Can automated unit testing replace static type checking?

Question

I've started to look into the whole unit testing/test-driven development idea, and the more I think about it, the more it seems to fill a similar role to static type checking. Both techniques provide a compile-time, rapid-response check for certain kinds of errors in your program. However, correct me if I'm wrong, but it seems that a unit test suite with full coverage would test everything static type checking would test, and then some. Or phrased another way, static type checks only go part of the way to "prove" that your program is correct, whereas unit tests will let you "prove" as much as you want (to a certain extent).

So, is there any reason to use a language with static type checking if you're using unit testing as well? A somewhat similar question was asked here, but I'd like to get into more detail. What specific advantages, if any, does static type checking have over unit tests? A few issues like compiler optimizations and intellisense come to mind, but are there other solutions for those problems? Are there other advantages/disadvantages I haven't thought of?

Answer 1

There is one immutable fact about software quality.

If it can't compile, it can't ship

In this rule, statically typed languages will win over dynamically typed languages.

Ok, yes this rule is not immutable. Web Apps can ship without compiling (I've deployed many test web apps that didn't compile). But what is fundamentally true is

The sooner you catch an error, the cheaper it is to fix

A statically typed language will prevent real errors from happening at one of the earliest possible moments in the software development cycle. A dynamic language will not. Unit Testing, if you are thorough to a super human level can take the place of a statically typed language.

However why bother? There are a lot of incredibly smart people out there writing an entire error checking system for you in the form of a Compiler. If you're concerned about getting errors sooner use a statically typed language.

Please do not take this post as a bashing of dynamic languages. I use dynamic languages daily and love them. They are incredibly expressive and flexible and allow for incredibly fanscinating program.s However in the case of early error reporting they do lose to statically typed languages.

Answer 2

For any reasonably sized project, you just cannot account for all situations with unit tests only.

So my answer is "no", and even if you manage to account for all situations, you've thereby defeated the whole purpose of using a dynamic language in the first place.

If you want to program type-safe, better use a type-safe language.