feat: impl std::error::Error for Frame #29

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Merged

andylokandy merged 9 commits into fast:main from andylokandy:into-error

Jan 30, 2026

+190 −0

CHANGELOG.md

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Expand Up		@@ -13,3 +13,4 @@ All significant changes to this project will be documented in this file.
		### New Features

		* This crate is now `no_std` compatible, while the `alloc` crate is still required for heap allocations. It is worth noting that `no_std` support is a nice-to-have feature, and can be dropped if it blocks other important features in the future. Before 1.0, once `exn` APIs settle down, the decision on whether to keep `no_std` as a promise will be finalized.
		* `Frame` now implements `std::error::Error`, and `Exn<E>` can be converted into `Box<dyn std::error::Error>`.

Cargo.lock

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Cargo.toml

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+Diff line change
@@ Expand Up / @@ -28,6 +28,7 @@ repository = "https://github.com/fast/exn" @@
     exn = { path = "exn" }
     # Crates.io dependencies
+    anyhow = { version = "1.0.100" }
     clap = { version = "4.5.20", features = ["derive"] }
     derive_more = { version = "2.1.0", features = ["full"] }
     insta = { version = "1.45.1" }
@@ Expand Down @@

examples/Cargo.toml

-Original file line number
+Diff line change
@@ Expand Up / @@ -38,10 +38,19 @@ path = "src/downcast.rs" @@
     name = "make-error"
     path = "src/make-error.rs"
+    [[example]]
+    name = "into-anyhow"
+    path = "src/into-anyhow.rs"
+    [[example]]
+    name = "into-std-error"
+    path = "src/into-std-error.rs"
     [package.metadata.release]
     release = false
     [dev-dependencies]
+    anyhow = { workspace = true }
     derive_more = { workspace = true }
     exn = { workspace = true }
@@ Expand Down @@

examples/src/into-anyhow.rs

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-Original file line number
+Diff line change
@@ -0,0 +1,75 @@
+    // Copyright 2025 FastLabs Developers
+    //
+    // Licensed under the Apache License, Version 2.0 (the "License");
+    // you may not use this file except in compliance with the License.
+    // You may obtain a copy of the License at
+    //
+    //     http://www.apache.org/licenses/LICENSE-2.0
+    //
+    // Unless required by applicable law or agreed to in writing, software
+    // distributed under the License is distributed on an "AS IS" BASIS,
+    // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    // See the License for the specific language governing permissions and
+    // limitations under the License.
+    //! # Anyhow Interoperate Example - Returning `anyhow::Result<_>`
+    //!
+    //! This example shows a common pattern:
+    //! - Using `exn::Result<T, E>` internally.
+    //! - At the boundary, convert `Exn<E>` into `anyhow::Error`.
+    use std::error::Error;
+    use derive_more::Display;
+    use exn::Result;
+    use exn::ResultExt;
+    fn main() -> anyhow::Result<()> {
+        app::run().map_err(convert_error)?;
+        Ok(())
+    }
+    fn convert_error<E: Error + Send + Sync + 'static>(err: exn::Exn<E>) -> anyhow::Error {
+        anyhow::Error::from_boxed(err.into())
+    }
+    mod app {
+        use super::*;
+        pub fn run() -> Result<(), AppError> {
+            let port = config::load_port().or_raise(|| AppError)?;
+            let _ = port;
+            Ok(())
+        }
+        #[derive(Debug, Display)]
+        #[display("failed to start app")]
+        pub struct AppError;
+        impl Error for AppError {}
+    }
+    mod config {
+        use super::*;
+        pub fn load_port() -> Result<u16, ConfigError> {
+            let raw = "not-a-number";
+            let port = raw
+                .parse::<u16>()
+                .or_raise(|| ConfigError(format!("PORT must be a number; got {raw:?}")))?;
+            Ok(port)
+        }
+        #[derive(Debug, Display)]
+        pub struct ConfigError(String);
+        impl Error for ConfigError {}
+    }
+    // Output when running `cargo run -p examples --example into-anyhow`:
+    //
+    // Error: failed to start app
+    //
+    // Caused by:
+    //     0: PORT must be a number; got "not-a-number"
+    //     1: invalid digit found in string

examples/src/into-std-error.rs

-Original file line number
+Diff line change
@@ -0,0 +1,71 @@
+    // Copyright 2025 FastLabs Developers
+    //
+    // Licensed under the Apache License, Version 2.0 (the "License");
+    // you may not use this file except in compliance with the License.
+    // You may obtain a copy of the License at
+    //
+    //     http://www.apache.org/licenses/LICENSE-2.0
+    //
+    // Unless required by applicable law or agreed to in writing, software
+    // distributed under the License is distributed on an "AS IS" BASIS,
+    // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    // See the License for the specific language governing permissions and
+    // limitations under the License.
+    //! # std::error::Error Interoperate Example - Returning `Result<_, Box<dyn std::error::Error>>`
+    //!
+    //! This example shows a common pattern:
+    //! - Using `exn::Result<T, E>` internally.
+    //! - At the boundary, convert `Exn<E>` into `Box<dyn std::error::Error>`.
+    use std::error::Error;
+    use derive_more::Display;
+    use exn::Result;
+    use exn::ResultExt;
+    fn main() -> std::result::Result<(), Box<dyn Error>> {
+        app::run()?;
+        Ok(())
+    }
+    mod app {
+        use super::*;
+        pub fn run() -> Result<(), AppError> {
+            let port = config::load_port().or_raise(|| AppError)?;
+            let _ = port;
+            Ok(())
+        }
+        #[derive(Debug, Display)]
+        #[display("failed to start app")]
+        pub struct AppError;
+        impl Error for AppError {}
+    }
+    mod config {
+        use super::*;
+        pub fn load_port() -> Result<u16, ConfigError> {
+            let raw = "not-a-number";
+            let port = raw
+                .parse::<u16>()
+                .or_raise(|| ConfigError(format!("PORT must be a number; got {raw:?}")))?;
+            Ok(port)
+        }
+        #[derive(Debug, Display)]
+        pub struct ConfigError(String);
+        impl Error for ConfigError {}
+    }
+    // Output when running `cargo run -p examples --example into-std-error`:
+    //
+    // Error: failed to start app, at examples/src/into-std-error.rs:36:40
+    // |
+    // |-> PORT must be a number; got "not-a-number", at examples/src/into-std-error.rs:55:14
+    // |
+    // |-> invalid digit found in string, at examples/src/into-std-error.rs:55:14

exn/src/impls.rs

-Original file line number
+Diff line change
@@ Expand Up / @@ -158,3 +158,29 @@ impl Frame { @@
             &self.children
         }
     }
+    impl Error for Frame {
+        fn source(&self) -> Option<&(dyn Error + 'static)> {
+            self.children
+                .first()
+                .map(|child| child as &(dyn Error + 'static))
+        }
+    }
+    impl<E: Error + Send + Sync + 'static> From<Exn<E>> for Box<dyn Error + 'static> {
+        fn from(exn: Exn<E>) -> Self {
+            Box::new(exn.frame)
+        }
+    }
+    impl<E: Error + Send + Sync + 'static> From<Exn<E>> for Box<dyn Error + Send + 'static> {
+        fn from(exn: Exn<E>) -> Self {
+            Box::new(exn.frame)
+        }
+    }
+    impl<E: Error + Send + Sync + 'static> From<Exn<E>> for Box<dyn Error + Send + Sync + 'static> {
+        fn from(exn: Exn<E>) -> Self {
+            Box::new(exn.frame)
+        }
+    }

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