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

11 files changed, 704 insertions, 4 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 78698b3..b9ca68b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -124,6 +124,58 @@ fn render_entry_show(_args: EntryShow, res: &mut LazyRes<ResLargeData>) {
 
 7. **\[core:comp\]** Added `Program::is_completing()` method to check whether the program is currently running in completion mode. This provides a convenient way to conditionally skip certain logic during completion generation, where those operations may be unnecessary or undesirable.
 
+8. **\[macros\]** Added the `pack_err!` macro for creating error structs with automatic `name` field.
+
+The `pack_err!` macro provides a concise way to define error types that implement `Groupped` and are automatically registered for inclusion in the program enum. The `name` field is automatically set to the snake_case version of the struct name at compile time.
+
+Two forms are supported:
+
+```rust
+// Simple form — generates a struct with only `name: String` and a `Default` impl:
+pack_err!(ErrorNotFound);
+
+// Typed form — generates a struct with `name: String` + `info: Type` and a `new(info)` constructor:
+pack_err!(ErrorNotDir = PathBuf);
+```
+
+For `pack_err!(ErrorNotFound)`, the generated code is:
+
+```rust
+#[derive(::mingling::Groupped)]
+pub struct ErrorNotFound {
+    name: String,
+}
+
+impl Default for ErrorNotFound {
+    fn default() -> Self {
+        Self {
+            name: "error_not_found".into(),
+        }
+    }
+}
+```
+
+For `pack_err!(ErrorNotDir = PathBuf)`:
+
+```rust
+#[derive(::mingling::Groupped)]
+pub struct ErrorNotDir {
+    name: String,
+    info: PathBuf,
+}
+
+impl ErrorNotDir {
+    pub fn new(info: PathBuf) -> Self {
+        Self {
+            name: "error_not_dir".into(),
+            info,
+        }
+    }
+}
+```
+
+This macro is only available with the `extra_macros` feature.
+
 #### **BREAKING CHANGES** (API CHANGES):
 
 1. **\[core\]** Changed the signature of `ProgramSetup::setup` from `fn setup(&mut self, program: &mut Program<C>) -> S` to `fn setup(self, program: &mut Program<C>)`, consuming `self` instead of taking a mutable reference. Correspondingly, `Program::with_setup` now accepts `S` by value (`&mut self, setup: S`) instead of by mutable reference (`&mut self, setup: &mut S`).
diff --git a/docs/example-pages/examples.json b/docs/example-pages/examples.json
index e7f0c0c..8e8719b 100644
--- a/docs/example-pages/examples.json
+++ b/docs/example-pages/examples.json
@@ -220,6 +220,23 @@
     ]
   },
   {
+    "id": "example-pack-err",
+    "name": "pack_err!",
+    "icon": "🛑",
+    "category": "macros",
+    "desc": "Demonstrates how to use the `pack_err!` macro to define error types with automatic `name` field (snake_case at compile time) and optional `info` field. Also shows `--json` serialization when `general_renderer` is enabled.\n",
+    "tags": [
+      "pack_err!",
+      "extra_macros",
+      "general_renderer",
+      "--json"
+    ],
+    "files": [
+      "src/main.rs",
+      "Cargo.toml"
+    ]
+  },
+  {
     "id": "example-panic-unwind",
     "name": "Panic Unwind",
     "icon": "💥",
@@ -258,9 +275,8 @@
       "injection"
     ],
     "files": [
-      "Cargo.toml",
       "src/main.rs",
-      "src/lib.rs"
+      "Cargo.toml"
     ]
   },
   {
@@ -289,8 +305,8 @@
       "extra_macros"
     ],
     "files": [
-      "Cargo.toml",
-      "src/main.rs"
+      "src/main.rs",
+      "Cargo.toml"
     ]
   },
   {
diff --git a/examples/example-pack-err/Cargo.lock b/examples/example-pack-err/Cargo.lock
new file mode 100644
index 0000000..36d4b6f
--- /dev/null
+++ b/examples/example-pack-err/Cargo.lock
@@ -0,0 +1,141 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "example-pack-err"
+version = "0.1.0"
+dependencies = [
+ "mingling",
+ "serde",
+]
+
+[[package]]
+name = "itoa"
+version = "1.0.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8f42a60cbdf9a97f5d2305f08a87dc4e09308d1276d28c869c684d7777685682"
