Cannot use generic type without instantiation error in Go

Generics are introduced in Go 1.18 version.

If the type definition includes type parameters, the type name called as a generic type.

If you forgot to instantiate a generic type you will get cannot use generic type without instantiation error in Go language.

type GenericSlice[T any] []T

In the above example I have created a genetic type called GenericSlice in Go.

We must instantiate Generic types before they used.

A generic type is instantiated by substituting type arguments for the type parameters.

g := GenericSlice[any]{} // Instantiation of Generic type
g = append(g, 1)
g = append(g, "golang")

for _, v := range g {
		fmt.Println(v)
}

But if you forgot to substitute the type arguments you will get cannot use generic type without instantiation error.

g := GenericSlice{} // No substitution of type arguments

//cannot use generic type GenericSlice[T any] without instantiation

The proper way to instantiate a generic type is to substituting the type arguments.

//Integer type
gInt := GenericSlice[int]{}

//Float Type
gFloat := GenericSlice[float64]{}

To instantiate the Go language generic type you can make use of make() function as well.

g := make(GenericSlice[any], 0)

Again if you miss substituting the type arguments you will get the same error in your Go program.

g := make(GenericSlice, 0)

Let’s go through the summary of the article.

1. In Go language, We must instantiate Generic types before they used.
2. A Go generic type is instantiated by substituting type arguments for the type parameters.
3. If you miss the instantiation you will get cannot use generic type without instantiation error in Go language.