rust Option/Result map() tricks

1 files changed, 41 insertions, 3 deletions

diff --git a/software/rust.page b/software/rust.page
index df99de4..4e8aa44 100644
--- a/software/rust.page
+++ b/software/rust.page
@@ -3,9 +3,10 @@ Rust
 
 ## Resources
 
-- [http://xion.io/post/code/rust-iter-patterns.html]()
-- [https://deterministic.space/rust-cli-tips.html]()
-- [https://manishearth.github.io/blog/2018/01/10/whats-tokio-and-async-io-all-about/]()
+- <http://xion.io/post/code/rust-iter-patterns.html>
+- <https://deterministic.space/rust-cli-tips.htm>
+- <https://manishearth.github.io/blog/2018/01/10/whats-tokio-and-async-io-all-about/>
+- <https://saghm.github.io/five-rust-things/>
 
 Optimization: use `RUSTFLAGS="-C target-cpu=native"` to take advantage of CPU
 special features.
@@ -22,3 +23,40 @@ Run tests with stdout output:
 
 To run tests with logging enabled (eg, with `env_logger`), make sure you add
 `env_logger::init()` to the test function itself.
+
+
+## map() and Result Ergonomics
+
+`.collect()` has some magical features! In addition to turning an iterator of
+`Item` into `Vec<Item>`, it will turn an iterator of `Result<Item>` into
+`Result<Vec<Item>>`. This makes it really useful for the end of functions.
+
+This is particularly useful for resolving some categories of "error handling in
+map closures": you can use `?` in the map closure as long as you wrap the happy
+path with `Ok()` and call collect on the outside. Eg:
+
+    let list: Vec<Item> = junk
+        .iter()
+        .map(|thing| Ok(Item {
+            a: thing.a,
+            b: fixup(thing.widget)?,
+        }))
+        .collect::Result<Vec<Item>>()?;
+
+What about when `map` over an `Option`? Eg:
+
+    let toy = Shiny {
+        a: 123,
+        b: component.map(|v| paint(v).expect("paint to succeed"),
+    };
+
+Should use match in this case:
+
+    let toy = Shiny {
+        a: 123,
+        b: match component {
+            None => None,
+            Some(v) => Some(paint(v)?),
+        },
+    };
+