# Ocaml interpreter

**URL:** <https://discuss.ocaml.org/t/ocaml-interpreter/14650>\
**Category:** Learning\
**Created:** [May 15, 2024, 10:45pm UTC](https://discuss.ocaml.org/t/ocaml-interpreter/14650 "2024-05-15T22:45:55Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![Peppe\_La\_Tosa](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/peppe_la_tosa/32/5300_2.png) [@Peppe\_La\_Tosa](https://discuss.ocaml.org/u/Peppe_La_Tosa)\
**Post date:** [May 15, 2024, 10:45pm UTC](https://discuss.ocaml.org/t/ocaml-interpreter/14650/1 "2024-05-15T22:45:55Z")

</div>

AST  
type expr =  
| EInt of int  
| EBool of bool  
| Var of string  
| Let of string \* expr \* expr | Prim of string \* expr \* expr | If of expr \* expr \* expr  
| Fun of string \* expr  
| Call of expr \* expr  
| GetInput of expr

type value =  
| Int of int  
| Bool of bool  
| Closure of string \* expr \* value env

Environment:

(\* environment _)  
type 'v env = (string \* 'v \* bool) list  
(_ binding _)  
let rec lookup env x =  
match env with  
| [] → failwith (x ^ “not found”)  
| (y, v, \_) :: r → if x = y then v else lookup r x  
(_ taintness of a variable \*) let rec t\_lookup env x =  
match env with  
| → failwith (x ^ “not found”)  
| (y, \_, t) :: r → if x = y then t else t\_lookup r x

Interpreter  
let rec eval (e : expr) (env:value env) (t : bool) : value \* bool = match e with  
| EInt n → (Int n, t)  
| EBool b → (Bool b, t)  
| Var x → (lookup env x, t\_lookup env x) | Prim (op, e1, e2) →   
begin  
let v1, t1 = eval e1 env t in let v2, t2 = eval e2 env t in  
match (op, v1, v2) with  
(\* taintness of binary ops is given by the OR of the taintness of the args _) | "_", Int i1, Int i2 → (Int (i1 \* i2), t1 || t2)  
| “+”, Int i1, Int i2 → (Int (i1 + i2), t1 || t2)  
| “-”, Int i1, Int i2 → (Int (i1 - i2), t1 || t2)  
| “=”, Int i1, Int i2 → (Bool (if i1 = i2 then true else false), t1 || t2)  
| “\<”, Int i1, Int i2 → (Bool (if i1 \< i2 then true else false), t1 || t2)  
| “\>”, Int i1, Int i2 → (Bool (if i1 \> i2 then true else false), t1 || t2)  
| \_, \_, \_ → failwith “Unexpected primitive.”  
end

| If (e1, e2, e3) → begin  
let v1, t1 = eval e1 env t in match v1 with  
| Bool true → let v2, t2 = eval e2 env t in (v2, t1 || t2) | Bool false → let v3, t3 = eval e3 env t in (v3, t1 || t3) | \_ → failwith “Unexpected condition.”  
end

| Fun (f\_param, f\_body) → (Closure (f\_param, f\_body, env), t) | Call (f, param) →   
let f\_closure, f\_t = eval f env t in begin  
match f\_closure with  
| Closure (f\_param, f\_body, f\_dec\_env) →   
let f\_param\_val, f\_param\_t = eval param env t in  
let env’ = (f\_param, f\_param\_val, f\_param\_t)::f\_dec\_env in  
let f\_res, t\_res = eval f\_body env’ t in (f\_res, f\_t || f\_param\_t || t\_res)  
| \_ → failwith “Function expected error” end  
| GetInput(e) → eval e env true

In the eval of the Call in the Interpreter there is a error, what is? Can you fix it?

