TRLS002: Use Bind instead of Map when lambda returns Result

Cause

A Map operation is used with a lambda function that returns Result<T>, which creates a double-wrapped Result<Result<T>>.

Rule Description

In Railway Oriented Programming:

Map transforms the value inside a Result<T> while keeping the Result wrapper
Bind (also called flatMap) is used when the transformation itself returns a Result<T>

Using Map when the lambda returns a Result<T> creates Result<Result<T>>, which is almost always unintended.

How to Fix Violations

Replace Map with Bind when the transformation function returns a Result<T>:

// ❌ Bad - Creates Result<Result<Customer>>
return emailResult.Map(email => Customer.Create(email, name));
//                 ^^^                          ^^^^^^^^^^^^^^
//                 Map                    Returns Result<Customer>

// ✅ Good - Properly flattens to Result<Customer>
return emailResult.Bind(email => Customer.Create(email, name));
//                 ^^^^
//                 Bind

Example

public Result<Order> CreateOrder(string emailStr, decimal amount)
{
    // ❌ Bad - Result<Result<Order>>
    return Email.TryCreate(emailStr)
        .Map(email => Order.Create(email, amount));
        // Order.Create returns Result<Order>
        // Map wraps it again → Result<Result<Order>>

    // ✅ Good - Result<Order>
    return Email.TryCreate(emailStr)
        .Bind(email => Order.Create(email, amount));
        // Bind flattens → Result<Order>
}

Code Fix

This diagnostic offers an automatic code fix that replaces Map with Bind.

When to Suppress Warnings

Do not suppress this warning. If you actually need Result<Result<T>> (extremely rare), make it explicit with a comment explaining why.

TRLS008 - Result is double-wrapped

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