Ask HN: What languages include implicit condition checking?

Not even sure if I'm using the right name. I've been doing a lot of precondition / type checking in PHP, and it occurred to me that it might be a cool language feature to bake into a language. For example, perhaps I could write `$i = count($keys) = count($values)` if I want $i to be set only on the conditions that $keys and $values are equi-lengthed array. An more advanced implementation could include auto checking a function's inputs, based on some set of rules defined in syntax or comments.