Replace GUI exercise with Logger (#1682)

This should be a bit simpler, and notably
* does not require trait objects, which per #1516 should be moved later
in the course
 * does not require a lot of futzing with string formatting

But all that hard work developing the GUI exercise is not for naught: it
remains in the "Modules" segment, where students will get a chance to
read some Rust code and reorganize it a little bit.

Fixes #1617.

R=mgeisler as the original author of the GUI exercise.

Cargo.lock

@@ -1355,6 +1355,10 @@ dependencies = [
  "syn 2.0.48",
 ]
 
+[[package]]
+name = "modules"
+version = "0.1.0"
+
 [[package]]
 name = "native-tls"
 version = "0.2.11"

Cargo.toml

@@ -13,6 +13,7 @@ members = [
   "src/std-types",
   "src/std-traits",
   "src/iterators",
+  "src/modules",
   "src/testing",
   "src/memory-management",
   "src/smart-pointers",

src/SUMMARY.md

@@ -78,7 +78,7 @@
   - [Traits](methods-and-traits/traits.md)
   - [Deriving](methods-and-traits/deriving.md)
   - [Trait Objects](methods-and-traits/trait-objects.md)
-  - [Exercise: GUI Library](methods-and-traits/exercise.md)
+  - [Exercise: Generic Logger](methods-and-traits/exercise.md)
     - [Solution](methods-and-traits/solution.md)
 - [Generics](generics.md)
   - [Generic Functions](generics/generic-functions.md)
@@ -167,7 +167,7 @@
   - [Filesystem Hierarchy](modules/filesystem.md)
   - [Visibility](modules/visibility.md)
   - [`use`, `super`, `self`](modules/paths.md)
-  - [Exercise: Modules for the GUI Library](modules/exercise.md)
+  - [Exercise: Modules for a GUI Library](modules/exercise.md)
     - [Solution](modules/solution.md)
 - [Testing](testing.md)
   - [Test Modules](testing/unit-tests.md)

src/methods-and-traits/exercise.md

@@ -1,75 +1,26 @@
 ---
-minutes: 30
+minutes: 20
 ---
 
-# Exercise: GUI Library
+# Exercise: Generic Logger
 
-Let us design a classical GUI library using our new knowledge of traits and
-trait objects. We'll only implement the drawing of it (as text) for simplicity.
+Let's design a simple logging utility, using a trait `Logger` with a `log`
+method. Code which might log its progress can then take an `&impl Logger`. In
+testing, this might put messages in the test logfile, while in a production
+build it would send messages to a log server.
 
-We will have a number of widgets in our library:
+However, the `StderrLogger` given below logs all messages, regardless of
+verbosity. Your task is to write a `VerbosityFilter` type that will ignore
+messages above a maximum verbosity.
 
-- `Window`: has a `title` and contains other widgets.
-- `Button`: has a `label`. In reality, it would also take a callback function to
-  allow the program to do something when the button is clicked but we won't
-  include that since we're only drawing the GUI.
-- `Label`: has a `label`.
-
-The widgets will implement a `Widget` trait, see below.
-
-Copy the code below to <https://play.rust-lang.org/>, fill in the missing
-`draw_into` methods so that you implement the `Widget` trait:
+This is a common pattern: a struct wrapping a trait implementation and
+implementing that same trait, adding behavior in the process. What other kinds
+of wrappers might be useful in a logging utility?
 
 ```rust,compile_fail
-// TODO: remove this when you're done with your implementation.
-#![allow(unused_imports, unused_variables, dead_code)]
-
 {{#include exercise.rs:setup}}
 
-// TODO: Implement `Widget` for `Label`.
-
-// TODO: Implement `Widget` for `Button`.
-
-// TODO: Implement `Widget` for `Window`.
+// TODO: Define and implement `VerbosityFilter`.
 
 {{#include exercise.rs:main}}
 ```
-
-The output of the above program can be something simple like this:
-
-```text
-========
-Rust GUI Demo 1.23
-========
-
-This is a small text GUI demo.
-
-| Click me! |
-```
-
-If you want to draw aligned text, you can use the
-[fill/alignment](https://doc.rust-lang.org/std/fmt/index.html#fillalignment)
-formatting operators. In particular, notice how you can pad with different
-characters (here a `'/'`) and how you can control alignment:
-
-```rust,editable
-fn main() {
-    let width = 10;
-    println!("left aligned:  |{:/<width$}|", "foo");
-    println!("centered:      |{:/^width$}|", "foo");
-    println!("right aligned: |{:/>width$}|", "foo");
-}
-```
-
-Using such alignment tricks, you can for example produce output like this:
-
-```text
-+--------------------------------+
-|       Rust GUI Demo 1.23       |
-+================================+
-| This is a small text GUI demo. |
-| +-----------+                  |
-| | Click me! |                  |
-| +-----------+                  |
-+--------------------------------+
-```

src/methods-and-traits/exercise.rs

@@ -1,4 +1,4 @@
-// Copyright 2022 Google LLC
+// Copyright 2024 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -14,124 +14,44 @@
 
