add --filename option

Sponsored-by: author

2 files changed, 37 insertions, 1 deletions

diff --git a/Cargo.toml b/Cargo.toml
index eb81e2f..16f9ea9 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,3 +7,4 @@ edition = "2021"
 
 [dependencies]
 clap = { version = "4.0.2", features = ["derive"] }
+thiserror = "1.0.37"
diff --git a/src/main.rs b/src/main.rs
index d1d7e5d..45ebbb6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,12 +1,47 @@
 use clap::Parser;
+use std::fs::read;
+use std::path::PathBuf;
 
 fn main() {
+    if let Err(e) = fallible_main() {
+        eprintln!("ERROR: {}", e);
+    }
+}
+
+fn fallible_main() -> Result<(), HelloError> {
     let args = Args::parse();
-    println!("hello, {}", args.whom);
+    println!("hello, {}", args.whom()?);
+    Ok(())
 }
 
 #[derive(Parser)]
 struct Args {
     #[clap(default_value = "world")]
     whom: String,
+
+    #[clap(short, long)]
+    filename: Option<PathBuf>,
+}
+
+impl Args {
+    fn whom(&self) -> Result<String, HelloError> {
+        if let Some(filename) = &self.filename {
+            let data = read(&filename)
+                .map_err(|e| HelloError::Read(filename.clone(), e))?;
+            let whom = String::from_utf8(data)
+                .map_err(|e| HelloError::Utf8(filename.clone(), e))?;
+            Ok(whom.trim().to_string())
+        } else {
+            Ok(self.whom.clone())
+        }
+    }
+}
+
+#[derive(Debug, thiserror::Error)]
+enum HelloError {
+    #[error("failed to read file {0}")]
+    Read(PathBuf, #[source] std::io::Error),
+
+    #[error("failed to parse file {0} as UTF-8")]
+    Utf8(PathBuf, #[source] std::string::FromUtf8Error),
 }