result-transformer

Composable, type‑safe transforms for Result<T, E> – with matching sync and async APIs.

result-transformer lets you describe how a Result should be rewritten without coupling that logic to your domain types or execution model. Implement a tiny trait – or let a one‑line macro do it for you – and drop the transformer anywhere you need it. Current version is 0.0.1. The API is not yet stable and may change.

Table of contents

• Why use result-transformer?
• Crate overview
• Quick start
  • Add the dependency
  • A minimal synchronous transformer
  • Async example
  • Manual implementation (no macros)
  • 3. (Optional) Build a transformer via a flow
• Feature matrix
• Project status
• License
• Contributing

Why use result‑transformer?

• Decoupled by design – success (OkTransformer) and error (ErrTransformer) paths are isolated, keeping concerns clear and testable.
• Symmetric sync / async – every concept exists in twin flavours. You can reuse the same design, but switching to async requires the corresponding _async traits and macros.
• Macro‑assisted ergonomics – declarative impl_*_transformer! and impl_*_transformer_via_*_flow! macros generate the boring glue while you focus on behaviour.
• Optional “flow / step” DSL – chain lightweight steps into a flow when you want a quick inline pipeline. (It’s a helper, not a requirement.)
• Tiny & feature‑gated – pull in only the capabilities and dependencies you actually use.

Crate overview

Quick start

1. Add the dependency

# Cargo.toml
result-transformer = { version = "0.0.1", features = ["core-sync", "core-sync-macros"] }

2. A minimal synchronous transformer

use result_transformer::sync::macros::*;

struct DoublePrefix;

/// doubles on success and prefixes errors – nothing more.
impl_ok_transformer! {
    impl_for      = DoublePrefix,
    input_ok      = i32,
    output_ok     = i32,
    transform_ok  = |v| v * 2
}

impl_err_transformer! {
    impl_for       = DoublePrefix,
    input_err      = &'static str,
    output_err     = String,
    transform_err  = |e| format!("E:{e}")
}

impl_result_transformer_via_self_parts! {
    impl_for  = DoublePrefix,
    input_ok  = i32,
    input_err = &'static str
}

use result_transformer::sync::ResultTransformer; // needed to access the `transform` method

assert_eq!(DoublePrefix.transform(Ok(21)), Ok(42));
assert_eq!(DoublePrefix.transform(Err("no")), Err("E:no".into()));

To use these transformers in an asynchronous context, enable the core-async feature and import items from the async_ module. The examples above will need to be rewritten with async/await and cannot be copied verbatim. The following snippet shows how to reuse the synchronous transformer by wrapping it with an asynchronous implementation.

Async example

use result_transformer::async_::macros::*;

impl_async_result_transformer_via_self_result_transformer! {
    impl_for  = DoublePrefix,
    input_ok  = i32,
    input_err = &'static str
}

async {
assert_eq!(DoublePrefix.transform_async(Ok(21)).await, Ok(42));
assert_eq!(DoublePrefix.transform_async(Err("no")).await, Err("E:no".into()));
}

Manual implementation (no macros)

This approach is handy when macros are unavailable or you need finer-grained control.

use result_transformer::sync::{
    OkTransformer, ErrTransformer, ResultTransformer
};

struct PlainTransformer;

impl OkTransformer<i32> for PlainTransformer {
    type OutputOk = i32;
    fn transform_ok(&self, ok: i32) -> Self::OutputOk {
        ok * 2
    }
}

impl ErrTransformer<&'static str> for PlainTransformer {
    type OutputErr = String;
    fn transform_err(&self, err: &'static str) -> Self::OutputErr {
        format!("E:{err}")
    }
}

impl ResultTransformer<i32, &'static str> for PlainTransformer {
    type OutputOk = <Self as OkTransformer<i32>>::OutputOk;
    type OutputErr = <Self as ErrTransformer<&'static str>>::OutputErr;

    fn transform(&self,
                 result: Result<i32, &'static str>)
                 -> Result<Self::OutputOk, Self::OutputErr> {
        match result {
            Ok(v) => Ok(self.transform_ok(v)),
            Err(e) => Err(self.transform_err(e)),
        }
    }
}

assert_eq!(PlainTransformer.transform(Ok(21)), Ok(42));
assert_eq!(PlainTransformer.transform(Err("no")), Err("E:no".into()));

The manual route makes the separation of concerns crystal‑clear – each trait lives on its own – and it requires zero compiler magic. In larger codebases you’ll probably reach for the macros to avoid repetition, but both styles interoperate seamlessly.

3. (Optional) Build a transformer via a flow

Note: To use flows, enable the flow-sync and flow-sync-macros features.

When you do want a small pipeline the flow / step DSL has your back. The example below chains two steps and then turns the resulting flow into a reusable transformer with impl_result_transformer_via_result_flow!.

use result_transformer::flow::sync::{
    step::map::ResultMapBothStep,
    macros::impl_result_transformer_via_result_flow,
};

// any value implementing `ResultFlow` can be used – a single step is already a valid flow!
let flow = ResultMapBothStep::new(|ok: i32| ok * 2, |err: &str| err.to_owned())
    // chaining the steps with `.then_result`
    .then_result(ResultMapBothStep::new(|ok| ok + 1, |err| format!("E:{err}")));

struct ViaFlow;

impl_result_transformer_via_result_flow! {
    impl_for   = ViaFlow,
    input_ok   = i32,
    input_err  = &'static str,
    output_ok  = i32,
    output_err = String,
    flow       = flow
}

Feature matrix

Pick exactly what you need and keep compile times down. Only core-sync is enabled by default. Combine features as needed.

Some features automatically enable their dependencies. For example, flow-async-macros will automatically enable flow-async and core-async. You can still list them explicitly if you want to be more precise.

Example: enabling the async execution model, flow DSL, and flow macros:

result-transformer = { version = "0.0.1", features = ["flow-async-macros"] }

Or, if you prefer to list everything explicitly:

result-transformer = { version = "0.0.1", features = ["core-async", "flow-async", "flow-async-macros"] }

Project status

This project is currently in early development (0.0.1) and not yet stable. APIs — especially in the flow crate — are subject to change.

License

Licensed under either of

• MIT license (LICENSE-MIT)
• Apache‑2.0 license (LICENSE-APACHE)

at your option.

Contributing

Bug reports, feature requests, and pull requests are always welcome.

I'm still new to GitHub and not very confident in English. For now, I'm doing my best with the help of ChatGPT, so I might misunderstand something. Thanks for your understanding!