How to define constants in Anchor

Question

Is there some special approach for defining constants in Anchor? I tried it normally just before the program ID:

pub const MY_CONST: &'static str = "some example str";

declare_id!("MY_PROGRAM_PUBKEY");

But then I try to verify if my constants were defined within some of my Anchor instructions:

msg! {"My const value: {:#?}", std::env::var("MY_CONST")};
msg! {"My const value: {:#?}", option_env!("MY_CONST")};

But I keep getting empty values, I guess it has something to do with the run time. Is there some "Anchor way" of defining constants or verify if they are actually defined?

Answer 1

I think you're mixing up constants and environment variables. You can use constants by just referring to them by name, they don't become environment variables.

For example:

use anchor_lang::prelude::*;

declare_id!("B5KGifZpdZqPtnC1qgkwjMZB2LBp9seXC5fJXPTN8x4W");

const THE_NUMBER: u8 = 42;

#[program]
mod hello_anchor {
    use super::*;
    pub fn initialize(ctx: Context<Initialize>) -> Result<()> {
        msg!("The number is: {}!", THE_NUMBER); // Message will show up in the tx logs
        Ok(())
    }
}

#[derive(Accounts)]
pub struct Initialize {
}

Transaction: https://explorer.solana.com/tx/2BxPY6pNU2d6C1hZfBdfvFvDqwxeB14m6LohwLCrZQ4eWnigqr3DUufiuuSyM7f3HavAothwBAZvtVQVAGfjA3Mw?cluster=devnet (notice "The number is: 42!" in the logs)

Example playground: https://beta.solpg.io/63f8a3cf67edfe0f001069d4