Toy language

{
  open Printf

  let create_hashtable size init =
    let tbl = Hashtbl.create size in
    List.iter (fun (key, data) -> Hashtbl.add tbl key data) init;
    tbl

  type token =
    | IF
    | THEN
    | ELSE
    | BEGIN
    | END
    | FUNCTION
    | ID of string
    | OP of char
    | INT of int
    | FLOAT of float
    | CHAR of char

  let keyword_table = 
    create_hashtable 8 [
      ("if", IF);
      ("then", THEN);
      ("else", ELSE);
      ("begin", BEGIN);
      ("end", END);
      ("function", FUNCTION)
    ]

}

let digit = ['0'-'9']
let id = ['a'-'z' 'A'-'Z']['a'-'z' '0'-'9']*

rule toy_lang = parse
  | digit+ as inum
  	{ let num = int_of_string inum in
	  printf "integer: %s (%d)\n" inum num;
	  INT num
	}
  | digit+ '.' digit* as fnum
  	{ let num = float_of_string fnum in
	  printf "float: %s (%f)\n" fnum num;
	  FLOAT num
	}
  | id as word
  	{ try
	    let token = Hashtbl.find keyword_table word in
	    printf "keyword: %s\n" word;
	    token
	  with Not_found ->
	    printf "identifier: %s\n" word;
	    ID word
	}
  | '+'
  | '-'
  | '*'
  | '/' as op
  	{ printf "operator: %c\n" op;
	  OP op
	}
  | '{' [^ '\n']* '}'	(* eat up one-line comments *)
  | [' ' '\t' '\n']	(* eat up whitespace *)
  	{ toy_lang lexbuf }
  | _ as c
  	{ printf "Unrecognized character: %c\n" c;
	  CHAR c
	}
  | eof
  	{ raise End_of_file }

{
  let rec parse lexbuf =
     let token = toy_lang lexbuf in
     (* do nothing in this example *)
     parse lexbuf

  let main () =
    let cin =
      if Array.length Sys.argv > 1
      then open_in Sys.argv.(1)
      else stdin
    in
    let lexbuf = Lexing.from_channel cin in
    try parse lexbuf
    with End_of_file -> ()

  let _ = Printexc.print main ()
}