Invalid recursive values should be a compiler error, not a runtime exception

[evancz]

Right now, Elm will not yell at you for writing invalid recursive values like ones = 1 :: ones where a value is defined directly in terms of itself. In a strict language like Elm, this means you'd spend an infinite amount of time building the entire structure!

Current Behavior

Right now, invalid code like this leads to a runtime exception. In some ways that is better than infinite looping forever, and these exceptions happen very reliably. It is not conditional on anything, you just need to evaluate the code. So it's not awful, but it is also something that beginners run into a decent amount.

Solution

Only permit recursion when there is at least one lambda between the definition and the usage. So x = x is not allowed, but fact n = if n <= 0 then 1 else n * fact (n - 1) is allowed because the definition implicitly introduces a lambda.

Timeline

This bug is kind of tricky to resolve, but it is not crazy. From a raw time efficiency standpoint, it is probably best to batch with work on Dead Code Elimination. I find it super annoying when people say "but Elm can have runtime exceptions" as if a compiler bug with a clear resolution is the same as runtime exceptions in JavaScript, so I'd like to do it sooner than DCE if possible.

Other Info

• Check out https://github.com/elm-lang/core/issues/305 which shows how the order of definitions can be important in generated code. Probably should be done together.
• Xavier has some very useful analysis here about how it works in OCaml.

[evancz]

This should be fixed in 0.18

[evancz]

Okay, writing something like x = x + 1 will now result in this message:

The link at the end points here.