# How to iterate over a union of slices passed in a generic function?

## Issue

I am testing out generics in go 1.18 and took a look at this example.
I would like to recreate that example but instead be able to pass in a slice of int or slice of float instead, and in the function I’ll just sum up everything in the slice.

This is when I ran into some issues just iterating the slice. This is what I tried:

``````
import "fmt"

// NumberSlice constraint
type NumberSlice interface {
[]int64 | []float64
}

// want: to range over n and print value of v

// got: cannot range over n (variable of type N constrained by NumberSlice)
// (N has no core type)
for _, v := range n {
fmt.Println(v)
}
}

func main() {
ints := []int64{1, 2}
}
``````

How do I accomplish this?

## Solution

A core type, for an interface (including an interface constraint) is defined as follows:

An interface T has a core type if one of the following conditions is
satisfied:

• There is a single type `U` which is the underlying type of all types in the type set of T

• or the type set of T contains only channel types with identical element type E, and all directional channels have the same direction.

Your interface constraint has no core type, because it has two underlying types: `[]int64` and `[]float64`.

Therefore you can’t use it where a core type is required. Notably `range` and `make`.

You can change the interface to require the base types, and then specify the slice in the function signature:

``````// still no core type...
type Number interface {
int64 | float64
}

// ...but the argument will be instantiated with either int64 or float64
for _, v := range n {
fmt.Println(v)
}
}
``````

This also works, but it’s way more verbose:

``````type NumberSlice[N int64 | float64] interface {
// one core type []N
~[]N
}

func add[S NumberSlice[N], N int64 | float64](n S) {
for _, v := range n {
fmt.Println(v)
}
}
``````