Rust learning: lessons, notes, and exercise crates

lessons/0006-errors-spec.rs

@@ -0,0 +1,104 @@
+// The 0006 specification, as executable tests. Do not edit this file — make it pass.
+// Copy to: tasks/tests/errors.rs
+// Run with: cargo test
+
+use std::error::Error;
+use tasks::command::Command;
+use tasks::error::TaskError;
+use tasks::store::Store;
+use tasks::task::Priority;
+
+fn args(list: &[&str]) -> Vec<String> {
+    list.iter().map(|s| s.to_string()).collect()
+}
+
+// ---------- one variant per way of failing ----------
+
+#[test]
+fn parse_errors_name_the_exact_failure() {
+    assert_eq!(Command::parse(&args(&[])).unwrap_err(), TaskError::NoCommand);
+    assert_eq!(
+        Command::parse(&args(&["fly"])).unwrap_err(),
+        TaskError::UnknownCommand("fly".to_string())
+    );
+    assert_eq!(
+        Command::parse(&args(&["add"])).unwrap_err(),
+        TaskError::MissingTitle
+    );
+    assert_eq!(
+        Command::parse(&args(&["add", "x", "urgent"])).unwrap_err(),
+        TaskError::BadPriority("urgent".to_string())
+    );
+    assert_eq!(
+        Command::parse(&args(&["done"])).unwrap_err(),
+        TaskError::MissingId
+    );
+}
+
+#[test]
+fn store_reports_which_id_was_missing() {
+    let mut store = Store::new();
+    store.add("only task", Priority::Low);
+    assert_eq!(store.complete(99).unwrap_err(), TaskError::NotFound(99));
+    assert_eq!(store.remove(7).unwrap_err(), TaskError::NotFound(7));
+}
+
+// ---------- Display: the sentence the user reads ----------
+
+#[test]
+fn messages_are_lowercase_and_unpunctuated() {
+    assert_eq!(TaskError::NoCommand.to_string(), "no command given");
+    assert_eq!(
+        TaskError::UnknownCommand("fly".to_string()).to_string(),
+        "unknown command: fly"
+    );
+    assert_eq!(TaskError::MissingTitle.to_string(), "add needs a title");
+    assert_eq!(
+        TaskError::MissingId.to_string(),
+        "this command needs a task id"
+    );
+    assert_eq!(
+        TaskError::BadPriority("urgent".to_string()).to_string(),
+        "unknown priority: urgent"
+    );
+    assert_eq!(TaskError::NotFound(9).to_string(), "no task with id 9");
+}
+
+// ---------- From + ? : the parse error is wrapped, not thrown away ----------
+
+#[test]
+fn a_bad_id_wraps_the_parse_error() {
+    let err = Command::parse(&args(&["done", "abc"])).unwrap_err();
+    assert!(matches!(err, TaskError::BadId(_)), "expected BadId");
+    assert!(
+        Command::parse(&args(&["remove", "-1"])).is_err(),
+        "a negative id is not a u32"
+    );
+}
+
+#[test]
+fn a_wrapped_error_is_reported_once_and_reachable() {
+    let err = Command::parse(&args(&["done", "abc"])).unwrap_err();
+    // Display says your sentence and does not repeat std's.
+    assert_eq!(err.to_string(), "task id must be a number");
+    // std's sentence is still reachable, through Error::source().
+    let inner = err.source().expect("BadId must expose its source");
+    assert_eq!(inner.to_string(), "invalid digit found in string");
+}
+
+#[test]
+fn errors_that_carry_nothing_have_no_source() {
+    assert!(TaskError::NoCommand.source().is_none());
+    assert!(TaskError::NotFound(1).source().is_none());
+}
+
+// ---------- the Error trait, and the bounds the ecosystem expects ----------
+
+#[test]
+fn task_error_is_a_real_error() {
+    fn assert_usable_as_error<E: Error + Send + Sync + 'static>() {}
+    assert_usable_as_error::<TaskError>();
+
+    let boxed: Box<dyn Error> = Box::new(TaskError::NotFound(1));
+    assert_eq!(boxed.to_string(), "no task with id 1");
+}