Table of Contents

TRLS019: Combine chain exceeds maximum supported tuple size

Cause

A chain of .Combine() calls produces a tuple with more than 9 elements, which exceeds the maximum supported by downstream methods like Bind, Map, Tap, and Match.

Rule Description

Combine aggregates multiple Result<T> values into a single result containing a tuple. Trellis supports tuples up to 9 elements. When a Combine chain exceeds this limit, downstream operations cannot destructure the result.

This rule fires as an Error because the code will fail to compile when you attempt to use Bind, Map, or other methods on the over-sized result.

How to Fix Violations

Group related validations into intermediate value objects or sub-results, then combine those groups:

// ❌ Bad - 10 elements exceeds the maximum of 9
var result = FirstName.TryCreate(request.FirstName)
    .Combine(LastName.TryCreate(request.LastName))
    .Combine(EmailAddress.TryCreate(request.Email))
    .Combine(PhoneNumber.TryCreate(request.Phone))
    .Combine(Age.TryCreate(request.Age))
    .Combine(Street.TryCreate(request.Street))
    .Combine(City.TryCreate(request.City))
    .Combine(State.TryCreate(request.State))
    .Combine(ZipCode.TryCreate(request.Zip))
    .Combine(CountryCode.TryCreate(request.Country)); // 10th element!

// ✅ Good - Group related fields into sub-objects
var personalInfo = FirstName.TryCreate(request.FirstName)
    .Combine(LastName.TryCreate(request.LastName))
    .Combine(EmailAddress.TryCreate(request.Email))
    .Combine(PhoneNumber.TryCreate(request.Phone))
    .Combine(Age.TryCreate(request.Age));

var address = Street.TryCreate(request.Street)
    .Combine(City.TryCreate(request.City))
    .Combine(State.TryCreate(request.State))
    .Combine(ZipCode.TryCreate(request.Zip))
    .Combine(CountryCode.TryCreate(request.Country));

var result = personalInfo
    .Combine(address)
    .Bind((personal, addr) => User.TryCreate(personal, addr));

Examples

Example 1: Order with Many Fields

// ❌ Bad - Too many fields in one chain
var order = ProductId.TryCreate(req.ProductId)
    .Combine(Quantity.TryCreate(req.Quantity))
    .Combine(Price.TryCreate(req.Price))
    .Combine(CurrencyCode.TryCreate(req.Currency))
    .Combine(FirstName.TryCreate(req.ShipFirstName))
    .Combine(LastName.TryCreate(req.ShipLastName))
    .Combine(Street.TryCreate(req.ShipStreet))
    .Combine(City.TryCreate(req.ShipCity))
    .Combine(ZipCode.TryCreate(req.ShipZip))
    .Combine(CountryCode.TryCreate(req.ShipCountry)); // 10 elements

// ✅ Good - Separate into logical groups
var lineItem = ProductId.TryCreate(req.ProductId)
    .Combine(Quantity.TryCreate(req.Quantity))
    .Combine(Money.TryCreate(req.Price, req.Currency));

var shippingAddress = FirstName.TryCreate(req.ShipFirstName)
    .Combine(LastName.TryCreate(req.ShipLastName))
    .Combine(Street.TryCreate(req.ShipStreet))
    .Combine(City.TryCreate(req.ShipCity))
    .Combine(ZipCode.TryCreate(req.ShipZip))
    .Combine(CountryCode.TryCreate(req.ShipCountry));

var order = lineItem
    .Combine(shippingAddress)
    .Bind((item, address) => Order.TryCreate(item, address));

Example 2: Boundary Case (9 Elements Is Valid)

// ✅ OK - 9 elements is the maximum supported
var result = Field1.TryCreate(a)
    .Combine(Field2.TryCreate(b))
    .Combine(Field3.TryCreate(c))
    .Combine(Field4.TryCreate(d))
    .Combine(Field5.TryCreate(e))
    .Combine(Field6.TryCreate(f))
    .Combine(Field7.TryCreate(g))
    .Combine(Field8.TryCreate(h))
    .Combine(Field9.TryCreate(i)); // 9 elements - OK
  • TRLS012 - Consider using Result.Combine
  • TRLS008 - Result is double-wrapped

See Also

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