On Sat, Apr 9, 2022 at 8:28 PM Aidan Hahn <aidanjacobh...@gmail.com> wrote:
>
> I am playing with the generics that were added in go 1.18 and noticed an odd
> discrepancy that I have not been able to find an authoritative answer to.
> Consider the following data type, as well as the function defined for it:
>
> type Node[T any] struct {
> Inner T
> Next *Node[any]
> }
>
> func (n *Node[any]) ListPreTraverse(f func(*Node[any])) {
> tmpN := interface{}(n)
> if tmpN, ok := tmpN.(*Node[*Node[any]]); ok {
> tmpN.Inner.ListPreTraverse(f)
> } else {
> f(n)
> }
> n.Next.ListPreTraverse(f)
> }
>
> This function is a simple attempt to iterate across a generic linked list and
> call a function on each node in the list (although in this case it may behave
> more like a tree if inner stores another node). When I went to compile this
> code I encountered the following error:
>
> test.go:64:25: cannot use f (variable of type func(*Node[any])) as type
> func(*Node[any]) in argument to n.Next.ListPreTraverse
>
> To me at least it seems that all the information to validate that the
> argument fits the type specified in the function call is there, defined in
> the function header. What am I missing?
The error message is confusing because the "any" used in
ListPreTraverse is shadowing the "any" used in Node. See
https://go.dev/doc/faq#types_in_method_declaration. You want to write
func (n *Node[T]) ListPreTraverse(f func(*Node[T])) {
But then you will see that your code has an instantiation cycle,
violating the restriction described at
https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#generic-types:
"A generic type can refer to itself in cases where a type can
ordinarily refer to itself, but when it does so the type arguments
must be the type parameters, listed in the same order. This
restriction prevents infinite recursion of type instantiation."
I don't really understand what you are trying to do here. The type
assertion doesn't make sense to me. Perhaps this is the code you
want:
type Node[T any] struct {
Inner T
Next *Node[T]
}
func (n *Node[T]) ListPreTraverse(f func(*Node[T])) {
f(n)
if n.Next != nil {
n.Next.ListPreTraverse(f)
}
}
Ian
--
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to golang-nuts+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/golang-nuts/CAOyqgcVk4M8XPgFTovFPa5KruoR9N289yRg%3Dg5jg_DX%3D7W93QA%40mail.gmail.com.
