global and local variables with scope

also reworked arg parsing, now has --debug flag

lib/interpreter.ml

@@ -1,20 +1,36 @@
 let ( let* ) = Result.bind
 
-open Expr
+open Environment
 open Error
+open Expr
 open Stmt
 open Value
 
 let value_of_literal (literal : literal) : Value.lox_value =
   match literal with String s -> String s | Number x -> Number x | Bool b -> Bool b | Nil -> Nil
 
-let rec interpret_expr (expr : expr_node) : (lox_value, runtime_error) result =
+let rec interpret_expr (env : environment) (expr : expr_node) : (lox_value, runtime_error) result =
   let { pos; expr } = expr in
   match expr with
   | Literal literal -> Ok (value_of_literal literal)
+  | Variable name -> (
+      let value_opt = Env.get env name in
+      match value_opt with
+      | Some x -> Ok x
+      | None ->
+          let msg = Printf.sprintf "name \"%s\" is not defined" name in
+          Error (RuntimeError.make pos msg))
+  | Assignment { name; expr } ->
+      if not (Env.is_defined env name) then
+        let msg = Printf.sprintf "tried to assign to undefined variable %s" name in
+        Error (RuntimeError.make pos msg)
+      else
+        let* value = interpret_expr env expr in
+        Env.update env name value;
+        Ok value
   | BinaryExpr { op; left; right } -> (
-      let* left = interpret_expr left in
-      let* right = interpret_expr right in
+      let* left = interpret_expr env left in
+      let* right = interpret_expr env right in
       match (left, op, right) with
       | String a, Plus, String b -> Ok (String (a ^ b))
       | Number x, Plus, Number y -> Ok (Number (x +. y))
@@ -43,7 +59,7 @@ let rec interpret_expr (expr : expr_node) : (lox_value, runtime_error) result =
           in
           Error { pos; msg })
   | UnaryExpr { op; expr } -> (
-      let* expr = interpret_expr expr in
+      let* expr = interpret_expr env expr in
       match (op, expr) with
       | Neg, Number x -> Ok (Number (-.x))
       | Not, Bool b -> Ok (Bool (not b))
@@ -54,14 +70,31 @@ let rec interpret_expr (expr : expr_node) : (lox_value, runtime_error) result =
           in
           Error (RuntimeError.make pos msg))
 
-let interpret_stmt (stmt : stmt_node) : (unit, runtime_error) result =
+let rec interpret_stmt (env : environment) (stmt : stmt_node) : (unit, runtime_error) result =
   let { pos; stmt } = stmt in
   ignore pos;
   match stmt with
+  | Expr expr ->
+      let* _ = interpret_expr env expr in
+      Ok ()
   | Print expr ->
-      let* value = interpret_expr expr in
+      let* value = interpret_expr env expr in
       print_endline (Value.string_of_lox_value value);
       Ok ()
-  | Expr expr ->
-      let* _ = interpret_expr expr in
-      Ok ()
+  | VarDecl { name; init } ->
+      let* init = Option.map (interpret_expr env) init |> Option.value ~default:(Ok Nil) in
+      let success = Env.define env name init in
+      if success then Ok ()
+      else
+        let msg = Printf.sprintf "Tried to define %s, but was already defined" name in
+        Error (RuntimeError.make pos msg)
+  | Block stmts ->
+      let env = Env.enter env in
+      let rec _interpret stmts =
+        match stmts with
+        | stmt :: tail ->
+            let* _ = interpret_stmt env stmt in
+            _interpret tail
+        | [] -> Ok ()
+      in
+      _interpret stmts