Concurrency in Go with Accounts

account.go

package main

import (
	"errors"
	"fmt"
)

type Account struct {
	balance     int
	deltaChan   chan int
	balanceChan chan int
	errChan     chan error
}

func NewAccount() (a *Account) {
	a = &Account{
		balance:     0,
		deltaChan:   make(chan int),
		balanceChan: make(chan int),
		errChan:     make(chan error),
	}
	go a.run()
	return
}

func (a *Account) Balance() int {
	return <-a.balanceChan
}

func (a *Account) Deposit(amount int) error {
	fmt.Printf("depositing %d\n", amount)
	a.deltaChan <- amount
	return <-a.errChan
}

func (a *Account) Withdraw(amount int) error {
	fmt.Printf("withdrawing %d\n", amount)
	a.deltaChan <- -amount
	return <-a.errChan
}

func (a *Account) applyDelta(amount int) error {
	newBalance := a.balance + amount
	if newBalance < 0 {
		return errors.New("Insufficient funds")
	}
	a.balance = newBalance
	fmt.Printf("new balance: %d\n", newBalance)
	return nil
}

func (a *Account) run() {
	var delta int
	for {
		select {
		case delta = <-a.deltaChan:
			a.errChan <- a.applyDelta(delta)
		case a.balanceChan <- a.balance:
			//Do nothing,
		}
	}
}

func main() {
	a := NewAccount()
	a.Deposit(1000)

	if a.Withdraw(1000) != nil {
		fmt.Printf("ERROR\n")
	}
	fmt.Printf("Balance: %d\n", a.Balance())

	if a.Withdraw(1000) != nil {
		fmt.Printf("ERROR\n")
	}
	fmt.Printf("Balance: %d\n", a.Balance())
}