I just started Advent of Code 2018. This is exercise 2 of day 1, and I can't understand why my code is so slow (4 minutes 37 seconds, compared to 200ms for a friend who did it in Clojure with the same computer).
Basically, it accumulates integers from a provided list of ints, named
input. Whenever the current value of the accumulator is equal to one of its previous values, it is returned. We might have to loop many times over the provided list before we find such an occurrence. Here is the code:
let main () = let rec my_slow_code lst history = if lst ==  then my_slow_code input history else let current = List.hd lst + List.hd history in if List.mem current history then current else my_slow_code (List.tl lst) (current :: history) in my_slow_code input 
I can't understand what mistake here makes such a slow code, maybe the use of List.mem?