[continuational]

Unit tests do not verify behavior, except for a few cases (out of a virtually infinite set of cases) that the programmer happened to think about.

Types do verify behavior - e.g. this function always returns an Int, that function only returns a UserId, this other function doesn't do any I/O.

[bluGill]

That is a very different behavior though. If my multiply function returns ADDs the input and returns an int it passes the type checker but is still wrong.

[morelisp]

But if your multiply function fails to handle some special case that the tests don't catch (negatives, overflow/saturation, denorms, ...) then the tests are also wrong.

What you really want is to be able to state properties, e.g. "for all x/y, mul(x, 0) = 0, mul(x, 1) = x, mul(x, 2) = x + x, mul(x, y) = mul(y, x)", and so on. And working with these kinds of statements (via e.g. quickcheck or whatever) might generate tests, but feels a lot more like working with types. You're making proofs, not examples.

[bluGill]

Which is why I have always (well for the last 5 years or so anyway) said that you want both because they catch different issues.