assignStmt = do
   s <- identifier
   symbol "="
   e <- expr
   symbol ";"
   return $ Assign s e

ifStmt = do
   keyword "if"
   cond <- (expr)
   s1 <- stmt
   keyword "else"
   s2 <- stmt
   return $ If cond s1 s2

whileStmt = do
   keyword "while"
   cond <- (expr)
   s <- stmt
   return $ While cond s

blockStmt = do
   symbol "{"
   stmts <- many stmt
   symbol "}"
   return $ Block stmts