Aigamo.Results

A fully-serializable, F#-like result pattern for C#.

Aigamo.Results models success and failure as a single Result<T, TError> value — a closed record with exactly two cases, Ok and Error — and gives you a full set of functional operations (Map, FlatMap, Fold, Combine, …) to transform and compose results without ever throwing. Unlike many result types, it round-trips cleanly through System.Text.Json, so a Result can cross a serialization boundary — an API response, a message queue, a cache — and come back intact.

Features

• Result<T, TError> with a strongly-typed error channel (not just a message or an exception)
• A closed hierarchy (Ok / Error) — the compiler knows there are no other cases
• Fully serializable with System.Text.Json (polymorphic, round-trips by value)
• Rich functional API: Map, MapError, FlatMap, FlatMapError, Fold, Tap, GetOr, Combine, Merge, MapEach, Flatten, Validate, Contains, and more
• Async all the way: every operation has Task-lifted overloads, so sync and async steps chain seamlessly
• Unit for value-less results (Result<Unit, TError>)
• Implicit conversions: return a value or an error directly and let it become a Result<T, TError>
• An optional analyzer to enforce implicit- or explicit-construction style across a project

Getting Started

1. Install the package

dotnet add package Aigamo.Results

Tip: To share the reference and drop the per-file using, add it to Directory.Build.props with a global Using:

<Project>

  <ItemGroup>
    <PackageReference Include="Aigamo.Results" />
    <Using Include="Aigamo.Results" />
  </ItemGroup>

</Project>

2. Create results

using Aigamo.Results;

Result<int, string> ok = Result.Ok<int, string>(42);
Result<int, string> error = Result.Error<int, string>("something went wrong");

A value or an error also converts implicitly, so you can often drop the factory call:

Result<int, string> ok = 42;                    // -> Ok(42)
Result<int, string> error = "something went wrong"; // -> Error("something went wrong")

A function that can fail returns its outcome instead of throwing:

static Result<int, string> Parse(string s) =>
	int.TryParse(s, out var value)
		? value                                 // implicit -> Ok(value)
		: $"'{s}' is not a number";             // implicit -> Error(...)

Note: implicit conversion is unavailable when T and TError are the same type (Result<string, string>) — the two conversions are ambiguous (CS0457). Use the explicit Result.Ok / Result.Error factories there.

3. Inspect the outcome

Fold collapses both cases into a single value:

string message = Parse("42").Fold(
	onOk: value => $"Parsed {value}",
	onError: error => $"Failed: {error}"
);

Match gives you the case objects directly, and helpers like GetOr, Contains, and Validate cover common checks:

int value = Parse("nope").GetOr(ifError: _ => 0); // 0
bool isFortyTwo = Parse("42").Contains(42);         // true
bool isPositive = Parse("42").Validate(x => x > 0); // true

Transforming and composing

// Map the success value; Error passes through untouched.
Result<string, string> label = Parse("42").Map(x => $"#{x}");

// FlatMap chains another fallible step, short-circuiting on the first Error.
Result<int, string> checked = Parse("42").FlatMap(x =>
	x >= 0 ? Result.Ok<int, string>(x) : Result.Error<int, string>("negative"));

// MapError transforms the error channel.
Result<int, MyError> typed = Parse("42").MapError(msg => new MyError(msg));

// Tap runs a side effect on success without changing the result.
Parse("42").Tap(x => Console.WriteLine(x));

Combine several results into a tuple (the first Error wins), or collapse a sequence:

// Combine: accumulate success values into a growing tuple.
Result<(int, int, int), string> combined = Parse("1")
	.Combine(_ => Parse("2"))
	.Combine(_ => Parse("3"));

// Merge: IEnumerable<Result<T, E>> -> Result<T[], E>
Result<int[], string> merged = new[] { "1", "2", "3" }.Select(Parse).Merge();

Async

Every operation ships Task-lifted overloads, so you can freely mix synchronous and asynchronous steps in one chain — the Task is threaded through for you:

Result<User, string> result = await LoadUserAsync(id)   // Task<Result<User, string>>
	.FlatMap(user => ValidateAsync(user))               // async step
	.Map(user => user.Name)                             // sync step
	.TapError(error => logger.LogWarning(error));       // async source, sync action

For each operation you get the overloads for a Task source, an async selector, and both at once.

Serialization

Result<T, TError> is annotated for System.Text.Json polymorphic serialization and round-trips by value:

using System.Text.Json;

var ok = Result.Ok<int, string>(42);
string json = JsonSerializer.Serialize(ok);
// {"$type":"Ok","ResultValue":42}

var back = JsonSerializer.Deserialize<Result<int, string>>(json);
// back == ok  (true)

An Error serializes as {"$type":"Error","ErrorValue":...}. No custom converter or configuration required.

Enforcing a conversion style

Whether to write Result.Ok(value) explicitly or lean on the implicit conversion is a matter of taste — so the package ships an analyzer that lets a project pick one and enforce it. Set the policy in .editorconfig:

[*.cs]
# choose one:
aigamo_results_conversion_style = implicit   # or: explicit

• implicit → ARS001 flags an explicit Result.Ok / Result.Error call where the value could convert implicitly, and the code fix removes the wrapper.
• explicit → ARS002 flags reliance on the implicit conversion, and the code fix wraps the value in Result.Ok / Result.Error.

When the key is unset, neither rule fires. Both default to warning severity and can be tuned per rule:

dotnet_diagnostic.ARS001.severity = suggestion

ARS001 is never reported for Result<T, T>, where implicit conversion is ambiguous.

Under the hood

Result<T, TError> is a closed record whose Ok/Error cases are nested with a private constructor, so the hierarchy cannot be extended elsewhere. The exhaustive Match method is generated by Aigamo.MatchGenerator, and the Task-lifted overloads are emitted by an internal [GenerateAsyncOverloads] source generator — the hand-written code only defines the synchronous core of each operation.

Inspiration

Aigamo.Results is inspired by Nut.Results. The goal here is a result type that keeps a strongly-typed error channel and serializes cleanly, with the functional surface generated rather than hand-maintained.

License

MIT