How to call a Go method on a struct type?

Question

I getting the error not enough arguments in call to method expression AccountService.Open when I run the following. Run here: https://play.golang.org/p/Z9y-QIcwNy

type AccountService interface {
    Open(no string, name string, openingDate time.Time) AccountService
}

type Account struct {
    Num      string
    Name     string
    OpenDate time.Time
    Balance  float64
}

type SavingsAccount struct {
    InterestRate float32
    Account
}

type CheckingAccount struct {
    TransactionFee float32
    Account
}

func (a SavingsAccount) Open(no string, name string, openingDate time.Time) AccountService {
    return SavingsAccount{
        Account: Account{Num: no,
            Name:     name,
            OpenDate: openingDate,
        },
        InterestRate: 0.9,
    }
}
func (a CheckingAccount) Open(no string, name string, openingDate time.Time) AccountService {
    return CheckingAccount{
        Account: Account{Num: no,
            Name:     name,
            OpenDate: openingDate,
        },
        TransactionFee: 0.15,
    }
}

func main() {
    aliceAcct := AccountService.Open("12345", "Alice", time.Date(1999, time.January, 03, 0, 0, 0, 0, time.UTC))

    fmt.Println("Alice's account =", aliceAcct)
}

Answer 1

The result of the method expression AccountService.Open is a function with with type func(AccountService, string, string, time.Time) AccountService. The code is missing the first argument. You can call it like this:

aliceAcct := AccountService.Open(CheckingAccount{}, "12345", "Alice", time.Date(1999, time.January, 03, 0, 0, 0, 0, time.UTC))

playground example

If you have no meaningful value to use as the receiver, then use a function instead:

func OpenCheckingAccount(no string, name string, openingDate time.Time) AccountService {
    ...
}

If you need to abstract account creation, then define the service as a function:

 type AccountService func(no string, name string, openingDate time.Time) AccountType
 func OpenSavings(no string, name string, openingDate time.Time) AccountType { ... }
 func OpenChecking(no string, name string, openingDate time.Time) AccountType { ... }

The OpenSavings and OpenChecking functions can be used as an AccountService.

You will want to use different types for the service and accounts. Here, I use AccountService and AccountType. AccountType is not a very good name, but Account is already used. AccountType might be defined as the following:

type AccountType interface {
   func Deposit(int)
   func Withdraw(int)
}