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.

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.

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

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);

Writing this is equivalent to providing an arbitrary third-party with the ability to crash your application with a user-unfriendly error message. Your program should have code paths to handle all error conditions. One of them can be { print("unexpected result from system call. exiting.") exit(); }