compile.ml

open Sexplib.Sexp
module Sexp = Sexplib.Sexp

(*
expr := <number>
     |  (<op> <expr>)
op   := inc | dec
*)


type op =
  | Inc
  | Dec

type expr =
  | ENum of int
  | EOp of op * expr

let rec sexp_to_expr (se : Sexp.t) : expr =
  match se with
    | Atom(s) -> ENum(int_of_string s)
    | List(sexps) ->
      match sexps with
        | [Atom("inc"); arg] -> EOp(Inc, sexp_to_expr arg)
        | [Atom("dec"); arg] -> EOp(Dec, sexp_to_expr arg)
        | _ -> failwith "Parse error"

let parse (s : string) : expr =
  sexp_to_expr (Sexp.of_string s)

open Printf

  let rec expr_to_instrs (e : expr) : string list =
    match e with
      | ENum(i) -> [sprintf "mov rax, %d" i]
      | EOp(op, e) ->
        let arg_exprs = expr_to_instrs e in
        match op with
          | Inc -> arg_exprs @ ["add rax, 1"]
          | Dec -> arg_exprs @ ["sub rax, 1"]

(* Compiles a source program string to an x86 string *)
let compile (program : string) : string =
  let ast = parse program in
  let instrs = expr_to_instrs ast in 
  let instrs_str = (String.concat "\n" instrs) in
  sprintf "
section .text
global our_code_starts_here
our_code_starts_here:
  %s
  ret\n" instrs_str;;

let () =
  let input_file = (open_in (Sys.argv.(1))) in
  let input_program = (input_line input_file) in
  let program = (compile input_program) in
  printf "%s\n" program;;