Closure and format!() syntax

1

Hi,

An interesting example:

    let sp = SpinnerBuilder::new("Long Running op!".into()).
        format(|sp, status|{
            format!("{spin} -- Currently working on: \'{status}\' -- {spin}",
                    spin = sp, status = status)
        }).start();

I can understand none of it :slight_smile: . What are names inside brackets? Rust has no string comprehension. What are format!("...", spin = sp, status = status) arguments, and what is = in this context?. I sort of guess why there are three pairs of braces, and only two arguments, yet can't actually understand it.

What is it all about?

2

Everything is documented in the official docs of the fmt module.

They are named placeholders which will be formatted. They correspond to the left-hand side of the assignment-looking parameters inside the format!(…) invocation.

That's just how the syntax of the format!() macro is defined. It accepts key-value pairs, so the key = value syntax is used to specify what values should be inserted wherever the corresponding placeholder occurs in the format string.

There are three uses of two arguments. You can use a single argument (named or not) more than once in a format string.

3

Rustc 1.58.0, due tomorrow, will https://github.com/rust-lang/rust/pull/90473/ . So with that, the example could even be written at:

let sp = SpinnerBuilder::new("Long Running op!".into())
    .format(|spin, status| format!("{spin} -- Currently working on: \'{status}\' -- {spin}"))
    .start();

Does this mean rust has string comprehension as of tomorrow? Maybe not, it's just macro processing, but close enough for me. :sunglasses:

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4

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