Attempting to compare two values of type V with Go generics not working

Question

Go version: go1.21rc2

I'm writing a function using generics in Go, that takes a value and returns true/false if the value is in the map.

In version 1 below, I don't understand why the two values can't be compared when they're of type any? But after casting to type any in version 2, it now works... I assume I've missed something obvious but I don't really understand this.

Version 1 (NOT WORKING):

func InValues[M ~map[K]V, K comparable, V any](m M, v V) bool {
    for _, x := range maps.Values(m) {
        if x == v {
            return true
        }
    }
    return false
}

Version 2 (WORKING):

func InValues[M ~map[K]V, K comparable, V any](m M, v V) bool {
    for _, x := range maps.Values(m) {
        if any(x) == any(v) {
            return true
        }
    }
    return false
}

Your second version _compiles_ and works for _some_ cases because interface values are comparable but probably isn't what you _want_. You basically cannot use any as constraint for V (you have to use comparable) and even then your code will be broken unless you _really_ do some hard work to make it correct (e.g. for float64). — Volker, Jul 02 '23 at 08:53

score 3 · Accepted Answer · answered Jul 01 '23 at 19:09

3

V needs to be comparable for == to be allowed.
The cast to any and then comparison works because then it uses the non generic == between any which is always allowed.

answered Jul 01 '23 at 19:09

Daniel

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There are no casts in Go, it's a conversion to any. – Volker Jul 02 '23 at 08:50
Isn't type casting the same thing? Converting one type to another? – Sam Wood Jul 03 '23 at 20:36

Attempting to compare two values of type V with Go generics not working

1 Answers1