Replacement

... show before/after table replacing each of { for, while and do...while }

Technical Details or Preliminary Wording, Etc

Label statements, goto statement. Etc.

Further Work

Initial results suggest a "computed goto" could also replace if and switch statements - thus completely replacing all control flow mechanisms with a single keyword. The committee should give me a kidney.