(* this file can be a "drop-in replacement" for interp.ml from hw1 *)
open Ast
(* all about _when_ things are computed, though hard to tell b/c
everything is functional in the metalanguage In particular, if we
float the fun h out, then our denotations are still deconstructing
abstract syntax -- basically a large-step semantics -- as we do it
here, our denotations make no use of abstract syntax, which is
cool! And look at if case -- we actually denote both sides! *)
(* See how our language without "functions" is denoted by OCaml functions
taking heaps *)
exception InterpUnimplemented
let rec denote_exp e =
match e with
Int i -> (fun h -> i)
| Var v -> (fun h -> h v)
| Plus(e1,e2) ->
let d1 = denote_exp e1 in
let d2 = denote_exp e2 in
(fun h -> (d1 h) + (d2 h))
| Times(e1,e2) ->
let d1 = denote_exp e1 in
let d2 = denote_exp e2 in
(fun h -> (d1 h) * (d2 h))
let rec denote_stmt s =
match s with
Skip -> (fun h -> h)
| Assign(v,e) ->
let d = denote_exp e in
(fun h ->
let c = d h in
fun x -> if x=v then c else h x)
| Seq(s1,s2) ->
let d1 = denote_stmt s1 in
let d2 = denote_stmt s2 in
(fun h -> d2 (d1 h)) (* i.e., d2 o d1 *)
| If(e,s1,s2) ->
let d1 = denote_exp e in
let d2 = denote_stmt s1 in
let d3 = denote_stmt s2 in
(fun h -> if (d1 h)==0 then (d2 h) else (d3 h))
| While(e,s) ->
let d1 = denote_exp e in
let d2 = denote_stmt s in
let rec f h = if (d1 h) > 0 then f (d2 h) else h in
f
| _ -> raise InterpUnimplemented
let denote_prog s =
let d = denote_stmt s in
fun () -> (d (fun x -> 0)) "ans"
let get_prog () =
let argv = Sys.argv in
let _ =
if Array.length argv != 2
then (prerr_string ("usage: " ^ argv.(0) ^ " [file-to-interpret]\n");
exit 1) in
let ch = open_in argv.(1) in
Parse.program Lex.lexer (Lexing.from_channel ch)
let _ =
let prog = get_prog () in
let translated_prog = denote_prog prog in
(* no further need for the AST module! *)
let result = translated_prog () in
print_string ("ans = " ^ string_of_int result ^ "\n")