How to handle cyclic reducer dependencies?

[matt-dewitt]

Hey everyone, got a unique TCA issue I'm trying to figure out. Working a dynamic form builder project and trying to figure out how to have two reducers that depend on each other. Below is a simplified example of what I'm trying to accomplish:

struct Form {
    var rootSection: SectionDefinition
}

struct SectionDefinition {
    var units: IdentifiedArrayOf<UnitDefinition>
}

enum UnitDefinition: Equatable {
    case section(SectionDefinition)
    case field(FieldDefinition)
}

This is my basic data model; a Form has a root section SectionDefinition and a section definition has a list of UnitDefinition enums. One of those unit types is also a section, since they can be nested. Trying to figure out how to set up my reducers to make it work:

typealias FormReducer = Reducer<Form, FormAction, FormEnvironment>

let formReducer: FormReducer = .combine(
    sectionReducer
        .pullback(
            state: \.rootSection,
            action: /FormAction.rootSection,
            environment: { $0.rootSection }
        )
)


typealias SectionReducer = Reducer<SectionDefinition, SectionAction, SectionEnvironment>

let sectionReducer: SectionReducer = .combine(
    unitReducer.forEach(
        state: \.units,
        action: /SectionAction.unit(id:action:),
        environment: { $0.unit }
    )
)

typealias UnitReducer = Reducer<UnitDefinition, UnitAction, UnitEnvironment>

let unitReducer: UnitReducer = .combine(
    sectionReducer
        .pullback(
            state: /UnitDefinition.section,
            action: /UnitAction.section,
            environment: { $0.section }
        ),
    fieldReducer
        .pullback(
            state: /UnitDefinition.field,
            action: /UnitAction.field,
            environment: { $0.field }
        )
)

The formReducer does a pullback on the root section using the section reducer, the section reducer does a forEach over the units using the unitReducer, but then the unit reducer also needs to do a pullback on the section reducer. This compiles fine but crashes at runtime. If I remove the sectionReducer.pullback from the unitReducer things mostly work, except for supporting nested sections.

Are there any tricks or tips for making this work?

Thanks in advance!

[matt-dewitt]

Got a good solution over on the Swift forums: https://forums.swift.org/t/cyclic-reducer-dependencies/56610/4