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

command_handler.go

package finishcard

import (
    "ace-life/hexagon/domain"
    "ace-life/hexagon/usecases"
)

type CommandHandler struct {
    drawReplacementCard  usecases.ForDrawingReplacementCards
    loadEventStream      usecases.ForLoadingEventStreams
    appendEventsToStream usecases.ForAppendingEventsToStreams
    eventStreamPrefix    string
}

func NewFinishCardCommandHandler(
    drawReplacementCard usecases.ForDrawingReplacementCards,
    loadEventStream usecases.ForLoadingEventStreams,
    appendEventsToStream usecases.ForAppendingEventsToStreams,
    eventStreamPrefix string,
) *CommandHandler {

    return &CommandHandler{
        drawReplacementCard:  drawReplacementCard,
        loadEventStream:      loadEventStream,
        appendEventsToStream: appendEventsToStream,
        eventStreamPrefix:    eventStreamPrefix,
    }
}

func (h *CommandHandler) Handle(command Command) error {
    streamID := domain.MustBuildStreamID(
        h.eventStreamPrefix,
        command.PlayerID.String(),
    )

    eventStream, loadErr := h.loadEventStream(streamID)
    if loadErr != nil {
        return loadErr
    }

    recordedEvent := FinishCard(
        eventStream,
        command,
        h.drawReplacementCard,
    )

    appendErr := h.appendEventsToStream(
        streamID,
        eventStream.StreamRevision(),
        recordedEvent,
    )
    if appendErr != nil {
        return appendErr
    }

    failureEvent, ok := recordedEvent.(domain.FailureEvent)
    if ok {
        return failureEvent.MapToError()
    }

    return nil
}