TRLS002 — Use Bind instead of Map when lambda returns Result

• Severity: Info
• Category: Trellis

What it detects

Flags Map and MapAsync when the transformation already returns Result<T>, Task<Result<T>>, or ValueTask<Result<T>>. It covers lambdas, method groups, and member-access method groups.

Why it matters

Map wraps the transformation result as a value, which leads to Result<Result<T>> or async equivalents. Bind and BindAsync flatten the pipeline correctly.

Warning

This rule is broader than a simple x => Result.Success(...) lambda. A method group like Map(ParseAsync) can trigger it too if ParseAsync already returns a Result.

Bad example

using Trellis;

static class Example
{
    public static Result<int> Bad()
    {
        var result = Result.Success("Ada");
        return result.Map(ParseLength);
    }

    static Result<int> ParseLength(string value) =>
        Result.Success(value.Length);
}

Good example

using Trellis;

static class Example
{
    public static Result<int> Good()
    {
        var result = Result.Success("Ada");
        return result.Bind(ParseLength);
    }

    static Result<int> ParseLength(string value) =>
        Result.Success(value.Length);
}

Code fix available

Yes — replaces Map with Bind, or MapAsync with BindAsync.

Configuration

Use standard Roslyn configuration if you need to suppress this rule in a specific scope.

dotnet_diagnostic.TRLS002.severity = none

#pragma warning disable TRLS002
// Intentional: documented exception or test-only pattern.
#pragma warning restore TRLS002

Tip

Use Map when you turn a success value into a plain value. Use Bind when you turn a success value into another Result.