Mixed type array in Go

[Tong Sun]

I need a data struct / solution that I can store mixed data-types into an array. How to architect that?

Details -- Consider the checkout point at the cashier, the checkout process receives the following string of instructions

1. purchase
   
2. coupon
   
3. purchase
   
4. purchase
   

I want to store all the above request data into an array, so I don't lost the sequence how they arrive, which in turn requests the array element to be either purchase or coupon.

How to make it happen? Thanks

[Josh Humphries]

You would typically define the array type as an array of some interface that is implemented by both purchase and coupon. You can then use a type-switch or type-assertion to determine/assert the actual type at runtime, on a per element basis.

If the two types do not share some interface (e.g. common methods), you could define the array as []interface{}. But this also allows code to accidentally add any kind of value to the array (not just a purchase or a coupon). So it may be better to create a marker interface -- an unexported interface with a no-op unexported marker method -- and make purchase and coupon both implement that interface.

For example:

type checkoutObject interface {

  checkoutObject()

}

type Purchase struct {

    // yadda yadda

}

func (p *Purchase) checkoutObject() {

    // no-op marker method

}

type Coupon struct {

    // yadda yadda

}

func (c *Coupon) checkoutObject() {

    // no-op marker method

}

// assert that Purchase and Coupon implement checkoutObject

var _ checkObject = (*Purchase)(nil)

var _ checkObject = (*Coupon)(nil)

// Now you can define your array like so:

var checkoutItems []checkoutObject

// The compiler will only let you add *Purchase and *Coupon

// values to the array.

// You can use the values at runtime like so:

for e := range checkoutItems {

    switch e := e.(type) {

    case *Purchase:

        // handle purchase

    case *Coupon:

        // handle coupon

    default:

        panic(fmt.Sprintf("unsupported type: %T", e))

    }

}

----
Josh Humphries
jh...@bluegosling.com

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[Tong Sun]

That's really comprehensive. Thanks a million Josh!

[Tong Sun]

On Thursday, January 4, 2018 at 10:59:35 AM UTC-5, Tong Sun wrote:

That's really comprehensive. Thanks a million Josh!

Hi Josh, FTA, 

I've archived your masterpiece as 

https://github.com/suntong/lang/blob/master/lang/Go/src/ds/MixedType.go

and

https://play.golang.org/p/lmmkTxVJht1

so that people can enjoy it right away.

Thanks again