Elm and Maximum call stack size exceeded

A-ha, this will be an easy blog post!

So perhaps your JavaScript console tells you Uncaught RangeError: Maximum call stack size exceeded.

That probably means your recursion isn’t tail-recursive. A recursive function calls itself. A tail-recursive function calls itself as its final step – it does not further modify the result of the recursive call.

A thing that caught me out just now, while f (a b) and f <| a b are functionally equivalent, the former is tail recursive while the latter is not.

From Basics.elm:

{-| Saying `f <| x` is exactly the same as `f x`.

It can help you avoid parentheses, which can be nice sometimes. Maybe you want
to apply a function to a `case` expression? That sort of thing.
-}
apL : (a -> b) -> a -> b
apL f x =
  f x

In f (a b), the a b part is evaluated first, and then f is called as the last thing. In f <| a b, the <| itself is application of a function, thus breaking the tail elimination as f is not the last thing called.

Elm says no runtime exceptions. Generally true, not too much of a stretch. But you still do have to watch out a tiny little bit…