Example for blog post: Domain-Driven Design vs. "Functional Core, Imperative Shell"

finish_card.go

package finishcard

import (
    "ace-life/errtypes"
    "ace-life/hexagon/domain"
    "ace-life/hexagon/domain/player"
    "ace-life/hexagon/events"
    "ace-life/hexagon/usecases"
)

func FinishCard(
    eventStream domain.EventStream,
    command Command,
    drawReplacementCard usecases.ForDrawingReplacementCards,
) domain.Event {

    hand := reconstituteFrom(eventStream)

    // check if the card-to-finish exists in the player's hand
    finishedCard, err := hand.CardBy(command.CardID)
    if err != nil {
        return events.BuildFinishCardFailed(
            command.PlayerID,
            command.CardID,
            errtypes.ResourceNotFound(err),
            command.MessageID,
            command.CurrentTime,
        )
    }

    // try to draw a uniqe replacement card
    replacementCard, err := drawReplacementCard(hand.CardIDs()...)
    if err != nil {
        return events.BuildFinishCardFailed(
            command.PlayerID,
            command.CardID,
            errtypes.ExternalConstraintNotFulfilled(err),
            command.MessageID,
            command.CurrentTime,
        )
    }

    // all fine
    return events.BuildCardFinished(
        command.PlayerID,
        finishedCard,
        replacementCard,
        command.MessageID,
        command.CurrentTime,
    )
}

func reconstituteFrom(eventStream domain.EventStream) player.Hand {
    var hand player.Hand

    for _, event := range eventStream.Events {
        switch actualEvent := event.(type) {
        case events.PlayerEnrolled:
            hand = player.BuildHand(
                actualEvent.Card1,
                actualEvent.Card2,
                actualEvent.Card3,
            )
        case events.CardFinished:
            hand = hand.ReplaceCard(
                actualEvent.FinishedCard.ID,
                actualEvent.ReplacementCard,
            )
        case events.CardRejected:
            hand = hand.ReplaceCard(
                actualEvent.RejectedCard.ID,
                actualEvent.ReplacementCard,
            )
        }
    }

    return hand
}