On Sun, Oct 1, 2023 at 7:30 PM Jon Watte <jwa...@gmail.com> wrote:
> I want to implement a generic function to "remove nil values from a
> slice." This should be able to remove nil instances of error, for example.
> So, let's say I have:
>
> var arg = []error{nil, errors.New("oh noes"), nil}
>
> Unfortunately, this doesn't work:
>
> func Compact[T comparable](s []T) T {
> ...
> if s[i] == nil {
>
> because "error" is not comparable.
>
Actually, error is comparable. See https://go.dev/play/p/ymBOc5JQyJT
The problem with your code is that "T comparable" guarantees that two
values of type T can be compared, but does not guarantee you can compare a
value of type T to nil. For example, int is a comparable type, but you
can't compare an int to nil. See https://go.dev/play/p/SETrraZ7vkR, which
produces the error message: ./prog.go:11:11: invalid operation: t != nil
(mismatched types T and untyped nil)
This also doesn't work:
>
> func Compact[T any](s []T) T {
> var zero T
> ...
> if s[i] == zero {
>
> because T is not a comparable constraint.
>
You can fix this by making T comparable instead of any. But then the
function will also remove 0 from int slices, "" from string slices, etc.
https://go.dev/play/p/NigMasu1aL1
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