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Comment by rolux | Hacker News Reader

When it comes to API design, a good rule of thumb is that parameters that can be either 0, 1 or 2 will cause bugs.

More generally: "when it comes to API design, a good rule of thumb is that parameters will cause bugs."

No, I was actually trying to be specific. There's usually nothing wrong with 0/1 or 1-9.

Empirically, 0/1/2 either means no/yes/yes-and-something-obscure-on-top-of-that, or, worse, as in this case, no/kind-of-but-not-really/yes.

It's akin to http://c2.com/cgi/wiki?ZeroOneInfinityRule. If you started out thinking it was just a switch, you should be very reluctant to add a third state without rethinking what you really need to express, or you end up with my favorite from TheDailyWTF:

  enum Bool { 
      True,
      False,
      FileNotFound
  };

This would have been much safer as separate AUTHENTICATE_CERT (which checks the CA sig and expiry and revocation) and VERIFY_CERT_COMMON_NAME options, both of which default to true.

Your more general adage is useless. The one you responded to is useful. A more useful, also general adage would be "the more choice in selecting parameters to an API, the more likely there will be bugs consuming that API."

In this case I think it wouldn't cause bugs if the effect of 1 and 2 would be switched. Then everybody would use it like a boolean and that would be fine, only the few people who really want the name-check disabled would use that option.

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