improve: error logging

Signed-off-by: kjuulh <contact@kjuulh.io>

crates/mad/examples/nested_errors/main.rs

@@ -0,0 +1,86 @@
+use async_trait::async_trait;
+use tokio_util::sync::CancellationToken;
+
+struct NestedErrorComponent {
+    name: String,
+}
+
+#[async_trait]
+impl notmad::Component for NestedErrorComponent {
+    fn name(&self) -> Option<String> {
+        Some(self.name.clone())
+    }
+
+    async fn run(&self, _cancellation: CancellationToken) -> Result<(), notmad::MadError> {
+        // Simulate a deeply nested error
+        let io_error = std::io::Error::new(
+            std::io::ErrorKind::PermissionDenied,
+            "access denied to /etc/secret",
+        );
+
+        Err(anyhow::Error::from(io_error)
+            .context("failed to read configuration file")
+            .context("unable to initialize database connection pool")
+            .context(format!("component '{}' startup failed", self.name))
+            .into())
+    }
+}
+
+struct AnotherFailingComponent;
+
+#[async_trait]
+impl notmad::Component for AnotherFailingComponent {
+    fn name(&self) -> Option<String> {
+        Some("another-component".into())
+    }
+
+    async fn run(&self, _cancellation: CancellationToken) -> Result<(), notmad::MadError> {
+        tokio::time::sleep(std::time::Duration::from_millis(100)).await;
+
+        Err(anyhow::anyhow!("network timeout after 30s")
+            .context("failed to connect to external API")
+            .context("service health check failed")
+            .into())
+    }
+}
+
+#[tokio::main]
+async fn main() {
+    tracing_subscriber::fmt()
+        .with_env_filter("mad=debug")
+        .init();
+
+    let result = notmad::Mad::builder()
+        .add(NestedErrorComponent {
+            name: "database-service".into(),
+        })
+        .add(AnotherFailingComponent)
+        .run()
+        .await;
+
+    match result {
+        Ok(()) => println!("Success!"),
+        Err(e) => {
+            eprintln!("\n=== Error occurred ===");
+            eprintln!("{}", e);
+
+            // Also demonstrate how to walk the error chain manually
+            if let notmad::MadError::AggregateError(ref agg) = e {
+                eprintln!("\n=== Detailed error chains ===");
+                for (i, error) in agg.get_errors().iter().enumerate() {
+                    eprintln!("\nComponent {} error chain:", i + 1);
+                    if let notmad::MadError::Inner(inner) = error {
+                        for (j, cause) in inner.chain().enumerate() {
+                            eprintln!("  {}. {}", j + 1, cause);
+                        }
+                    }
+                }
+            } else if let notmad::MadError::Inner(ref inner) = e {
+                eprintln!("\n=== Error chain ===");
+                for (i, cause) in inner.chain().enumerate() {
+                    eprintln!("  {}. {}", i + 1, cause);
+                }
+            }
+        }
+    }
+}