[skydhash]

A comment "this CANNOT happen" has no value on itself. Unless you've formally verified the code (including its dependencies) and have the proof linked, such comments may as well be wishes and prayers.

Yes, sometimes, the compiler or the hardware have bugs that violate the premises you're operating on, but that's rare. But most non pure algorithms (side effects and external systems) have documented failure cases.

[JohnFen]

> A comment "this CANNOT happen" has no value on itself.

I think it does have some value: it makes clear an assumption the programmer made. I always appreciate it when I encounter comments that clarify assumptions made.

[addaon]

But if you spell that `assert(false)` instead of as a comment, the intent is equally clear, but the behavior when you're wrong is well-defined.

[JohnFen]

I agree that including that assert along with the comment is much better. But the comment alone is better than nothing, so isn't without value.

[eterm]

Better yet, `assert(false, message)`, with the message what you would have written in the comment.

[addaon]

`assert(false)` is pronounced "this can never happen." It's reasonable to add a comment with /why/ this can never happen, but if that's all the comment would have said, a message adds no value.

[eterm]

Oh I agree, literally `assert(false, "This cannot happen")` is useless, but ensuring message is always there encourages something more like, `assert(false, "This implies the Foo is Barred, but we have the Qux to make sure it never is")`.

Ensuring a message encourages people to state the assumptions that are violated, rather than just asserting that their assumptions (which?) don't hold.

[breatheoften]

what language are we talking about? If it's cpp then the pronounciation depends on compiler flags (perhaps inferred from CMAKE_BUILD_TYPE)

[andrewf]

Swap the parameters around for C++ and similar langs where `assert(a, b)` evaluates the same as `(void) a; assert(b)`.

[fwip]

I think you might have missed that they threw an exception right under the comment.

[zffr]

At least on iOS, asserts become no-ops on release builds

[josephg]

It really depends on the language you use. Personally I like the way rust does this:

- assert!() (always checked),

- debug_assert!() (only run in debug builds)

- unreachable!() (panics)

- unsafe unreachable_unchecked() (tells the compiler it can optimise assuming this is actually unreachable)

- if cfg!(debug_assertions) { … } (Turns into if(0){…} in release mode. There’s also a macro variant if you need debug code to be compiled out.)

This way you can decide on a case by case basis when your asserts are worth keeping in release mode.

And it’s worth noting, sometimes a well placed assert before the start of a loop can improve performance thanks to llvm.

[addaon]

> debug_assert!() (only run in debug builds)

debug_assert!() (and it's equivalent in other languages, like C's assert with NDEBUG) is cursed. It states that you believe something to be true, but will take no automatic action if it is false; so you must implement the fallback behavior if your assumption is false manually (even if that fallback is just fallthrough). But you can't /test/ that fallback behavior in debug builds, which means you now need to run your test suite(s) in both debug and release build versions. While this is arguably a good habit anyway (although not as good a habit as just not having separate debug and release builds), deliberately diverging behavior between the two, and having tests that only work on one or the other, is pretty awful.

[josephg]

I hear you, but sometimes this is what I want.

For example, I’m pretty sure some complex invariant holds. Checking it is expensive, and I don’t want to actually check the invariant every time this function runs in the final build. However, if that invariant were false, I’d certainly like to know that when I run my unit tests.

Using debug_assert is a way to do this. It also communicates to anyone reading the code what the invariants are.

If all I had was assert(), there’s a bunch of assertions I’d leave out of my code because they’re too expensive. debug_assert lets me put them in without paying the cost.

And yes, you should run unit tests in release mode too.

[addaon]

You can (and probably should) undef NDEBUG even for release builds.

[AdieuToLogic]

>> A comment "this CANNOT happen" has no value on itself.

> I think it does have some value: it makes clear an assumption the programmer made.

To me, a comment such as the above is about the only acceptable time to either throw an exception (in languages which support that construct) or otherwise terminate execution (such as exiting the process). If further understanding of the problem domain identifies what was thought impossible to be rare or unlikely instead, then introducing use of a disjoint union type capable of producing either an error or the expected result is in order.

