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)
}

Possible duplicate of not enough arguments in call to method expression — John Scott Stensland
– John Scott Stensland, Commented Jul 25, 2017 at 23:00
This happens because you're trying to transfer your knowledge of other languages into GO. You need to unlearn. — Inanc Gumus
– Inanc Gumus, Commented Jun 3, 2018 at 12:30

score 7 · Accepted Answer · 2017-01-04 03:53:47Z

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)
}

Didn't know about the method expression. The error message is clear now. Wish the compiler generated a little bit more verbose/clear error message.
Account is an embedded type within SavingsAccount or CheckingAccount structs. How can OpenSavings(...) and OpenChecking() return Account type?

tier1 · Accepted Answer · 2017-01-04 02:45:34Z

4

Try this

acct := SavingsAccount{}
aliceAcct:= acct.Open("12345", "Alice", time.Date(1999, time.January, 03, 0, 0, 0, 0, time.UTC))
fmt.Println("Alice's account =", aliceAcct)

You need to create an instance of a struct that properly implements the AccountService interface

--Edit Working example: play.golang.org

answered Jan 4, 2017 at 2:45

tier1

6,4636 gold badges47 silver badges76 bronze badges

1 Comment

Aravind Yarram Over a year ago

Is there an alternate way to organize the code so that I can avoid creating a dummy instance of the struct just to call the methods? Also, why does the error say that an argument is missing?

Zan Lynx · Accepted Answer · 2017-01-04 02:58:57Z

2

Most Go packages do one of two things:

Have a package level function such as OpenSavingsAccount that returns an open, initialized SavingsAccount.
Have a default variable in the package named DefaultSavingsAccount and duplicate the SavingsAccount methods at the package level where they operate on the DefaultSavingsAccount. This is a lot of function duplication but all of the logic stays in the methods. And if the DefaultSavingsAccount is not const and is an interface type it can be replaced with another implementation.

answered Jan 4, 2017 at 2:58

Zan Lynx

54.7k11 gold badges85 silver badges135 bronze badges

2 Comments

Aravind Yarram Over a year ago

So the general Idiom in Go is to declare package level constructor functions to initialize structs and use interfaces for other behavior say deposit, credit & transfer? Also, can you please post an e.g. for second point

Zan Lynx Over a year ago

@AravindR.Yarram: See DefaultClient at golang.org/pkg/net/http/#pkg-variables

Collectives™ on Stack Overflow

How to call a Go method on a struct type?

3 Answers 3

2 Comments

1 Comment

2 Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

1 Comment

2 Comments

Linked

Related