I was puzzled by a strange problem my student experienced in “List Ops” exercise in OCaml. So I downloaded the exercise myself and wrote the following (incorrect on purpose) code
let length _ = 0 let reverse _ =  let map ~f _ =  let filter ~f _ =  let fold ~init ~f = function |  -> init | l -> List.hd l |> f init let append _ _ =  let concat _ = 
Obviously the tests fail locally. Please notice that there are no loops or recursion in the code. And when I submitted the code to Exercism here is what I see:
Your tests timed out. This might mean that there was an issue in our infrastructure, but more likely it suggests that your code is running slowly. Is there an infinite loop or something similar? Please check your code, and if nothing seems to be wrong, try running the tests again.
I have tried to re-run the tests, with the same result. What do I miss?