Question

I am learning standard ML and I keep getting this error and I am not sure why?

Here is the code and the error:

> fun in_list(element, list) = 
    if hd(list) = element then true
    else val tempList = List.drop(list, 1);
    in_list(element, tempList);
# # Error-Expression expected but val was found
Static Errors

I know there has to be something wrong with the syntax that I am trying.

Answer 1

You need to wrap val values in a let..in..end block.

fun in_list(element, list) = 
    if hd(list) = element then true
    else 
        let
           val tempList = List.drop(list, 1)
        in
           in_list(element, tempList)
        end

Moreover, hd and drop aren’t recommended to decompose a list. You should use pattern-matching instead.

fun in_list(element, x::xs) = 
    if x = element then true
    else in_list(element, xs)

There is a base case with empty list missing, and you can use orelseto replace if x = element then true .... I leave them for you as suggestions.