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<div class="post-metadata">

**Author:** ![deca3](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/deca3/32/6109_2.png) [@deca3](https://discuss.ocaml.org/u/deca3)\
**Post date:** [June 26, 2024, 11:37pm UTC](https://discuss.ocaml.org/t/ocaml-interpreter/14650/2 "2024-06-26T23:37:06Z")

</div>

Could you describe the error more in details?  
and give some test code, where the error appears?  
(also please copy-past code inside a code block with three backticks ```, before and after the code)

```ocaml

type expr =
  | EInt of int
  | EBool of bool
  | Var of string
  | Let of string * expr * expr
  | Prim of string * expr * expr
  | If of expr * expr * expr
  | Fun of string * expr
  | Call of expr * expr
  | GetInput of expr

(* environment *)
type 'v env = (string * 'v * bool) list

type value =
  | Int of int
  | Bool of bool
  | Closure of string * expr * value env

(* binding *)
let rec lookup env x =
  match env with
  | [] -> failwith (x ^ "not found")
  | (y, v, _) :: r -> if x = y then v else lookup r x

(* taintness of a variable *)
let rec t_lookup env x =
  match env with
  | [] -> failwith (x ^ "not found")
  | (y, _, t) :: r -> if x = y then t else t_lookup r x

(* Interpreter *)
let rec eval (e : expr) (env:value env) (t : bool) : value * bool =
  match e with
  | EInt n -> (Int n, t)
  | EBool b -> (Bool b, t)
  | Var x -> (lookup env x, t_lookup env x)
  | Prim (op, e1, e2) ->
    begin
      let v1, t1 = eval e1 env t in
      let v2, t2 = eval e2 env t in
      match (op, v1, v2) with
      (* taintness of binary ops is given by the OR of the taintness of the args *)
      | "", Int i1, Int i2 -> (Int (i1 * i2), t1 || t2)
      | "+", Int i1, Int i2 -> (Int (i1 + i2), t1 || t2)
      | "-", Int i1, Int i2 -> (Int (i1 - i2), t1 || t2)
      | "=", Int i1, Int i2 -> (Bool (if i1 = i2 then true else false), t1 || t2)
      | "<", Int i1, Int i2 -> (Bool (if i1 < i2 then true else false), t1 || t2)
      | ">", Int i1, Int i2 -> (Bool (if i1 > i2 then true else false), t1 || t2)
      | _, _, _ -> failwith "Unexpected primitive."
    end
    | If (e1, e2, e3) -> begin
        let v1, t1 = eval e1 env t in
        match v1 with
        | Bool true -> let v2, t2 = eval e2 env t in (v2, t1 || t2)
        | Bool false -> let v3, t3 = eval e3 env t in (v3, t1 || t3)
        | _ -> failwith "Unexpected condition."
        end
    | Fun (f_param, f_body) -> (Closure (f_param, f_body, env), t)
    | Call (f, param) ->
        let f_closure, f_t = eval f env t in
        begin
          match f_closure with
          | Closure (f_param, f_body, f_dec_env) ->
              let f_param_val, f_param_t = eval param env t in
              let env' = (f_param, f_param_val, f_param_t) :: f_dec_env in
              let f_res, t_res = eval f_body env' t in
              (f_res, f_t || f_param_t || t_res)
          | _ -> failwith "Function expected error"
        end
    | GetInput(e) -> eval e env true
    | Let (_, _, _) -> assert false

```

The pattern matching was not exhaustive, I had to add the last line to avoid a warning.

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<div class="post-metadata">

**Author:** ![R\_Huxton](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ocaml.org/r_huxton/32/4216_2.png) [@R\_Huxton](https://discuss.ocaml.org/u/R_Huxton)\
**Post date:** [June 27, 2024, 8:40am UTC](https://discuss.ocaml.org/t/ocaml-interpreter/14650/3 "2024-06-27T08:40:17Z")

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OK, so presumably you need to diagnose this bug and fix it for an assignment.

Tips:

- paste each chunk into utop one at a time with `;;` on the end to evaluate them.
- deal with any errors as they occur before moving on to the next chunk.