+
+[[package]]
+name = "just_fmt"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5454cda0d57db59778608d7a47bff5b16c6705598265869fb052b657f66cf05e"
+
+[[package]]
+name = "memchr"
+version = "2.8.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "88904434abc2901f197fe8cc55f0445e7ded921dba5911dad2e2b39b48e663c4"
+
+[[package]]
+name = "mingling"
+version = "0.2.0"
+dependencies = [
+ "mingling_core",
+ "mingling_macros",
+ "serde",
+]
+
+[[package]]
+name = "mingling_core"
+version = "0.2.0"
+dependencies = [
+ "just_fmt",
+ "serde",
+ "serde_json",
+]
+
+[[package]]
+name = "mingling_macros"
+version = "0.2.0"
+dependencies = [
+ "just_fmt",
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "proc-macro2"
+version = "1.0.106"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8fd00f0bb2e90d81d1044c2b32617f68fcb9fa3bb7640c23e9c748e53fb30934"
+dependencies = [
+ "unicode-ident",
+]
+
+[[package]]
+name = "quote"
+version = "1.0.45"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "41f2619966050689382d2b44f664f4bc593e129785a36d6ee376ddf37259b924"
+dependencies = [
+ "proc-macro2",
+]
+
+[[package]]
+name = "serde"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9a8e94ea7f378bd32cbbd37198a4a91436180c5bb472411e48b5ec2e2124ae9e"
+dependencies = [
+ "serde_core",
+ "serde_derive",
+]
+
+[[package]]
+name = "serde_core"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "41d385c7d4ca58e59fc732af25c3983b67ac852c1a25000afe1175de458b67ad"
+dependencies = [
+ "serde_derive",
+]
+
+[[package]]
+name = "serde_derive"
+version = "1.0.228"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d540f220d3187173da220f885ab66608367b6574e925011a9353e4badda91d79"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "serde_json"
+version = "1.0.150"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e8014e44b4736ed0538adeecded0fce2a272f22dc9578a7eb6b2d9993c74cfb9"
+dependencies = [
+ "itoa",
+ "memchr",
+ "serde",
+ "serde_core",
+ "zmij",
+]
+
+[[package]]
+name = "syn"
+version = "2.0.118"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1b9ae57f904213ebb649ce6895b8a66c66f0203b9319718f69a5612a065b1422"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "unicode-ident"
+version = "1.0.24"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e6e4313cd5fcd3dad5cafa179702e2b244f760991f45397d14d4ebf38247da75"
+
+[[package]]
+name = "zmij"
+version = "1.0.21"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b8848ee67ecc8aedbaf3e4122217aff892639231befc6a1b58d29fff4c2cabaa"
diff --git a/examples/example-pack-err/Cargo.toml b/examples/example-pack-err/Cargo.toml
new file mode 100644
index 0000000..883fc89
--- /dev/null
+++ b/examples/example-pack-err/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "example-pack-err"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+serde = { version = "1.0.228", features = ["derive"] }
+
+[dependencies.mingling]
+path = "../../mingling"
+features = [
+    "general_renderer",
+    "extra_macros",
+]
+
+[workspace]
diff --git a/examples/example-pack-err/page.toml b/examples/example-pack-err/page.toml
new file mode 100644
index 0000000..5534236
--- /dev/null
+++ b/examples/example-pack-err/page.toml
@@ -0,0 +1,10 @@
+[example]
+id = "example-pack-err"
+name = "pack_err!"
+icon = "🛑"
+category = "macros"
+desc = """
+Demonstrates how to use the `pack_err!` macro to define error types with automatic `name` field (snake_case at compile time) and optional `info` field. Also shows `--json` serialization when `general_renderer` is enabled.
+"""
+tags = ["pack_err!", "extra_macros", "general_renderer", "--json"]
+files = ["src/main.rs", "Cargo.toml"]
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();
+}
diff --git a/examples/test-examples.toml b/examples/test-examples.toml
index 8d2cf0d..4a50ab1 100644
--- a/examples/test-examples.toml
+++ b/examples/test-examples.toml
@@ -207,3 +207,23 @@ expect.result = "Hello, Alice!"
 command = "greet Alice -r 5"
 expect.exit-code = 0
 expect.result = "Hello, Alice, Alice, Alice, Alice, Alice!"
