And more stream-2025-04-16.md.

Author: dkorolev

stream-2025-04-16.md

@@ -159,3 +159,65 @@ fn main() {
     o = None;
 }
 ```
+
+Matching and error handling: [link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=9a97bfd6781f5f31a0225da44df65814).
+
+```rust
+use std::error::Error;
+use std::io::{self, Write};  // Note: nontrivial import of `Write` for `flush` to work!
+use std::fmt::{Display, Formatter};
+
+#[derive(Debug)]
+struct FooError;
+
+impl Display for FooError {
+    fn fmt(&self, ostream: &mut Formatter<'_>) -> Result<(), std::fmt::Error> {
+        write!(ostream, "triggered the FooError")
+    }
+}
+    
+impl Error for FooError {}
+
+// Note: no need in `Debug` since `match` analyzes every option, no generic `{:?}` printing.
+// #[derive(Debug)]
+enum HalfResult {
+    Halved(i32),
+    Negative
+}
+
+// Note to C++ folks: `Box<dyn Error>` is literally `std::unique_ptr<ErrorBaseClass>` =)
+// Since there's more than one error type to handle.
+fn half(s: &str) -> Result<Option<HalfResult>, Box<dyn Error>> {
+    if s == "foo" {
+        Err(Box::new(FooError))
+    } else {
+        // Question mark to early-return if parse failed.
+        let x = s.parse::<i32>()?;
+        println!("Parsed as i32({x}).");
+        if x < 0 {
+            Ok(Some(HalfResult::Negative))
+        } else if x % 2 != 0 {
+            Ok(None)
+        } else {
+            Ok(Some(HalfResult::Halved(x / 2)))
+        }
+    }
+}
+
+fn main() -> Result<(), Box<dyn Error>> {
+    print!("Enter a number: ");
+    io::stdout().flush()?;
+    let mut input = String::new();
+    io::stdin().read_line(&mut input)?;
+    println!("");
+
+    match half(input.trim()) {
+        Ok(Some(HalfResult::Halved(v))) => println!("OK, halved: {v}."),
+        Ok(Some(HalfResult::Negative)) => println!("Was negative."),
+        Ok(None) => println!("Gotta be odd."),
+        Err(e) => println!("Error: {e}.")
+    }
+
+    Ok(())
+}
+```