// since typechecking is usually syntax-directed, we can implement a typechecker as a recognizer

ometa LittleTypechecker {
  typecheck = term:t spaces end                          -> t,
  term      = "0"                                        -> "Int"
            | "1"                                        -> "Int"
            | "true"                                     -> "Bool"
            | "false"                                    -> "Bool"
            | "isZero" term:t
                ?(t == "Int")                            -> "Bool"
            | "if" term:t1 "then" term:t2 "else" term:t3
                ?(t1 == "Bool")
                ?(t2 == t3)                              -> t2
}

program = "
           if isZero 1
           then if true
                then 0
                else 1
           else 1
          "
LittleTypechecker.matchAll(program, "typecheck")