Most of the time, "this CANNOT happen" falls into the category of "it happens, but rarely" and is best addressed with types and verified by the compiler.

[dllthomas]

Importantly, specifying reasoning can have communicative value while falling very far short of formal verification. Personally, I also try to include a cross reference to the things that could allow "this" to happen were they to change.

[skydhash]

Such comments rot so rapidly that they're an antipattern. Such assumptions are dangerous and I would point it out in a PR.

[LegionMammal978]

Do you not make such a tacit assumption every time you index into an array (which in almost all languages throws an exception on bounds failure)? You always have to make assumptions that things stay consistent from one statement to the next, at least locally. Unless you use formal verification, but hardly anyone has the time and resources for that.

[lmm]

> Do you not make such a tacit assumption every time you index into an array (which in almost all languages throws an exception on bounds failure)?

Yes, which is one reason why decent code generally avoids doing that.

[MobiusHorizons]

Are you saying decent code avoids indexing into arrays? Or are you saying it avoids doing so without certainty the bounds checks will succeed?

[lmm]

Decent code generally avoids indexing into arrays at all; if it does so then it does so in ways where the bound checks are so certain to succeed that you can usually explain it to the compiler (e.g. split an array into slices and access those slices).

[skydhash]

If such an error happens, that would be a compiler bug. Why? Because I usually do checks against the length of the array or have it done as part of the standard functions like `map`. I don't write such assumptions unless I'm really sure about the statements, and even then I don't.

[tosapple]

How does one defend against cosmic rays?

Keep two copies or three like RAID?

Edit: ECC ram helps for sure, but what else?

[well_ackshually]

>How does one defend against cosmic rays?

Unless you are in the extremely small minority of people who would actually be affected by it (in which case your company would already have bought ECC ram and made you work with three isolated processes that need to agree to proceed): you don't. You eat shit, crash and restart.

[JonChesterfield]

You run equivalent or equal calculations simultaneously on N computers and take majority wins, aircraft control or distributed filesystem style.

[LegionMammal978]

> or have it done as part of the standard functions like `map`.

Which are all well and good when they are applicable, which is not always 100% of the time.

> Because I usually do checks against the length of the array

And what do you have your code do if such "checks" fail? Throw an assertion error? Which is my whole point, I'm advocating in favor of sanity-check exceptions.

Or does calling them "checks" instead of "assumptions" magically make them less brittle from surrounding code changes?

[skydhash]

A comment have no semantic value to the code. Having code that check for stuff is different from writing comments as they are executed by the machine. Not read by other humans.

[awesome_dude]

Do you really have code that's

if array.Len > 2 { X = Y[1] }

For every CRUD to that array?

That seems... not ideal

[skydhash]

Yes. Unless there’s some statement earlier that verify that the array has 2 items. It’s quick to do, so why not do it?

[threethirtytwo]

False it has value. It’s actually even better to log it or throw an exception. print(“this cannot happen.”)

If you see it you immediately know the class of error is purely a logic error the programmer made a programming mistake. Logging it makes it explicit your program has a logic bug.

What if you didn’t log it? Then at runtime you will have to deduce the error from symptoms. The log tells you explicitly what the error is.

[AnimalMuppet]

Worse: You may created the proof. You may have linked to the proof. But if anyone has touched any of the code involved since then, it still has no value unless someone has re-done the proof and linked that. (Worse, it has negative value, because it can mislead.)

[skydhash]

Not really. A quick git blame (or alternative) will give you the required information about the validity of such proof.

[dullcrisp]

You must have some git plugin I haven’t heard about.

[skydhash]

Git blame will show the commit and the date for each line. It’s easy to verify if the snippet has changed since the comment. i use Emacs and it’s builtin vc package that color code each block.

[AnimalMuppet]

But you need the snippet and, potentially, the entire call tree (both up and down).

[dullcrisp]

And anything that can affect relevant state, any dependencies that may have changed, validations to input that may have been modified; it’s hard to know without knowing what assumptions the assertion is based on.