From 47ea4b27c60e271b7ed1058957c88eb0363ba5f7 Mon Sep 17 00:00:00 2001
From: Dmitry Boulytchev <dboulytchev@gmail.com>
Date: Wed, 14 Feb 2018 16:07:03 +0300
Subject: [PATCH] Restored the implementation

---
 src/Embedding.ml |  4 ++--
 src/Expr.ml      | 34 +++++++++++++++++++++++++++++-----
 2 files changed, 31 insertions(+), 7 deletions(-)

diff --git a/src/Embedding.ml b/src/Embedding.ml
index 5946027df..d52fbdf25 100644
--- a/src/Embedding.ml
+++ b/src/Embedding.ml
@@ -33,8 +33,8 @@ let y = !"y"
 let z = !"z"
 let t = !"t"
 
-(* Voila; comment this out before submitting the solution *)
+(* Voila; comment this out before submitting the solution 
 let _ =
   List.iter (fun e -> Printf.printf "eval s (%s) = %d\n" (show(expr) e) (eval s e)) [x+y*z- !?3; t-z+y && x]
-
+*)
                    
diff --git a/src/Expr.ml b/src/Expr.ml
index c694a2a11..7292c06f0 100644
--- a/src/Expr.ml
+++ b/src/Expr.ml
@@ -1,4 +1,3 @@
-!!!
 (* Simple expressions: syntax and semantics *)
 
 (* Opening a library for generic programming (https://github.com/dboulytchev/GT).
@@ -35,20 +34,45 @@ let update x v s = fun y -> if x = y then v else s y
 (* An example of a non-trivial state: *)                                                   
 let s = update "x" 1 @@ update "y" 2 @@ update "z" 3 @@ update "t" 4 empty
 
-(* Some testing; comment this definition out when submitting the solution. *)
+(* Some testing; comment this definition out when submitting the solution. 
 let _ =
   List.iter
     (fun x ->
        try  Printf.printf "%s=%d\n" x @@ s x
        with Failure s -> Printf.printf "%s\n" s
     ) ["x"; "a"; "y"; "z"; "t"; "b"]
-
+*)
+    
 (* Expression evaluator
 
      val eval : state -> expr -> int
  
    Takes a state and an expression, andreturns the value of the expression in 
    the given state.
-*)
-let eval = failwith "Not implemented yet"
+ *)
+let rec eval st expr =
+  let to_func op =
+    let bti   = function true -> 1 | _ -> 0 in
+    let itb b = b <> 0 in
+    let (|>) f g   = fun x y -> f (g x y) in
+    match op with
+    | "+"  -> (+)
+    | "-"  -> (-)
+    | "*"  -> ( * )
+    | "/"  -> (/)
+    | "%"  -> (mod)
+    | "<"  -> bti |> (< )
+    | "<=" -> bti |> (<=)
+    | ">"  -> bti |> (> )
+    | ">=" -> bti |> (>=)
+    | "==" -> bti |> (= )
+    | "!=" -> bti |> (<>)
+    | "&&" -> fun x y -> bti (itb x && itb y)
+    | "!!" -> fun x y -> bti (itb x || itb y)
+    | _    -> failwith (Printf.sprintf "Unknown binary operator %s" op)
+  in
+  match expr with
+  | Const n -> n
+  | Var   x -> st x
+  | Binop (op, x, y) -> to_func op (eval st x) (eval st y)