(* File Expr/Parse.fs *)
(* Lexing and parsing of simple expressions using fslex and fsyacc *)

module Parse

open System
open System.IO
open System.Text
open Microsoft.FSharp.Text.Lexing
open Absyn

type sinstr =
  | SCstI of int                        (* push integer           *)
  | SVar of int                         (* push variable from env *)
  | SAdd                                (* pop args, push sum     *)
  | SSub                                (* pop args, push diff.   *)
  | SMul                                (* pop args, push product *)
  | SPop                                (* pop value/unbind var   *)
  | SSwap                               (* exchange top and next  *)

(* Plain parsing from a string, with poor error reporting *)

let fromString (str : string) : expr =
    let lexbuf = Lexing.LexBuffer<char>.FromString(str)
    try
      ExprPar.Main ExprLex.Token lexbuf
    with
      | exn -> let pos = lexbuf.EndPos
               failwithf "%s near line %d, column %d\n"
                  (exn.Message) (pos.Line+1) pos.Column

(* Parsing from a text file *)

let fromFile (filename : string) =
    use reader = new StreamReader(filename)
    let lexbuf = Lexing.LexBuffer<char>.FromTextReader reader
    try
      ExprPar.Main ExprLex.Token lexbuf
    with
      | exn -> let pos = lexbuf.EndPos
               failwithf "%s in file %s near line %d, column %d\n"
                  (exn.Message) filename (pos.Line+1) pos.Column


let compString s : list<sinstr> =
    let rec r e (a:list<sinstr>) (x:int) =
        match e with
        | CstI(i) -> SCstI(i)::a
        | Var(s) -> SVar(x)::a
        | Let(s,e1,e2) -> (r e1 a 0)@(r e2 a 0)@SSwap::SPop::a
        | Prim("+",e1,e2) -> (r e1 a x)@(r e2 a (x+1))@SAdd::a
        | Prim("-",e1,e2) -> (r e1 a x)@(r e2 a (x+1))@SSub::a
        | Prim("*",e1,e2) -> (r e1 a x)@(r e2 a (x+1))@SMul::a
    r (fromString s) [] 0