I feel like this shouldn't have been so difficult

2021/src/lib.rs

@@ -1,15 +1,47 @@
-use std::io::{BufReader, BufRead};
+use std::io::Read;
+use std::str::FromStr;
 use std::fs::File;
 
-use color_eyre::eyre;
+use color_eyre::{eyre};
 
 
-pub fn iter_nums(filename: &str) -> eyre::Result< impl Iterator<Item=eyre::Result<i32>> > {
-    let file = File::open(filename)?;
-    let reader = BufReader::new(file);
+pub fn load(filename: &str) -> eyre::Result<String> {
+    let mut file = File::open(filename)?;
+    let mut data = String::new();
+    file.read_to_string(&mut data)?;
+    Ok(data)
+}
+
+
+pub struct LineParser<'a, T: FromStr> {
+    it: std::str::Lines<'a>,
+    convert: fn(&str) -> Result<T, <T as FromStr>::Err>
+}
+
+
+impl<'a, T: FromStr> Iterator for LineParser<'a, T> {
+    type Item = Result<T, <T as FromStr>::Err>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.it.next().map(|s|
+            (self.convert)(s)
+        )
+    }
+}
+
+
+// define a trait so that we can attach it to String
+pub trait ParseLines {
+    fn parse_lines<T: FromStr>(&self) -> LineParser<'_, T>;
+}
+
+
+impl ParseLines for String {
+    fn parse_lines<T: FromStr>(&self) -> LineParser<'_, T> {
+        LineParser {
+            it: self.lines(),
+            convert: str::parse::<T>
+        }
+    }
+}
 
-    let it = reader.lines().map(|line| 
-        Ok(line?.parse::<i32>()?)
-    );
-    Ok(it)
-}

2021/src/main.rs

@@ -1,7 +1,7 @@
 use color_eyre::eyre;
 
 mod lib;
-use lib::iter_nums;
+use lib::{load, ParseLines};
 
 
 
@@ -11,10 +11,12 @@ fn main() -> eyre::Result<()> {
 
 
 fn day_one() -> eyre::Result<()> {
+    let data = load("data/01.txt")?;
+
     let mut single_increases = 0;
     let mut window_increases = 0;
     let mut prev = (None, None, None);
-    for res in iter_nums("data/01.txt")? {
+    for res in data.parse_lines::<i32>() {
         let n = res?;
         if let Some(p) = prev.2 {
             // apparently we can't combine "if let" and regular boolean conditions
@@ -36,3 +38,8 @@ fn day_one() -> eyre::Result<()> {
     println!("Two: {}", window_increases);
     Ok(())
 }
+
+
+// fn day_two() -> eyre::Result<()> {
+
+// }