+++ /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