+
+[[test.example-pack-err]]
+command = "find"
+expect.exit-code = 0
+expect.result = "Search path not provided"
+
+[[test.example-pack-err]]
+command = "find --json"
+expect.exit-code = 0
+expect.result = '{"name":"error_not_found"}'
+
+[[test.example-pack-err]]
+command = "find Cargo.toml"
+expect.exit-code = 0
+expect.result = "Not a directory: Cargo.toml"
+
+[[test.example-pack-err]]
+command = "find Cargo.toml --json"
+expect.exit-code = 0
+expect.result = '{"name":"error_not_dir","info":"Cargo.toml"}'
diff --git a/mingling/src/example_docs.rs b/mingling/src/example_docs.rs
index f4a27bd..b1d693b 100644
--- a/mingling/src/example_docs.rs
+++ b/mingling/src/example_docs.rs
@@ -1550,6 +1550,131 @@ pub mod example_implicit_dispatcher {}
 /// gen_program!();
 /// ```
 pub mod example_lazy_resources {}
+/// 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"}
+///  ```
+///
+/// Source code (./Cargo.toml)
+/// ```toml
+/// [package]
+/// name = "example-pack-err"
+/// version = "0.1.0"
+/// edition = "2024"
+///
+/// [dependencies]
+/// serde = { version = "1.0.228", features = ["derive"] }
+///
+/// [dependencies.mingling]
+/// path = "../../mingling"
+/// features = [
+///     "general_renderer",
+///     "extra_macros",
+/// ]
+///
+/// [workspace]
+/// ```
+///
+/// Source code (./src/main.rs)
+/// ```ignore
+/// 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();
+/// }
+/// ```
+pub mod example_pack_err {}
 /// Example Panic Unwind
 ///
 ///  > This example introduces how to catch Panic in the Mingling program loop
diff --git a/mingling/src/lib.rs b/mingling/src/lib.rs
index 7b96d34..f1232d3 100644
--- a/mingling/src/lib.rs
+++ b/mingling/src/lib.rs
@@ -99,6 +99,9 @@ pub mod macros {
     pub use mingling_macros::node;
     /// Used to create a wrapper type for use with `Chain` and `Renderer`
     pub use mingling_macros::pack;
+    /// Used to create an error struct with automatic `name` field
+    #[cfg(feature = "extra_macros")]
+    pub use mingling_macros::pack_err;
     #[cfg(feature = "comp")]
     /// Internal macro for '`gen_program`' used to finally generate the completion structure
     pub use mingling_macros::program_comp_gen;
@@ -192,6 +195,9 @@ pub mod prelude {
     pub use crate::macros::gen_program;
     /// Re-export of the `pack` macro for creating wrapper types.
     pub use crate::macros::pack;
+    /// Re-export of the `pack_err` macro for creating error types.
+    #[cfg(feature = "extra_macros")]
+    pub use crate::macros::pack_err;
     /// Re-export of the `r_print` macro for printing within a renderer context.
     pub use crate::macros::r_print;
     /// Re-export of the `r_println` macro for printing with a newline within a renderer
diff --git a/mingling_macros/src/lib.rs b/mingling_macros/src/lib.rs
index 97dd824..408450a 100644
--- a/mingling_macros/src/lib.rs
+++ b/mingling_macros/src/lib.rs
@@ -158,6 +158,8 @@ mod help;
 mod node;
 mod pack;
 #[cfg(feature = "extra_macros")]
+mod pack_err;
+#[cfg(feature = "extra_macros")]
 mod program_setup;
 mod render;
 mod renderer;
@@ -319,6 +321,74 @@ pub fn pack(input: TokenStream) -> TokenStream {
     pack::pack(input)
 }
 
+/// Creates an error struct with a `name: String` field and optional `info: Type` field.
+///
+/// This macro provides a concise way to define error types that implement `Groupped`
+/// and are registered for inclusion in the program enum.
+///
+/// The `name` field is automatically set to the snake_case version of the struct name
+/// at compile time.
+///
+/// # Syntax
+///
+/// Two forms are supported:
+///
+/// ```rust,ignore
+/// // Simple form — generates a struct with only `name: String` and a `Default` impl:
+/// pack_err!(ErrorNotFound);
+///
+/// // Typed form — generates a struct with `name: String` + `info: Type` and a `new(info)` constructor:
+/// pack_err!(ErrorNotDir = PathBuf);
+/// ```
+///
+/// # Generated code
+///
+/// For `pack_err!(ErrorNotFound)`:
+///
+/// ```rust,ignore
+/// #[derive(::mingling::Groupped)]
+/// pub struct ErrorNotFound {
+///     name: String,
+/// }
+///
+/// impl Default for ErrorNotFound {
+///     fn default() -> Self {
+///         Self {
+///             name: "error_not_found".into(),
+///         }
+///     }
+/// }
+/// ```
+///
+/// For `pack_err!(ErrorNotDir = PathBuf)`:
+///
+/// ```rust,ignore
+/// #[derive(::mingling::Groupped)]
+/// pub struct ErrorNotDir {
+///     name: String,
+///     info: PathBuf,
+/// }
+///
+/// impl ErrorNotDir {
+///     pub fn new(info: PathBuf) -> Self {
+///         Self {
+///             name: "error_not_dir".into(),
+///             info,
+///         }
+///     }
+/// }
+/// ```
+///
+/// When the `general_renderer` feature is enabled, the struct also gets
+/// `#[derive(serde::Serialize)]`.
