My rust macro doesn't want to accept this let statement

Question

I've been trying to make a macro that functions similar to Python's input function.

Rather than write the stdin completely every time I wanted to automate it somewhat, and combine println! so I could kill 2 birds with one stone.

Essenitally, if someone passes in an argument it'll print a string then ask for input, if they don't it will just ask for input from the terminal.

#[macro_export]
macro_rules! input {
    ($a:expr) => {
        println!("{}", $a);
        let mut input = String::new();
        std::io::stdin().read_line(&mut input).unwrap();

        return $input
    };

    (_) => {
        let mut input = String::new();
        std::io::stdin().read_line(&mut $input).unwrap();

        return $input
    };
}

I keep getting an error on the let statement and just don't know how to continue because I don't know the macro syntax well.

I posted the whole code block beacuse on the second match expression I was trying to make a match for when there were no arguments but i'm not sure if I did it correctly.

Sometimes the error messages brought me to github pages and I encounter randomed bugs, so i'm just confused how to continue furhter

It would be most appreciated if someone could help me fix the let statement, and i'd like to apologize for any inconvience.

What is `return $input` intending? Your variable is called `input`. — tadman, Jan 28 '23 at 21:47
macro expansion ignores token `let` and any following the usage of `input!` is likely invalid in expression context — Thetrue kingofwaffles, Jan 28 '23 at 21:47
I feel like the `return` is wrong; a macro isn't a function so it's not something you return from. Instead, it evaluates to a block of code — cadolphs, Jan 28 '23 at 21:47
`$input` isn't a thing. `input` is the variable you captured that value into, and presumably should `return`. Get rid of all `$input` and replace with `input`. — tadman, Jan 28 '23 at 21:48
@tadman I got rid of all the $ for input and removed the return's but the error text is the same — Thetrue kingofwaffles, Jan 28 '23 at 21:52

score 3 · Accepted Answer · edited Jan 30 '23 at 13:32

3

#[macro_export]
macro_rules! input {
    ($a:expr) => {{
        println!("{}", $a);
        let mut input = String::new();
        std::io::stdin().read_line(&mut input).unwrap();
        input
    }};

    (_) => {{
        let mut input = String::new();
        std::io::stdin().read_line(&mut $input).unwrap();

        input
    }};
}

fn main() {
    let foo = input!("Blargh");
}

You need the extra curly braces. The first pair is to enclose what the macro should expand to. The second pair is because the macro should expand to a code block.

Then we get rid of return because a macro isn't a function. Instead, it just expands to some code, and the value of a code block is just the last expression (without a ;).

edited Jan 30 '23 at 13:32

cafce25

15,907
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answered Jan 28 '23 at 21:53

cadolphs

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Excellent work. – tadman Jan 28 '23 at 21:54
Thank you, I was really confused the moment I made contact with the macro syntax and you were really helpful @cadolphs, thank you. I'll give you the checkmark when Stack Overflow lets me. – Thetrue kingofwaffles Jan 28 '23 at 21:57
@cadolphs After seeing how it actually works, I think i'ma just use the regular print command honestly, maybe flush at the end if I can figure it out, thank you though. – Thetrue kingofwaffles Jan 28 '23 at 22:00
Maybe what you want _is_ just a regular function? – cadolphs Jan 29 '23 at 01:00

My rust macro doesn't want to accept this let statement

1 Answers1