# Returning errors from actions

Sometimes it can be useful to have action code that is able to return an error instead of being expected to produce a value of type T directly. This happens because we usually cannot reject all invalid input just by using grammar rules, or rather, the work to do so would be too much.

Even in our calculator example, you can see that we're "cheating" the system: Our grammar accepts an unlimited number of digits in the input, but the result is parsed as a i32. This is an issue because the maximum number that i32 can represent is 2147483647. Try giving it a bigger number and it will panic because it always expects the i32 conversion to succeed.

If you are familiar with Rust's error handling story, you might think that we can just make Num return an Option<i32> or even Result<i32, ...>, and you would be right. However, that is not necessary, because if we look at the type of ExprParser::parse(), we can see that it already returns a Result<i32, ParseError>. So the goal is to "hook" into this existing error machinery and create action code that can return errors.

LALRPOP supports this very easily by defining action code with =>? instead of =>. The returned value is then assumed to be a Result<T, ParseError> instead of a plain T:

Num: i32 = {
r"[0-9]+" =>? i32::from_str(<>)
.map_err(|_| ParseError::User {
error: "number is too big"
})
};


In addition, we have to add use lalrpop_util::ParseError; to the top of the file so that we have access to the ParseError type. You can find the full source as calculator6.lalrpop. This allows you to nicely handle the errors:


#![allow(unused)]
fn main() {
#[macro_use] extern crate lalrpop_util;

lalrpop_mod!(pub calculator6);

#[test]
fn calculator6() {
// Number is one bigger than std::i32::MAX
let expr = calculator6::ExprsParser::new().parse("2147483648");
assert!(expr.is_err());
}
}


No panics!

You can even go a step further and define your own error type, for example an enum with all possible errors. This allows you to distinguish between different errors more easily, without relying on strings.

For that, let's say we want to define two errors: One if the input number was too big, and another one if the input number was not even - we're changing the calculator to only accept even numbers for now.

We first define our error enum in main.rs:


#![allow(unused)]
fn main() {
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
pub enum Calculator6Error {
InputTooBig,
OddNumber,
}
}


Then we import it into our grammar and tell LALRPOP to use it as the user error type, so we change the top of the file to:

use std::str::FromStr;
use ast::{Expr, Opcode};

use super::Calculator6Error;

use lalrpop_util::ParseError;

grammar;

extern {
type Error = Calculator6Error;
}
...


We can also change the rule for Num to make use of our new error:

Num: i32 = {
r"[0-9]+" =>? i32::from_str(<>)
.map_err(|_| ParseError::User {
error: Calculator6Error::InputTooBig
})
.and_then(|i| if i % 2 == 0 {
Ok(i)
} else {
Err(ParseError::User {
error: Calculator6Error::OddNumber
})
})
};


And finally we can see if it works:


#![allow(unused)]
fn main() {
#[macro_use] extern crate lalrpop_util;

lalrpop_mod!(pub calculator6b);

#[test]
fn calculator6b() {
use lalrpop_util::ParseError;

let expr = calculator6b::ExprsParser::new().parse("2147483648");
assert!(expr.is_err());
assert_eq!(expr.unwrap_err(), ParseError::User { error: Calculator6Error::InputTooBig });

let expr = calculator6b::ExprsParser::new().parse("3");
assert!(expr.is_err());
assert_eq!(expr.unwrap_err(), ParseError::User { error: Calculator6Error::OddNumber });
}
}


There we go! You can find the full grammar in calculator6b.lalrpop.