I remember from twenty years ago, Amazon recommended a book for dementia patients, related to purchase of an AS/400 manual. Because of the name similarity between the IT author and other book's illustrator. We couldn't move one step forward.
mkose@casper:~/Desktop$ ./elm repl
---- Elm 0.19.1 -----------------------------------------
> Basics.modBy 0 3
Error: Cannot perform mod 0. Division by zero error.
I believe there are a couple of things in the standard library which should have returned a Maybe (or even better taken a NonZero as an argument). The community Extra libraries tend to offer safe versions of these cases, so it's easy to avoid if you know about it.
Another gotcha that will give a runtime error is the stack overflow if you have too many recursions in a function. You can get around it by using builtins like maps and folds, or refactoring for tail-calls.
Since recursion is often taught to FP beginners as the go-to replacement for loops without explaining TCO, I think many users are surprised by this early on, and with just the compiler they would otherwise have no warning of this without knowing how and when to test for it.
I believe Elm-review is coming out with a method to warn about these cases, so it will be detectable with standard tools.
On the whole, though, these are the few avoidable edge cases, and the promise does hold the rest of the time.
My team is working on a very large Elm codebase for over 3 years now, we got a runtime error in production only once (stack overflow when parsing an unusually large file, made it tail call optimized to get rid of it).
I will consider it sticking to it's promise of no runtime errors.
