From 23892acf6d597fa495cdca2278300ea566840ae6 Mon Sep 17 00:00:00 2001
From: Hyeonung Baek <wiserlix@gmail.com>
Date: Sat, 12 Feb 2022 03:18:00 +0900
Subject: [PATCH] Refactor env to list

---
 eval.ml | 48 +++++++++++++++++-------------------------------
 main.ml |  9 ++++-----
 2 files changed, 21 insertions(+), 36 deletions(-)

diff --git a/eval.ml b/eval.ml
index aedc480..1985479 100644
--- a/eval.ml
+++ b/eval.ml
@@ -14,10 +14,7 @@ type value =
 and expr = Ast.t
 
 (* environment for eval *)
-and env = {
-  vars : (string, value) Hashtbl.t;
-  parent : env option;
-}
+and env = (string * value) list
 
 exception No_operation
 exception Too_many_arguments
@@ -80,26 +77,16 @@ end
 module Env = struct
   type t = env
 
-  let init_global () = {
-    vars = Hashtbl.create 100;
-    parent = None;
-  }
+  let empty = []
 
-  let make parent = {
-    vars = Hashtbl.create 100;
-    parent = Some parent;
-  }
+  let get_opt e name =
+    List.assoc_opt name e
 
-  let rec get_opt e name =
-    match Hashtbl.find_opt e.vars name with
-    | None -> Option.bind e.parent (fun p -> get_opt p name)
-    | Some _ as v -> v
+  let bind v e =
+    v::e
 
-  let set e name value =
-    Hashtbl.replace e.vars name value
-
-  let add_seq e seq =
-    Hashtbl.add_seq e.vars seq
+  let bind_seq seq e =
+    List.of_seq seq @ e
 end
 
 (* operators *)
@@ -275,9 +262,8 @@ let rec eval env ast =
         | Some v -> v
       end
     | Letin (v, e, f) ->
-      let nenv = Env.make env in
-      Env.set nenv v (aux e);
-      eval nenv f
+      let env = Env.bind (v, aux e) env in
+      eval env f
 
     | Unary (op, t) ->
       let t = aux t in
@@ -299,9 +285,8 @@ let rec eval env ast =
           begin match args with
             | [] -> f
             | a::args ->
-              let nenv = Env.make env in
-              Env.set nenv var (aux a);
-              eval nenv @@ Apply (e, args)
+              let env = Env.bind (var, aux a) env in
+              eval env @@ Apply (e, args)
           end
         | External f ->
           let args = List.map aux args in
@@ -333,9 +318,10 @@ let rec eval env ast =
   in
   aux ast
 
-let eval_top env ast =
+let eval_top env_ref ast =
   match ast with
   | Let (var, e) ->
-    let v = eval env e in
-    Env.set env var v; var, v
-  | ast -> "-", eval env ast
+    let v = eval !env_ref e in
+    env_ref := Env.bind (var, v) !env_ref;
+    var, v
+  | ast -> "-", eval !env_ref ast
diff --git a/main.ml b/main.ml
index 0fa1b4c..77654f2 100644
--- a/main.ml
+++ b/main.ml
@@ -28,10 +28,9 @@ let stdlib = [
   |> List.to_seq
   |> Seq.map (fun v -> v, External v)
 
+(* global environment *)
 let g =
-  let g = Env.init_global () in
-  Env.add_seq g stdlib;
-  g
+  ref @@ Env.bind_seq stdlib Env.empty
 
 (* read-eval-print *) 
 let rep env : unit =
@@ -44,14 +43,14 @@ let rep env : unit =
   match v with
   | Nop -> ()
   | _ ->
-    Env.set env "ans" v;
+    g := Env.bind ("ans", v) !g;
     printf "%s: %s = %s\n"
       var (Type.to_string @@ Value.typeof v) (Value.to_string v)
 
 exception Reset_line (* used to indicate ^C is pressed *)
 
 let init_repl () =
-  Env.set g "ans" (Int 0);
+  g := Env.bind ("ans", Int 0) !g;
   (* treat Ctrl-C as to reset line *)
   let reset_line _ = raise Reset_line in
   Sys.(set_signal sigint (Signal_handle reset_line))