Add code

example_9/.gitignore

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+/target

example_9/Cargo.toml

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+[package]
+name = "example_9"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+anyhow = "1.0.86"

example_9/src/main.rs

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+use anyhow::{Context, Result};
+use std::num::NonZero;
+
+enum CustomOption<T> {
+    Some(T),
+    None,
+}
+enum CustomResult<T, E> {
+    Ok(T),
+    Err(E),
+}
+
+struct CustomVec {}
+
+impl CustomVec {
+    fn new() -> Self {
+        Self {}
+    }
+}
+
+pub trait Findable<T> {
+    fn find(&self, fun: T) -> Option<u32>;
+}
+
+impl<F> Findable<F> for CustomVec
+where
+    F: Fn(i32) -> bool,
+{
+    fn find(&self, fun: F) -> Option<u32> {
+        todo!()
+    }
+}
+fn do_stuff() -> Result<NonZero<i32>> {
+    let my_vec = CustomVec::new();
+
+    // v is Option<&T>, not &T -- cannot use without checking for None
+    let v = my_vec.find(|t| t >= 42);
+
+    // n is Result<i32, ParseIntError> -- cannot use without checking for Err
+    let n = "42".parse::<u32>();
+
+    // ? suffix is "return Err if Err, otherwise unwrap Ok"
+    let n = "42".parse::<u32>()?;
+
+    let n = NonZero::new(43).context("oopie")?;
+    Ok(n)
+}
+
+fn main() {
+    do_stuff();
+}