[Gadiguibou]

You can also often encode those checks in the type system like in the Option example near the start of the article.

[RHSeeger]

Assuming your type system is robust enough to support such things. That's not the case for many mainstream languages.

[sfink]

In that case, use DEBUG-only asserts. They're not foolproof (they're only as good as your test suite), but they're pretty damn good.