From 2be0b7f794239c92141842717b76b364914e21d4 Mon Sep 17 00:00:00 2001
From: Hyeonung Baek <wiserlix@gmail.com>
Date: Fri, 21 Jan 2022 00:17:01 +0900
Subject: [PATCH] Add let

---
 ast.ml    |  5 +++++
 eval.ml   |  4 ++++
 parser.ml | 17 +++++++++++++++++
 token.ml  |  2 ++
 4 files changed, 28 insertions(+)

diff --git a/ast.ml b/ast.ml
index 0f19d60..ae18fa3 100644
--- a/ast.ml
+++ b/ast.ml
@@ -99,6 +99,7 @@ end
 type t =
   | Value of Value.t
   | Var of string
+  | Let of string * t
   | Binop of t * Binop.t * t
   | Set_binop_pre of Binop.t * t
   | Get_binop_pre of Binop.t
@@ -117,6 +118,10 @@ let print ast =
   let rec aux = function
     | Value n -> pv n
     | Var v -> pr "%s" v
+    | Let (v, e) ->
+      pr "(let %s " v;
+      aux e;
+      pr ")"
     | Binop (left, op, right) -> begin
         let op = Binop.to_string op in
         pr "(%s " op; aux left; pr " "; aux right; pr ")";
diff --git a/eval.ml b/eval.ml
index 6f2bd25..51b071b 100644
--- a/eval.ml
+++ b/eval.ml
@@ -31,6 +31,10 @@ let eval vars ast =
     | Binop (l, op, r) ->
       let l = aux l and r = aux r in
       binop op l r
+    | Let (var, e) ->
+      let v = aux e in
+      Hashtbl.replace vars var v;
+      v
     | Set_binop_pre (op, l) ->
       let l =
         match aux l with
diff --git a/parser.ml b/parser.ml
index 40d1246..1492822 100644
--- a/parser.ml
+++ b/parser.ml
@@ -78,6 +78,14 @@ let token tok seq =
     if x = tok then x, seq
     else expected @@ Token.to_string tok
 
+let any_ident seq =
+  match seq () with
+  | Seq.Nil -> expected "ident"
+  | Seq.Cons (x, seq) -> begin match x with
+      | Token.Ident id -> id, seq
+      | _ -> unexpected_token x
+    end
+
 let idents set seq =
   match seq () with
   | Seq.Nil ->
@@ -122,6 +130,7 @@ let rec expr pre seq =
   seq |> oneof [
     level;
     assoc;
+    let_value;
     value @> binop pre;
   ]
 
@@ -151,6 +160,14 @@ and assoc seq =
   else
     failwith "Parser.assoc"
 
+(* let_value := "let" ident "=" expr *)
+and let_value seq =
+  let _, seq = ident "let" seq in
+  let id, seq = any_ident seq in
+  let _, seq = token Token.Equal seq in
+  let e, seq = expr min_int seq in
+  Let (id, e), seq
+
 (* value := int | float | ( expr ) *)
 and value seq =
   match seq () with
diff --git a/token.ml b/token.ml
index 1e83fc8..3647f3c 100644
--- a/token.ml
+++ b/token.ml
@@ -10,6 +10,7 @@ type t =
   | Percent
   | LParen
   | RParen
+  | Equal
 
 let tokens = ref [
   "+", Plus;
@@ -20,6 +21,7 @@ let tokens = ref [
   "%", Percent;
   "(", LParen;
   ")", RParen;
+  "=", Equal;
 ]
 
 let to_string = function