Assert(): A Modern How To

An overlapping technique (covering some but not all of the circumstances you'd use assert) is to take a parse-dont-validate approach, and essentially encode the fact that an assertion has been applied to a value in its type.

How ergonomic this is will vary by language, but the general idea would be to apply the assertion logic in some sort of constructor, then prevent any operations which would break the invariant going forward. The simplest way to protect this being by making the value immutable where possible.

Users of the value who care about the invariant being true can then specify in their types that they want a non-empty-collection or a foo-id or whatever it may be, rather than asking for the wider type, then asserting.

Personally I'm not much of a fan of assert()'s... at least in the language I use the most (C++). It's not that I don't think you should validate the input ranges of parameters to functions its that in general exceptions are better.

> Can assertions be used in production?

Yes

> What should I be asserting on?

Invariants

> Can I customize how assert behaves?

It should abort the program, logging the stack and whatever the context you pass into in, printf-style. If it doesn't abort, it just buries the issue of the program being in incorrect internal state. It should never be OK.

I love to combine assertions with « restartability ».

If you’re program has entered an unknown, failed state, just restart it from a known state.

Even better if you can divide a complex system in sub modules that can recover independently without bringing down the entire system.

Something like Erlang supervision tree. Or at least a systemd Restart=always service.

if your program is mostly stateless, and « restartable », it becomes fault tolerant, and you can use assertions liberally and easily avoid unknown/bad states.

Invariants can be enforced, and correctness preserved. But an important question remains when an assertion is triggered, why invariants were violated?

We need to preserve context, and decide to handle or not this case. That is easy to forget in code that is assertions oriented.

If we're trying to outline a future for assertions, I think it would help to situate them among the other mechanisms we have for ensuring correctness and explain where assertions have the right tradeoffs. For example, what unique need does a production assertion API satisfy that a normal conditional throw does not? Are there cases where production assertions are still necessary even when invariants are established through type constructors?

Interesting article about a worthy topic, even though I disagree with some it. Side note: I expected to see mention of design by contract and function preconditions/invariants/postconditions.

I don't think the article is well founded. Before you can discuss usage you need to establish the semantics for assert(). The author touches on this in the introduction but then leaves the details unexamined. In particular I'd need to know: can assertions be disabled (as with C/C++ NDEBUG)? does the project have a policy of leaving asserts in production builds? or are asserts only ever enabled for development and testing. if used in production, do you care about the overhead of checking assertions in performance critical code? if an assertion is hit does it always log and panic/terminate? or does it throw a catchable exception? What is the runtime context of the code: is it a server process with a supervision tree? is the failure paradigm "let it crash"? is it an interactive program where the only supervisor is the user? is it a use-case where a program crash is undesirable and/or safety critical? is it a library with unknown use-cases? Are the developers in full control of the program inputs and outputs that trigger asserts?

As a general principle for layered systems, when there is a policy decision to be made, lower-level code should delegate upwards to higher levels, which should implement the policy. Throwing an exception or returning an error code is frequently better than terminating (if you squint, crashing out to the supervision tree is more like throwing an exception than it is like terminating.)

> Correctness - All possible function input and output values which do not have full value coverage should have assertions covering them.

Only if you control all of the callers. Library users would prefer an invalid parameters error/exception.

> Safety - When performing operations that can have unwanted, known, or unknown side effects, assertions should be used to prevent those conditions from happening.

Why assertions? If it is safety critical, shouldn't these checks be mandatory?

> Development - Use an assertion to enforce assumptions on values and state. These assertions can optionally be compiled out of code when coupled with proper testing.

This is where preconditions/postconditions/invariants come in. If safety is important you probably want to leave them in place. A runtime contract violation should enter a fail-safe state.

> Documentation - When writing code, use assertions as self documenting guardrails around your logic. Use assertions to enforce values and state which might be unclear from documentation or hard to decipher from reading code.

I do this, but in this case you either need to be 100% sure that the exception won't get hit, 100% sure that the exception won't make it into production builds, or okay with production crashes.

  > var error = system_call(...);
  > assert(!error);

Not this again ...

Assertions should only be thought of as predicates on state space to ensure program correctness. Everything else is just a corollary.

Some relevant past comments of mine here - https://news.ycombinator.com/item?id=48358691

There's a whole big contentious point that the author completely ignored: using assertions in tests (like unit-tests). Some unit testing frameworks expect their users to use assertions to do the job, others are very much against it because they want to separate between the failures of the system under test from failures of the test. (If that matters, I'm in the later camp).

* * *

I also think that the article confuses the how assert works at present (in some languages. Obviously, not Prolog, for example :D), and how he wants it to work. Sometimes his reasoning for doing one thing or the other is based on how assert works today, and sometimes it's based on how he wishes for it to work. Both have merit, but put together don't make much sense.

As for me, I think that the bullet points the author gives for the "proper" use of assertions need to be covered by different tools. Especially if the program is to be compiled with optimizations. I don't think there can be a general rule to tell if an assertion should stay at runtime or not. Sometimes it will depend on the knowledge about the environment in which the program will run. So, you'd need "persistent assertions" and "transient assertions" for the lack of a better word, where "persistent assertion" is functionally an exception, it just checks the same thing as the "transient assertion" would, so it makes sense that they are both called "assertions".