Go: use slice of different numeric types as field of a struct

Question

I am writing a collector that collects metrics and stores in structs that looks something like this:

type Metric struct {
  Name string
  Data []float64
}

However for some metrics, it does not make sense to use float64, since their values are unsigned integers. Any idea how I could use different numeric types for the Data field?

I could use Data []interface{}, but then I won't be able to use indexing on the array elements.

(For clarity: I don't need different types in one slice, like a list in Python: my slice has to be strongly typed, but I want to be able to change the type of the slice.)

What about defining another Metric struct, that stores data as type uint? Besides, why shouldn't you be able to use indexing on a slice of interfaces? [-> example](https://play.golang.org/p/oSiuSjCYNCx) — FObersteiner, Mar 12 '21 at 08:58
You cannot "change the type of the slice" just like you can't change the type of any other value. Use multiple fields or different struct types, as suggested above. — Peter, Mar 12 '21 at 09:38
thanks for the replies, I am trying now to define an interface "vector" and work with that. @MrFuppes, thanks, you're right, but doesn't this make the slice dynamically typed? performance is critical for my project, so I really want the same performance as with native slices. — vgratian, Mar 12 '21 at 09:41
For clarity, I wouldn't use a slice of interface if I had only two types (and I know which). Regarding the interface slice, the slice would be of interface type (so basically static); but each element could hold reference to a different type. Compared to a list in Python, a list is an object, it doesn't have a type in that sense. — FObersteiner, Mar 12 '21 at 09:52

Eli Bendersky · Accepted Answer · 2021-03-12T13:46:05.533

For a full solution to this, you'll have to wait until generics lands in Go (potentially in 1.18): https://blog.golang.org/generics-proposal

With generics, you'd be able to have a generic Metric type that can either hold float64 or unsigned, and you could instantiate each of them separately.

E.g. (generics-enabled playgorund):

type Metric[T any] struct {
  Name string
  Data []T
}

func main() {
  mf := Metric[float64]{"foo", []float64{12.24, 1.1, 2.22}}
  mu := Metric[uint32]{"bar", []uint32{42, 2}}

  fmt.Println(mf)
  fmt.Println(mu)
}

Note that [T any] means that the type held in Data is unconstrained. You can constrain it to types with certain characteristics, or to a hardcoded list like float64, uint32 if you prefer.

In the meanwhile, there are some options:

float64 can represent a lot of integers; at least all 32-bit ones (see Representing integers in doubles)
You can use Data []interface{}, but it's rather wasteful. There should be no problem indexing into this slice, but you'll have to have type asserts whenever you work with it. It's costly both memory-wise and runtime performance-wise; something that can really matter for metrics.
You can have two versions of Metric, with code duplication (and use code generation to help, if needed).

Go: use slice of different numeric types as field of a struct

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