demonstrate "refactor error away" pattern

src/lib.rs

@@ -1,40 +1,9 @@
-//! This crate contains the implementation for `rwc` (rust word count), named
-//! after the `wc -w` Unix utility.
-//!
-//! This is intended as an example of how I approach error handling in Rust
-//! applications with a focus of different error handling strategies for
-//! application and library code.
-//!
-//! Code inside this file demonstrates application code, which annotates errors
-//! with context using [`anyhow`].
-//!
-//! [`anyhow`]: ../anyhow/index.html
-
-#[cfg(test)]
-#[macro_use]
-extern crate assert_matches;
-
-pub mod wordcounter;
-
-use std::env;
-use std::fs::File;
-
-use crate::wordcounter::count_words;
-use anyhow::Context;
-
-/// The run function is called from `main()` in [`rwc`].
-///
-/// It annotates errors from fallible functions (like `File::open` and
-/// `count_words`) with `.context()` before propagating them upwards, where
-/// `main()` will eventually handle reporting of the error.
-///
-/// [`rwc`]: ../rwc/index.html
-pub fn run() -> anyhow::Result<()> {
-    for filename in env::args().skip(1).collect::<Vec<String>>() {
-        let mut reader = File::open(&filename).context(format!("unable to open '{}'", filename))?;
-        let wordcount =
-            count_words(&mut reader).context(format!("unable to count words in '{}'", filename))?;
-        println!("{} {}", wordcount, filename);
+pub fn count_words(lines: &mut dyn Iterator<Item = String>) -> u32 {
+    let mut wordcount = 0;
+    for line in lines {
+        for _word in line.split_whitespace() {
+            wordcount += 1;
+        }
     }
-    Ok(())
+    wordcount
 }

src/main.rs

@@ -1,13 +1,28 @@
-//! This crate provides the `rwc` (rust word count) binary, named after the `wc
-//! -w` Unix utility.
-//!
-//! This is intended as an example of how I approach error handling in Rust
-//! applications with a focus of different error handling strategies for
-//! application and library code.
+#![feature(iter_map_while)]
+use std::{
+    env,
+    fs::File,
+    io::{BufRead, BufReader},
+};
 
-fn main() {
-    if let Err(err) = wordcount::run() {
-        eprintln!("Error: {:?}", err);
-        std::process::exit(1);
+use anyhow::Context;
+
+fn main() -> anyhow::Result<()> {
+    for filename in env::args().skip(1).collect::<Vec<String>>() {
+        let file = File::open(&filename).context(format!("unable to open '{}'", filename))?;
+        let reader = BufReader::new(file);
+
+        let mut err = None;
+        let mut iter = reader
+            .lines()
+            .map_while(|line| line.map_err(|e| err = Some(e)).ok());
+        let wordcount = wordcount::count_words(&mut iter);
+        if let Some(err) = err {
+            return Err(
+                anyhow::Error::new(err).context(format!("unable to count words in '{}'", filename))
+            );
+        }
+        println!("{} {}", wordcount, filename);
     }
+    Ok(())
 }