revert statement

^Syntax
RevertStatement :
   revert Expression^?

The revert statement is denoted with the keyword revert. Evaluating a revert statement will cause to revert all state changes made by the call and return with an revert error to the caller. A revert statement may be followed by an expression that evaluates to a struct in which case the struct is encoded as revert data as defined by EIP-838.

An example of a revert statement without revert data:

contract Foo {
    fn transfer(self, to: address, value: u256) {
        if not self.in_whitelist(addr: to) {
            revert
        }
        // more logic here
    }

    fn in_whitelist(self, addr: address) -> bool {
        return false
    }
}

An example of a revert statement with revert data:

struct ApplicationError {
    pub code: u8
}

contract Foo {
    pub fn transfer(self, to: address, value: u256) {
        if not self.in_whitelist(addr: to) {
            revert ApplicationError(code: 5)
        }
        // more logic here
    }

    fn in_whitelist(self, addr: address) -> bool {
        return false
    }
}