Add macro implementation exercise

config.json

@@ -589,6 +589,18 @@
         "higher-order functions"
       ]
     },
+    {
+      "uuid": "29583cc6-d56d-4bee-847d-93d74e5a30e7",
+      "slug": "macros",
+      "core": false,
+      "unlocked_by": null,
+      "difficulty": 4,
+      "topics": [
+        "macros",
+        "macros-by-example",
+        "hashmap"
+      ]
+    },
     {
       "uuid": "94f040d6-3f41-4950-8fe6-acf0945ac83d",
       "slug": "allergies",

exercises/macros/.gitignore

@@ -0,0 +1,3 @@
+# Ignore Cargo.lock if creating a library
+# More information here http://doc.crates.io/guide.html#cargotoml-vs-cargolock
+Cargo.lock

exercises/macros/.meta/description.md

@@ -0,0 +1,29 @@
+Macros are a powerful part of a Rust programmer's toolkit, and [macros by example](https://doc.rust-lang.org/reference/macros-by-example.html) are a relatively simple way to access this power. Let's write one!
+
+## Context
+
+What is a macro? [Wikipedia](https://en.wikipedia.org/wiki/Macro_(computer_science)) describes it thus:
+
+> A macro (short for "macroinstruction", from Greek μακρός 'long') in computer science is a rule or pattern that specifies how a certain input sequence (often a sequence of characters) should be mapped to a replacement output sequence (also often a sequence of characters) according to a defined procedure. The mapping process that instantiates (transforms) a macro use into a specific sequence is known as macro expansion.
+
+Illuminating! But to be more concrete, macros are a special syntax which allows you to generate code at compile time. Macros can be used compile-time calculation, but more often they're just another way to abstract your code. For example, you've probably already used `println!()` and `vec![]`. These each take an arbitrary number of arguments, so you can't express them as simple functions. On the other hand, they always expand to some amount of absolutely standard Rust code. If you're interested, you can use the [cargo expand](https://github.com/dtolnay/cargo-expand) subcommand to view the results of macro expansion in your code.
+
+For further information about macros in Rust, The Rust Book has a [good chapter](https://doc.rust-lang.org/book/first-edition/macros.html) on them.
+
+## Problem Statement
+
+You can produce a `Vec` of arbitrary length inline by using the `vec![]` macro. However, Rust doesn't come with a way to produce a [`HashMap`](https://doc.rust-lang.org/std/collections/struct.HashMap.html) inline. Rectify this by writing a `hashmap!()` macro.
+
+For example, a user of your library might write `hashmap!('a' => 3, 'b' => 11, 'z' => 32)`. This should expand to the following code:
+
+```rust
+{
+   let mut hm = HashMap::new();
+   hm.insert('a', 3);
+   hm.insert('b', 11);
+   hm.insert('z', 32);
+   hm
+}
+```
+
+Note that the [`maplit` crate](https://crates.io/crates/maplit) provides a macro which perfectly solves this exercise. Please implement your own solution instead of using this crate; please make an attempt on your own before viewing its source.

exercises/macros/.meta/metadata.yml

@@ -0,0 +1,3 @@
+---
+blurb: "Implement a macro using macros-by-example"
+source: "Peter Goodspeed-Niklaus"

exercises/macros/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "macros"
+version = "0.1.0"
+authors = ["Peter Goodspeed-Niklaus <peter.r.goodspeedniklaus@gmail.com>"]
+
+[dependencies]

