Go How to check type of product

<br>
I have model Product with field Type.
<br>
Something like this:

type ProductType string

var (
	PtRouteTransportation    ProductType = "ProductRT"
	PtOnDemandTransportation ProductType = "ProductDT"
	PtExcursion              ProductType = "ProductEX"
	PtTicket                 ProductType = "ProductTK"
	PtQuote                  ProductType = "ProductQT"
	PtGood                   ProductType = "ProductGD"
)

type Product struct {
    ...
	Type ProductType
    ...
}

In Create function I have type form param:

type := req.Form.Get("type")

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Question: how to check is type valid?
<br>
<br>
Simplest way is:

if type != PtRouteTransportation && type != PtOnDemandTransportation && ...

but what I supposed to do if Product will have 100 types?
<br>
<br>
How to do this in go way?

Instead of using a type alias to a basic type, why not using a type alias to a private type (meaning a struct you cannot initialize outside your package)

See this example.

type ProductType productType

type productType struct {
	name string
}

var (
	PtRouteTransportation    ProductType = ProductType(productType{"ProductRT"})
	PtOnDemandTransportation ProductType = ProductType(productType{"ProductDT"})
	PtExcursion              ProductType = ProductType(productType{"ProductEX"})
	PtTicket                 ProductType = ProductType(productType{"ProductTK"})
	PtQuote                  ProductType = ProductType(productType{"ProductQT"})
	PtGood                   ProductType = ProductType(productType{"ProductGD"})
)

func printProductType(pt ProductType) {
	fmt.Println(pt.name)
}
func main() {
	fmt.Println("Hello, playground")
	// printProductType("test") // cannot use "test" (type string) as type ProductType in argument to printProductType
	printProductType(PtRouteTransportation)
}

It means you cannot use any other values than the public ones defined in the var section.

If you manage to pass a value to the printProductType(pt ProductType), your value is always a valid one.

For the dynamic checking part, that OneOfOne addresses in his answer, I would add a function GetProductType(name string) ProductType which:

• check if the name is a valid one
• return one of the official ProductType instances defined in the var section above.

That way, the rest of your code always work with an official ProductType value (and not with a 'string' which happens to be matching the right value)

Really the simplest is to use a map, not as fast as constants but if if you have to test against a large set, it's the most convenient way.

Also since it's pre-allocated, it's thread-safe, so you won't have to worry about locks, unless you add to it at runtime.

var (
	ptTypes = map[string]struct{}{
		"ProductRT": {},
		"ProductDT": {},
		"ProductEX": {},
		"ProductTK": {},
		"ProductQT": {},
		"ProductGD": {},
	}

)

func validType(t string) (ok bool) {
	_, ok = ptTypes[t]
	return
}