 // ANCHOR: solution
 // ANCHOR: setup
-pub trait Widget {
-    /// Natural width of `self`.
-    fn width(&self) -> usize;
+use std::fmt::Display;
 
-    /// Draw the widget into a buffer.
-    fn draw_into(&self, buffer: &mut dyn std::fmt::Write);
+pub trait Logger {
+    /// Log a message at the given verbosity level.
+    fn log(&self, verbosity: u8, message: impl Display);
+}
 
-    /// Draw the widget on standard output.
-    fn draw(&self) {
-        let mut buffer = String::new();
-        self.draw_into(&mut buffer);
-        println!("{buffer}");
+struct StderrLogger;
+
+impl Logger for StderrLogger {
+    fn log(&self, verbosity: u8, message: impl Display) {
+        eprintln!("verbosity={verbosity}: {message}");
     }
 }
 
-pub struct Label {
-    label: String,
-}
-
-impl Label {
-    fn new(label: &str) -> Label {
-        Label { label: label.to_owned() }
-    }
-}
-
-pub struct Button {
-    label: Label,
-}
-
-impl Button {
-    fn new(label: &str) -> Button {
-        Button { label: Label::new(label) }
-    }
-}
-
-pub struct Window {
-    title: String,
-    widgets: Vec<Box<dyn Widget>>,
-}
-
-impl Window {
-    fn new(title: &str) -> Window {
-        Window { title: title.to_owned(), widgets: Vec::new() }
-    }
-
-    fn add_widget(&mut self, widget: Box<dyn Widget>) {
-        self.widgets.push(widget);
-    }
-
-    fn inner_width(&self) -> usize {
-        std::cmp::max(
-            self.title.chars().count(),
-            self.widgets.iter().map(|w| w.width()).max().unwrap_or(0),
-        )
-    }
+fn do_things(logger: &impl Logger) {
+    logger.log(5, "FYI");
+    logger.log(2, "Uhoh");
 }
 // ANCHOR_END: setup
 
-impl Widget for Window {
-    fn width(&self) -> usize {
-        // Add 4 paddings for borders
-        self.inner_width() + 4
-    }
-
-    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
-        let mut inner = String::new();
-        for widget in &self.widgets {
-            widget.draw_into(&mut inner);
-        }
-
-        let inner_width = self.inner_width();
-
-        // TODO: after learning about error handling, you can change
-        // draw_into to return Result<(), std::fmt::Error>. Then use
-        // the ?-operator here instead of .unwrap().
-        writeln!(buffer, "+-{:-<inner_width$}-+", "").unwrap();
-        writeln!(buffer, "| {:^inner_width$} |", &self.title).unwrap();
-        writeln!(buffer, "+={:=<inner_width$}=+", "").unwrap();
-        for line in inner.lines() {
-            writeln!(buffer, "| {:inner_width$} |", line).unwrap();
-        }
-        writeln!(buffer, "+-{:-<inner_width$}-+", "").unwrap();
-    }
+/// Only log messages up to the given verbosity level.
+struct VerbosityFilter<L: Logger> {
+    max_verbosity: u8,
+    inner: L,
 }
 
-impl Widget for Button {
-    fn width(&self) -> usize {
-        self.label.width() + 8 // add a bit of padding
-    }
-
-    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
-        let width = self.width();
-        let mut label = String::new();
-        self.label.draw_into(&mut label);
-
-        writeln!(buffer, "+{:-<width$}+", "").unwrap();
-        for line in label.lines() {
-            writeln!(buffer, "|{:^width$}|", &line).unwrap();
+impl<L: Logger> Logger for VerbosityFilter<L> {
+    fn log(&self, verbosity: u8, message: impl Display) {
+        if verbosity <= self.max_verbosity {
+            self.inner.log(verbosity, message);
         }
-        writeln!(buffer, "+{:-<width$}+", "").unwrap();
-    }
-}
-
-impl Widget for Label {
-    fn width(&self) -> usize {
-        self.label.lines().map(|line| line.chars().count()).max().unwrap_or(0)
-    }
-
-    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
-        writeln!(buffer, "{}", &self.label).unwrap();
     }
 }
 
 // ANCHOR: main
 fn main() {
-    let mut window = Window::new("Rust GUI Demo 1.23");
-    window.add_widget(Box::new(Label::new("This is a small text GUI demo.")));
-    window.add_widget(Box::new(Button::new("Click me!")));
-    window.draw();
+    let l = VerbosityFilter { max_verbosity: 3, inner: StderrLogger };
+    do_things(&l);
 }
 // ANCHOR_END: main

src/modules/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "modules"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[[bin]]
+name = "modules"
+path = "exercise.rs"

src/modules/exercise.md

@@ -1,16 +1,15 @@
 ---
-minutes: 20
+minutes: 15
 ---
 
-# Exercise: Modules for the GUI Library
+# Exercise: Modules for a GUI Library
 
-In this exercise, you will reorganize the GUI Library exercise from the "Methods
-and Traits" segment of the course into a collection of modules. It is typical to
-put each type or set of closely-related types into its own module, so each
-widget type should get its own module.
+In this exercise, you will reorganize a small GUI Library implementation. This
+library defines a `Widget` trait and a few implementations of that trait, as
+well as a `main` function.
 
