A taste of NOM (II)

Parser combinator revisit

Jacob Xie published on
7 min, 1400 words

Categories: Post

Tags: Rust


Hello everyone! It has been a while since I've written the last blog post, and today's topic is an extension of the previous NOM parser combinator. The main idea is to create a custom error type for NOM, and makes it wrapped by a higher level enum type who serves a lib crate. From the previous post, we've learned that each NOM parser or combinator is a generic function who accepts generic types of input I, output O and error type E. Generally, we use the given type IResult<I, O, E> as our function's return type, and which means to let the compiler to determine the error type E for us. While we go deeper into the error type E, we will see that it is actually an enum Err<E> who carries a generic type E, and furthermore, the variant Error(E), which denotes an recoverable error, should be the most common error type while we are coding. Briefly speaking, in my expectation, once we wrapped the Err<E> into custom error type and makes it to satisfy some requirements (implement several traits), we can use ? operation in our functions to handle the error.


Ok, let's see how the official example works with custom error type:

pub struct CustomError(String);

impl<'a> From<(&'a str, ErrorKind)> for CustomError {
  fn from(error: (&'a str, ErrorKind)) -> Self {
    CustomError(format!("error code was: {:?}", error))

impl<'a> ParseError<&'a str> for CustomError {
  fn from_error_kind(_: &'a str, kind: ErrorKind) -> Self {
    CustomError(format!("error code was: {:?}", kind))

  fn append(_: &'a str, kind: ErrorKind, other: CustomError) -> Self {
    CustomError(format!("{:?}\nerror code was: {:?}", other, kind))

Two implementations are required: the former one is a From trait which allows us to convert our custom error type from a tuple, whose first element is the input type I and the second element is an enum ErrorKind indicated which parser returned an error; the latter one is a ParseError trait which is required by the error type of a NOM parser. There are four methods in ParseError<I> trait:

pub trait ParseError<I>: Sized {
  /// Creates an error from the input position and an [ErrorKind]
  fn from_error_kind(input: I, kind: ErrorKind) -> Self;

  /// Combines an existing error with a new one created from the input
  /// position and an [ErrorKind]. This is useful when backtracking
  /// through a parse tree, accumulating error context on the way
  fn append(input: I, kind: ErrorKind, other: Self) -> Self;

  /// Creates an error from an input position and an expected character
  fn from_char(input: I, _: char) -> Self {
    Self::from_error_kind(input, ErrorKind::Char)

  /// Combines two existing errors. This function is used to compare errors
  /// generated in various branches of `alt`.
  fn or(self, other: Self) -> Self {

From above, we know that from_char and or has default implementations, and the rest two methods from_error_kind and append has Self as their return type. Apparently, the easiest way for user to design a custom error type is to convert and save the incoming parameters input: I and kind: ErrorKind as String, which shown above as pub struct CustomError(String).

Let's take a closer look on the following functions' signature to see the difference between default error type and custom error type:

fn test_with_default_error(input: &str) -> IResult<&str, &str> {

fn test_with_custom_error(input: &str) -> IResult<&str, &str, CustomError> {

With the custom error, we are explicitly telling the compiler that we want to use the custom error type CustomError instead of the default error type. The next question is how do we use it in our own Result type. First of all, we are going to create an enum type TasteNomError to handle variates errors, and don't forget a From trait for the further implicit conversion:

#[derive(Error, Debug)]
pub enum TasteNomError {

    #[error("sql error: {0}")]

    ParseInt(#[from] std::num::ParseIntError),

impl From<nom::Err<CustomError>> for TasteNomError {
    fn from(error: nom::Err<CustomError>) -> Self {

Here I imported a new dependency thiserror, which is a very useful tool when writing ones own lib crate. For more information, go visit thiserror.

thiserror = "1"

For a better demonstration, let's assume a real world problem is turning a Sql connection string "mysql://root:root@localhost:3306/test" into a struct SqlConnInfo:

#[derive(Debug, Clone)]
pub enum SqlBuilder {

impl FromStr for SqlBuilder {
    type Err = TasteNomError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        match s {
            "mysql" => Ok(SqlBuilder::Mysql),
            "postgres" => Ok(SqlBuilder::Postgres),
            "sqlite" => Ok(SqlBuilder::Sqlite),
            _ => Err(TasteNomError::Sql(format!("unknown database type: {}", s))),

pub struct SqlConnInfo {
    pub driver: SqlBuilder,
    pub username: String,
    pub password: String,
    pub host: String,
    pub port: u32,
    pub database: String,

impl SqlConnInfo {
    pub fn new(
        driver: SqlBuilder,
        username: &str,
        password: &str,
        host: &str,
        port: u32,
        database: &str,
    ) -> SqlConnInfo {
        SqlConnInfo {
            username: username.to_owned(),
            password: password.to_owned(),
            host: host.to_owned(),
            database: database.to_owned(),

