Why does this type error keep occurring?

Question

I have an ocaml function in which I am pattern matching several cases. I keep getting a type error and I can't see how to deal with it. Any help would be appreciated. This is the error message I'm getting : "File "eval.ml", line 40, characters 18-35: Error: This expression has type Env.symEntry". Line 40 is the line with "let l = binding_of env x in"

(* eval_expr: expr -> proc_state -> env -> store -> value *)
(* eval_expr: expr -> proc_state -> env -> store -> int *)
let rec eval_expr (expr:expr) (ps:proc_state) (env:environment) (store:store) : int = match expr with
      Add (e1, e2) -> 
      let r1 = eval_expr e1 ps env store in
      let r2 = eval_expr e2 ps env store in
      r1 + r2
  |  Sub (e1, e2) ->
      let r1 = eval_expr e1 ps env store in
      let r2 = eval_expr e2 ps env store in
      r1 - r2
  |  Mul (e1, e2) ->
      let r1 = eval_expr e1 ps env store in
      let r2 = eval_expr e2 ps env store in
      r1 * r2  
  |  Div (e1, e2) ->
      let r1 = eval_expr e1 ps env store in
      let r2 = eval_expr e2 ps env store in
      r1 / r2
  |  Neg (e1) ->
      let r1 = eval_expr e1 ps env store in
      r1 * -1
  | IntConst i -> i

     (* - variable reads: handle the Id constructor from expr. 
  use binding_of and 
      value_at *)

  | Id(x) ->
    let l = binding_of env  x in
    let i = value_at store l in i;;

Here is the code for some of the functions and types I used :

module IntMap = Map.Make(struct type t = int let compare = compare end);;
type store = int IntMap.t;;
let value_at store loc = IntMap.find loc store;;
let insert_value store loc v =  IntMap.add loc v store;;


let enter_scope env scope = scope::env;;
let exit_scope env = List.tl env;;
let current_scope env = List.hd env;;

type expr = (* Expressions in C-flat *)

  (* int operations *)
  Add of expr * expr
| Sub of expr * expr
| Mul of expr * expr
| Div of expr * expr
| Neg of expr

  (* constants/variables *)
| Id of string
| IntConst of int

  (* array/pointer operations *)
| At of expr * expr
| Deref of expr
| AddressOf of expr

  (* function *)
| Call of string * expr list

  (* pre- & post-increment *)
| Pre of expr
| Post of expr
;;



let binding_of env name = 
  let lookup name symTab = List.assoc name symTab in
  try 
    lookup name (List.hd env)
  with Not_found ->
    lookup name (List.hd (List.tl env))
;;

Jeffrey Scofield Jeffrey Scofield · Accepted Answer · 2015-05-05T02:05:28

You’re not showing enough of your definitions to see what’s happening on the line you mention.

My guess is that binding_of_env doesn’t return a location, but value_at expects a location. Possibly you need to extract the location from the binding.

Why does this type error keep occurring?

2 Answers