Creating Custom Errors

In the counter program, you used errors provided by solana_program's ProgramError enum that provides comprehensive but somewhat generic errors. These errors may work for some programs; however, some programs, for example, this voting program, may require more specific errors.

For example, you cannot use ProgramError::InvalidInstructionData to handle scenarios where the voting window is closed or when the client passes less than 2 poll options, you need more descriptive errors.

To create custom errors, define a program-specific error enum and convert it to ProgramError so you can return precise failures from your instructions.

Like so:

use solana_program::program_error::ProgramError;
use thiserror::Error;

#[derive(Debug, Error)]
pub enum PollError {
    #[error("You need at least 2 poll options")]
    NotEnoughPollOptions,

    #[error("End Slot cannot be less than start slot")]
    InvalidSlotTime,

    #[error("Voting window for this poll has closed")]
    VotingWindowClosed,

    #[error("Voting window for this poll is still open")]
    VotingWindowStillOpen,

    #[error("This poll has been closed")]
    PollAlreadyClosed,

    #[error("Input data exceeds max length")]
    InvalidDataLength,
}

impl From<PollError> for ProgramError {
    fn from(value: PollError) -> Self {
        ProgramError::Custom(value as u32)
    }
}

thiserror::Error gives each variant a readable message for logs and tests. Now you can use this enum as you’d ProgramError.