+///
+/// This macro is only available with the `extra_macros` feature.
+#[cfg(feature = "extra_macros")]
+#[proc_macro]
+pub fn pack_err(input: TokenStream) -> TokenStream {
+    pack_err::pack_err(input)
+}
+
 /// Early-returns an error from a `Result`, converting the `Ok` branch to a
 /// `ChainProcess`.
 ///
diff --git a/mingling_macros/src/pack_err.rs b/mingling_macros/src/pack_err.rs
new file mode 100644
index 0000000..dd7b083
--- /dev/null
+++ b/mingling_macros/src/pack_err.rs
@@ -0,0 +1,143 @@
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{Ident, Token, Type, parse_macro_input};
+
+/// Converts a PascalCase/UpperCamelCase identifier string to snake_case.
+///
+/// Examples:
+/// - `ErrorNotFound` → `"error_not_found"`
+/// - `ErrorNotDir` → `"error_not_dir"`
+/// - `FileIO` → `"file_io"`
+/// - `XMLParser` → `"xml_parser"`
+fn to_snake_case(ident: &str) -> String {
+    let mut result = String::new();
+    let mut prev_is_upper = false;
+
+    for (i, c) in ident.chars().enumerate() {
+        if c.is_uppercase() {
+            if i > 0 && !prev_is_upper {
+                result.push('_');
+            }
+            for lower_c in c.to_lowercase() {
+                result.push(lower_c);
+            }
+            prev_is_upper = true;
+        } else {
+            result.push(c);
+            prev_is_upper = false;
+        }
+    }
+
+    result
+}
+
+enum PackErrInput {
+    /// pack_err!(ErrorNotFound)
+    Simple { type_name: Ident },
+    /// pack_err!(ErrorNotDir = PathBuf)
+    Typed {
+        type_name: Ident,
+        inner_type: Box<Type>,
+    },
+}
+
+impl syn::parse::Parse for PackErrInput {
+    fn parse(input: syn::parse::ParseStream) -> syn::Result<Self> {
+        let type_name: Ident = input.parse()?;
+
+        if input.peek(Token![=]) {
+            input.parse::<Token![=]>()?;
+            let inner_type: Type = input.parse()?;
+            Ok(PackErrInput::Typed {
+                type_name,
+                inner_type: Box::new(inner_type),
+            })
+        } else {
+            Ok(PackErrInput::Simple { type_name })
+        }
+    }
+}
+
+#[allow(clippy::too_many_lines)]
+pub fn pack_err(input: TokenStream) -> TokenStream {
+    let parsed = parse_macro_input!(input as PackErrInput);
+
+    match parsed {
+        PackErrInput::Simple { type_name } => {
+            let name_str = type_name.to_string();
+            let snake_name = to_snake_case(&name_str);
+
+            #[cfg(not(feature = "general_renderer"))]
+            let derive = quote! {
+                #[derive(::mingling::Groupped)]
+            };
+
+            #[cfg(feature = "general_renderer")]
+            let derive = quote! {
+                #[derive(::mingling::Groupped, ::serde::Serialize)]
+            };
+
+            let expanded = quote! {
+                #derive
+                pub struct #type_name {
+                    /// The snake_case name of this error, automatically set at compile time.
+                    name: String,
+                }
+
+                impl ::std::default::Default for #type_name {
+                    fn default() -> Self {
+                        Self {
+                            name: #snake_name.into(),
+                        }
+                    }
+                }
+
+                ::mingling::macros::register_type!(#type_name);
+            };
+
+            expanded.into()
+        }
+        PackErrInput::Typed {
+            type_name,
+            inner_type,
+        } => {
+            let name_str = type_name.to_string();
+            let snake_name = to_snake_case(&name_str);
+
+            #[cfg(not(feature = "general_renderer"))]
+            let derive = quote! {
+                #[derive(::mingling::Groupped)]
+            };
+
+            #[cfg(feature = "general_renderer")]
+            let derive = quote! {
+                #[derive(::mingling::Groupped, ::serde::Serialize)]
+            };
+
+            let expanded = quote! {
+                #derive
+                pub struct #type_name {
+                    /// The snake_case name of this error, automatically set at compile time.
+                    name: String,
+                    /// Additional context info for this error.
+                    info: #inner_type,
+                }
+
+                impl #type_name {
+                    /// Creates a new error with the given info.
+                    /// The `name` field is automatically set to the snake_case of the struct name.
+                    pub fn new(info: #inner_type) -> Self {
+                        Self {
+                            name: #snake_name.into(),
+                            info,
+                        }
+                    }
+                }
+
+                ::mingling::macros::register_type!(#type_name);
+            };
+
+            expanded.into()
+        }
+    }
+}