Go-State: Lightweight & Reflect-Based State Machine

Go-State is a thread-safe, generic state machine for Go that leverages reflection to eliminate boilerplate. It allows you to dispatch events based on their types and manage state transitions with minimal friction.

✨ Features

• Type-Safe Handlers: Define handlers using specific event structs without manual type assertions.
• Generic State: Support for any custom state data structure using Go Generics ([T any]).
• Parallel Composition: Run multiple handlers for the same event concurrently with automatic context cancellation on failure.
• Thread-Safe: Built-in sync.Mutex to ensure consistent status transitions.
• Context-Aware: Full support for context.Context across all handlers.

📦 Installation

go get github.com/israelsodanoa/state

🚀 Quick Start

1. Define your State and Events

type OrderState struct {
    ID int
}

type PaymentConfirmed struct {
    Amount float64
}

2. Setup the Machine

func main() {
    ctx := context.Background()

    // Initialize with a state type and initial status
    sm := &state.StateMachine[OrderState]{
        State:  OrderState{ID: 42},
        Status: "PENDING",
    }

    // Create a handler for a specific event type
    confirmHandler := state.When(sm, func(ctx context.Context, e PaymentConfirmed) error {
        fmt.Printf("Processing payment of $%.2f\n", e.Amount)
        return nil
    })

    // Map the handler to a status transition
    sm.TransitionTo("COMPLETED", confirmHandler)

    // 3. Publish an event
    // The library automatically finds the handler for the 'PaymentConfirmed' type
    err := sm.Pub(ctx, PaymentConfirmed{Amount: 150.00})
    if err != nil {
        log.Fatal(err)
    }

    fmt.Println("New Status:", sm.Status) // Output: New Status: COMPLETED
}

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🛠 Advanced Usage

Parallel Handler Composition

One of the most powerful features of Go-SM is the Compose function. It allows you to trigger multiple side effects (like logging, analytics, or notifications) for a single event in parallel.

logHandler := state.When(sm, func(ctx context.Context, e PaymentConfirmed) error {
    log.Println("Logging to database...")
    return nil
})

emailHandler := state.When(sm, func(ctx context.Context, e PaymentConfirmed) error {
    log.Println("Sending confirmation email...")
    return nil
})

// Compose them! They will run in separate goroutines.
// If one fails, the context for the others is cancelled.
combined := state.Compose(logHandler, emailHandler)

sm.TransitionTo("READY_FOR_SHIPPING", combined)

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📚 API Overview

StateMachine[T]

The core engine. It holds your custom state T and a slice of EventTransition.

• Pub(ctx, data): Matches the type of data to a registered handler and executes it.
• TransitionTo(status, handler): Registers which status the machine should move to after a handler successfully executes.

When[T, E](sm, handlerFn)

A helper to register a handler function.

• T: Your state type.
• E: The event struct type you want to listen for.
• Safety: This will panic if you try to register two different handlers for the same event type on the same machine.

Compose(handlers...)

Takes multiple EventHandler instances and returns a single wrapped handler.

• Execution: All handlers run in their own goroutine.
• Error Handling: Returns the first error encountered and cancels the execution of the remaining handlers in that group.

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⚖️ License

Distributed under the MIT License. See LICENSE for more information.