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Language Design: Drop break and continue

Published on 2022-12-10.

TL;DR: Optimize for the common case, not the exotic ones.

First of all: The argument is not that break and continue in loops aren’t …

  • useful
  • convenient
  • sometimes the best option
  • …

That’s not the argument being made. The argument that is being made is that break and continue are …

… optimizing for an infrequent special case …

Consider a codebase that contains 1000 loops.

Out of those 1000, 900 loops aren’t using break or continue.

Of the remaining 100 loops, perhaps 90 loops use break, and 10 loops use continue.

Of those 90 loops with breaks, 80 are easily convertible to equivalent code not using break.

Of the 10 loops with continues maybe 5 are easily convertible.

.------------------------------------------------. | all loops | | | | +---+ | | | | | |<---loops with break | +---+ | | |<---loops with continue '--------------------------------------------+---'

This means that out of 1000 loops, supporting break and continue focuses on making 1.5% of the loops more convenient, to the detriment of the other 98.5% of the loops.

… while worsening the general case!

What’s the detriment? The loss of the ability to read the head of the loop and know what’s going on (e. g. when the loop terminates), as the body of every loop could contain a break or continue:

while true { // is it really an endless loop? only way to find out is reading the whole loop body!
  ...
  if isNewLine() {
    break;
  }
}

This inability is so ingrained in people, that they cannot fathom the mental load that gets removed when they do not have to keep “this loop may contain a break or continue” in the back of their head:

while !isNewLine { // loop head shows immediately when the loop terminates
  ...
  isNewLine = true
}

Conclusion

Dropping break and continue removes mental load from 98.5% of the loops that don’t use them, with the pain of having a few loops that are now more painful to write.

That’s a good trade-off.