Add `pack_err!` macro for error structs with automatic name field

1 files changed, 101 insertions, 0 deletions

diff --git a/examples/example-pack-err/src/main.rs b/examples/example-pack-err/src/main.rs
new file mode 100644
index 0000000..72fecd6
--- /dev/null
+++ b/examples/example-pack-err/src/main.rs
@@ -0,0 +1,101 @@
+//! Example `pack_err!`
+//!
+//! > This example demonstrates how to use the `pack_err!` macro to define error types
+//! > with automatic `name` field (set to snake_case at compile time) and optional `info` field.
+//! > Also demonstrates `--json` serialization when `general_renderer` is enabled.
+//!
+//! Run:
+//! ```bash
+//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find
+//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find --json
+//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find Cargo.toml
+//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find Cargo.toml --json
+//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find src
+//! cargo run --manifest-path examples/example-pack-err/Cargo.toml --quiet -- find src --json
+//! ```
+//!
+//! Output:
+//! ```plaintext
+//! Search path not provided
+//! {"name":"error_not_found"}
+//! Not a directory: Cargo.toml
+//! {"name":"error_not_dir","info":"Cargo.toml"}
+//! Found directory: src
+//! {"inner":"src"}
+//! ```
+
+use mingling::prelude::*;
+use mingling::setup::GeneralRendererSetup;
+use std::path::PathBuf;
+
+dispatcher!("find", CMDFind => EntryFind);
+
+// --------- IMPORTANT ---------
+// `pack_err!` is a convenient macro for defining error types.
+//
+//     Simple form:         pack_err!(ErrorNotFound);
+//     Typed form:          pack_err!(ErrorNotDir = PathBuf);
+//
+// The simple form generates a struct with `name: String` and `impl Default`.
+//   name = "error_not_found"  (automatically snake_cased at compile time)
+//
+// The typed form additionally generates `pub fn new(info)`.
+//   name = "error_not_dir"
+//
+// When `general_renderer` is enabled, the struct also gets
+// `#[derive(serde::Serialize)]` for --json / --yaml output.
+// --------- IMPORTANT ---------
+
+// Simple form — name = "error_not_found"
+pack_err!(ErrorNotFound);
+
+// Typed form — name = "error_not_dir"
+pack_err!(ErrorNotDir = PathBuf);
+
+// Success type using traditional pack!
+pack!(ResultPath = PathBuf);
+
+#[chain]
+fn handle_find(args: EntryFind) -> Next {
+    let Some(path_str) = args.inner.first().cloned() else {
+        // No path provided → use the simple error form (Default)
+        return ErrorNotFound::default().to_render();
+    };
+
+    let path = PathBuf::from(&path_str);
+    if path.is_dir() {
+        // Is a directory → success
+        ResultPath::new(path).to_render()
+    } else {
+        // Not a directory (or doesn't exist) → use the typed error form
+        ErrorNotDir::new(path).to_render()
+    }
+}
+
+/// Renders the successful result with the found directory path.
+#[renderer]
+fn render_result_path(path: ResultPath) {
+    r_println!("Found directory: {}", path.display());
+}
+
+/// Renders the error when no search path is provided.
+#[renderer]
+fn render_error_not_found(_: ErrorNotFound) {
+    r_println!("Search path not provided");
+}
+
+/// Renders the error when the given path is not a directory.
+#[renderer]
+fn render_error_not_dir(err: ErrorNotDir) {
+    r_println!("Not a directory: {}", err.info.display());
+}
+
+gen_program!();
+
+fn main() {
+    let mut program = ThisProgram::new();
+    // Add GeneralRendererSetup to support --json / --yaml flags
+    program.with_setup(GeneralRendererSetup);
+    program.with_dispatcher(CMDFind);
+    let _ = program.exec();
+}