watt: turn into a library

watt/main.rs

@@ -0,0 +1,57 @@
+use std::{
+  fmt::Write as _,
+  io,
+  io::Write as _,
+  process,
+};
+
+use yansi::Paint as _;
+
+fn main() {
+  let Err(error) = watt::main() else {
+    return;
+  };
+
+  let mut err = io::stderr();
+
+  let mut message = String::new();
+  let mut chain = error.chain().rev().peekable();
+
+  while let Some(error) = chain.next() {
+    let _ = write!(
+      err,
+      "{header} ",
+      header = if chain.peek().is_none() {
+        "error:"
+      } else {
+        "cause:"
+      }
+      .red()
+      .bold(),
+    );
+
+    String::clear(&mut message);
+    let _ = write!(message, "{error}");
+
+    let mut chars = message.char_indices();
+
+    let _ = match (chars.next(), chars.next()) {
+      (Some((_, first)), Some((second_start, second)))
+        if second.is_lowercase() =>
+      {
+        writeln!(
+          err,
+          "{first_lowercase}{rest}",
+          first_lowercase = first.to_lowercase(),
+          rest = &message[second_start..],
+        )
+      },
+
+      _ => {
+        writeln!(err, "{message}")
+      },
+    };
+  }
+
+  process::exit(1);
+}