Question

I have this SML code. I don’t know why I cannot compile this :

fun score = 
    let
       val sum = 3; (* error at this line : SYNTAX ERROR : inserting LPAREN *)
       if sum div 2 > 0
       then sum = 0
       else sum = 1
       (*some other code*)
    in
       sum (* I want to return sum after some steps of calculation *)
    end

Answer 1

There are more issues with your code, than jacobm points out.
You are also missing a function argument. Functions in SML always takes one argument. For example

fun score () = 
    let val sum = 3
        val sum = if sum div 2 > 0
                  then sum = 0
                  else sum = 1
    in
       sum
    end

However this still doesn’t make much sense. since the expressions sum = 0 and sum = 1 evaluates to a Boolean.

A let-expression is used to make some local declarations which are only visible inside the in ... end part. Thus the calculations you wan’t to do with sum, should probably be done inside the in ... end part, unless you wan’t to express it as a means of a function.

One such example is

fun score () = 
    let val sum = 3
    in
      if sum div 2 > 0
      then ...
      else ...
    end

If we look at the syntax of a let-expression, it probably makes more sense

let
  <declaration>
in
  <expr> ; ... ; <expr>
end

Since if-then-else is an expression, it can’t be in the “declarations part” by itself.