Below is a non-tail-recursive function to compute the sum of a list of integers.

```
let rec sum = function
| [] -> 0
| h::t -> h + (sum t)
```

The last operation the function performs is the addition. Thus, the function isn't tail-recursive.

Below is a tail-recursive version of the same function.

```
let sum l =
let rec aux acc = function
| [] -> acc
| h::t -> aux (acc+h) t
in
aux 0 l
```

Here, the `aux`

function is tail-recursive: the last operation it performs is calling itself. As a consequence, the latter version of `sum`

can be used with lists of any length.