So in the course I’m taking we’re trying to write an interpreter in f#. And for this we have to make a small calculator.
For this I was handed out this code:

type Value = INT of int

type Binop = BPLUS | BMINUS | BTIMES

type Rop = RSUM | RPROD | RMAX | RARGMAX

type vname = string

type Exp =
  | CONSTANT   of Value
  | VARIABLE   of vname
  | OPERATE    of Binop * Exp * Exp
  | LET_IN     of vname * Exp * Exp
  | OVER_LIST  of Rop * Exp list //must be non-empty
  | OVER_RANGE of Rop * vname * Exp * Exp * Exp

(* An association list mapping variable names to their values. *)
type SymTab = (vname * Value) list

(* Inserts a variable into the variable table. *)
let bind v k vtab = (v, k) :: vtab : SymTab

(* Look up the value of a variable. *)
let rec lookup v = function
  | (v', k) :: vtab -> if v = v' then k else lookup v vtab
  | _ -> failwith ("unbound variable: " + v)

(* EVALUATION *)

(*****************************************************
 * TODO: Task 2: complete the definition of `eval`. You may also
 * define any auxiliary functions as you see fit, to help express `eval`
 * as cleanly and concisely as possible.
 ****************************************************)

(* The evaluation function. *)
let rec eval (vtab : SymTab) (e : Exp) : Value =
  match e with
    | CONSTANT n -> n
    | VARIABLE v -> failwith "case for VARIABLE not handled"

Where we have to define variable and a couple of other functions ourselves.

For Variable my idea was to use pattern matching like they did during the lectures.

So something like this

| VARIABLE v -> 
      match lookup v vtab with
      | Some x -> x
      | None -> failwith "Unknown variable."

So if lookup gives a variable, I just return that and if it doesn’t I return an error saying it doesnt exist.

However when I do this I get
“This expression was expected to have type
‘Value’
but here has type
”a option’ “

Please don’t just write the solution but if you can give some hints as to why this is not working I would be very happy.