-If you no longer have your version, that's fine - refer back to the
-[provided solution](../methods-and-traits/solution.html).
+It is typical to put each type or set of closely-related types into its own
+module, so each widget type should get its own module.
 
 ## Cargo Setup
 
@@ -23,8 +22,16 @@ cd gui-modules
 cargo run
 ```
 
-Edit `src/main.rs` to add `mod` statements, and add additional files in the
-`src` directory.
+Edit the resulting `src/main.rs` to add `mod` statements, and add additional
+files in the `src` directory.
+
+## Source
+
+Here's the single-module implementation of the GUI library:
+
+```rust
+{{#include exercise.rs:single-module}}
+```
 
 <details>
 

src/modules/exercise.rs

@@ -0,0 +1,132 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// ANCHOR: single-module
+pub trait Widget {
+    /// Natural width of `self`.
+    fn width(&self) -> usize;
+
+    /// Draw the widget into a buffer.
+    fn draw_into(&self, buffer: &mut dyn std::fmt::Write);
+
+    /// Draw the widget on standard output.
+    fn draw(&self) {
+        let mut buffer = String::new();
+        self.draw_into(&mut buffer);
+        println!("{buffer}");
+    }
+}
+
+pub struct Label {
+    label: String,
+}
+
+impl Label {
+    fn new(label: &str) -> Label {
+        Label { label: label.to_owned() }
+    }
+}
+
+pub struct Button {
+    label: Label,
+}
+
+impl Button {
+    fn new(label: &str) -> Button {
+        Button { label: Label::new(label) }
+    }
+}
+
+pub struct Window {
+    title: String,
+    widgets: Vec<Box<dyn Widget>>,
+}
+
+impl Window {
+    fn new(title: &str) -> Window {
+        Window { title: title.to_owned(), widgets: Vec::new() }
+    }
+
+    fn add_widget(&mut self, widget: Box<dyn Widget>) {
+        self.widgets.push(widget);
+    }
+
+    fn inner_width(&self) -> usize {
+        std::cmp::max(
+            self.title.chars().count(),
+            self.widgets.iter().map(|w| w.width()).max().unwrap_or(0),
+        )
+    }
+}
+
+impl Widget for Window {
+    fn width(&self) -> usize {
+        // Add 4 paddings for borders
+        self.inner_width() + 4
+    }
+
+    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
+        let mut inner = String::new();
+        for widget in &self.widgets {
+            widget.draw_into(&mut inner);
+        }
+
+        let inner_width = self.inner_width();
+
+        // TODO: Change draw_into to return Result<(), std::fmt::Error>. Then use the
+        // ?-operator here instead of .unwrap().
+        writeln!(buffer, "+-{:-<inner_width$}-+", "").unwrap();
+        writeln!(buffer, "| {:^inner_width$} |", &self.title).unwrap();
+        writeln!(buffer, "+={:=<inner_width$}=+", "").unwrap();
+        for line in inner.lines() {
+            writeln!(buffer, "| {:inner_width$} |", line).unwrap();
+        }
+        writeln!(buffer, "+-{:-<inner_width$}-+", "").unwrap();
+    }
+}
+
+impl Widget for Button {
+    fn width(&self) -> usize {
+        self.label.width() + 8 // add a bit of padding
+    }
+
+    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
+        let width = self.width();
+        let mut label = String::new();
+        self.label.draw_into(&mut label);
+
+        writeln!(buffer, "+{:-<width$}+", "").unwrap();
+        for line in label.lines() {
+            writeln!(buffer, "|{:^width$}|", &line).unwrap();
+        }
+        writeln!(buffer, "+{:-<width$}+", "").unwrap();
+    }
+}
+
+impl Widget for Label {
+    fn width(&self) -> usize {
+        self.label.lines().map(|line| line.chars().count()).max().unwrap_or(0)
+    }
+
+    fn draw_into(&self, buffer: &mut dyn std::fmt::Write) {
+        writeln!(buffer, "{}", &self.label).unwrap();
+    }
+}
+
+fn main() {
+    let mut window = Window::new("Rust GUI Demo 1.23");
+    window.add_widget(Box::new(Label::new("This is a small text GUI demo.")));
+    window.add_widget(Box::new(Button::new("Click me!")));
+    window.draw();
+}