From b5518e9b16f2aef3c7ce62b945318d2dab048554 Mon Sep 17 00:00:00 2001
From: gendx <gendx@users.noreply.github.com>
Date: Fri, 3 Feb 2023 15:29:49 +0000
Subject: [PATCH] Add a solution for day 1's bonus question with generics.
(#338)
---
src/exercises/day-1/solutions-morning.md | 35 ++++++++++++++++++++++--
1 file changed, 33 insertions(+), 2 deletions(-)
diff --git a/src/exercises/day-1/solutions-morning.md b/src/exercises/day-1/solutions-morning.md
index d5aed0db..6b218a64 100644
--- a/src/exercises/day-1/solutions-morning.md
+++ b/src/exercises/day-1/solutions-morning.md
@@ -9,8 +9,39 @@
```
### Bonus question
-It honestly doesn't work so well. It might seem that we could use a slice-of-slices (`&[&[i32]]`) as the input type to transpose and thus make our function handle any size of matrix. However, this quickly breaks down: the return type cannot be `&[&[i32]]` since it needs to own the data you return.
+It requires more advanced concepts. It might seem that we could use a slice-of-slices (`&[&[i32]]`) as the input type to transpose and thus make our function handle any size of matrix. However, this quickly breaks down: the return type cannot be `&[&[i32]]` since it needs to own the data you return.
-You can attempt to use something like `Vec<Vec<i32>>`, but this doesn't work very well either: it's hard to convert from `Vec<Vec<i32>>` to `&[&[i32]]` so now you cannot easily use `pretty_print` either.
+You can attempt to use something like `Vec<Vec<i32>>`, but this doesn't work out-of-the-box either: it's hard to convert from `Vec<Vec<i32>>` to `&[&[i32]]` so now you cannot easily use `pretty_print` either.
+
+Once we get to traits and generics, we'll be able to use the [`std::convert::AsRef`][1] trait to abstract over anything that can be referenced as a slice.
+
+```rust
+use std::convert::AsRef;
+use std::fmt::Debug;
+
+fn pretty_print<T, Line, Matrix>(matrix: Matrix)
+where
+ T: Debug,
+ // A line references a slice of items
+ Line: AsRef<[T]>,
+ // A matrix references a slice of lines
+ Matrix: AsRef<[Line]>
+{
+ for row in matrix.as_ref() {
+ println!("{:?}", row.as_ref());
+ }
+}
+
+fn main() {
+ // &[&[i32]]
+ pretty_print(&[&[1, 2, 3], &[4, 5, 6], &[7, 8, 9]]);
+ // [[&str; 2]; 2]
+ pretty_print([["a", "b"], ["c", "d"]]);
+ // Vec<Vec<i32>>
+ pretty_print(vec![vec![1, 2], vec![3, 4]]);
+}
+```
In addition, the type itself would not enforce that the child slices are of the same length, so such variable could contain an invalid matrix.
+
+[1]: https://doc.rust-lang.org/std/convert/trait.AsRef.html