F# error FS0588: The block following this 'let' is unfinished. Every code block is an expression and must have a result

Question

I am tasked with finishing an interpreter in F#, but I'm having some trouble, as I im getting the error: error FS0588: The block following this 'let' is unfinished. Every code block is an expression and must have a result. 'let' cannot be the final code element in a block. Consider giving this block an explicit result.

Its been a long time since last time I programmed I F#.

The following is my code. I have a helper function inside my eval function, called OperateAux. It gets called in the pattern matching, when it matches e with OPERATE. It should then call OperateAux, and calculate the given expression. The error I'm getting is at line: let OperateAux (op:BINOP) (e1:EXP) (e2:EXP) : VALUE = so I guess somehow my helper function isn't finished, I just cant figure out where.

let rec eval (vtab : SymTab) (e : EXP) : VALUE =
  match e with
    | CONSTANT n -> n
    | VARIABLE v -> lookup v vtab 
    | OPERATE (op, e1, e2) -> OperateAux op e1 e2//(eval vtab e1) (eval vtab e2) 
    | LET_IN (var, e1, e2) -> failwith "case for LET_IN not handled"
    | OVER (rop, var, e1, e2, e3) -> failwith "case for OVER not handled"

  let OperateAux (op:BINOP) (e1:EXP) (e2:EXP) : VALUE = 
    let (INT e1) = eval vtab e1
    let (INT e2) = eval vtab e2 
    match op with 
      | BPLUS -> (e1+e2) 
      | BMINUS  -> (e1-e2)
      | BTIMES -> (e1*e2)
      | _ -> ()

Here is some types, I'm not sure if they are relevant for this question, but for good measure I'll show them.

type VALUE = INT of int

type BINOP = BPLUS | BMINUS | BTIMES

type RANGEOP = RSUM | RPROD | RMAX | RARGMAX

type EXP =
  | CONSTANT  of VALUE
  | VARIABLE  of string
  | OPERATE   of BINOP * EXP * EXP
  | LET_IN    of string * EXP * EXP
  | OVER      of RANGEOP * string * EXP * EXP * EXP

(* A list mapping variable names to their values. *)
type SymTab = (string * VALUE) list

Answer 1

Nevermind, I figured it out. You have to "initialise" your helper function before actually calling it. So the helper function operateAux should come before the pattern matching which calls it.