Question

I have to write a function, that returns true if a given list is sorted in ascending order. The empty and 1-element lists are sorted. Also, [5,12,12] should return true.

I’ve written a function that seems to work:

let rec isSorted (l: int list) = 
    match l with
    | [] -> true
    | [x] -> true
    | [x;y] -> x <= y
    | x::y::xr -> if x > y then false else isSorted([y] @ xr);

But it seems a bit off… I’m thinking there must be an easier way to do this? I hate that I have to match 4 cases, but I cant figure out how to make it any smarter.

Any better solutions?

Answer 1

Well, never say

[y] @ xr

when

y :: xr

will do just as well. (In general, @ is a code smell.)

Kinda nitpicky, but the last line could be

| x::((y::_)as t) -> if x > y then false else isSorted(t)

and save you from doing any allocation.

Now, do you need the third case? What happens if you remove it?