desk_calc.peg

%{
let printf = Printf.printf
let vars = Array.make 26 0
%}

stmt    : - e=Expr EOL                  { printf "%d\n" e }
        | ( !EOL . )* EOL               { printf "error\n" }
;
Expr    : i=ID ASSIGN s=Sum             { vars.(i) <- s; s }
        | s=Sum                         { s }
;
Sum     : l=Product
             rs=( PLUS  r=Product       { fun v -> v + r }
                | MINUS r=Product       { fun v -> v - r }
                )*                      { List.fold_right (fun f v -> f v) rs l }
;
Product : l=Value
             rs=( TIMES  r=Value        { fun v -> v * r }
                | DIVIDE r=Value        { fun v -> v / r }
                )*                      { List.fold_right (fun f v -> f v) rs l }
;
Value   : i=NUMBER                      { i }
        | i=ID !ASSIGN                  { vars.(i) }
        | OPEN i=Expr CLOSE             { i }
;
NUMBER  : < [0-9]+ >    -               { int_of_string yytext };
ID      : < [a-z]  >    -               { Char.(code yytext.[0] - code 'a') };
ASSIGN  : '='           - ;
PLUS    : '+'           - ;
MINUS   : '-'           - ;
TIMES   : '*'           - ;
DIVIDE  : '/'           - ;
OPEN    : '('           - ;
CLOSE   : ')'           - ;

-       : [ \t]* ;
EOL     : '\n' | '\r\n' | '\r' | ';' | !. {""};

%%

let () =
  let rec loop () =
    let input = input_line stdin in
    let result = stmt (string_view_of_string input) in
    match result with
    | Ok _ -> flush stdout; loop ()
    | Error msg -> Printf.eprintf "Error: %s\n" msg
  in loop ()