On Wed, Apr 27, 2022 at 3:56 PM will....@gmail.com
<will.fau...@gmail.com> wrote:
>
> Do slices and maps have compatible type sets regarding the range operator?
>
> I was trying to iterate through either, where slice keys are the indexes,
> such that this would work:
>
> type KV[K comparable, V any] interface {
> ~[]V | map[K]V
> }
>
> func f[KV2 KV[K, V], K comparable, V any](kv KV2) {
> for k, v := range kv { // line 12
> fmt.Println(k, v)
> }
> }
>
> func main() {
> f[map[string]string, string, string](map[string]string{"a": "b", "c":
> "d"}) // works
> f[[]string, int, string]([]string{"e", "f"}) // error
> }
>
> I get this error:
>
> ./prog.go:12:20: cannot range over kv (variable of type KV2 constrained by
> KV[K, V]) (KV2 has no core type)
>
> Play link: https://go.dev/play/p/jSfaEaTmcQ1
>
> I'm having trouble understanding this error message. Is this saying that maps
> and slices don't have a core type because they're not compatible for the
> range operator?
Currently a for/range statement using a value whose type is a type
parameter is only permitted if the type parameter constraint gives it
a core type. Basically, a core type is an optional tilde followed by
a single type. It is not a union. See
https://go.dev/ref/spec#Core_types.
So the compiler is telling you that the type parameter KV2 has no core
type, so for/range is not permitted.
We impose this restriction to simplify the meaning of for/range, and
to avoid having to figure out what it would mean to range over a type
parameter whose constraint is []byte | map[int]string | chan float64.
It is possible that we will lift this restriction in a future release.
Ian
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