exercises/macros/README.md

@@ -0,0 +1,70 @@
+# Macros
+
+Macros are a powerful part of a Rust programmer's toolkit, and [macros by example](https://doc.rust-lang.org/reference/macros-by-example.html) are a relatively simple way to access this power. Let's write one!
+
+## Context
+
+What is a macro? [Wikipedia](https://en.wikipedia.org/wiki/Macro_(computer_science)) describes it thus:
+
+> A macro (short for "macroinstruction", from Greek μακρός 'long') in computer science is a rule or pattern that specifies how a certain input sequence (often a sequence of characters) should be mapped to a replacement output sequence (also often a sequence of characters) according to a defined procedure. The mapping process that instantiates (transforms) a macro use into a specific sequence is known as macro expansion.
+
+Illuminating! But to be more concrete, macros are a special syntax which allows you to generate code at compile time. Macros can be used compile-time calculation, but more often they're just another way to abstract your code. For example, you've probably already used `println!()` and `vec![]`. These each take an arbitrary number of arguments, so you can't express them as simple functions. On the other hand, they always expand to some amount of absolutely standard Rust code. If you're interested, you can use the [cargo expand](https://github.com/dtolnay/cargo-expand) subcommand to view the results of macro expansion in your code.
+
+For further information about macros in Rust, The Rust Book has a [good chapter](https://doc.rust-lang.org/book/first-edition/macros.html) on them.
+
+## Problem Statement
+
+You can produce a `Vec` of arbitrary length inline by using the `vec![]` macro. However, Rust doesn't come with a way to produce a [`HashMap`](https://doc.rust-lang.org/std/collections/struct.HashMap.html) inline. Rectify this by writing a `hashmap!()` macro.
+
+For example, a user of your library might write `hashmap!('a' => 3, 'b' => 11, 'z' => 32)`. This should expand to the following code:
+
+```rust
+{
+   let mut hm = HashMap::new();
+   hm.insert('a', 3);
+   hm.insert('b', 11);
+   hm.insert('z', 32);
+   hm
+}
+```
+
+Note that the [`maplit` crate](https://crates.io/crates/maplit) provides a macro which perfectly solves this exercise. Please implement your own solution instead of using this crate; please make an attempt on your own before viewing its source.
+
+## Rust Installation
+
+Refer to the [exercism help page][help-page] for Rust installation and learning
+resources.
+
+## Writing the Code
+
+Execute the tests with:
+
+```bash
+$ cargo test
+```
+
+All but the first test have been ignored.  After you get the first test to
+pass, remove the ignore flag (`#[ignore]`) from the next test and get the tests
+to pass again.  The test file is located in the `tests` directory.   You can
+also remove the ignore flag from all the tests to get them to run all at once
+if you wish.
+
+Make sure to read the [Modules](https://doc.rust-lang.org/book/second-edition/ch07-00-modules.html) chapter if you
+haven't already, it will help you with organizing your files.
+
+## Feedback, Issues, Pull Requests
+
+The [exercism/rust](https://github.com/exercism/rust) repository on GitHub is the home for all of the Rust exercises. If you have feedback about an exercise, or want to help implement new exercises, head over there and create an issue. Members of the [rust track team](https://github.com/orgs/exercism/teams/rust) are happy to help!
+
+If you want to know more about Exercism, take a look at the [contribution guide](https://github.com/exercism/docs/blob/master/contributing-to-language-tracks/README.md).
+
+[help-page]: http://exercism.io/languages/rust
+[modules]: https://doc.rust-lang.org/book/second-edition/ch07-00-modules.html
+[cargo]: https://doc.rust-lang.org/book/second-edition/ch14-00-more-about-cargo.html
+
+## Source
+
+Peter Goodspeed-Niklaus
+
+## Submitting Incomplete Solutions
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

exercises/macros/example.rs

@@ -0,0 +1,16 @@
+// Ignoring the README's injunction, this is heavily based on the implementation from maplit.
+// Original source is at https://github.com/bluss/maplit/blob/master/src/lib.rs#L27-L60
+
+#[macro_export]
+macro_rules! hashmap {
+    ($($key:expr => $value:expr,)+) => { hashmap!($($key => $value),+) };
+    ($($key:expr => $value:expr),*) => {
+        {
+            let mut _map = ::std::collections::HashMap::new();
+            $(
+                _map.insert($key, $value);
+            )*
+            _map
+        }
+    };
+}

exercises/macros/src/lib.rs

@@ -0,0 +1,6 @@
+#[macro_export]
+macro_rules! hashmap {
+    () => {
+        unimplemented!()
+    };
+}

exercises/macros/tests/macros.rs

@@ -0,0 +1,62 @@
+#[macro_use]
+extern crate macros;
+
+use std::collections::HashMap;
+
+#[test]
+fn test_empty() {
+    let expected: HashMap<usize, usize> = HashMap::new();
+    let computed: HashMap<usize, usize> = hashmap!();
+    assert_eq!(computed, expected);
+}
+
+#[test]
+#[ignore]
+fn test_no_trailing_comma() {
+    let mut expected = HashMap::new();
+    expected.insert(1, "one");
+    expected.insert(2, "two");
+    assert_eq!(hashmap!(1 => "one", 2 => "two"), expected);
+}
+
+#[test]
+#[ignore]
+fn test_trailing_comma() {
+    let mut expected = HashMap::new();
+    expected.insert('h', 89);
+    expected.insert('a', 1);
+    expected.insert('s', 19);
+    expected.insert('h', 8);
+    assert_eq!(
+        hashmap!(
+            'h' => 89,
+            'a' => 1,
+            's' => 19,
+            'h' => 8,
+        ),
+        expected
+    );
+}
+
+#[test]
+#[ignore]
+fn test_nested() {
+    let mut expected = HashMap::new();
+    expected.insert("non-empty", {
+        let mut subhashmap = HashMap::new();
+        subhashmap.insert(23, 623);
+        subhashmap.insert(34, 21);
+        subhashmap
+    });
+    expected.insert("empty", HashMap::new());
+    assert_eq!(
+        hashmap!(
+            "non-empty" => hashmap!(
+                23 => 623,
+                34 => 21
+            ),
+            "empty" => hashmap!()
+        ),
+        expected
+    );
+}