Make the code a mystery for the reader:
{:"?", ...} -> ???
:unexpected -> :"???"
{:terminalcase, ...} -> fn -> ??? end
D4no0
sodapopcan
I really like ???.
I originally liked the idea of case! but I feel that @kwando’s argument against it completely kills it for me:
It’s not necessarily that I find it ugly but those examples illustrate that it could be downright confusing. Just making a typo could lead to an error that is potentially hard to spot. ??? is very clear.
dimitarvp
Even though I giggled at the good jokes in the comments following yours, I’d probably still consider Rust’s unreachable! or todo!.
Copying the second example from the unreachable! page just to illustrate (because it’s not a directly translated abstraction):
fn divide_by_three(x: u32) -> u32 { // one of the poorest implementations of x/3
for i in 0.. {
if 3*i < i { panic!("u32 overflow"); }
if x < 3*i { return i-1; }
}
unreachable!("The loop should always return");
}
In Elixir’s case we might have todo!() at the place of your ???.
sodapopcan
dimitarvp
And it’s easy to search for. Though ??? is easy to search for as well.
But to your point, todo!'s idea is exactly to nag you. In the case of Elixir exhaustive pattern matching is not enforced at compile-time so you can leave yourself a nagging reminder to come back to the code later.
josevalim
Creator of Elixir
I like the unreachable! variation as well, definitely clearer, albeit a bit too long. And I like that the compiler will warn if it is sure it can be reachable, so it is something to consider in addition to the type system design (and a good reason why we need something in addition to raise). Productive convo!
dimitarvp
Yep, that was my idea: to complement the currently considered approach, not to replace it.
sodapopcan
Greppability is actually a good point. Perhaps a bit of a bikeshed but is it easier to remember the case statement argument (like case! do_the_thing() do or whatever) or to get a sea of unreachable/???/todos when searching for these where you can’t remember the message or didn’t write a very meaningful one? You may even have duplicate messages.
Ah yes, right. I was thinking that if I’m not dealing with it now I probably never will 
adw632
I wouldn’t mind something like oops! or bad! as a terser indicator of something unexprected.
It’s one character more than ??? but if we want to support other semantics like todo! then it may provide a shorter option than unreachable!
bartblast
Creator of Hologram