This is quite a simple problem that we only need to split the string into serval parts like this patter "{driver}://{username}:{password}@{host}:{port}/{database}". Just simply use separated_pair method is enough:

fn take_driver_and_rest(input: &str) -> IResult<&str, (&str, &str)> {
    separated_pair(alpha1, tag("://"), rest)(input)

fn take_username_and_rest(input: &str) -> IResult<&str, (&str, &str)> {
    separated_pair(alphanumeric1, tag(":"), rest)(input)

fn take_password_and_rest(input: &str) -> IResult<&str, (&str, &str)> {
    separated_pair(alphanumeric1, tag("@"), rest)(input)

fn take_host_and_rest(input: &str) -> IResult<&str, (&str, &str)> {
    separated_pair(alpha1, tag(":"), rest)(input)

fn take_port_and_database(input: &str) -> IResult<&str, (&str, &str)> {
    separated_pair(digit1, tag("/"), rest)(input)

And their unit tests:

fn driver_and_rest() {
    let foo = take_driver_and_rest("mysql://root:root@localhost:3306/test");

fn username_and_rest() {
    let foo = take_username_and_rest("root:root@localhost:3306/test");

fn password_and_rest() {
    let foo = take_password_and_rest("root@localhost:3306/test");

fn host_and_rest() {
    let foo = take_host_and_rest("localhost:3306/test");

fn port_and_db() {
    let foo = take_port_and_database("3306/test");

Now, we can combine them together:

type ResultInfo1<'a> = (&'a str, (&'a str, (&'a str, (&'a str, (&'a str, &'a str)))));

fn get_conn_info1(value: &str) -> IResult<&str, ResultInfo1, CustomError> {
    let f_port_and_database = separated_pair(digit1, tag("/"), alphanumeric1);
    let f_host_and_rest = separated_pair(alpha1, tag(":"), f_port_and_database);
    let f_password_and_rest = separated_pair(alphanumeric1, tag("@"), f_host_and_rest);
    let f_username_and_rest = separated_pair(alphanumeric1, tag(":"), f_password_and_rest);
    let mut f_driver_and_rest = separated_pair(alpha1, tag("://"), f_username_and_rest);



type ResultInfo2<'a> = (&'a str, ((&'a str, &'a str), ((&'a str, &'a str), &'a str)));

fn get_conn_info2(value: &str) -> IResult<&str, ResultInfo2, CustomError> {
    let f_host_and_port = separated_pair(alpha1, tag(":"), digit1);
    let f_address_and_database = separated_pair(f_host_and_port, tag("/"), alphanumeric1);
    let f_username_and_password = separated_pair(alphanumeric1, tag(":"), alphanumeric1);
    let f_user_and_rest = separated_pair(f_username_and_password, tag("@"), f_address_and_database);
    let mut f_driver_and_rest = separated_pair(alpha1, tag("://"), f_user_and_rest);


The next thing is to use our own Result<T, TasteNomError> instead of IResult<&str, T, CustomError>. Before we heading further, let's do a little work to make sure that the extracted &str patterns can be converted to SqlConnInfo:

type ConnStrPattern<'a> = (
    &'a str,
    (&'a str, ((&'a str, &'a str), ((&'a str, &'a str), &'a str))),

impl<'a> TryFrom<ConnStrPattern<'a>> for SqlConnInfo {
    type Error = TasteNomError;

    fn try_from(source: ConnStrPattern<'a>) -> Result<Self, Self::Error> {
        let (_, (driver, ((username, password), ((host, port), database)))) = source;


Finally, here comes the final step:

type GeneralResult<T> = Result<T, TasteNomError>;

fn get_conn_info3(value: &str) -> GeneralResult<SqlConnInfo> {
    let f_host_and_port = separated_pair(take_until1(":"), tag(":"), digit1);
    let f_address_and_database = separated_pair(f_host_and_port, tag("/"), alphanumeric1);
    let f_username_and_password = separated_pair(alphanumeric1, tag(":"), alphanumeric1);
    let f_user_and_rest = separated_pair(f_username_and_password, tag("@"), f_address_and_database);
    let mut f_driver_and_rest = separated_pair(alpha1, tag("://"), f_user_and_rest);

    let res = f_driver_and_rest(value)?;


Thanks to what we did before: impl From<nom::Err<CustomError>> for TasteNomError, which let us to use ? operation to convert nom::Err<CustomError> to TasteNomError. after being through all the works, the line let res = f_driver_and_rest(value)?; finally can be compiled!

Don't forget the unit test:

fn conn_info() {
  const CONN1: &str = "mysql://root:root@localhost:3306/test";
  const CONN2: &str = "mysql://root:root@";

  let foo = get_conn_info3(CONN1);

  let foo = get_conn_info3(CONN2);

The full code is in my Github page custom_error.rs and database_conn.rs, and you are more than welcome to leave a message for me. That's all for today, until next time! 👋