--- /dev/null
+(* evaluation2.ml: evaluation-based interpreter *)
+
+type literal = Num of int | Bool of bool (* intersection of values and Constant terms *)
+type primFunction = Succ | Pred | IsZero | Leq (* | Leq_partially_applied of int *)
+
+(* these have to be declared later, inside the Env modules ...
+type value = LiteralV of literal | Closure of lambdaTerm * env
+type bound_value = value (* for now, "x" is bound to the same type of thing that Var "x" expresses, but in later stages that won't always be true *)
+*)
+
+type constant = LiteralC of literal | FunctC of primFunction
+type identifier = string
+
+type lambdaTerm = Constant of constant | Var of identifier | Abstract of identifier * lambdaTerm | App of lambdaTerm * lambdaTerm | IfThenElse of lambdaTerm * lambdaTerm * lambdaTerm | Let of identifier * lambdaTerm * lambdaTerm
+
+module type Env = sig
+ type env
+ type value = LiteralV of literal | Closure of lambdaTerm * env
+ type bound_value = value
+ val empty: env
+ val push: identifier -> bound_value -> env -> env
+ val lookup: identifier -> env -> bound_value option
+end
+
+module Env1: Env = struct
+ type env = (identifier * bound_value) list
+ and value = LiteralV of literal | Closure of lambdaTerm * env
+ and bound_value = value
+ let empty = []
+ let push ident value env = (ident,value)::env
+ let rec lookup ident' env = match env with
+ | [] -> None
+ | (ident,value)::_ when ident = ident' -> Some value
+ | _::env' -> lookup ident' env'
+end
+
+module Env2: Env = struct
+ type env = identifier -> bound_value option
+ and value = LiteralV of literal | Closure of lambdaTerm * env
+ and bound_value = value
+ let empty = fun _ -> None
+ let push ident value env = fun ident' -> if ident = ident' then Some value else env ident'
+ let lookup ident' env = env ident'
+end
+
+open Env1
+
+exception Stuck of lambdaTerm
+
+let rec eval (term:lambdaTerm) (r:env) : value =
+ match term with
+ | IfThenElse(test, yes, no) -> (match eval test r with
+ | LiteralV(Bool true) -> eval yes r
+ | LiteralV(Bool false) -> eval no r
+ | LiteralV lit -> raise (Stuck (IfThenElse(Constant(LiteralC lit),yes,no)))
+ | Closure(term, _) -> raise (Stuck (IfThenElse(term,yes,no))))
+ | Let(bound_ident, arg, body) -> (match eval arg r with
+ | value -> eval body (push bound_ident value r))
+ | App(head, arg) -> (match eval head r with
+ | LiteralV lit -> raise (Stuck (App(Constant(LiteralC lit), arg)))
+ | Closure (Abstract(bound_ident, body), saved_r) -> eval body (push bound_ident arg saved_r) (* FIX ME *)
+ | Closure (Constant (FunctC Leq), saved_r) -> failwith "not yet implemented"
+ | Closure (Constant (FunctC (_ as prim)), saved_r) ->
+ (match (prim, eval arg r) with
+ | (Succ, LiteralV(Num n)) -> LiteralV(Num (n+1))
+ | (Pred, LiteralV(Num n)) -> LiteralV(Num (if n = 0 then 0 else n-1))
+ | (IsZero, LiteralV(Num n)) -> LiteralV(Bool (n=0))
+ | (_, LiteralV lit) -> raise (Stuck (App(Constant(FunctC prim), Constant(LiteralC lit))))
+ | (_, Closure(term, _)) -> raise (Stuck (App(Constant(FunctC prim), term))))
+ | Closure (term, _) -> raise (Stuck (App(term, arg))))
+ | Var ident -> (match lookup ident r with
+ | Some v -> v
+ | None -> raise (Stuck term)) (* free variables are stuck *)
+ | Constant (LiteralC lit) -> LiteralV lit
+ | Constant (FunctC _) -> Closure(term, empty) (* primFunctions evaluate as Closures with empty environments *)
+ | Abstract (_,_) -> Closure(term, r) (* Abstracts evaluate as Closures with the current environment; a more efficient implementation would save only that part of the environment that binds variables that are free in the Abstract *)
+
+let rec check_numbers (term:lambdaTerm) : unit =
+ match term with
+ | Constant(LiteralC(Num n)) when n < 0 -> failwith ("Bad Number: " ^ string_of_int n)
+ | Constant _ -> ()
+ | Var _ -> ()
+ | Abstract(_, body) -> check_numbers body
+ | App(head, arg) -> let () = check_numbers head
+ in check_numbers arg
+ | Let(_, arg, body) -> let () = check_numbers arg
+ in check_numbers body
+ | IfThenElse(test, yes, no) -> let () = check_numbers test
+ in let () = check_numbers yes
+ in check_numbers no
+
+let evaluate (term:lambdaTerm) : value =
+ (* scan to verify that term doesn't have any Const(Num (negative)) *)
+ let () = check_numbers term
+ (* evaluate starting with empty env *)
+ in eval term empty