Rust transpose method

use std::str::FromStr;
use anyhow::Context;

fn add(a: Option<&str>, b: Option<&str>) -> anyhow::Result<f32> {
    let a = a.map(f32::from_str).transpose()?.context("not a value")?;
    let b = b.map(f32::from_str).transpose()?.context("not a value")?;
    Ok(a + b)
}

fn main() {
    println!("{:?}", add(Some("12"), Some("1.2")));
    println!("{:?}", add(Some("a"), Some("1.2")));
    println!("{:?}", add(None, Some("1.2")));
}

Ok(13.2)
Err(invalid float literal)
Err(not a value)