How to concatenate list values in OCaml

Question

If I have a function

let rec function n = 
   if n<0 then [] 
   else n-2 @ function n-2 ;;

I get an error saying that the expression function n-2 is a list of int but it is expecting an int.

How do I concatenate the values to return all the n-2 values above zero as a list?

I cannot use the List module to fold. Thanks

Answer 1

Your title asks how to concatenate lists, but your question seems rather different.

To concatenate lists, you can use the @ operator. In many cases, code that depends on this operator is slower than it needs to be (something to keep in mind for later :-).

Here are some things I see wrong with the code you give:

a. You can't name a function function, because function is a keyword in OCaml.

b. If you use the @ operator, you should have lists on both sides of it. As near as I can see, the thing on the left in your code is not a list.

c. Function calls have higher precedence than infix operators. So myfun n - 2 is parsed as (myfun n) - 2. You probably want something closer to myfun (n - 2).

Even with these changes, your code seems to generate a list of integers that are 2 apart, which isn't what you say you want. However, I can't understand what the function is actually supposed to return.

Answer 2

It seems like you are not concatenating lists, but concatenating ints instead. This is done by the :: operator. So your code would look like:

     else (n-2)::(fun (n-2))

Although I could see this function possibly not producing the desired output if you put in negative numbers. For example if you pass through n = 1, n-2 will evaluate to -1 which is less than zero.